ut testing section 6 selected applications & techniques

Section 6: Selected Applications & Techniques

Content: Section 6: Selected Applications & Techniques

6.1: Defects & Discontinuities6.2: Rail Inspection 6.3: Weldments (Welded Joints)6.4: Pipe & Tube6.5: Echo Dynamic6.6: Technique Sheets6.7: Material Properties-Elastic Modulus Measurements6.8: High Temperature Ultrasonic Testing6.9: Thickness Gauging6.10: In-Service Inspection

Continues next page….

6.11: Casting6.12: Inspection of bonded Joints6.13: Corrosion Monitoring6.14: Crack Monitoring6.15: Residual Stress Measurements6.16: Bond Testing

Appendix: (Non-exam)6.App-1: TOFD Introduction

6.1: Defects & Discontinuities

6.1.1 Casting Defects & Discontinuities

Casting Defects & Discontinuities

Casting Defects & Discontinuities- A Cold Shut is caused when a molten metal is poured over solidified metal without fusing.

Casting Defects & Discontinuities – Hot tear or shrinkage crack forms when the molten section of unequal thickness solidified and the shrinkage stress tear the partially molten apart.

Micro-shrinkage is usually many small subsurface holes that appear at the gate of casting / can also occur when molten metal must flow from a thin section into thicker section of casting.

Blow hole are small hole at the surface of the casting caused by gas which comes from the mold itself. (wet sand mould forming steam resulting in blow-hole)

Porosity is caused by entrapped gas. It is usually subsurface or surfacedepending on the mold design.

Casting Defects & Discontinuities- Hot Tear

Casting Defects & Discontinuities- Blister

Casting Defects & Discontinuities- Porosity

Casting Defects & Discontinuities - Mismatch

Casting Defects & Discontinuities- Cold Shut

Casting Defects & Discontinuities- Missrun

Casting Defects & Discontinuities- Misrun

Casting Defects & Discontinuities- Blow Hole

Casting Defects & Discontinuities- Gas Porosity

Casting Defects & Discontinuities- Porosity

Casting Defects & Discontinuities- Cold Shut

Casting Defects & Discontinuities- Shrinkage Cavity

Casting Defects & Discontinuities- Assorted

6.1.2 Processing Defects & Discontinuities

Processing Defects & Discontinuities

Salute to the Steel Workers!

Processing Defects & Discontinuities- Lamination formed when the casting defects are flatten during rolling, forging, extrusion or other mechanical working processes.

Processing Defects & Discontinuities- Stringers formed when the billet is rolled into shape the casting non metallic inclusions are squeezed into long and thinner inclusions.

Processing Defects & Discontinuities- Forging lap is caused by folding of metal on the surface, usually when some of the metal is squuaed ot between the two dies.

Processing Defects & Discontinuities- Forging burst is a rupture causes by forging at improper temperature. The burst may be internal or external.

Processing Defects & Discontinuities

Q9: The preferred method of ultrasonically inspecting a complex-shape forging:

A. Is an automated immersion test of the finished forging using instrument containing a calibrated attenuator in conjunction with a C-scan recorder

B. Combined thorough inspection of the billet prior to forging with a careful inspection of the finished part in all areas where shape permit

C. Is a manual contact test of the finished part

D. Is an automated immersion test of the billet prior to forging

6.1.3 Welding Defects & Discontinuities

Welding Defects & Discontinuities

Welding Defects & Discontinuities- Incomplete Penetration

Welding Defects & Discontinuities- Slag Inclusion

Welding Defects & Discontinuities- Cluster Porosity

Welding Defects & Discontinuities- Lack of Sidewall Fusion (with Slag entrapped)

Welding Defects & Discontinuities- Wagon Track (slag inclusion at hot pass)

Welding Defects & Discontinuities- Burn Thru

Welding Defects & Discontinuities- Offset with LOP

Welding Defects & Discontinuities- Excessive Penetration

Welding Defects & Discontinuities- Internal (Root) Under Cut

Welding Defects & Discontinuities- Transverse Crack

Welding Defects & Discontinuities- Tungsten Inclusion

Welding Defects & Discontinuities- Root Pass Porosity

6.1.4 Service Induced Defects & Discontinuities

Service Induced Defects & Discontinuities

http://failure-analysis.info/2010/05/analyzing-material-fatigue/

Service Induced Defects & Discontinuities- Fatigue Cracks

Figure 4-24 – In a carbon steel sample, metallographic section through a thermal fatigue crack indicates origin at the toe of an attachment weld. Mag. 50X, etched.

Figure 4-26 – Metallographic cross-section of a superheated steam outlet that failed from thermal fatigue. Unetched.

Figure 4-36 – Weld detail used to join a carbon steel elbow (bottom) to a weld overlaid pipe section (top) in high pressure wet H2S service. Sulfide stress cracking (SSC) occurred along the toe of the weld (arrow), in a narrow zone of high hardness.

Figure 4-37 – High magnification photomicrograph of SSC in pipe section shown in Figure 4-36.

Figure 4-38 – Failure of DMW joining 1.25Cr-0.5Mo to Alloy 800H in a Hydro-dealkylation (HAD) Reactor Effluent Exchanger. Crack propagation due to stresses driven at high temperature of 875°F (468°C) and a hydrogen partial pressure of 280 psig (1.93 MPa).

Figure 4-57 – Vibration induced fatigue of a 1-inch socket weld flange in a thermal relief system shortly after startup.

Figure 4-58 – Cross-sectional view of the crack in the socket weld in Figure 4-57.

Figure 5-1 – Localized amine corrosion at the weld found in piping from reboiler to regenerator tower in an MEA unit. Many other similar cases found, some going as deep as half thickness. They were originally found and mistaken as cracks with shear wave UT inspection.

Figure 5-2 – Hot Lean Amine Corrosion of Carbon Steel:

Figure 5-3 – Preferential weld corrosion in lean amine (Reference 5)

Figure 5-46 – Overhead interstage knockout drum vapor outlet nozzle.

Figure 5-47 – Carbonate cracking adjacent to a weld (Reference 6).

Figure 5-48 – Metallographic sample showing intergranular carbonate cracking developed after 6 months service (Reference 6).lean amine (Reference 5)

Figure 5-49 – Most cracks originate in base metal but this weldment contained a crack that originated at the root and propagated through the weld metal. Other cracks appear to have initiated in the HAZ (Reference 7).

