penetrant testing

Penetrant Testing

Introduction and History ofPenetrant Inspection

Liquid penetration inspection is a method that is used to reveal surface breaking flaws by bleedout of a colored or fluorescent dye from the flaw. The technique is based on the ability of a liquid to be drawn into a "clean" surface breaking flaw by capillary action. After a period of time called the "dwell," excess surface penetrant is removed and a developer applied. This acts as a "blotter." It draws the penetrant from the flaw to reveal its presence. Colored (contrast) penetrants require good white light while fluorescent penetrants need to be used in darkened conditions with an ultraviolet "black light".

A very early surface inspection technique involved the rubbing of carbon black on glazed pottery, whereby the carbon black would settle in surface cracks rendering them visible. Later it became the practice in railway workshops to examine iron and steel components by the "oil and whiting" method. In this method, a heavy oil commonly available in railway workshops was diluted with kerosene in large tanks so that locomotive parts such as wheels could be submerged. After removal and careful cleaning, the surface was then coated with a fine suspension of chalk in alcohol so that a white surface layer was formed once the alcohol had evaporated. The object was then vibrated by being striked with a hammer, causing the residual oil in any surface cracks to seep out and stain the white coating. This method was in use from the latter part of the 19th century through to approximately 1940, when the magnetic particle method was introduced and found to be more sensitive for the ferromagnetic iron and steels.

A different (though related) method was introduced in the 1940's, where the surface under examination is coated with a lacquer, and after drying the surface is vibrated by hitting with a hammer, for example. This causes the brittle lacquer layer to crack generally around surface defects. The brittle lacquer (stress coat) has been used primarily to show the distribution of stresses in a part and not finding defects.

Many of these early developments were carried out by Magnaflux in Chicago, IL, USA in association with the Switzer Bros., Cleveland, OH, USA. More affective penetrating oils containing highly visible (usually red) dyes were developed by Magnaflux to enhance flaw detection capability. This method, known as the visible or color contrast dye penetrant method, is still used quite extensively today. In 1942, Magnaflux introduced the Zyglo system of penetrant inspection where fluorescent dyes were added to the liquid penetrant. These dyes would then fluoresce when exposed to ultraviolet light (sometimes referred to as "black light") rendering indications from cracks and other surface flaws more readily visible to the inspectors' eyes.

http://www.ndt-ed.org/EducationResources/CommunityCollege/PenetrantTest/Introduction/Keywords/pt4.htm




Why a Penetrant Inspection Improves the Detectability of Flaws

(LPI) offers over an unaided visual inspection is that it makes defects easier to see for the inspector. There are basically two ways that a penetrant inspection process makes flaws more easily seen. First, LPI produces a flaw indication that is much larger and easier for the eye to detect than the flaw itself. Many flaws are so small or narrow that they are undetectable by the unaided eye. Due to the physical features of the eye, there is a threshold below which objects cannot be resolved. This threshold of visual acuity is around 0.003 inch for a person with 20/20 vision.

The second way that LPI improves the detectability of a flaw is that it produces a flaw indication with a high level of contrast between the indication and the background which also helps to make the indication more easily seen. When a visible dye penetrant inspection is performed, the penetrant materials are formulated using a bright red dye that provides for a high level of contrast between the white developer that serves as a background as well as to pull the trapped penetrant from the flaw. When a fluorescent penetrant inspection is performed, the penetrant materials are formulated to glow brightly and to give off light at a wavelength that the eye is most sensitive to under dim lighting conditions.

Additional information on the human eye can be found by following the links below.

Visual Acuity of the Human Eye

The eye has a visual acuity threshold below which an object will go undetected. This threshold varies from person to person, but as an example, the case of a person with normal 20/20 vision can be considered. As light enters the eye through the pupil, it passes through the lens and is projected on the retina at the back of the eye. Muscles called extraocular muscles, move the eyeball in the orbits and allow the image to be focussed on the central retinal or fovea.

The retina is a mosaic of two basic types of photoreceptors, rods, and cones. Rods are sensitive to blue-green light with peak sensitivity at a wavelength of 498 nm, and are used for vision under dark or dim conditions. There are three types of cones that give us our basic color vision and they are L-cones (red) with a peak sensitivity of 564 nm, M-cones (green) with a peak sensitivity of 533 nm, and S-cones (blue) with a peak sensitivity of 437 nm.

Cones are highly concentrated in a region near the center of the retina called the fovea region. The maximum concentration of cones is roughly 180,000 per square mm in the fovea region and this density decreases rapidly outside of the fovea to a value of less than 5,000 per square mm. Note the blind spot caused by the optic nerve which is void photoreceptors.

The standard definition of normal visual acuity (20/20 vision) is the ability to resolve a spatial pattern separated by a visual angle of one minute of arc. Since one degree contains sixty minutes, a visual angle of one minute of arc is 1/60 of a degree. The spatial resolution limit is derived from the fact that one degree of a scene is projected across 288 micrometers of the retina by the eye's lens.

In this 288 micrometers dimension, there are 120 color sensing cone cells packed. Thus, if more than 120 alternating white and black lines are crowded side-by-side in a single degree of viewing space, they will appear as a single gray mass to the human eye. With a little trigonometry it is possible to calculate the resolution of the eye at a specific distance away from the lens of the eye.

For the case of normal visual acuity the angle Theta is 1/60 of a degree. By bisecting this angle we have a right triangle with angle Theta/2 that is 1/120 of a degree. Using this right triangle it is easy to calculate the distance X/2 for a given distance d.

X/2 = d (tan Theta/2)

When visually inspecting an object for a defect such as a crack the distance (d) might be around 12 inches. This would be a comfortable viewing distance. At 12 inches, the normal visual acuity of the human eye is 0.00349 inch. What this means is that if you had alternating black and white lines that were all 0.00349 inch wide, it would appear to most people as a mass of solid gray.

Contrast Sensitivity

When conducting a visible dye penetrant inspection, the contrast sensitivity of the eye is important. Contrast sensitivity is a measure of how faded or washed out an image can be before it becomes indistinguishable from a uniform field. It has been experimentally determined that the minimum discernible difference in gray scale level that the eye can detect is about 2% of full brightness. Contrast sensitivity is a function of the size or spatial frequency of the features in the image. However, this is not a direct relationship as larger objects are not always easier to see than smaller objects as contrast is reduced as demonstrated by the image below.

In the image below, the luminance of pixels is varied sinusoidally in the horizontal direction. The spatial frequency increases exponentially from left to right. The contrast also varies logarithmically from 100% at the bottom to about 0.5% at the top. The luminance of peaks and troughs remains constant along a given horizontal path through the image. If the detection of contrast was dictated solely by image contrast, the alternating bright and dark bars should appear to have equal height everywhere in the image. However, the bars seem to be taller in the middle of the image.

The Human Eye's Response to Light

The three curves in the figure above shows the normalized response of an average human eye to various amounts of ambient light. The shift in sensitivity occurs because two types of photoreceptors, cones and rods, are responsible for the eye's response to light. The curve on the right shows the eye's response under normal lighting conditions and this is called the photopic response. The cones respond to light under these conditions. As mentioned previously, cones are composed of three different photo pigments that enable color perception. This curve peaks at 555 nanometers, which means that under normal lighting conditions, the eye is most sensitive to a greenish yellow color. When the light levels drop to near total darkness, the response of the eye changes significantly as shown by the scotopic response curve on the left. At this level of light, the rods are most active and the human eye is more sensitive to any amount of light that is present, but is less sensitive to the range of color. Rods are highly sensitive to light but are comprised of a single photo pigment, which accounts for the loss in ability to discriminate color. At this very low light level, sensitivity to blue, violet, and ultraviolet is increased, but sensitivity to yellow and red is reduced. The heavier curve in the middle represents the eye's response at the ambient light level found in a typical inspection booth. This curve peaks at 550 nanometers, which means the eye is most sensitive to yellowish green color at this light level. Fluorescent penetrant inspection materials are designed to fluoresce at around 550 nanometers to produce optimal sensitivity under dim lighting conditions

Basic Processing Steps of a Liquid Penetrant Inspection

1. Surface Preparation: One of the most critical steps of a liquid penetrant inspection is the surface preparation. The surface must be free of oil, grease, water, or other contaminants that may prevent penetrant from entering flaws. The sample may also require etching if mechanical operations such as machining, sanding, or grit blasting have been performed. These and other mechanical operations can smear the surface of the sample, thus closing the defects.

2 Application: Once the surface has been thoroughly cleaned and dried, the penetrant material is applied by spraying, brushing, or immersing the parts in a penetrant bath.

3 Penetrant Dwell: The penetrant is left on the surface for a sufficient time to allow as much penetrant as possible to be drawn from or to seep into a defect. Penetrant dwell time is the total time that the penetrant is in contact with the part surface. Dwell times are usually recommended by the penetrant producers or required by the specification being followed. The times vary depending on the application, penetrant materials used, the material, the form of the material being inspected, and the type of defect being inspected. Minimum dwell times typically range from 5 to 60 minutes. Generally, there is no harm in using a longer penetrant dwell time as long as the penetrant is not allowed to dry.

4. Excess Penetrant Removal: This is a most delicate part of the inspection procedure because the excess penetrant must be removed from the surface of the sample while removing as little penetrant as possible from defects. Depending on the penetrant system used, this step may involve cleaning with a solvent, direct rinsing with water, or first treated with an emulsifier and then rinsing with water

5 Developer Application: A thin layer of developer is then applied to the sample to draw penetrant trapped in flaws back to the surface where it will be visible. Developers come in a variety of forms that may be applied by dusting (dry powdered), dipping, or spraying (wet developers).

6 Indication Development: The developer is allowed to stand on the part surface for a period of time sufficient to permit the extraction of the trapped penetrant out of any surface flaws. This development time is usually a minimum of 10 minutes and significantly longer times may be necessary for tight cracks.

7 Inspection: Inspection is then performed under appropriate lighting to detect indications from any flaws which may be present.

8. Clean Surface: The final step in the process is to thoroughly clean the part surface to remove the developer from the parts that were found to be acceptable.

Common Uses of Liquid Penetrant Inspection

Liquid penetrant inspection (LPI) is one of the most widely used nondestructive evaluation (NDE) methods. Its popularity can be attributed to two main factors, which are its relative ease of use and its flexibility. LPI can be used to inspect almost any material provided that its surface is not extremely rough or porous. Materials that are commonly inspected using LPI include the following:

Metals (aluminum, copper, steel, titanium, etc.) Glass

Many ceramic materials

Rubber

Plastics

LPI offers flexibility in performing inspections because it can be applied in a large variety of applications ranging from automotive spark plugs to critical aircraft components. Penetrant material can be applied with a spray can or a cotton swab to inspect for flaws known to occur in a specific area or it can be applied by dipping or spraying to quickly inspect large areas. At right, visible dye penetrant being locally applied to a highly loaded connecting point to check for fatigue cracking.

Penetrant inspection systems have been developed to inspect some very large components. In this picture, DC-10 banjo fittings are being moved into a penetrant inspection system at what used to be the Douglas Aircraft Company's Long Beach, California facility. These large machined aluminum forgings are used to support the number 3 engine in the tail of a DC-10 aircraft.