6.2: Rail Inspection

Rail Inspection

One of the major problems that railroads have faced since the earliest days is the prevention of service failures in track. As is the case with all modes of high-speed travel, failures of an essential component can have serious consequences. The North American railroads have been inspecting their most costly infrastructure asset, the rail, since the late 1920's. With increased traffic at higher speed, and with heavier axle loads in the 1990's, rail inspection is more important today than it has ever been. Although the focus of the inspection seems like a fairly well-defined piece of steel, the testing variables present are significant and make the inspection process challenging.Rail inspections were initially performed solely by visual means. Of course,

visual inspections will only detect external defects and sometimes the subtle signs of large internal problems.

The need for a better inspection method became a high priority because of a derailment at Manchester, NY in 1911, in which 29 people were killed and 60 were seriously injured. In the U.S. Bureau of Safety's (now the National Transportation Safety Board) investigation of the accident, a broken rail was determined to be the cause of the derailment. The bureau established that the rail failure was caused by a defect that was entirely internal and probably could not have been detected by visual means. The defect was called a transverse fissure (example shown on the bottom). The railroads began investigating the prevalence of this defect and found transverse fissures were widespread.

Transverse Fissure

One of the methods used to inspect rail is ultrasonic inspection. Both normal- and angle-beam techniques are used, as are both pulse-echo and pitch-catch techniques. The different transducer arrangements offer different inspection capabilities. Manual contact testing is done to evaluate small sections of rail but the ultrasonic inspection has been automated to allow inspection of large amounts of rail.

Fluid filled wheels or sleds are often used to couple the transducers to the rail. Sperry Rail Services, which is one of the companies that perform rail inspection, uses Roller Search Units (RSU's) comprising a combination of different transducer angles to achieve the best inspection possible. A schematic of an RSU is shown below.

Techniques: Wheel Probe

Techniques: Examples of axles with outside bearings of the Deutsche Bundesbahn. (a) Of goods truck; (b) axle with roller bearing, bearing ring not removed; c same with additional brake disc

Techniques: (c) same with additional brake disc

6.3: Weldments (Welded Joints)

6.3.1: UT of Weldments (Welded Joints)

The most commonly occurring defects in welded joints are porosity, slag inclusions, lack of side-wall fusion, lack of inter-run fusion, lack of root penetration, undercutting, and longitudinal or transverse cracks. With the exception of single gas pores all the defects listed are usually well detectable by ultrasonics. Most applications are on low-alloy construction quality steels, however, welds in aluminum can also be tested. Ultrasonic flaw detection has long been the preferred method for nondestructive testing in welding applications. This safe, accurate, and simple technique has pushed ultrasonics to the forefront of inspection technology.

Ultrasonic weld inspections are typically performed using a straight beam transducer in conjunction with an angle beam transducer and wedge. A straight beam transducer, producing a longitudinal wave at normal incidence into the test piece, is first used to locate any laminations in or near the heat-affected zone. This is important because an angle beam transducer may not be able to provide a return signal from a laminar flaw.

UT of Weldments (Welded Joints)

0 20 40 60 80 100

s

aa'

d

x

a = s sinßa = s sinß

a' = a - xa' = a - x

d' = s cosßd' = s cosß

d = 2T - t'd = 2T - t'

s

Lack of fusionWork piece with welding

F ß = probe angles = sound patha = surface distancea‘ = reduced surface distanced‘ = virtual depthd = actual depthT = material thickness

ß

UT Calculator

Flaw Detection- Depth Determination

The second step in the inspection involves using an angle beam transducer to inspect the actual weld. Angle beam transducers use the principles of refraction and mode conversion to produce refracted shear or longitudinal waves in the test material. [Note: Many AWS inspections are performed using refracted shear waves. However, material having a large grain structure, such as stainless steel may require refracted longitudinal waves for successful inspections.] This inspection may include the root, sidewall, crown, and heat-affected zones of a weld. The process involves scanning the surface of the material around the weldment with the transducer. This refracted sound wave will bounce off a reflector (discontinuity) in the path of the sound beam. With proper angle beam techniques, echoes returned from the weld zone may allow the operator to determine the location and type of discontinuity.

T = Plate Thickness

ϴ = Shear wave angle

LEG = T/Cos ϴ, V path= 2 x LEG.

Skip = 2.T Tan ϴ

https://www.mandinasndt.com/index.php?option=com_content&view=article&id=32%253Aut-angle-beam-calculator&catid=12%253Atools&Itemid=18

https://www.nde-ed.org/GeneralResources/Formula/AngleBeamFormula/AngleBeamTrig.htm

Flaw Detection- Triangulations of reflector

ϴ = Refracted angle T= Thickness LEG1=LEG2= T/Cos ϴ

V PATH= 2x LEG= 2T/Cos ϴ SKIP= 2.T Tan ϴ

ϴ

Flaw Detection- Triangulations of reflector

ϴ = Refracted angle T= Thickness Surface Distance= S.Sin ϴ

Depth= S.Cos ϴ

ϴ

To determine the proper scanning area for the weld, the inspector must first calculate the location of the sound beam in the test material. Using the refracted angle, beam index point and material thickness, the V-path and skip distance of the sound beam is found. Once they have been calculated, the inspector can identify the transducer locations on the surface of the material corresponding to the crown, sidewall, and root of the weld.

6.3.2 Weld Scanning

Expert at works

Typical Scanning Patterns:Typically the weld should be inspected in the 1st or 2nd leg (1st Skip).

Typically scanning patterns

Weld Scanning

Echo Dynamic- Position of Defects

Sometimes it will be possible to differentiate between these 2 defects simply by plotting their position within the weld zone:

Echo Dynamic- Position of Defects

Plate Weld Scanning

Practice Makes Perfect

52. One of the most apparent characteristics of a discontinuity echo, as opposed to a non-relevant indication is:

(a) Lack of repeatability(b) Sharp, distinct signal(c) Stable position with fixed transducer position(d) High noise level

58. What useful purpose may be served by maintaining grass on the baseline?(a) To estimate casting grain size(b) To provide a reference for estimating signal to noise ratio(c) To verify adequate coupling to the test piece(d) All of the above

Practice Makes Perfect

62. Which of the following conditions would be most likely to cause strong, interfering surface waves?

(a) High frequency transducers(b) Testing on a small diameter surface(c) Testing on a flat surface(d) Testing on a curved surface with a contoured wedge and transducer

6.4: Pipe & Tube

Pipe & Tube

Experts at work

Pipe Scanning

Pipe Scanning

48.59o max 30o

max

Pipe Scanning

Pipe Scanning- thickness/OD ratio

Pipe Scanning- thickness/OD ratio

When the t/OD ratio = .2 , t=.2OD, ID=OD-2t= OD-.4OD= .6OD

ϴ max = Sin-1(ID/OD), ϴ max = Sin-1(0.6), ϴ max = 37° Max.