Liquid penetrant inspection is used to inspect of flaws that break the surface of the sample. Some of these flaws are listed below:

Fatigue cracks Quench cracks

Grinding cracks

Overload and impact fractures

Porosity

Laps

Seams

Pin holes in welds

Lack of fusion or braising along the edge of the bond line

As mentioned above, one of the major limitations of a penetrant inspection is that flaws must be open to the surface. To learn more about the advantages and disadvantages of LPI proceed to the next page.

Advantages and Disadvantages of Penetrant Testing

Like all nondestructive inspection methods, liquid penetrant inspection has both advantages and disadvantages. The primary advantages and disadvantages when compared to other NDE methods are summarized below.

Primary Advantages

The method has high sensitive to small surface discontinuities. The method has few material limitations, i.e. metallic and nonmetallic, magnetic

and nonmagnetic, and conductive and nonconductive materials may be inspected.

Large areas and large volumes of parts/materials can be inspected rapidly and at low cost.

Parts with complex geometric shapes are routinely inspected.

Indications are produced directly on the surface of the part and constitute a visual representation of the flaw.

Aerosol spray cans make penetrant materials very portable.

Penetrant materials and associated equipment are relatively inexpensive.

Primary Disadvantages

Only surface breaking defects can be detected. Only materials with a relative nonporous surface can be inspected.

Precleaning is critical as contaminants can mask defects.

Metal smearing from machining, grinding, and grit or vapor blasting must be removed prior to LPI.

The inspector must have direct access to the surface being inspected.

Surface finish and roughness can affect inspection sensitivity.

Multiple process operations must be performed and controlled.

Post cleaning of acceptable parts or materials is required.

Chemical handling and proper disposal is required.

Advantages and Disadvantages of Penetrant Testing

Like all nondestructive inspection methods, liquid penetrant inspection has both advantages and disadvantages. The primary advantages and disadvantages when compared to other NDE methods are summarized below.

Primary Advantages

The method has high sensitive to small surface discontinuities. The method has few material limitations, i.e. metallic and nonmetallic, magnetic

and nonmagnetic, and conductive and nonconductive materials may be inspected.

Large areas and large volumes of parts/materials can be inspected rapidly and at low cost.

Parts with complex geometric shapes are routinely inspected.

Indications are produced directly on the surface of the part and constitute a visual representation of the flaw.

Aerosol spray cans make penetrant materials very portable.

Penetrant materials and associated equipment are relatively inexpensive.

Primary Disadvantages

Only surface breaking defects can be detected. Only materials with a relative nonporous surface can be inspected.

Precleaning is critical as contaminants can mask defects.

Metal smearing from machining, grinding, and grit or vapor blasting must be removed prior to LPI.

The inspector must have direct access to the surface being inspected.

Surface finish and roughness can affect inspection sensitivity.

Multiple process operations must be performed and controlled.

Post cleaning of acceptable parts or materials is required.

Chemical handling and proper disposal is required.

Penetrant Testing Materials

The penetrant materials used today are much more sophisticated than the kerosene and whiting first used by railroad inspectors near the turn of the 20th century. Today's penetrants are carefully formulated to produce the level of sensitivity desired by the inspector. To perform well, a penetrant must possess a number of important characteristics. A penetrant must

spread easily over the surface of the material being inspected to provide complete and even coverage.

be drawn into surface breaking defects by capillary action.

remain in the defect but remove easily from the surface of the part.

remain fluid so it can be drawn back to the surface of the part through the drying and developing steps.

be highly visible or fluoresce brightly to produce easy to see indications.

must not be harmful to the material being tested or the inspector.

All penetrant materials do not perform the same and are not designed to perform the same. Penetrant manufactures have developed different formulations to address a variety of inspection applications. Some applications call for the detection of the smallest defects possible and have smooth surface where the penetrant is easy to remove. In other applications the rejectable defect size may be larger and a penetrant formulated to find larger flaws can be used. The penetrants that are used to detect the smallest defect will also produce the largest amount of irrelevant indications.

Penetrant materials are classified in the various industry and government specifications by their physical characteristics and their performance. Aerospace Material Specification (AMS) 2644, Inspection Material, Penetrant, is now the primary specification used in the USA to control penetrant materials. Historically, Military Standard 25135, Inspection Materials, Penetrants, has been the primary document for specifying penetrants but this document is slowly being phased out and replaced by AMS 2644. Other specifications such as ASTM 1417, Standard Practice for Liquid Penetrant Examinations, may also contain information on

the classification of penetrant materials but they are generally referred back to MIL-I-25135 or AMS 2644.

Penetrant materials come in two basic types. These types are listed below:

Type 1 - Fluorescent Penetrants Type 2 - Visible Penetrants

Fluorescent penetrants contain a dye or several dyes that fluoresce when exposed to ultraviolet radiation. Visible penetrants contain a red dye that provides high contrast against the white developer background. Fluorescent penetrant systems are more sensitive than visible penetrant systems because the eye is drawn to the glow of the fluorescing indication. However, visible penetrants do not require a darkened area and an ultraviolet light in order to make an inspection. Visible penetrants are also less vulnerable to contamination from things such as cleaning fluid that can significantly reduce the strength of a fluorescent indication.

Penetrants are then classified by the method used to remove the excess penetrant from the part. The four methods are listed below:

Method A - Water Washable Method B - Post Emulsifiable, Lipophilic

Method C - Solvent Removable

Method D - Post Emulsifiable, Hydrophilic

Water washable (Method A) penetrants can be removed from the part by rinsing with water alone. These penetrants contain some emulsifying agent (detergent) that makes it possible to wash the penetrant from the part surface with water alone. Water washable penetrants are sometimes referred to as self-emulsifying systems. Post emulsifiable penetrants come in two varieties, lipophilic and hydrophilic. In post emulsifiers, lipophilic systems (Method B), the penetrant is oil soluble and interacts with the oil-based emulsifier to make removal possible. Post emulsifiable, hydrophilic systems (Method D), use an emulsifier that is a water soluble detergent which lifts the excess penetrant from the surface of the part with a water wash. Solvent removable penetrants require the use of a solvent to remove the penetrant from the part.

Penetrants are then classified based on the strength or detectability of the indication that is produced for a number of very small and tight fatigue cracks. The five sensitivity levels are shown below:

Level ½ - Ultra Low Sensitivity Level 1 - Low Sensitivity

Level 2 - Medium Sensitivity

Level 3 - High Sensitivity

Level 4 - Ultra-High Sensitivity

The major US government and industry specifications currently rely on the US Air Force Materials Laboratory at Wright-Patterson Air Force Base to classify penetrants into one of the five sensitivity levels. This procedure uses titanium and Inconel specimens with small surface cracks produced in low cycle fatigue bending to classify penetrant systems. The brightness of the indication produced is measured using a photometer.

The sensitivity levels and the test procedure used can be found in Military Specification MIL-I-25135 and Aerospace Material Specification 2644, Penetrant Inspection Materials.

An interesting note about the sensitivity levels is that only four levels were originally planned but when some penetrants were judged to have sensitivities significantly less than most others in the level 1 category, the ½ level was created. An excellent historical summary of the development of test specimens for evaluating the performance of penetrant materials can be found in the following reference.

Penetrants

The industry and military specification that control the penetrant materials and their use all stipulate certain physical properties of the penetrant materials that must be met. Some of these requirements address the safe use of the materials, such as toxicity, flash point, and corrosiveness, and other requirements address storage and contamination issues. Still others delineate properties that are thought to be primarily responsible for the performance or sensitivity of the penetrants. The properties of penetrant materials that are controlled by AMS 2644 and MIL-I-25135E include flash point, surface wetting capability, viscosity, color, brightness, ultraviolet stability, thermal stability, water tolerance, and removability.

More information on how some of these properties can affect penetrant testing can be found by following these links.

Surface EnergyDensity or Specific GravityViscosityColor and Fluorescence BrightnessDimensional Threshold of FluorescenceUltraviolet and Thermal StabilityRemovability

http://www.ndt-ed.org/EducationResources/CommunityCollege/PenetrantTest/PTMaterials/removability.htm

http://www.ndt-ed.org/EducationResources/CommunityCollege/PenetrantTest/PTMaterials/stabilityofpenetrants.htm

http://www.ndt-ed.org/EducationResources/CommunityCollege/PenetrantTest/PTMaterials/dimensionalthreshold.htm

http://www.ndt-ed.org/EducationResources/CommunityCollege/PenetrantTest/PTMaterials/colorandfluorescents.htm

http://www.ndt-ed.org/EducationResources/CommunityCollege/PenetrantTest/PTMaterials/viscosity.htm

http://www.ndt-ed.org/EducationResources/CommunityCollege/PenetrantTest/PTMaterials/specificgravity.htm

http://www.ndt-ed.org/EducationResources/CommunityCollege/PenetrantTest/PTMaterials/surfaceenergy.htm

Surface Energy (Surface Wetting Capability)

As previously mentioned, one of the important characteristics of a liquid penetrant material is its ability to freely wet the surface of the object being inspected. At the liquid-solid surface interface, if the molecules of the liquid have a stronger attraction to the molecules of the solid surface than to each other (the adhesive forces are stronger than the cohesive forces), then wetting of the surface occurs. Alternately, if the liquid molecules are more strongly attracted to each other and not the molecules of the solid surface (the cohesive forces are stronger than the adhesive forces), then the liquid beads-up and does not wet the surface of the part.

One way to quantify a liquid's surface wetting characteristics is to measure the contact angle of a drop of liquid placed on the surface of the subject object. The contact angle is the angle formed by the solid/liquid interface and the liquid/vapor interface measured from the side of the liquid. See the figure below. Liquids wet surfaces when the contact angle is less than 90 degrees. For a penetrant material to be effective, the contact angle should be as small as possible. In fact, the contact angle for most liquid penetrants is very close to zero degrees.

Wetting ability of a liquid is a function of the surface energies of the solid-gas interface, the liquid-gas interface, and the solid-liquid interface. The surface energy across an interface or the surface tension at the interface is a measure of the energy required to form a unit area of new surface at the interface. The intermolecular bonds or cohesive forces between the molecules of a liquid cause surface tension. When the liquid encounters another substance, there is usually an attraction between the two materials. The adhesive forces between the liquid and the second substance will compete against the cohesive forces of the liquid. Liquids with weak cohesive bonds and a strong attraction to another material (or the desire to create adhesive bonds) will tend to spread over the second material. Liquids with strong cohesive bonds and weaker adhesive forces will tend to bead-up or form a droplet when in contact with the second material.

In liquid penetrant testing, there are usually three surface interfaces involved, the solid-gas interface, the liquid-gas interface, and the solid-liquid interface. For a liquid to spread over the surface of a part, two conditions must be met. First, the surface energy of the solid-gas interface must be greater than the combined surface energies of the liquid-gas and the solid-liquid interfaces. Second, the surface energy of the solid-gas interface must exceed the surface energy of the solid-liquid interface.