For the sound path to scans the inner face the maximum shear angle shall be 37° Max. Therefore 45° /60° /70° probe can not scan the pipe inner face.

Pipe Scanning- Contact Methods

Q: Calculate the maximum shear wave angle and the range for 360°revolution scanning when the shear wave angle is 45°. Given that the OD=6” Thickness=3/4”

Answer: (a)The maximum shear wave angle ϴ = Sin-1(ID/OD) = Sin-1(2.25/3)ϴ = 48.6° Max.

(b) ?

Answer part B

b

a

a/Sin A = b/Sin B

2.25/ Sin 45 = b / Sin B, 3.182= b/ Sin B,

c = a.Sin B, Sin B= c/a

3.182= b/c x 2.25, b/c= 1.414c

Q35: During immersion testing of pipe or tubing the incident longitudinal wave angle must be limited to a narrow range. The reason for the upper limit is:

(a) To avoid complete reflection of ultrasound from the test piece(b) To prevent formation of Rayleigh waves(c) To prevent formation of shear waves(d) To avoid saturating the test piece with ultrasound

Q35: Which of the following may result in a narrow rod if the beam divergence results in a reflection from a side of the test piece before the sound wave reaches the back surface:

A. Multiple indications before the first back reflectionB. Indications from multiple surface reflectionsC. Conversion from longitudinal mode to shear modeD. Loss of front surface indications

6.5: Echo Dynamic

Expert at works

6.5.1 Basic echodynamic pattern of reflectors

Echo Dynamic of Discontinuity- Non-destructive testing of welds -Ultrasonic testing - Characterization of indications in welds; German version EN 1713:1998 + A1:2002

Basic echodynamic pattern of reflectorsC.1 Pattern 1Point-like reflector response, figure C.1. At any probe position the A-scan show a single sharp echo. As the probe is moved this rises in amplitude smoothly to a single maximum before falling smoothly to noise level.

1

32

4

5

6

7

C.1 Pattern 1 Point-like reflector

C.2 Pattern 2Extended (elongated) smooth reflector respond, figure C.2. At any probe position the A-scan shows a single sharp echo. When the ultrasound beam is moved over the reflector the echo rises smoothly to a plateau and is maintained with minor variation in magnitude up to 4 dB, until the beam moves off the reflector, when the echo fall smoothly to noise level.

C.2 Pattern 2Extended (elongated) smooth reflector

C.2 Pattern 2Extended (elongated) smooth reflector (figure modified to depict obliquely oriented planar face)

Extended (elongated) smooth reflector-planar face obliquely oriented

C.3 Pattern 3Extended (elongated) rough reflector response. There are two variants of this pattern, depending upon the angle of incident of the probe beam on the reflector.

C.3 Pattern 3aExtended (elongated) rough reflector response. Near normal incidence, figure C.3a At any probe position the A-scan shows a single but rugged echo. As the probe moved this may undergo large (>+/- 6dB) random fluctuation in amplitude. The fluctuation are caused by reflection from the different facets of the reflector and by interference of waves scattered from the groups of facets.

C.3 Pattern 3aExtended (elongated) rough reflector response.

C.3 Pattern 3b Oblique incidence, travelling echo pattern, figure C.3 b At any probe position, the A-scan shows an extended train of signals (subsidiary peaks) within a bell-shaped pulse envelope. As the probe is moved each subsidiary peak travels through the pulse envelop, rising to its own maximum toward the center envelop and then falling. The overall signal may shown large (>+/-6dB) random fluctuation in amplitude.

C.3 Pattern 3b Oblique incidence, travelling echo pattern

C.4 Pattern 4 Multiple reflector respond, figure C.4. At any probe position the A-scan shows a cluster of signal which may or may not be well resolved in range. As the probe is moved the signals rise and fall at random but the signal from each separate reflector element ,if resolved, shows pattern 1 respond.

C.4 Pattern 4 Multiple reflector respond

Echodynamic- Change of echo height and echo shape when the direction of irradiation is changed. (a) On flat or linear flaw; (b) on rounded flaw

Echodynamic- Differences between the indications of inclusions and cracks, drawn schematically and exaggerated for greater clarity. a Inclusions; b flake cracks. The echoes of the more distantflaws, because of divergence and attenuation of the sound beam, are rather weak

Break TimeBreak Time

Echo Dynamic of Discontinuity- Flaw detection

Echo Dynamic of Discontinuity- Flaw Detection

Echo Dynamic of Discontinuity- Flaw detections

Echo Dynamic of Discontinuity- Improper flaw orientation

Echo Dynamic of Discontinuity- Reflection angle

Echo Dynamic of Discontinuity- Angles of reflection

Echo Dynamic of Discontinuity- Perfect flaw orientation

Echo Dynamic of Discontinuity- Vertical near surface flaw

Echo Dynamic of Discontinuity- Tandem Techniques

Echo Dynamic

Echo Dynamic- Root Concavity

Echo Dynamic

Echo Dynamic

Crack

Echo Dynamic- Broad indication with low amplitude

Echo Dynamic- Shaper indication and higher amplitude than porosity

Echo Dynamic

Echo DynamicThreadlike defects, point defects and flat planar defects orientated near-normal to the beam axis all produce an echo response which has a single peak

Echo DynamicThe echo response from a large slag inclusion or a rough crack is likely to have multiple peaks:

Echo DynamicIn case “a” it will be difficult to determine whether the defect is slag or a crack. “Rotational- Swivel” or “orbital” probe movements may help:

Echo DynamicTypical Echo Dynamic Patterns

Q. A smooth flat discontinuity whose major plane is not perpendicular to the direction of sound propagation may be indicated by:

A. An echo amplitude comparable in magnitude to the back surface reflectionB. A complete loss of back surface reflectionC. An echo amplitude larger in magnitude than the back surface reflectionD. All of the above

Q183. In immersion testing, irrelevant or false indications caused by contoured surfaces are likely to result in a:

A. Broad base indicationB. Peaked indicationC. Hashy signalD. Narrow based indication

Q24. During inspection of a parallel sided machined forging using straight beam immersion techniques, a diminishing back reflection in a localized area in the absence of a defect indication would least likely represent:

A. A course grain structuresB. A small non-metallic stringer C. A defect oriented at a severe angle to the entry surfaceD. A large inclusion.