A penetrant's wetting characteristics are also largely responsible for its ability to fill a void. Penetrant materials are often pulled into surface breaking defects by capillary action. The capillary force driving the penetrant into the crack is a function of the surface tension of the liquid-gas interface, the contact angle, and the size of the defect opening. The driving force for the capillary action can be expressed as the following formula:

Force = 2 r LG cos

Where:

r = radius of the crack opening (2pr is the line of contact between the liquid and the solid tubular surface.)LG = liquid-gas surface tension = contact angle

Since pressure is the force over a given area, it can be written that the pressure developed, called the capillary pressure, is

Capillary Pressure = (2 LG cos)/ r

The above equations are for a cylindrical defect but the relationships of the variables are the same for a flaw with a noncircular cross section. Capillary pressure equations only apply when there is simultaneous contact of the penetrant along the entire length of the crack opening and a liquid front forms that is an equidistant from the surface. A liquid penetrant surface could take-on a complex shape as a consequence of the various deviations from flat parallel walls that an actual crack could have. In this case, the expression for pressure is

Capillary Pressure = 2( SG - s SL)/r = 2 /r

Where:

SG = the surface energy at the solid-gas interface. SL = the surface energy at the solid-liquid interface. r = the radius of the opening. = the adhesion tension (SG - SL).

Therefore, at times, it is the adhesion tension that is primarily responsible for a penetrant's movement into a flaw and not the surface energy of the liquid-gas interface. Adhesion tension is the force acting on a unit length of the wetting line from the direction of the solid. The wetting performance of the penetrant is degraded when adhesion tension is the primary driving force.

It can be seen from the equations in this section, that the surface wetting characteristics (defined by the surface energies) are important penetrant characteristics for filling the flaw. The liquid penetrant will continue to fill the void until an opposing force balances the capillary pressure. This force is usually the pressure of trapped gas in the void as most flaws are open only at the surface of the part. Since the gas originally in the flaw volume cannot escape through the layer of penetrant, the gas is compressed near the closed end of the flaw. Since the contact angle for penetrants is very close to zero, other methods have been devised to make relative comparisons of the wetting characteristics of these liquids. One method is to measure the height that a liquid reaches in a capillary tube. However, the solid interface in this method is usually glass and may not accurately represent the surface that the penetrant inspection will be performed on. Another method of comparative evaluation is to measure after a set time has elapsed, the radius, the diameter, or the area of a spot formed when a drop of penetrant is placed on the test surface. However, using this method, other factors are also acting in the comparison. These methods include the density, viscosity, and volatility of the liquid, which do not enter into the capillarity equations, but may have an effect on the inspection as discussed in the related pages.

Density or Specific Gravity

The density or the specific gravity of a penetrant material probably has a slight to negligible effect on the performance of a penetrant. The gravitational force acting on the penetrant liquid can be working in cooperation with or against the capillary force depending on the orientation of the flaw during the dwell cycle. When the gravitational pull is working against the capillary rise the strength of the force is given by the following equation:

Force = r2hpg

Where:

r = radius of the crack opening h = height of penetrant above its free surface p = density of the penetrant g = acceleration due to gravity

When the direction of capillary flow is in the same direction as the force of gravity, the added force driving the penetrant into the flaw is given by the formula shown below:

Force = hAp

Where:

h = the height of the penetrant column A = cross-sectional area of the opening P = density of the penetrant

Increasing the specific gravity by decreasing the volume percent of solvent in the solution will increase the penetration speed.

Viscosity

Viscosity has little effect on the ability of a penetrant material to enter a defect but it does have an effect on speed at which the penetrant fills a defect. The equation for a cylindrical void and an elliptical void are shown below:

Cylindrical VoidFill time = (2l2)/ rcos LG

Elliptical VoidFill time = [(2l2) / LGcos] * [a2+b2 / (a+b)ab]

Where: l = defect depth = viscosity

r = radius of the crack opening LG = liquid-gas surface tension

= contact anglea = flaw widthb = flaw length

From these equations, it can be seen that fill time is directly proportional to penetrant viscosity. While it has no real bearing on this discussion, it should be noted that the two equations do not take into account entrapped gas that could be present in a closed end capillary.

Color and Fluorescent Brightness

Penetrant Color and Fluorescence

The color of the penetrant material is of obvious importance in visible dye penetrant inspection, as the dye must provide good contrast against the developer or part being inspected. Remember from our earlier discussion of contrast sensitivity that generally the higher the contrast, the easier objects are to see. The dye used in visible dye penetrant is usually vibrant red but other colors can be purchased for special applications.

When fluorescent materials are involved, the effect of color and fluorescence is not so straightforward. LPI materials fluoresce because they contain one or more dyes that absorb electromagnetic radiation over a particular wavelength and the absorption of photons leads to changes in the electronic configuration of the molecules. Since the molecules are not stable at this higher energy state, they almost immediately re-emit the energy. There is some energy loss in the process causing the photons to be re-emitted at a slightly longer wavelength, which is in the visible range. The radiation absorption and emission could take place a number of times until the desired color and brightness is achieved. Two different fluorescent colors can be mixed to interact by a mechanism called cascading. The emission of visible light by this process involves one dye absorbing ultraviolet radiation to emit a band of radiation that makes a second dye glow. Since the human eye is the most commonly used sensing device, most penetrants are designed to fluoresce as close as possible to the eyes' peak response.

Penetrant Brightness

Fluorescent brightness was erroneously once thought to be the controlling factor with respect to flaw detection sensitivity. Measurements have been made to evaluate the intrinsic brightness of virtually all commercially available penetrants and they all have about the same brightness. Intrinsic brightness values are determined for thick liquid films and the dimensional threshold of fluorescence (discussed on the next page) is a more important property. The measurement of fluorescent brightness is detailed in ASTM E-1135, "Standard Test Method for Comparing the Brightness of Fluorescent Penetrants."

Why Things Fluoresce

Fluorescence is the process wherein a molecule absorbs a photon of radiant energy at a particular wavelength and then quickly re-emits the energy at a slightly longer wavelength. It is the rapid and short-term re-emittance of energy that distinguishes fluorescence from phosphorescence. Phosphorescence is usually the result of a chemical reaction which sustains the release of energy for a significant period of time. Fluorescence was first described in the sixteenth century and was probably observed long before that time since a large number of plant and animal products fluoresce.

The phenomenon of fluorescence requires a short lesson in quantum mechanics which explains why fluorescence was not understood until the twentieth century. In the nineteenth century, Huygen's wave theory of light had replaced Newton's concept of the particulate nature of light and fluorescence was one of the embarrassing phenomena which simply could not be explained by use of the wave theory. The wave theory, as with most classical physics, generally assumes change to be a continuous process with no abrupt changes. Near the beginning of the twentieth century, Max Planck suggested that energy changes might occur in a stepwise manner. This concept forms the basis of quantum mechanics and Einstein applied the quantum concept of energy to light and revived the idea of the particulate nature of light. Planck formalized the relationship with the equation shown below:

E= h/v

Where:

E = energy h = a constant v = the frequency of light

This equations shows that the size of the energy steps change with the frequency or wavelength of the light. Einstein introduced the term photon to describe the smallest increments of light.

In our current model of the atom, protons and neutrons are found in the nucleus and electrons are found spinning around outside the nucleus. Electrons spin and rotate around the nucleus billions of times a second. According to modern theory, electrons are arranged in energy levels as they rotate around the nucleus. When electrons gain or lose energy, they jump between energy levels as they are rotating around the nucleus. As electrons gain energy, they move to the third, or outer, level and as they lose energy, they move to the inner or first energy level. Since the energy of the system is restricted to certain energy values, the atom is

said to be quantized. In the animated image below, it can be seen that the electrons move to a different energy state only when a specific amount of energy is added to or removed from the system.

Another way of illustrating this point is with an energy diagram as presented below. This diagram shows the quantized energy levels for an atom. Each energy level corresponds to a quantum state of the atom. The lowest energy state is called the ground state and is the E0 line in the diagram. If energy is added to the system an electron or electrons will jump to a higher level and the atom is said to be at an excited state. The upward arrow in the illustration represents a quantum jump of the atom from the ground state to second excited state. Depending on the amount of energy input into the atom, the energy jump could have been to any of the levels. However, the jump must be to one of the levels shown as the atom cannot have an intermediate value of energy. Atoms will generally be in their ground state.

When considering fluorescence, energy must be considered at a molecular level. When molecules form, two or more atoms form an association where the energy of the molecule is lower than that of the constituent atoms when they were separate. The total energy of the molecule is the sum of the energies holding the nuclei together and the energy of the chemical bonds holding the molecule together. Molecules have rotational, vibrational and electronic (due to the electrons) energy . It is the vibrational and electronic energy of the molecule that contributes to fluorescence. Molecules, like atoms, will generally be in their ground state. Molecules can move to a greater energy state only when energy is added to their system. One of the ways a molecule can gain energy is by absorbing light. If a molecule absorbs light, the energy of the light must equal to the energy required to put the molecule in one of the higher energy states. When a molecule reaches an excited state, it does not stay there for very long but quickly returns to a lower energy state either by emitting light or by colliding with another atomic particle. When an molecule emits light, the energy of that light is equal to the energy difference between the quantum levels that molecules has moved between.

Fluorescent penetrant materials usually contain several dye compounds that are especially suited for the production of fluorescence.

Dimensional Threshold of Fluorescence

The dimensional threshold of fluorescence is a property that is not currently controlled by the specifications but appears to largely determine the sensitivity of a fluorescent penetrant. A.L. Walters and R. C. McMaster conducted an experiment that led to the understanding of this condition. Two optically flat plates of glass were clamped tightly together. A drop of fluorescent penetrant was placed at the interface of the plates. The penetrant could be seen migrating in between the plates but when exposed to black light, no fluorescence was seen. The phenomenon was not fully understood until 1960 when Alburger introduced the concept of thin-film transition of fluorescent response.

The dimensional magnitudes of typical crack defects correspond to the dimensional thresholds of fluorescence response which are characteristic of the available penetrant. Alternately stated, the degree of fluorescence response, under a given intensity of ultraviolet radiation, is dependent on the absorption of ultraviolet radiation, which in turn depends on dye concentration and film thickness. Therefore, the ability of a penetrant to yield an indication depends primarily on its ability to fluoresce as a very thin film. The performance of penetrants based on the physical constraints of the dyes can be predicted using Beer's Law equation. This equation does not hold true when very thin layers are involved but works well to establish general relationships between variables.

I = Io x e-Ct

Where:

I = Transmitted light intensityIo = Incident light intensitye = Base of natural log (2.71828) = Absorption coefficient per unit of concentrationC = Dye concentrationt = Thickness of the absorbing layer trolled to a certain degree by the concentration of the fluorescent tracer dye in the penetrant.

This equation states that the intensity of the transmitted energy is directly proportional to the intensity of the incident light and varies exponentially with the thickness of the penetrant layer and its dye concentration. Therefore, when the dye concentration is increased, the brightness of the thin layer of penetrant generally increases. However, the dye concentration can only be increased so much before it starts to have a negative effect of brightness. A Meniscus-Method Apparatus can be used to measure the dimensional threshold of fluorescence.