Q46. Which best describes a typical display of a crack whose major surface is perpendicular to the ultrasound beam?

A. A broad indicationB. A sharp indicationC. A indication will not show due to improper orientationD. A broad indication with high amplitude

Q46. A smooth flat discontinuities whose major plane is not perpendicular to the direction of sound propagation may be indicated by:

A. An echo amplitude comparable in magnitude to the back surface reflectionB. A complete loss of back surface reflectionC. An echo amplitude larger in magnitude than the back surface reflectionD. All of the above

6.6: Technique Sheets

Expert at works

Hanger Pin Testing using Shear Wavehttp://www.fhwa.dot.gov/publications/research/infrastructure/structures/04042/index.cfm#toc

Physical Dimension

Reporting: Basic Pin Information

Reporting: Scanning Report – Top of Pin

Reporting: Scanning Report – Bottom of Pin

Mock-Up

Reporting: Basic Pin Information

Hanger Pin Testing using Shear Wave

Pitch and Catch Methods- Echo Dynamic

Pitch and Catch Methods- Set-up

Pitch and Catch Methods- Echo Dynamic

6.7: Material Properties-Elastic Modulus Measurements

6.7.1 Determination of Microstructural Differences

Ultrasonic methods can be used to determine microstructural differences in metals. For this, contact testing with the pulse-echo technique is used. The testing can be either the measurement of (1) ultrasonic attenuation or the (2) measurement of bulk sound velocity.

6.7.2 The attenuation method

The attenuation method is based on the decay of multiple echoes from test piece surfaces. Once a standard is established, other test pieces can be compared to it by comparing the decay of these echoes to an exponential curve. This test is especially suited for the microstructural control of production parts, in which all that is necessary is to determine whether or not the parts conform to a standard. An example of the use of ultrasonic attenuation in the determination of differences in microstructure is the control of graphite-flake size in gray iron castings, which in turn controls tensilestrength. In one application, a water-column search unit that produced a pulsed beam with a frequency of 2.25 MHz was used to test each casting across an area of the casting wall having uniform thickness and parallel front and back surfaces.

A test program had been first carried out to determine the maximum size of graphite flakes that could be permitted in the casting and still maintain a minimum tensile strength of 200 MPa (30 ksi). Then, ultrasonic tests were made on sample castings to determine to what intensity level the second back reflection was lowered by the attenuation effects of graphite flakes larger than permitted. Next, a gate was set on the ultrasonic instrument in the region of the second back reflection, and an alarm was set to signal whenever the intensity of this reflection was below the allowable level. The testing equipment was then integrated into an automatic loading conveyor, where the castings were 100% inspected and passed or rejected before any machining operation.

6.7.3 Velocity Measurements

Velocity Measurements When considering the compressional and shear wave velocities given in Table 1, there may be small deviations for crystalline materials because of elastic anisotropy. This is important and particularly evident in copper, brass, and austenitic steels. The following example illustrates the variation of sound velocity with changes in the microstructure of leaded free-cutting brass.

6.7.4 Elastic Modulus Measurement

Application:

Measurement on Young's Modulus and Shear Modulus of Elasticity, and Poisson's ratio, in non-dispersive isotropic engineering materials.

Background:

1. Young's Modulus of Elasticity is defined as the ratio of stress (force per unit area) to corresponding strain (deformation) in a material under tension or compression.

2. Shear Modulus of Elasticity is similar to the ratio of stress to strain in a material subjected to shear stress.

3. Poisson's Ratio is the ratio of transverse strain to corresponding axial strain on a material stressed along one axis.

http://www.olympus-ims.com/en/applications/elastic-modulus-measurement/http://www.olympus-ims.com/en/applications/?347[search][sCategoryId][1166017122]=1166017163&347[search][submit]=Search

Elastic Modulus Measurement – Young’s Modulus & Shear Modulus

http://en.wikipedia.org/wiki/Shear_modulus

Elastic Modulus Measurement- Poisson Ratio

These basic material properties, which are of interest in many manufacturing and research applications, can be determined through computations based on measured sound velocities and material density.

Sound velocity can be easily measured using ultrasonic pulse-echo techniques with appropriate equipment. The general procedure outlined below is valid for any (1) homogeneous, (2) isotropic, (3) non-dispersive material (velocity does not change with frequency).

This includes most common metals, industrial ceramics, and glasses as long as cross sectional dimensions are not close to the test frequency wavelength. Rigid plastics such as polystyrene and acrylic can also be measured, although they are more challenging due to higher sound attenuation.

Keyword:non-dispersive material (velocity does not change with frequency).

Rubber cannot be characterized ultrasonically because of its high dispersion and nonlinear elastic properties. Soft plastics similarly exhibit very high attenuation in shear mode and as a practical matter usually cannot be tested.

In the case of anisotropic materials, elastic properties vary with direction, and so do longitudinal and/or shear wave sound velocity. Generation of a full matrix of elastic moduli in anisotropic specimens typically requires six different sets of ultrasonic measurements.

Porosity or coarse granularity in a material can affect the accuracy of ultrasonic modulus measurement since these conditions can cause variations in sound velocity based on grain size and orientation or porosity size and distribution, independent of material elasticity.

Keyword:anisotropic materials, elastic properties vary with direction

Equipment:

The velocity measurements for modulus calculation are most commonly made with precision thickness gages such as models 38DL PLUS and 45MG with Single Element software, or a flaw detector with velocity measurement capability such as the EPOCH series instruments. Pulser/receivers such as the Model 5072PR or 5077PR can also be used with an oscilloscope or waveform digitizer for transit time measurements.