Ultraviolet and Thermal Stability of Penetrant Indications

Exposure to intense ultraviolet light and elevated temperatures can have a negative effect on fluorescent penetrant indications. Fluorescent materials can loose their brightness after a period of exposure to high intensity UV light. One study measured the intensity of fluorescent penetrant indications of a sample that was subjected to multiple UV exposure cycles. Each cycle consisted of 15 minutes of 800 microwatts/cm² UV light and 2.5 minutes of 1500 microwatts/cm² UV light. Two penetrants were tested in the study, water washable, level 3 and a post emulsifiable, level 4. The results from the study showed that the indications from both penetrants faded with increased UV exposure. After eight exposure cycles the brightness of the indications was less than one half their original values.

At an elevated temperature, penetrants can experience heat degradation or "heat fade." Excessive heat

1. evaporates the more volatile constituents which increases viscosity and adversely affects the rate of penetration.2. alters wash characteristics.3. "boils off" chemicals that prevent separation and gelling of water soluble penetrants.4. kills the fluorescence of tracer dyes.

This fourth degradation mechanism involves the molecules of the penetrant materials. The phenomenon of fluorescence involves electrons that are delocalized in a molecule. These electrons are not specifically associated with a given bond between two atoms. When a molecule takes up sufficient energy for the excitation source, the delocalized bonding electrons rise to a higher electronic state. After excitation, the electrons will normally lose energy and return to the lowest energy state. This loss of energy can involve a "radiative" process such as fluorescence or "non-radiative" processes. Non-radiative processes include relaxation by molecular collisions, thermal relaxation, and chemical reaction. Heat causes the number of molecular collisions to increase which results in more collision relaxation and less fluorescence.

This explanation is only valid when the part and the penetrant are at an elevated temperature. When the materials cool, the fluorescence will return. However, while exposed to elevated temperatures, penetrant solutions dry faster. As the molecules become more closely packed in the dehydrated solution, collision relaxation increases and fluorescence decreases. This effect has been called "concentration quenching" and experimental data shows that as the dye concentration is increased, fluorescent brightness initially increases but reaches a peak and then begins to decrease. Airflow over the surface on the part will also speed evaporation of the liquid carrier, so it should be kept to a minimum to prevent a loss of brightness. Generally thermal damage occurs when fluorescent penetrant materials are heated above 71C (160F). It should be noted that the sensitivity of an FPI inspection can be improved if a part is heated prior to applying the penetrant material that is used, but the temperature should be kept below 71C (160F). Some high temperature penetrants in use today are formulated with dyes with high melting points, which improves resistance to heat damage. The penetrants also have high

boiling points and the heat related problems are greatly reduced. However, a loss of brightness can still take place when the penetrant is exposed to elevated temperatures over an extended period of time. When one heat resistant formulation was tested, a 20 % reduction was measured after the material was subjected to 163C (325F) for 273 hours. The various types of fluorescent dyes commonly employed in today's penetrant materials begin decomposition at 71C (160F), and when the temperature approaches 94C (200F) there is almost total attenuation of fluorescent brightness of the total composition and sublimation of the fluorescent dyestuffs.

Removability

Removing the penetrant from the surface of the sample without removing it from the flaw is one of the most critical operations of the penetrant inspection process. The penetrant must be removed from the sample surface as completely as possible to limit background fluorescence. In order for this to happen, the adhesive forces of the penetrant must not be so strong that they can not be broken by the removal methods used. However, in order for the a penetrant to have good surface wetting characteristics the adhesive forces, which are the forces of attraction between the penetrant and the solid surface being inspected, must be stronger than the cohesive forces, which are the forces holding the liquid together. Proper formulation of the penetrant materials provides the correct balancing of these forces.

Another consideration in the formulation of the penetrant liquid is that it should not easily commingle and become diluted by the cleaning solution. Dilution of the penetrant liquid will affect the concentration of the dye and reduce the dimensional threshold of fluorescence

Emulsifiers

When removal of the penetrant from the defect due to over-washing of the part is a concern, a post emulsifiable penetrant system can be used. Post emulsifiable penetrants require a separate emulsifier to break the penetrant down and make it water washable. Most penetrant inspection specifications classify penetrant systems into four methods of excess penetrant removal. These are listed below:

1. Method A: Water-Washable 2. Method B: Post Emulsifiable, Lipophilic

3. Method C: Solvent Removable

4. Method D: Post Emulsifiable, Hydrophilic

Method C relies on a solvent cleaner to remove the penetrant from the part being inspected. Method A has emulsifiers built into the penetrant liquid that makes it possible to remove the excess penetrant with a simple water wash. Method B and D penetrants require an additional processing step where a separate emulsification agent is applied to make the excess penetrant more removable with a water wash. Lipophilic emulsification systems are oil-based materials that are supplied in ready-to-use form. Hydrophilic systems are water-based and supplied as a concentrate that must be diluted with water prior to use . Lipophilic emulsifiers (Method B) were introduced in the late 1950's and work with both a chemical and mechanical action. After the emulsifier has coated the surface of the object, mechanical action starts to remove some of the excess penetrant as the mixture drains from the part. During the emulsification time, the emulsifier diffuses into the remaining penetrant and the resulting mixture is easily removed with a water spray.

Hydrophilic emulsifiers (Method D) also remove the excess penetrant with mechanical and chemical action but the action is different because no diffusion takes place. Hydrophilic emulsifiers are basically detergents that contain solvents and surfactants. The hydrophilic emulsifier breaks up the penetrant into small quantities and prevents these pieces from recombining or reattaching to the surface of the part. The mechanical action of the rinse water removes the displaced penetrant from the part and causes fresh remover to contact and lift newly exposed penetrant from the surface.

The hydrophilic post emulsifiable method (Method D) was introduced in the mid 1970's and since it is more sensitive than the lipophilic post emulsifiable method it has made the later method virtually obsolete. The major advantage of hydrophilic emulsifiers is that they are less sensitive to variation in the contact and removal time. While emulsification time should be controlled as closely as possible, a variation of one minute or more in the contact time will have little effect on flaw detectability when a hydrophilic emulsifier is

used. However, a variation of as little as 15 to 30 seconds can have a significant effect when a lipophilic system is used.

Developers

The role of the developer is to pull the trapped penetrant material out of defects and to spread the developer out on the surface of the part so it can be seen by an inspector. The fine developer particles both reflect and refract the incident ultraviolet light, allowing more of it to interact with the penetrant, causing more efficient fluorescence. The developer also allows more light to be emitted through the same mechanism. This is why indications are brighter than the penetrant itself under UV light. Another function that some developers performs is to create a white background so there is a greater degree of contrast between the indication and the surrounding background.

Developer Forms

The AMS 2644 and Mil-I-25135 classify developers into six standard forms. These forms are listed below:

1. Form a - Dry Powder 2. Form b - Water Soluble

3. Form c - Water Suspendible

4. Form d - Nonaqueous Type 1 Fluorescent (Solvent Based)

5. Form e - Nonaqueous Type 2 Visible Dye (Solvent Based)

6. Form f - Special Applications

The developer classifications are based on the method that the developer is applied. The developer can be applied as a dry powder, or dissolved or suspended in a liquid carrier. Each of the developer forms has advantages and disadvantages.

Dry Powder

Dry powder developer is generally considered to be the least sensitive but it is inexpensive to use and easy to apply. Dry developers are white, fluffy powders that can be applied to a thoroughly dry surface in a number of ways. The developer can be applied by dipping parts in a container of developer, or by using a puffer to dust parts with the developer. Parts can also be placed in a dust cabinet where the developer is blown around

and allowed to settle on the part. Electrostatic powder spray guns are also available to apply the developer. The goal is to allow the developer to come in contact with the whole inspection area.

Unless the part is electrostatically charged, the powder will only adhere to areas where trapped penetrant has wet the surface of the part. The penetrant will try to wet the surface of the penetrant particle and fill the voids between the particles, which brings more penetrant to the surface of the part where it can be seen. Since dry powder developers only stick to the part where penetrant is present, the dry developer does not provide a uniform white background as the other forms of developers do. Having a uniform light background is very important for a visible inspection to be effective and since dry developers do not provide one, they are seldom used for visible inspections. When a dry developer is used, indications tend to stay bright and sharp since the penetrant has a limited amount of room to spread.

Water Soluble

As the name implies, water soluble developers consist of a group of chemicals that are dissolved in water and form a developer layer when the water is evaporated away. The best method for applying water soluble developers is by spraying it on the part. The part can be wet or dry. Dipping, pouring, or brushing the solution on to the surface is sometimes used but these methods are less desirable. Aqueous developers contain wetting agents that cause the solution to function much like dilute hydrophilic emulsifier and can lead to additional removal of entrapped penetrant. Drying is achieved by placing the wet but well drained part in a recalculating warm air dryer with the temperature held between 70 and 75°F. If the parts are not dried quickly, the indications will will be blurred and indistinct. Properly developed parts will have an even, pale white coating over the entire surface.

Water Suspendible

Water suspendible developers consist of insoluble developer particles suspended in water. Water suspendible developers require frequent stirring or agitation to keep the particles from settling out of suspension. Water suspendible developers are applied to parts in the same manner as water soluble developers. Parts coated with a water suspendible developer must be forced dried just as parts coated with a water soluble developer are forced dried. The surface of a part coated with a water suspendible developer will have a slightly translucent white coating.

Nonaqueous

Nonaqueous developers suspend the developer in a volatile solvent and are typically applied with a

spray gun. Nonaqueous developers are commonly distributed in aerosol spray cans for portability. The solvent tends to pull penetrant from the indications by solvent action. Since the solvent is highly volatile, forced drying is not required. A nonaqueous developer should be applied to a thoroughly dried part to form a slightly translucent white coating.

Special Applications

Plastic or lacquer developers are a special developers that are primarily used when a permanent record of the inspection is requi

Preparation of Part

One of the most critical steps in the penetrant inspection process is preparing the part for inspection. All coatings, such as paints, varnishes, plating, and heavy oxides must be removed to ensure that defects are open the surface of the part. If the parts have been machined, sanded, or blasted prior to the penetrant inspection, it is possible that a thin layer of metal may have smeared across the surface and closed off defects. It is even possible for metal smearing to occur as a result of cleaning operations such as grit or vapor blasting. This layer of metal smearing must be removed before inspection.

Contaminants

Coatings, such as paint, are much more elastic than metal and will not fracture even though a large defect may be present just below the coating. The part must be thoroughly cleaned as surface contaminates can prevent the penetrant from entering a defect. Surface contaminants can also lead to a higher level of background noise since the excess penetrant may be more difficult to remove.

Common coatings and contaminates that must be removed include: paint, dirt, flux, scale, varnish, oil, etchant, smut, plating, grease, oxide, wax, decals, machining fluid, rust, and residue from previous penetrant inspections.

Some of these contaminants would obviously prevent penetrant from entering defects and it is, therefore, clear that they must be removed. However, the impact of other contaminants such as the residue from previous penetrant inspections is less clear, but they can have a disastrous affect on the inspection. Take the link below to review some of the research that has been done to evaluate the effects of contaminants on LPI sensitivity.

A good cleaning procedure will remove all contamination from the part and not leave any residue that may interfere with the inspection process. It has been found that some alkaline cleaners can be detrimental to the penetrant inspection process if they have silicates in concentrations above 0.5 percent. Sodium metasilicate, sodium silicate, and related compounds can adhere to the surface of parts and form a coating that prevents penetrant entry into cracks. Researchers in Russia have also found that some domestic soaps and commercial detergents can clog flaw cavities and reduce the wettability of the metal surface, thus, reducing the sensitivity of the penetrant. Conrad and Caudill found that media from plastic media blasting was partially responsible for loss of LPI indication strength. Microphotographs of cracks after plastic media blasting showed media entrapment in addition to metal smearing.

It is very important that the material being inspected has not been smeared across its own surface during machining or cleaning operations. It is well recognized that machining, honing, lapping, hand sanding, hand scraping, shot peening, grit blasting, tumble deburring, and peening operations can cause a small amount of the material to smear on the surface of some materials. It is perhaps less recognized that some cleaning operations, such as steam cleaning, can also cause metal smearing in the softer materials. Take the link below to learn more about metal smearing and its affects on LPI

Material Smear and Its Removal

Material smearing can have a very detrimental effect on a LPI inspection as defects that are normally open to the surface can partially or completely be covered over. Some of the processes that can cause material to smear include machining, honing, lapping, sanding, scraping, shot peening, grit blasting, tumble deburring, and peening operations. When high pressure is used some cleaning operations, such as vapor and steam cleaning, can also cause material to smear in the softer materials. Softer materials such as plastics and aluminum alloys are most prone to smearing but many other materials, such as steel, titanium and Inconel alloys, have also been shown to smear. To evaluate the effect of a process on liquid penetrant inspection, cracked specimens are typically inspected before and after performing the potentially smearing operation and a comparison between the inspection results are made. It must be noted that under carefully controlled conditions, material smear can be avoided. Whenever parts have been mechanically processed prior to LPI, an evaluation should be performed to determine if flaw detectability has been compromised. If material smearing is a problem, an etching process can be used to remove the smeared material prior to inspection. The curves below illustrate the effect that metal smearing can have on the probability of detection for a defect and how etching the sample surface improves detectability.

The curve on the left shows the probability of detecting a crack versus crack length for as-machined aluminum specimens. The curve on the right shows the POD for the same aluminum specimens after their surfaces had been etched. Comparing the crack lengths

where the curves reach a POD level of 90%, it can be seen that in the as-machined condition, the crack length would need to be 0.4 inches long. However, when the surface is etched, cracks under 0.10 inch can be detected with a 90 % probability.

Removal of Material Smearing

Etching of the specimens can return the flaw to the pre-mechanical processing level of detectability. The amount of material that must be removed by the etching process depends on the amount of material that has been smeared and should be determined experimentally. Volume two of the Nondestructive Testing Handbook provides a great deal of information on material smearing and the amount of etching required to remover the damage. The handbook includes a number of photographs such as the set below that graphically show the effects of metal smearing.

Left Image: Original fluorescent penetrant inspection pattern in a quench cracked aluminum sample.

Center Image: Fluorescent penetrant inspection pattern after sanding with 240 grit paper.

Right Image: Fluorescent penetrant inspection pattern after etching to remove 0.0003 inch.

When an etchant is used, it must be properly removed from the part before applying penetrant. Experts in the penetrant field warn that acid and caustic entrapment from a prepenetrant etch can have disastrous effects on the penetrant inspection. Careful cleaning of both acid and caustic etches before penetrant inspection is highly recommended. There are several other risks to the parts being processed when an etchant is used. First, since the etching process is removing metal from the surface of the part, the minimum dimensional tolerances of the part must be considered. A second possible risk is that the etching process could have an effect on the material properties of the part. The chemical etchant used should uniformly remove material from the surface and should not etch microstructural features (such as grain boundaries) preferentially. Ideally, a study should be conducted to evaluate the effects of the etching process (or other chemical process) on the mechanical properties and performance of the component

Selection of a Penetrant Technique

The selection of a liquid penetrant system is not a straightforward task. There are a variety of penetrant systems and developer types that are available for use, and one set of penetrant materials will not work for all applications. Many factors must be considered when selecting the penetrant materials for a particular application. These factors include the sensitivity required, materials cost, number of parts and size of area requiring inspection, and portability.

When sensitivity is the primary consideration for choosing a penetrant system, the first decision that must be made is whether to use fluorescent dye penetrant, or visible dye penetrant. Fluorescent penetrants are generally more capable of producing a detectable indication from a small defect because the human eye is more sensitive to a light indication on a dark background and the eye is naturally drawn to a fluorescent indication. The graph below presents a series of curves that show the contrast ratio required for a spot of a certain diameter to be seen. The curves show that for indications spots larger than 0.076 mm (0.003 inch) in diameter, it does not really matter if it is a dark spot on a light background or a dark spot on a light background. However, when a dark indication on a light background is further reduced in size, it is no longer detectable even though contrast is increased. Furthermore, with a light indication on a dark background, indications down to 0.003 mm (0.0001 inch) were detectable when the contrast between the flaw and the background was high enough. From this data, it can be seen why a fluorescent penetrant offers an advantage over visible penetrant for finding very small defects. Data presented by De Graaf and De Rijk supports this statement. They inspected "Identical" fatigue cracked specimens using a red dye penetrant and a fluorescent dye penetrant. The fluorescent penetrant found 60 defects while the visible dye was only able to find 39 of the defects.

Penetrant Application and Dwell Time

The penetrant material can be applied in a number of different ways which include spraying, brushing, or immersing the parts in a penetrant bath. The method of penetrant application has little effect on the inspection sensitivity but an electrostatic spraying method is reported to produce slightly better results than other methods. Once the part is covered in penetrant it must be allowed to dwell so the penetrant has time to enter any defect present.

There are basically two dwell mode options, immersion-dwell (keeping the part immersed in the penetrant during the dwell period) or drain-dwell (letting the part drain during the dwell period). Prior to a study by Sherwin, the immersion-dwell mode was generally considered to be more sensitive but recognized to be less economical because more penetrant was washed away and emulsifiers were contaminated more rapidly. The reasoning for thinking this method was more sensitive was that the penetrant was more migratory and more likely to fill flaws when kept completely fluid and not allowed to loose volatile constituents by evaporation. However, Sherwin showed that if the specimens are allowed to drain-dwell, the sensitivity is higher because the evaporation increases the dyestuff concentration of the penetrant on the specimen. As pointed-out in section 3.1.5, sensitivity increases as the dyestuff concentration increases. Sherwin also cautions that the samples being inspected should be placed outside the penetrant tank wall so that vapors from the tank do not accumulate and dilute the dyestuff concentration of the penetrant on the specimen.

-- Vaerman, J., Fluorescent Penetrant Inspection, Quantified Evolution of the Sensitivity Versus Process Deviations, Proceedings of the 4th European Conference on Nondestructive Testing, Pergamon Press, Maxwell House, Fairview Park, Elmsford, New York, Volume 4, September 1987, pp. 2814-2823.

-- Sherwin, A.G., Establishing Liquid Penetrant Dwell Modes, Materials Evaluation, Vol. 32, No. 3, March 1974, pp. 63-67.

Penetrant Dwell Time

Penetrant dwell time is the total time that the penetrant is in contact with the part surface. The dwell time is important because it allows the penetrant the time necessary to be drawn or to seep into a defect. Dwell times are usually recommended by the penetrant producers or required by the specification being followed. The time required to fill a flaw depends on a number of variables which include the following:

The surface tension of the penetrant. The contact angle of the penetrant.

The dynamic shear viscosity of the penetrant, which can vary with the diameter of the capillary. The viscosity of a penetrant in microcapillary flaws is higher than its viscosity in bulk, which slows the infiltration of the tight flaws.

The atmospheric pressure at the flaw opening.

The capillary pressure at the flaw opening.

The pressure of the gas trapped in the flaw by the penetrant.

The radius of the flaw or the distance between the flaw walls.

The density or specific gravity of the penetrant.

Microstructural properties of the penetrant.

The ideal dwell time is often determined by experimentation and is often very specific to a particular application. For example, AMS 2647A requires that the dwell time for all aircraft and engine be at least 20 minutes while the ASTM E1209 only requires a 5 minute dwell time for parts made of titanium and other heat resistant alloys. Generally, there is no harm in using a longer penetrant dwell time as long as the penetrant is not allowed to dry.

The following tables summarize the dwell time requirements of several commonly used specifications. The information provided below is intended for general reference and no guarantee is made about its correctness or currentness. Please consult the specifications for the actual dwell time requirements.

Some Research Results on Dwell Time

An interesting point that Deutsch makes about dwell time is that if the elliptical flaw has a length to width ratio of 100 it will take the penetrant nearly ten times longer to fill than a cylindrical flaw with the same volume.

-- Deutsch, S. A, Preliminary Study of the Fluid Mechanics of Liquid Penetrant Testing, Journal of Research of the National Bureau of Standards, Vol. 84, No. 4, July-August 1979, pp. 287-291.

Lord and Holloway looked for the optimum penetrant dwell time required for detecting several types of defects in titanium. Both a level 2 post-emulsifiable fluorescent penetrant (Magnaflux ZL-2A penetrant and ZE-3 emulsifier) and a level 2 water washable penetrant (Tracer-Tech P-133A penetrant) were included in the study. The effect of the developer was a variable in the study and nonaqueous wet, aqueous wet, and dry developers were included. Specimens were also processed using no developer. The specimen defects included stress corrosion cracks, fatigue cracks and porosity. As expected, the researchers found that the optimal dwell time varied with the type of defect and developer used. The following table summarizes some of the findings.

Penetrant Removal Process

The penetrant removal procedure must effectively remove the penetrant from the surface of the part without removing an appreciable amount of entrapped penetrant from the defect. If the removal process extracts penetrant from the flaw, the flaw indication will be reduced by a proportional amount. If the penetrant is not effectively removed from the part surface, the contrast between the indication and the background will be reduced. As discussed in Contrast Sensitivity Section, as the contrast increases so does visibility of the indication.

Removal Method

Penetrant systems are classified into four methods of excess penetrant removal. These include the following:

1. Method A: Water-Washable 2. Method B: Post Emulsifiable, Lipophilic

3. Method C: Solvent Removable

4. Method D: Post Emulsifiable, Hydrophilic

Method C, Solvent Removable, is used primarily for inspecting small localized areas as this method requires hand wiping the surface with a cloth moistened with the solvent remover, and this process is too labor intensive for most production situations. Of the three production penetrant inspection methods, Method A, Water-Washable, is the most economical to apply. Water-washable or self-emulsifiable penetrants contain an emulsifier as an integral part of the formulation. The excess penetrant may be removed from the object surface, with a simple water rinse. These materials have the property of forming relatively viscous gels upon contact with water, which results in the formation of gel-like plugs in surface openings. While they are completely soluble in water, given enough contact time, the plugs offer a brief period of protection against rapid wash removal. Thus, water-washable penetrant systems provide ease of use and a high level of sensitivity.