This test also requires two transducers appropriate to the material being tested, for pulse-echo sound velocity measurement in longitudinal and shear modes. Commonly used transducers include an M112 or V112 broadband longitudinal wave transducer (10 MHz) and a V156 normal incidence shear wave transducer (5 MHz). These work well for many common metal and fired ceramic samples. Different transducers will be required for very thick, very thin, or highly attenuating samples. Some cases may also require use of through transmission techniques, with pairs of transducers positioned on opposite sides of the part. It is recommended that in all cases the user consult Olympus for specific transducer recommendations and assistance with instrument setup.

The test sample may be of any geometry that permits clean pulse/echo measurement of sound transit time through a section on thickness. Ideally this would be a sample at least 0.5 in. (12.5 mm) thick, with smooth parallel surfaces and a width or diameter greater than the diameter of the transducer being used. Caution must be used when testing narrow specimens due to possible edge effects that can affect measured pulse transit time. Resolution will be limited when very thin samples are used due to the small changes in pulse transit time across short sound paths. For that reason we recommend that samples should be at least 0.2 in. (5 mm) thick, preferably thicker. In all cases the thickness of the test sample must be precisely known.

Keywords:1. Caution must be used when testing narrow specimens due to possible

edge effects that can affect measured pulse transit time. 2. Resolution will be limited when very thin samples are used due to the

small changes in pulse transit time across short sound paths.

Testing Procedure: Equipment Used.

Measure the (1) longitudinal and (2) shear wave sound velocity of the test piece using the appropriate transducers and instrument setup.

The shear wave measurement will require use of a specialized high viscosity couplant such as our SWC. A Model 38DL PLUS a 45MG thickness gage can provide a direct readout of material velocity based on an entered sample thickness, and an EPOCH series flaw detector can measure velocity through a velocity calibration procedure. In either case, follow the recommended procedure for velocity measurement as described in the instrument's operating manual. If using a pulser/receiver, simply record the round-trip transit time through an area of known thickness with both longitudinal and shear wave transducers, and compute:

Question: For measurement of shear wave velocity is normal incident transverse wave used? (hint by the used of highly viscous couplant requirement)

Testing Procedure: Velocity Measurements & Calculations

Velocity= Distance / ( ½ Round trip traverse time)

Convert units as necessary to obtain velocities expressed as inches per second or centimeters per second. (Time will usually have been measured in microseconds, so multiply in/uS or cm/uS by 106 to obtain in/S or cm/S.) The velocities thus obtained may be inserted into the following equations.

Poisson Ratio (v) =

Young’s Modulus =

Shear Modulus =

Velocity & Equations

Poisson Ratio (v) =

Young’s Modulus (E) =

Shear Modulus (G) = ,

VL, VS = Longitudinal and Shear Velocityv = Poisson ratiop = Material density

Note on units: If sound velocity is expressed in cm/S and density in g/cm3, then Young's modulus will be expressed in units of dynes/cm2. If English units of in/S and lbs/in3 are used to compute modulus in pounds per square inch (PSI), remember the distinction between "pound" as a unit of force versus a unit of mass. Since modulus is expressed as a force per unit area, when calculating in English units it is necessary to multiply the solution of the above equation by a mass/force conversion constant of (1 / Acceleration of Gravity) to obtain modulus in PSI. Alternately, if the initial calculation is done in metric units, use the conversion factor 1 psi = 6.89 x 104 dynes/cm2. Another alternative is to enter velocity in in/S, density in g/cm 3, and divide by a conversion constant of 1.07 x 104 to obtain modulus in PSI.

6.8: High Temperature Ultrasonic Testing

Experts at work

1.0 Background:

Although most ultrasonic flaw detection and thickness gauging is performed at normal environmental temperatures, there are many situations where it is necessary to test a material that is hot. This most commonly happens in process industries, where hot metal pipes or tanks must be tested without shutting them down for cooling, but also includes manufacturing situations involving hot materials, such as extruded plastic pipe or thermally molded plastic immediately after fabrication, or testing of metal ingots or castings before they have fully cooled. Conventional ultrasonic transducers will tolerate temperatures up to approximately 50° C or 125° F. At higher temperatures, they will eventually suffer permanent damage due to internal disbonding caused by thermal expansion. If the material being tested is hotter than approximately 50° C or 125° F, then high temperature transducers and special test techniques should be employed.

http://www.olympus-ims.com/en/applications/high-temperature-ultrasonic-testing/

This application note contains quick reference information regarding selection of high temperature transducers and couplants, and important factors regarding their use. It covers conventional ultrasonic testing of materials at temperatures up to approximately 500°C or 1000°F. In research applications involving temperatures higher than that, highly specialized waveguide techniques are used. They fall outside the scope of this note.

Testing Methods used:

Methods used to increase the useful range for high temperature application are:

■ Delay Line■ High temperature Couplants■ Testing Techniques & Equipment Requirements

Temperature Limitation:Conventional ultrasonic transducers 50°C

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http://www.cc6uu.com/science/article/raiders/2407

High Temperature Conventional UT-Good Till & No-More.

2.0 Methods used for H.Temperature Scanning

2.1 Transducers- H.Temperature Delay Line Material

Panametrics-NDT high temperature transducers fall into two categories,

■ dual element transducers and ■ delay line transducers.

In both cases, the delay line material (which is internal in the case of duals) serves as thermal insulation between the active transducer element and the hot test surface.

For design reasons, there are no high temperature contact or immersion transducers in the standard product line. High temperature duals and delay line transducers are available for both thickness gaging and flaw detection applications. As with all ultrasonic tests, the best transducer for a given application will be determined by specific test requirements, including the material, the thickness range, the temperature, and in the case of flaw detection, the type and size of the relevant flaws.

(1a) Thickness gaging

The most common application for high temperature thickness gaging is corrosion survey work, the measurement of remaining metal thickness of hot pipes and tanks with corrosion gages such as Models 38DL PLUS and 45MG. Most of the transducers that are designed for use with Olympus corrosion gages are suitable for high temperature use. The commonly used D790 series transducers can be used on surfaces as hot as 500° C or 930° F. For a complete list of available corrosion gauging duals that includes temperature specifications, see this link: Corrosion Gage Duals.