When removal of the penetrant from the defect due to over-washing of the part is a concern, a post-emulsifiable penetrant system can be used. Post-emulsifiable penetrants require a separate emulsifier to break the penetrant down and make it water washable. The emulsifier is usually applied by dipping the object. Hydrophilic emulsifiers may also be sprayed on the object but spraying is not recommended for lipophilic emulsifiers because it can result in non-uniform emulsification if not properly applied. Brushing the emulsifier on to the part is not recommended because the bristles of the brush may force emulsifier into discontinuities causing the entrapped penetrant to be removed. The emulsifier is allowed sufficient time to react with the penetrant on the surface of the part

http://www.ndt-ed.org/EducationResources/CommunityCollege/PenetrantTest/Introduction/contrastsensitivity.htm

but not given time to make its way into defects to react with the trapped penetrant. The penetrant that has reacted with the emulsifier is easily cleaned away. Controlling the reaction time is of essential importance when using a post-emulsifiable system. If the emulsification time is too short, an excessive amount of penetrant will be left on the surface leading to high background levels. If the emulsification time is too long, the emulsifier will react with the penetrant entrapped in discontinuities making it possible to deplete the amount needed to form an indication.

The hydrophilic post emulsifiable method (Method D) is more sensitive than the lipophilic post emulsifiable method (Method B). Since these methods are generally only used when very high sensitivity is needed, Method D is almost always used making Method B virtually obsolete. The major advantage of hydrophilic emulsifiers is that they are less sensitive to variation in the contact and removal time. While emulsification time should be controlled as closely as possible, a variation of one minute or more in the contact time will have little effect on flaw detectability when a hydrophilic emulsifier is used, but a variation of as little as 15 to 30 seconds can have a significant effect when a lipophilic system is used. Using an emulsifier involves adding a couple of steps to the penetrant process and ,therefore, slightly increases the cost of an inspection. When using an emulsifier, the penetrant process includes the following steps (extra steps in bold): 1. pre-clean part, 2. apply penetrant and allow to dwell, 3. pre-rinse to remove first layer of penetrant, 4. apply hydrophilic emulsifier,and allow contact for specified time, 5. rinse to remove excess penetrant, 6. dry part, 7. apply developer and allow part to develop, and 8. inspect.

Rinse Method and Time for Water-Washable Penetrants

The method used to rinsing the excess penetrant from the object surface and the time of the rinse should be controlled so as to prevent over-washing. It is generally recommended that a coarse spray rinse or an air agitated, immersion wash tank be used. When a spray is being used, it should be directed at a 45° angle to the part surface so as to not force water directly into any discontinuities that may be present. The spray or immersion time should be kept to a minimum through frequent inspections of the remaining background level. When using

Hand Wiping of Solvent Removable Penetrants

When a solvent removable penetrant is used, care must also be taken to carefully remove the penetrant from the part surface while removing as little as possible from the flaw. The first step in this cleaning procedure is to dry wipe the surface of the part in one direction using a white lint free cotton rag. One dry pass in one direction is all that should be used to remove as much penetrant as possible. Next, the surface should be wiped with one pass in one direction with a cleaner moistened rag. One dry pass followed by one damp pass is all that is recommended. Additional wiping my sometimes be necessary but keep in mind that with every additional wipe, some of the entrapped penetrant will be removed and inspection sensitivity will be reduced.

To study the effects of the wiping process, Japanese researchers manufactured a test specimen out of acrylic plates that allowed them to view the movement of the penetrant in a narrow cavity. The sample consisted of two pieces of acrylic with two thin sheets of

vinyl clamped between as spaces. The plates were clamped in the corners and all but one of the edges sealed. The unsealed edge acted as the flaw. The clearance between the plates varied from 15 microns (0.059 inch) at the clamping points to 30 microns (0.118 inch) at the midpoint between the clamps. The distance between the clamping points is believed to be 30 mm (1.18 inch).

Although the size of the flaw represented by this specimen is large, an interesting observation was made. They found that when the surface of the specimen was wiped with a dry cloth, penetrant was blotted and removed from the flaw at the corner areas were the clearance between the plate was least. When the penetrant at the side areas was removed, penetrant moved horizontally from the center area to the ends of the simulated crack where capillary forces are stronger. Therefore, across the crack length, the penetrant surface has a parabola-like shape where the liquid is at the surface in the corners but depressed in the center. This shows that each time the cleaning cloth touches the edge of a crack, penetrant is lost from the defect. This also explains why the bleedout of an indication is often largest at the corners of cracks.

Use and Selection of a Developer

The use of developer is almost always recommended. One study reported that the output from a fluorescent penetrant could be multiplied by up to seven times when a suitable powder developer was used. Another study showed that the use of developer can have a dramatic effect on the probability of detection (POD) of an inspection. When a Haynes Alloy 188, flat panel specimen with a low-cycle fatigue crack was inspected without a developer, a 90 % POD was never reached with crack lengths as long as 19 mm (0.75 inch). The operator detected only 86 of 284 cracks and had 70 false-calls. When a developer was used, a 90 % POD was reached at 2 mm (0.077 inch), with the inspector identifying 277 of 311 cracks with no false-calls. However in some Some authors have reported that in special situations the use of a developer may actually reduce sensitivity. These situations primarily occur when large, well defined defects are being inspected on a surface that contains many nonrelevant indications that cause excessive bleedout.

Type of Developer Used and Method of Application

Nonaqueous developers are generally recognized as the most sensitive when properly applied. There is less agreement on the performance of dry and aqueous wet developers but the aqueous developers are usually considered more sensitive. Aqueous wet developers form a finer matrix of particles that is more in contact with the part surface. However, if the thickness of the coating becomes too great, defects can be masked. Also aqueous wet developers can cause leaching and blurring of indications when used with water washable penetrants. The relative sensitivities of developers and application techniques as ranked in Volume II of the Nondestructive Testing Handbook are shown in the table below. There is general industry agreement with this table, but some industry experts feel that water suspendible developers are more sensitive than water-soluble developers.

Sensitivity ranking of developers per the Nondestructive Testing Handbook. Sensitivity Ranking (highest to lowest) Developer Form Application Technique.

Ranking12345678910

Developer FormNonaqueous Wet Solvent

Plastic Filmwater-soluble

Water Suspendiblewater-soluble

Water SuspendibleDryDryDryDry

Method of ApplicationSpraySpraySpraySpray

ImmersionImmersion

Dust Cloud (Electrostatic)Fluidized Bed

Dust Cloud (Air Agitation)Immersion (Dip)

The following TABLE lists the main advantages and disadvantages of the various developer types.

Developer Advantages Disadvantages

Dry

Indications tend to remain brighter and more distinct over time

Easily to apply

Does not form contrast background so cannot be used with visible systems

Difficult to assure entire part surface has been coated

Soluble

Ease of coating entire part

White coating for good contrast can be produced which work well for both visible and fluorescent systems

Coating is translucent and provides poor contrast (not recommended for visual systems)

Indications for water washable systems are dim and blurred

Suspendible

Ease of coating entire part

Indications are bright and sharp


Indications weaken and become diffused after time

Nonaqueous

Very portable

Easy to apply to readily accessible surfaces


Indications show-up rapidly and are well defined

Provides highest sensitivity

Difficult to apply evenly to all surfaces

More difficult to clean part after inspection

To review a summary of some of the research that has been done on developer usage and performance, take this link.

Process Control of Temperature

The temperature of the penetrant materials and the part being inspected can have an effect on the results. Temperatures from 27 to 49oC (80 to 120oF) are reported in the literature to produce optimal results. Many specification allow testing in the range of 4 to 52oC (40 to 125oF). A tip to remember is that surfaces that can be touched for an extended period of time without burning the skin are generally below 52oC (125oF).

Since the surface tension of most materials decrease as the temperature increases, raising the temperature of the penetrant will increase the wetting of the surface and the capillary forces. Of course, the converse is also true and lowing the temperature will have a negative effect on the flow characteristics. Raising the temperature will also raise the speed of evaporation of penetrants, which can have a positive or negative effect on sensitivity. The impact will be positive if the evaporation serves to increase the dye concentration of the penetrant trapped in a flaw up to the concentration quenching point and not beyond. Higher temperatures and more rapid evaporation will have a negative effect if the dye concentration is caused to exceed the concentration quenching point or the flow characteristics are changed to the point where the penetrant does not readily flow.

The method of processing a hot part was once commonly employed. Parts were either heated or processed hot off the production line. In its day, this served to increase inspection sensitivity by increasing the viscosity of the penetrant. However, the penetrant materials used today have 1/2 to 1/3 the viscosity of the penetrants on the market in the 1960's and 1970's. Heating the part prior to inspection is no longer necessary and no longer recommended.

Quality Control of Penetrant

The quality of a penetrant inspection is highly dependent on the quality of the penetrant materials used. Only products meeting the requirements of an industry specification, such as AMS 2644, should be used. The performance of a penetrant can be affected by contamination and aging. Contamination by another liquid will change the surface tension and contact angle of the solution, and virtually all organic dyes deteriorate over time resulting in a loss of color or fluorescent response. Therefore, regular checks must be performed to insure that the material performance has not degraded.

When the penetrant is first received from the manufacturer, a sample of the fresh solution should be collected and stored as a standard for future comparison. The standard specimen should be stored in an opaque glass or metal, sealed container. Penetrants that are in-use should be compared regularly against the standard specimen to detect changes in color, odor and consistency. When using fluorescent penetrants, a brightness comparison per the requirements of ASTM E 1417 is also often required. This check involves placing a drop of the standard and the in-use penetrants on a piece of Whatman #4 filter paper and making a side by side comparison of the brightness of the two spots under UV light.

Additionally, the water content of water washable penetrants must be checked regularly. When water contaminates oil-based penetrants, the surface tension and contact angle of the mixture will increase since water has a higher surface tension than most oil-based penetrants In self-emulsifiable penetrants, water contamination can produce a gel break or emulsion inversion when the water concentration becomes high enough. The formation of the gel is an important feature during the washing processes but must be avoided until the stage in the process. Data indicates that the water contamination must be significant (greater than 10%) for gel formation to occur. Most specification limit water contamination to around 5% to be conservative. Non-water-based, water washable penetrants are checked using the procedure specified in ASTM D95 or ASTM E 1417. Water-based, water washable penetrants are checked with a refractometer. The rejection criteria is different for different penetrants so the requirements of the qualifying specification or the manufacturer's instructions must be consulted.

Application of the Penetrant

The application of the penetrant is the step of the process that requires the least amount of control. As long as the surface being inspected receive a generous coat penetrant, it really doesn't matter how the penetrant is applied. Generally, the application method is an economic or convenience decision.

It is important that the part be thoroughly cleaned and dried. Any contaminates or moisture on the surface of the part or within a flaw can prevent the penetrant material from entering the defect. The part should also be cool to the touch. The recommended range of temperature is 4 to 52 degrees centigrade.

Quality Control of Dwell

The dwell step involves allowing the test component to sit for a sufficient period of time for the penetrant to fill any surface breaking defects that happen to be present. There are basically two dwell modes, immersion-dwell and drain-dwell. The drain-dwell mode has been show to produce the most sensitive results. The only really quality control required in the dwell step of the process is to ensure that a minimum dwell time is reached. Dwell times are usually recommended by the penetrant producer or required by the specification being followed. There is no harm in allow a penetrant to dwell longer than the minimum time as long as the penetrant is not allowed to dry on the part.

Quality Control of Emulsifier Bath

Quality control of the emulsifier bath should be performed per the requirements of the applicable specification. The information provided here may not reflect the requirement of current specifications and is provided here for general education purposes only.

Lipophilic Emulsifiers

Lipophilic emulsifiers are miscible with penetrants in all concentrations. However, if the concentration of penetrant contamination in the emulsifier becomes too great, the mixture will not function affectively as a remover. AMS 2644 requires that lipophilic emulsifiers be capable of 20% penetrant contamination without a reduction in performance. AMS 2647A requires the emulsifier to be replaced when its cleaning action is less than that of new material.

Since lipophilic emulsifiers are oil-based, they have a limited tolerance for water. When the tolerance level is reached, the emulsifier starts to thicken and will eventually form a gel as more water is added. AMS 2644 requires that lipophilic emulsifiers be formulated to function adequately with at least 5% water contamination and AMS 2647A requires that lipophilic emulsifiers be replaced when the water concentration reaches 5%.

Hydrophilic Emulsifiers

Hydrophilic emulsifiers have less tolerance for penetrant contamination. The penetrant tolerance varies with emulsifier concentration and the type of contaminating penetrant. In some cases, as little as 1% by volume of penetrant contamination can seriously affect the performance of an emulsifier. One penetrant manufacture reports that 1 to 1.5% penetrant contamination will affect solution with a 10% concentration of emulsifier. As the emulsifier concentration increases in the solution, the penetrant contamination tolerance also increases and a solution with a 30% emulsifier concentration can tolerate from 5 to 8.5% penetrant contamination. The percentage of added penetrant required to destroy washability of the emulsifier can be measured and an oil tolerance index is commonly used to compare the tolerance of different emulsifiers to contamination by penetrants. AMS 2647A requires that the emulsification bath be discarded if penetrant is noted floating on the surface or adhering to the sides of the tank.

Water contamination is not as much of a concern with hydrophilic emulsifiers as they are miscible with water. However, it is very important that the emulsifier solution be kept at the proper concentration.

It should also be noted that penetrant dragout and, thus, level of possible emulsifier contamination by the penetrant is dependent on the type of material being processed. Tests have shown that on both polished and grit blasted surfaces, aluminum and stainless steel parts had a greater drag-out than titanium parts.

Emulsifier Concentration and Contact Time

The optimal emulsifier contact time is dependent on a number of variables that include the emulsifier used, the emulsifier concentration, the surface roughness of the part being inspected, and other factors. Usually some experimentation is required to select the proper emulsifier contact time. The emulsifier used must be matched to the penetrant material. For method D penetrant systems the concentration of the emulsifier should not exceed the percentage specified by the supplier and if working to a specification should not exceed the concentration specified. Since the emulsifier is mixed with water, which is prone to evaporation, it is recommended that the starting concentration be less than that recommended by the supplier. One penetrant manufacture recommends the following starting concentrations:

20% if the maximum concentration is 30% 13% if the maximum is 20%

6.5% if the maximum is 10%.

Some Research on Emulsifier Concentration and Contact Time

Vaerman reported on the effect of emulsifier concentration on sensitivity. He varied the contact time of a lipophilic emulsifier and compared the results to those from a 5% concentrate hydrophilic emulsifier with a three minute contact time. For a normal contact time of 45 seconds, the lipophilic emulsifier was found to average nearly 18% less sensitive over the range of crack depths (10 to 50 microns). The loss of sensitivity

increased rapidly as the lipophilic contact time was increased in steps to 5 minutes. Also as expected, the decrease in sensitivity increased with increasing crack size.

Vaerman also looked at the effect of hydrophilic emulsifier concentration. It was found that increasing the concentration for 5% by volume to 33 percent, decreased sensitivity by 15% when a three minute contact time was used. When a contact time of one minute was used the decrease in sensitivity was just over nine percent.

Ref: -- Vaerman, J., Fluorescent Penetrant Inspection, Quantified Evolution of the Sensitivity Versus Process Deviations, Proceedings of the 4th European Conference on Non- Destructive Testing, September 1987, Pergamon Press, Maxwell House, Fairview Park, Elmsford, New York, Volume 4, pp. 2814-2823.

Hyam also reports on the effect of the emulsifier concentration and contact time. Both hydrophilic and lipophilic removers were tested. The results showed that as the concentration of the emulsifier was increased from 2.5% to 20%, sensitivity decreased. The contact time was shown to have little effect on the hydrophilic system tested (up to 20 minutes) but to have a significant effect on the lipophilic system with sensitivity decreasing as contact time was increased from 2 to 10 minutes.

Quality Control of Wash Temperature and Pressure

The wash temperature and pressure and time are three parameters that are typically controlled in penetrant inspection process specification. A coarse spray or an immersion wash tank with air agitation is often used. When the spray method is used, the water pressure is usually limited to 276 kN/m2 (40 psi). The temperature range of the water is usually specified as a wide range (ex. 10 to 38C (50 to 100 F) in AMS 2647A.) A low-pressure, coarse water spray will force less water into flaws to dilute and/or remove trapped penetrant and weaken the indication. The temperature will have an effect on the surface tension of the water and warmer water will have more wetting action than cold water. Warmer water temperatures may also make emulsifiers and detergent more effective. The wash time should only be as long as necessary to decrease the background to an acceptable level. Frequent visual checks of the part should be made to determine when the part has be adequately rinsed.

Summary of Research on Wash Method Variables

Vaerman evaluated the effect that rinse time had on one high sensitivity water-washable penetrant and two post-emulsifiable penetrants (one medium and one high sensitivity). The evaluation was conducted using TESCO panels numerous cracks ranging in depth from 5 to 100 microns deep. A 38 percent decrease in sensitivity for the water-washable penetrant was seen when the rinse time was increased from 25 to 60 seconds. When the rinse times of two post-emulsifiable penetrants were increased from 20 to 60 seconds, a loss in sensitivity was seen in both cases but it was much reduced from the loss seen with the water-washable system. The relative sensitivity loss over the range of crack depths was 13 percent for the penetrant with medium sensitivity and roughly percent for the high sensitivity penetrant.

-- Vaerman, J., Fluorescent Penetrant Inspection, Quantified Evolution of the Sensitivity Versus Process Deviations, Proceedings of the 4th European Conference on Non-Destructive Testing, Pergamon Press, Maxwell House, Fairview Park, Elmsford, New York, Volume 4, September 1987, pp. 2814-2823.

In a 1972 paper by N.H. Hyam, the effects of the rinse time on the sensitivity of two level 4 water-washable penetrants are examined. It was reported that sensitivity decreased as spray-rinse time increased and that one of the penetrants was more affected by rinse time than the others. Alburger, points-out that some conventional fluorescent dyes are slightly soluble in water and can be leached out during the washing processes.

-- Hyam, N. H., Quantitative Evaluation of Factors Affecting the Sensitivity of Penetrant Systems, Materials Evaluation, Vol. 30, No. 2, February 1972, pp. 31-38.

Brittian evaluated the effect of wash time on a water-washable, level 4 penetrant (Ardrox 970P25) and found that indication brightness decreases rapidly in the first minute of wash and then slowed. The brightness value dropped from a relative value of 1100 to approximately 500 in the first minute and then continued to decrease nearly linearly to a value of 200 after five minutes of wash. Brittian concluded that wash time for water-washable systems should be kept to a minimum.

Brittain, P.I., Assessment of Penetrant Systems by Fluorescent Intensity, Proceedings of the 4th European Conference on Nondestructive Testing, Vol. 4, Published by Perganon Press, 1988, pp. 2814-2823.

Robinson and Schmidt used a Turner fluorometer to evaluate the variability that some of the processing steps can produce in the brightness of indications. To find out how much effect the wash procedure had on sensitivity, Tesco cracked, chrome-plated panels, were processed a number of times using the same materials but three different wash methods. The washing methods included spraying the specimens with a handheld nozzle, holding the specimens under a running tap, and using a washing machine that controlled the water pressure, temperature, spray pattern and wash time. The variation in indication brightness readings between five trials was reported. The variation was 16 percent for the running tap water, 14 percent for the handheld spray nozzle and 4.5 percent for the machine wash.

Quality Control of Drying Process

The temperature used to dry parts after the application of aqueous wet developer or prior to the application of a dry powder or a nonaqueous wet developer, must be controlled to prevent "cooking" of the penetrant in the defect. High drying temperature can affect penetrants in a couple of ways. First, some penetrants can fade at high temperatures due to dye vaporization or sublimation. Second, high temperatures can cause the penetrant to dry in the the flaw preventing it from migrating to the surface to produce an indication. To prevent harming the penetrant material, drying temperature should be kept to under 71 degree centigrade.

The drying should be limited to the minimum length of time necessary to thoroughly dry the component being inspected.

Quality Control of Developer

The function of the developer is very important in a penetrant inspection. It must draw out of the discontinuity a sufficient amount of penetrant to form an indication, and it must spread the penetrant out on the surface to produce a visible indication. In a fluorescent penetrant inspection, the amount of penetrant brought to the surface must exceed the dye's thin film threshold of fluorescence of the indication will not fluoresce. Additionally, the developer makes fluorescent indications appear brighter than indications produced with the same amount of dye but without the developer.

In order to accomplish these functions, a developer must adhere to the part surface and result in a uniform, highly porous layer with many paths for the penetrant to be moved due to capillary action. Some developers are applied wet and other dry, but the desired end result is always a uniform, highly porous, surface layer. Since the quality control requirements for each of the developer types is slightly different, they will be covered individually.

Dry Powder Developer

A dry powder developer should be checked daily to ensure that it is fluffy and not caked. It should be similar to fresh powdered sugar and not granulated like powered soup. It should also be relatively free from specks of fluorescent penetrant material from previous inspection. This check is performed by spreading a sample of the developer out and examining it under UV light. If there are ten or more fluorescent specks in an 10 cm diameter area, the batch should be discarded. Apply a light coat of the developer by immersing the test component or dusting the surface. After the development time, excessive powder can be removed by gently blowing on the surface with air not exceeding 35 kPa or 5 psi.

Wet Soluble/Suspendible Developer

Wet soluble developer must be completely dissolved in the water and wet suspendible developer must be thoroughly mixed prior to application. The concentration of powder in the carrier solution must be controlled in these developers. The concentration should be checked at least weekly using a hydrometer to make sure it meets the manufacturer's specification. To check for contamination, the solution should be examined weekly using both white light and UV light. If a scum is present or the solution fluoresces, it should be replaced. Some specification require that a clean aluminum panel be dipped in the developer, dried, and examined for indications of contamination by fluorescent penetrant materials.These developers are apply immediately after the final wash. A uniform coating should be applied by spraying, flowing or immersion of the component. They should never be applied with a brush. Care should be taken to avoid a heavy accumulation of the developer solution in crevices and recesses. Prolonged contact of the component with the developer solution should be avoided in order to minimize dilution or removal of the penetrant from discontinuities.