For precision thickness gauging applications using the Models 38DL PLUS or Model 45MG with Single Element software ,such as hot plastics, any of the standard Micro-scan delay line transducers in the M200 series (including gage default transducers M202, M206, M207, and M208) can be equipped with high temperature delay lines. DLHT-1, -2, and -3 delay lines may be used on surfaces up to 260° C or 500° F. DLHT-101, -201, and -301 delay lines may be used on surfaces up to 175° C or 350° F. These delay lines are listed in the Delay Line Option Chart.

In challenging applications requiring low frequency transducers for increased penetration, the Videoscan Replaceable Face Transducers and appropriate high temperature delay lines can also be used with 38DL PLUS and 45MG thickness gages incorporating the HP (high penetration) software option. Custom transducer setups will be required. Standard delay lines for this family of transducers can be used in contact with surfaces as hot as 480° C or 900° F. For a full list of transducers and delay lines, see this link: Replaceable Face Transducers.

(1b) Flaw detection

As in high temperature thickness gaging applications, high temperature flaw detection most commonly uses dual element or delay line transducers. All standard Panametrics-NDT flaw detection duals offer high temperature capability. Fingertip, Flush Case, and Extended Range duals whose frequency is 5 MHz or below may be used up to approximately 425° C or 800° F, and higher frequency duals (7.5 and 10 MHz) may be used up to approximately 175° C or 350° F. For a full list of transducers in this category, see this link: Flaw Detection Duals.

All of the Videoscan Replaceable Face Transducers can be used with appropriate high temperature delay lines in flaw detection applications. The available delay lines for this family of transducers can be used in contact with surfaces as hot as 480° C or 900° F. For a full list of transducers and delay lines suitable for various maximum temperatures, see this link: Replaceable Face Transducers.

Applications involving thin materials are often best handled by the delay line transducers in the V200 series (most commonly the V202, V206, V207, and V208), any of which can be equipped with high temperature delay lines. DLHT-1, -2, and -3 delay lines may be used on surfaces up to 260° C or 500°F. DLHT-101, -201, and -301 delay lines may be used on surfaces up to 175°C or 350° F. These transducers and delay lines are listed on the Delay Line Transducer List.

We also offers special high temperature wedges for use with angle beam transducers, the ABWHT series for use up to 260° C or 500° F and the ABWVHT series for use up to 480° C or 900° F. Detailed information on available sizes is available from the Sales Department.

2.2 High Temperature Couplants

Most common ultrasonic couplants such as propylene glycol, glycerin, and ultrasonic gels will quickly vaporize if used on surfaces hotter than approximately 100° C or 200° F. Thus, ultrasonic testing at high temperatures requires specially formulated couplants that will remain in a stable liquid or paste form without boiling off, burning, or releasing toxic fumes. It is important to be aware of the specified temperature range for their use, and use them only within that range. Poor acoustic performance and/or safety hazards may result from using high temperature couplants beyond their intended range.

At very high temperatures, even specialized high temperature couplants must be used quickly since they will tend to dry out or solidify and no longer transmit ultrasonic energy. Dried couplant residue should be removed from the test surface and the transducer before the next measurement.

Note that normal incidence shear wave coupling is generally not possible at elevated temperatures because commercial shear wave couplants will liquifyand lose the very high viscosity that is necessary for transmission of shear waves.

We offer two types of high temperature couplant:

■ Couplant E - Ultratherm Recommended for use between 500° and 970° F (260° to 520° C)

■ Couplant G - Medium Temperature Couplant Recommended for use attemperatures up to 600° F (315° C).

For a complete list of couplants available from Olympus, along with further notes on each, please refer to the application note on Ultrasonic Couplants.

Keyword:

Note that normal incidence shear wave coupling is generally not possible at elevated temperatures because commercial shear wave couplants will liquifyand lose the very high viscosity that is necessary for transmission of shear waves.

http://www.olympus-ims.com/en/applications/normal-incidence-shear-wave-transducers/http://static5.olympus-ims.com/data/Flash/shear_wave.swf?rev=3970http://www.olympus-ims.com/en/ultrasonic-transducers/shear-wave/

2.3 Test Techniques

The following factors should always be taken into consideration in establishing a test procedure for any high temperature application:

Transducer Time of Contacts

All standard high temperature transducers are designed with a duty cycle in mind. Although the delay line insulates the interior of the transducer, lengthy contact with very hot surfaces will cause significant heat buildup, and eventually permanent damage to the transducer if the interior temperature becomes hot enough. For most dual element and delay line transducers, the recommended duty cycle for surface temperatures between approximately 90° C and 425° C (200° F to 800° F) is no more than ten seconds of contact with the hot surface (five seconds is recomended), followed by a minimum of one minute of air cooling. Note that this is guideline only; the ratio of contact time to cooling time becomes more critical at the upper end of a given transducer's specified temperature range.

As a general rule, if the outer case of the transducer becomes too hot to comfortably hold with bare fingers, then the interior temperature of the transducer is reaching a potentially damaging temperature and the transducer must be allowed to cool down before testing continues.

Some users have employed water cooling to accelerate the cooling process, however Olympus publishes no official guidelines for water cooling and its appropriateness must be determined by the individual user

Keyword:

■ 10 second contact follows by 60 second air cooling■ Water cooling is not guarantee by Olympus NDT

Coupling Technique: The combination of transducer duty cycle requirements and the tendency of couplants to solidify or boil off at the upper end of their usable thickness range requires quick work on the part of the operator. Many users have found the best technique to be to apply a drop of couplant to the face of the transducer and then press the transducer firmly to the test surface, without twisting or grinding it (which can cause transducer wear). Any dried couplant residue should be removed from the transducer tip between measurements.

2.4 Equipment Functions

Freeze Function

Olympus Epoch series flaw detectors and all thickness gages have freeze functions that can be used to freeze the displayed waveform and reading. The freeze function is very useful in high temperature measurements because it allows the operator to capture a reading and quickly remove the transducer from the hot surface. With gages, the fast screen update mode should be used to help minimize contact time.

High Gain Boost

Gain Boost: The 38DL PLUS and 45MG gages have user adjustable gain boost functions, as do all Epoch series flaw detectors. Because of the higher attenuation levels associated with high temperature measurements, it is often useful to increase gain before making measurements.