Solvent Suspendible

Solvent suspendible developers are typically supplied in an sealed aerosol spray can. Since the developer solution is in a sealed vessel, direct check of the solution are not possible. However, the way that the developer is dispensed must be monitored. The spray developer should produce a fine, even coating on the surface of the part. Make sure the can is well shaken and apply a thin coating to a test article. If the spray produces spatters or other an uneven coating the can should be discarded. When applying a solvent suspendible developer, it is up to the inspector to control the thickness of the coating. When a visible penetrant system, the developer coating must be thick enough to provide a white contrasting background but not heavy enough to mask indications. When using a fluorescent penetrant system, a very light coating should be used. The developer should be applied under white light condition and should appear evenly transparent.

Development Time

Part should be allowed to develop for a minimum of 10 minutes and no more than 2 hours before inspecting.

Quality Control of Lighting

After a component has been properly processed, it is ready for inspection. While automated vision inspection systems are sometimes used, the focus here will be on inspection performed visually by a human inspector as this is the dominate method. Proper lighting is of great importance when visually inspecting a surface for a penetrant indication. Obviously, the lighting requirements are different for an inspection conducted using a visible dye penetrant than they are for an inspection conducted using a fluorescent dye penetrant. The lighting requirements for each of these techniques, as well as how light measurements are made, is discussed below.

Lighting for Visible Dye Penetrant Inspections

When using a visible penetrant, the intensity of the white light is of principal importance. Inspections can be conducted using natural lighting or artificial lighting. When using natural lighting, it is important to keep in mind that daylight varies from hour to hour so inspector must stay constantly aware on the lighting conditions and make adjustment when needed. To improve uniformity in lighting from one inspection to the next, the use of artificial lighting is recommended. Artificial lighting should be white whenever possible and white flood or halogen lamps are most commonly used. The light intensity is required to be 100 foot-candles at the surface being inspected. It is advisable to choose a white light wattage that will provide sufficient light, but avoid excessive reflected light that could distract from the inspection.

Lighting for Fluorescent Penetrant Inspections

When a fluorescent penetrant is being employed, the ultraviolet illumination and the visible light inside the inspection booth is important. Penetrant dyes are excited by the UV of 365-nm wavelength and emit visible light somewhere in the green-yellow range between 520 and 580 nm. The source of ultraviolet light (UV) is often a mercury arc lamp with a filter. The lamps emit many wavelengths and a filter is used to remove all but the UV and a small amount of visible light between 310 and 410 nm. Visible light of wavelengths above 410 nm interferes with contrast, and UV emissions below 310 nm include some hazardous wavelengths.

Standards and procedures require verification of lens condition and light intensity. Black lights should never be used with a cracked filter as output of white light and harmful black light will be increased. The cleanliness of the filter should also be checked as a coating of solvent carrier, oils, or other foreign materials can reduce the intensity by up to as much as 50%. The filter should be checked visually and cleaned as necessary before warm-up of the light.

Since fluorescent brightness is linear with respect to ultraviolet excitation, a change in the intensity of the light (from age or damage) and a change in the distance of the light source from the surface being inspected will have a direct impact on the inspection. For UV lights used in component evaluations, the normally accepted intensity is 1000 microwatts per square centimeter when measured at 15 inches from the filter face (requirements can vary from 800 to 1200). The required check should be performed when a new bulb is installed, at startup of the inspection cycle, if a change in intensity is noticed, or every eight hours of continuous use. Regularly checking the intensity of UV lights is very important because bulbs loose intensity over time. In fact, a bulb that is near the end of its operating life will often have an intensity of only 25 percent of its original output. Black light intensity will also be affected by voltage variations. A bulb that produces acceptable intensity at 120 volts will produce significantly less at 110 volts. For this reason it is important to provide constant voltage to the light. Also, most UV light must be warmed up prior to use and should be on for at least 15 minutes before beginning an inspection.

When performing a fluorescent penetrant inspection, it is important to keep white light to a minimum as it will significantly reduce the inspectors ability to detect fluorescent indications. Light levels of less than 2 fc are required by most procedures with some procedures requiring less than 0.5 fc at the inspection surface. Procedures require a check and documentation of ambient white light in the inspection area. When checking black light intensity at 15 inches a reading of the white light produced by the black light may be required to verify white light is being removed by the filter.

Light Measurement

Light intensity measurements are made using a radiometer. A radiometer is an instrument that translate light energy into an electrical current. Light striking a silicon photodiode detector causes a charge to build up between internal layers. When an external circuit isconnected to the cell, an electrical current is produced. This current is linear with respect to incident light. Some radiometers have the ability to measure both black and white light, while others require a separate sensor for each measurement. Whichever type is used, the sensing area should be clean and free of any materials that could reduce or obstruct light reaching the sensor. Radiometers are relatively unstable instruments and readings often change considerable over time. Therefore, they should be calibrated at least every six months.

Ultraviolet light measurements should be taken using a fixture to maintain a minimum distance of 15 inches from the filter face to the sensor. The sensor should be centered in the light field to obtain and record the highest reading. UV spot lights are often focused, so intensity readings will vary considerable over a small area. White lights are seldom focused and depending on the wattage, will often produce in excess of the 100 fc at 15 inches. Many specifications do not require the white light intensity check to be conducted at a specific distance.

System Performance Check

System performance checks involve processing a test specimen with known defects to determine if the process will reveal discontinuities of the size required. The specimen must be processed following the same procedure used to process production parts. A system performance check is typically required daily, at the reactivation of a system after maintenance or repairs, or any time the system is suspected of being out of control. As with penetrant inspections in general, results are directly dependent on the skill of the operator and, therefore, each operator should process a panel.

The ideal specimen is a production item that has natural defects of the minimum acceptable size. Some specification delineate the type and size of the defects that must be present in the specimen and detected. Surface finish is will affect washability so the check specimen should have the same surface finish as the production parts being processed. If penetrant systems with different sensitivity levels are being used, there should be a separate specimen for each system.

There are some universal test specimens that can be used if a standard part is not available. The most commonly used test specimen is the TAM or PSM panel. These panel are usually made of stainless steel that has been chrome plated on one half and surfaced finished on the other half to produced the desired roughness. The chrome plated section is impacted from the back side to produce a starburst set of cracks in the chrome. There are five impacted areas to produce range of crack sizes. Each panel has a characteristic “signature” and variances in that signature are indications of process variance. Panel patterns as well as brightness are indicators of process consistency or variance.

Care of system performance check specimens is critical. Specimens should be handled carefully to avoid damage. They should be cleaned thoroughly between uses and storage in a solvent is generally recommended. Before processing a specimen, it should be inspected under UV light to make sure that it is clean and not already producing an indication.

Nature of the Defect

The nature of the defect can have a large affect on sensitivity of a liquid penetrant inspection. Sensitivity is defined as the smallest defect that can be detected with a high degree of reliability. Typically, the crack length at the sample surface is used to define size of the defect. A survey of any probability-of-detection curve for penetrant inspection will quickly lead one to the conclusion that crack length has a definite affect on sensitivity. However, the crack length alone does not determine whether a flaw will be seen or go undetected. The volume of the defect is likely to be the more important feature. The flaw must be of sufficient volume so that enough penetrant will bleed back out to a size that is detectable by the eye or that will satisfy the dimensional thresholds of fluorescence.

Above is an example of fluorescent penetrant inspection probability of detection (POD) curve from the Nondestructive Evaluation (NDE) Capabilities Data Book. Please note that this curve is specific to one set of inspection conditions and should not be interpreted to apply to other inspection situations.

In general, penetrant inspections are more effective at finding

small round defects than small linear defects. Small round defects are generally easier to detect for several reasons. First, they are typically volumetric defects that can trap significant amounts of penetrant. Second, round defects fill with penetrant faster than linear defects. One research effort found that elliptical flaw with length to width ratio of 100, will take the penetrant nearly 10 times longer to fill than a cylindrical flaw with the same volume.

deeper flaws than shallow flaws. Deeper flaws will trap more penetrant than shallow flaws, and they are less prone to over washing.

flaws with a narrow opening at the surface than wide open flaws. Flaws with narrow surface openings are less prone to over washing.

flaws on smooth surfaces than on rough surfaces. The surface roughness of the part primarily affects the removability of a penetrant. Rough surfaces tend to trap more penetrant in the various tool marks, scratches, and pits that make up the surface. Removing the penetrant from the surface of the part is more difficult and a higher level of background fluorescence or over washing may occur.

flaws with rough fracture surfaces than smooth fracture surfaces. The surface roughness that the fracture faces is a factor in the speed at which a penetrant enters a defect. In general, the penetrant spreads faster over a surface as the surface roughness increases. It should be noted that a particular penetrant may spread slower than others on a smooth surface but faster than the rest on a rougher surface.

flaws under tensile or no loading than flaws under compression loading. In a 1987 study at the University College London, the effect of crack closure on detectability was evaluated. Researchers used a four-point bend fixture to place tension and compression loads on specimens that were fabricated to contain fatigue cracks. All cracks were detected with no load and with tensile loads placed on the parts. However, as compressive loads were placed on the parts, the crack length steadily decreased as load increased until a load was reached when the crack was no longer detectable.

Health and Safety Precautions in Liquid Penetrant Inspection

When proper health and safety precautions are followed, liquid penetrant inspection operations can be completed without harm to inspection personnel. However, there are a number of health and safety related issues that must be addressed. Since each inspection operation will have its own unique set of health and safety concerns that must be addressed, only a few of the most common concerns will be discussed here.

Chemical Safety

Whenever chemicals must be handled, certain precautions must be taken as directed by the material safety data sheets (MSDS) for the chemicals. Before working with a chemical of any kind, it is highly recommended that the MSDS be reviewed so that proper chemical safety and hygiene practices can be followed. Some of the penetrant materials are flammable and, therefore, should be used and stored in small quantities. They should only be used in a well ventilated area and ignition sources avoided. Eye protection should always be worn to prevent contact of the chemicals with the eyes. Many of the chemicals used contain detergents and solvents that can dermatitis. Gloves and other protective clothing should be warn to limit contact with the chemicals.

Ultraviolet Light Safety

Ultraviolet (UV) light or "black light" as it is sometimes called, has wavelengths ranging from 180 to 400 nanometers. These wavelengths place UV light in the invisible part of the electromagnetic spectrum between visible light and X-rays. The most familiar source of UV radiation is the the sun and is necessary in small doses for certain chemical processes to occur in the body. However, too much exposure can be harmful to the skin and eyes. Excessive UV light exposure can cause painful sunburn, accelerate wrinkling and increase the risk of skin cancer. UV light can cause eye inflammation, cataracts, and retinal damage.

Because of their close proximity, laboratory devices, like UV lamps, deliver UV light at a much higher intensity than the sun and, therefore, can cause injury much more quickly. The greatest threat with UV light exposure is that the individual is generally unaware that the damage is occurring. There is usually no pain associated with the injury until several hours after the exposure. Skin and eye damage occurs at wavelengths around 320 nm and shorter which is well below the 365 nm wavelength, where penetrants are designed to fluoresce. Therefore, UV lamps sold for use in LPI application are almost always filtered to remove the harmful UV wavelengths. The lamps produce radiation at the harmful wavelengths so it is essential that they be used with the proper filter in place and in good condition.

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