3.0 High Temperature Testing and Variability

3.1 Velocity Variation:

Sound velocity in all materials changes with temperature, slowing down as the material heats up. Accurate thickness gaging of hot materials always requires velocity recalibration. In steel, this velocity change is approximately 1% per 55°C or 100°F change in temperature. (The exact value varies depending on the alloy.) In plastics and other polymers, this change is much greater, and can approach 50% per 55°C or 100°F change in temperature up to the melting point. If a temperature/velocity plot for the material is not available, then a velocity calibration should be performed on a sample of the test material at the actual test temperature. The temperature compensation software function in the 38DL PLUS gage can be used to automatically adjust velocity for known elevated temperatures based on a programmed temperature/velocity constant.

Keyword:■ Velocity change of -1% (minus) per 55°C or 100°F change in temperature■ Temperature versus velocity plot

Keyword:

■ Velocity change of -1% (minus) per 55°C or 100°F change in temperature■ Temperature versus velocity plot

3.2 Zero Recalibration:

When performing thickness gaging with dual element transducers, remember that the zero offset value for a given transducer will change as it heats up due to changes in transit time through the delay line. Thus, periodic re-zeroing is necessary to maintain measurement accuracy. With Olympus corrosion gages this can be quickly and easily done through the gage's auto-zero function; simply press the 2nd Function > DO ZERO keys.

3.3 Increased Attenuation:

Sound attenuation in all materials increases with temperature, and the effect is much more pronounced in plastics than in metals or ceramics. In typical fine grain carbon steel alloys, attenuation at 5 MHz at room temperature is approximately 2 dB per 100 mm one-way sound path (equivalent to a round trip path of 50 mm each way). At 500°C or 930°C, attenuation increases to approximately 15 dB per 100 mm of sound path. This effect can require use of significantly increased instrument gain when testing over long sound paths at high temperature, and can also require adjustment to distance/amplitude correction (DAC) curves or TVG (Time Varied Gain) programs that were established at room temperature.

Temperature/attenuation effects in polymers are highly material dependent, but will be typically be several times greater than the above numbers for steel. In particular, long high temperature delay lines that have heated up may represent a significant source of total attenuation in a test.

Keyword:

In typical fine grain carbon steel alloys, attenuation at 5 MHz at room temperature is approximately 2 dB per 100 mm one-way sound path (equivalent to a round trip path of 50 mm each way).

At 500°C or 930°C, attenuation increases to approximately 15 dB per 100 mm of sound path.

3.4 Angular Variation in Wedges:

With any high temperature wedge, sound velocity in the wedge material will decrease as it heats up, and thus the refracted angle in metals will increase as the wedge heats up. If this is of concern in a given test, refracted angle should be verified at actual operating temperature. As a practical matter, thermal variations during testing will often make precise determination of the actual refracted angle difficult.

Keyword:As a practical matter, thermal variations during testing will often make precise determination of the actual refracted angle difficult.

Discussion: An offshore installation of Topside to Jacket Legs, hot conventional Ultrasonic Testing at elevated temperature below 500 C was proposed. What are the critical information to be reviewed?

Hints:High temperature testing methods used & limitationsVariability due to high temperature & concerns

6.9: Dimension-Measurement Applications

6.9.1 Dimension-Measurement Applications

Ultrasonic inspection methods can be used for measurement of metal thickness. These same methods can also be used to monitor the deterioration of a surface and subsequent thinning of a part due to wear or corrosion and to determine the position of a solid object or liquid material in a closed metallic cavity.

6.9.2 Thickness measurements

are made using pulse-echo techniques. Resonance techniques were also used in the past, but have become obsolete. The results can be read on an oscilloscope screen or on a meter, or they can be printed out. Also, the same data signals can be fed through gates to operate sorting or marking devices or to sound alarms. Resonance thickness testing was most often applied to process control inspection where opposite sides of the test pieces are smooth and parallel, such as in the inspection of hollow extrusions, drawn tubes, tube bends, flat sheet and plate, or electroplated parts.

The maximum frequency that can be used for the test determines the minimum thickness that can be measured. The maximum thickness that can be measured depends on such test conditions as couplant characteristics, test frequency, and instrument design and on material type, metallurgical condition, and surface roughness.

Pulse-echo thickness gages with a digital readout are widely used for thickness measurement. Pulse-echo testing can measure such great thickness that it can determine the length of a steel reinforcing rod in a concrete structure, provided one end of the rod is accessible for contact by the search unit. Although pulse-echo testing is capable of measuring considerable thicknesses, near-field effects make the use of pulse-echo testing ineffective on very thin materials.

6.9.3 Position measurements

Position measurements of solid parts or liquid materials in closed metallic cavities are usually made with pulse echo type equipment. One technique is to look for changes in back reflection intensity as the position of the search unit is changed. In one variation of this technique, the oil level in differential housings was checked to see if the automated equipment used to put the oil in the housing on an-assembly line had malfunctioned. The test developed for this application utilized a dual-gated pulse-echo system that employed a 1.6-MHz immersion-type search unit with a thin, oil filled rubber gland over its face. The search unit was automatically placed against the outside surface of the housing just below the proper oil level, as shown in Fig. 60(a).

With oil at the correct level, sufficient beam energy was transmitted across the boundary between the housing wall and the oil to attenuate the reflected beam so that multiple back reflections were all contained in the first gate (Fig. 60b). The lack of oil at the correct level allowed the multiple back reflections to spill over into the second gate (Fig. 60c). Thus, the test was a fail-safe test that signaled "no test" (no signal in the first gate), "go" (signals in the first gate only), and "no go" (signals in both gates).

Fig. 60 Method of determining correct oil level in on automobile differential housing by use of an ultrasonic pulse-echo system. See text.

In another position measurement system, a set of two contact-type 4-MHz search units was utilized in a through transmission pitch-catch arrangement to determine the movement of a piston in a hydraulic oil accumulator as both precharge nitrogen-gas pressure and standby oil pressure varied (Fig. 61). The two search units were placed 180° apart on the outside surface of the accumulator wall at a position on the oil side of the piston, as shown in Fig. 61.

When a high energy pulse was sent from the transmitting unit, the beam was able to travel straight through the oil, and a strong signal was picked up by the receiving unit. However, as the search units were moved toward the piston (see locations drawn in phantom in Fig. 61), the sloping sides of the recess in the piston bottom deflected the beam so that very little signal was detected by the receiving unit.

Fig. 61 Setup for determining the position of a piston in a hydraulic oil accumulator by use of two contact search units utilizing a through transmission arrangement

Q144. A thin sheet may be inspected with the ultrasonic wavw direction normal to the surface by observing:

A. The amplitude of the front surface reflectionB. The multiple reflection patternC. All front surface reflectionD. None of the above

6.10: In-Service Inspection

In-Service Inspection

The methods described above are applied in the course of and immediately after the production process and are therefore called production tests. To survey highly stressed parts, especially in power plants, repeated tests or in-service inspections are becoming more and more important. In these inspections any defects identified earlier but not being a cause for rejection can be observed for any changes caused by the service conditions. In addition service-produced defects must be detected, these being mainly cracks caused by thermal shock, fatigue or creep, or by corrosion attack.

In-Service Inspection- Testing for fatigue cracks on crankshafts and crankpins. a Without bore; b with bore

In-Service Inspection- Oblique or skewed fatigue cracks on crankpins

In-Service Inspection- (a) Crack test on press columns, pump rods, etc.(b) Crack test on thread in the shadow of a sound beam; schematic screen picture above

In-Service Inspection- (a) Probe for detecting fatigue cracks in turbine discs (design Krautkriimer-Branson) (b) Detection of cracks in riveted turbine blades

In-Service Inspection- (a) Testing methods for conical defects in a bolt(b) Testing for fatigue cracks in bolts

In-Service Inspection- (a) Cross-section through a leaf spring for railway cars with quenching crack showing testing with small angle probe or normal probe. The use of surface waves is unfavorable due to roughness(b)Testing a helical spring for quenching cracks, using surface waves

6.11: Casting

Casting

In castings flaw detection is almost exclusively concerned with manufacturing defects and only rarely as in-service inspection. Suitable testing techniques and the subsequent evaluation of indications in castings is very different from the testing of forged and worked material so that the differences must not be forgotten or difficulties can occur. In-service inspection, as in the case of forgings, depends on the local stresses and the piece geometry so it is not necessary to treat it specially in this section.

Casting- Typical casting defects and their detection methods

Casting

Casting- Detection of shrinkage cavities with normal and angle probes

6.12: Bonded Joint

Inspection of Bonded Joints

If the shape of a joint is favorable, ultrasonic inspection can be used to determine the soundness of joints bonded either adhesively or by any of the various metallurgical methods, including brazing and soldering. Both pulse-echo and resonance techniques have been used to evaluate bond quality in brazed joints.

A babbitted sleeve bearing is a typical part having a metallurgical bond that is ultrasonically inspected for flaws. The bond between babbitt and backing shell is inspected with a straight-beam pulse-echo technique, using a contact-type search unit applied to the outside of the steel shell. A small-diameter search unit is used to ensure adequate contact with the shell through the couplant. Before inspection, the outside of the steel shell and the inside of the cast babbitt liner are machined to a maximum surface roughness of 3.20 μm (125 μ in.) (but the liner is not machined to final thickness).

During inspection, the oscilloscope screen normally shows three indications: the initial pulse, a small echo from the bond line (due to differences in acoustical impedance of steel and babbitt), and the back reflection from the inside surface of the liner. Regions where the bond line indication is minimum are assumed to have an acceptable bond. Where the bond line signal increases, the bond is questionable. Where there is no back reflection at all from the inside surface of the liner (babbitt), there is no bond.

Inspection of other types of bonded joints is often done in a manner similar to that described above for babbitted bearings. An extensive discussion of the ultrasonic inspection of various types of adhesive-bonded joints (including two-component lap joints, three component sandwich structures, and multiple-component laminated structures) is available in the article "Adhesive-Bonded Joints" in this Volume.

6.13: Corrosion Monitoring

Corrosion Monitoring

Ultrasonic inspection can be used for the in situ monitoring of corrosion by measuring the thickness of vessel walls with ultrasonic thickness gages. The advantage of this method is that internal corrosion of a vessel can be monitored without penetration.

There are, however, some disadvantages. Serious problems may exist in equipment that has a metallurgically bonded internal lining, because it is not obvious from which surface the returning signal will originate. A poor surface finish, paint, or a vessel at high or low temperature may also complicate the use of contact piezoelectric transducers (although this difficulty might be addressed by noncontact in situ inspection with an EMA transducer).

Despite these drawbacks, ultrasonic thickness measurements are widely used to determine corrosion rates. To obtain a corrosion rate, a series of thickness measurements is made over an interval of time, and the metal loss per unit time is determined from the measurement samples. Hand-held ultrasonic thickness gages are suitable for these measurements and are relatively easy to use.

However, depending on the type of transducer used, the ultrasonic thickness method can overestimate metal thicknesses when the remaining thickness is under approximately 1.3 mm (0.05 in.). Another corrosion inspection method consists of monitoring back-surface roughness with ultrasonic techniques. The following example describes an application of this method in the monitoring of nuclear waste containers.

6.14: Crack Monitoring

Crack Monitoring

Laboratory and in-service monitoring of the initiation and propagation of cracks that are relatively slow growing (such as fatigue cracks, stress-rupture cracks, and stress-corrosion cracks) has been accomplished with ultrasonic techniques. An example of the ultrasonic detection of stress-rupture cracks resulting from creep in reformer-furnace headers is given in the article "Boilers and Pressure Vessels" in this Volume. A relatively new and improved approach for monitoring the growth of cracks is done with ultrasonic imaging techniques.

Monitoring of fatigue cracks in parts during laboratory tests and while in service in the field has been extensively done using ultrasonic techniques. Reference 13 describes the use of surface waves to detect the initiation of cracks in cylindrical compression-fatigue test pieces having a circumferential notch. The surface waves, which were produced by four angle-beam search units on the circumference of each test piece, were able to follow the contour of the notch and detect the cracks at the notch root.

Monitoring the crack-growth rate was accomplished by periodically removing the cracked test piece from the stressing rig and measuring the crack size by straight-beam, pulse-echo immersion inspection. It was found necessary to break open some of the cracked test pieces (using impact at low temperature) and visually measure the crack to establish an accurate calibration curve of indication height versus crack size.

The use of pulse-echo techniques for monitoring fatigue cracks in pressure vessels in laboratory tests is described in Ref 14. These techniques use several overlapping angle-beam (shear wave) search units, which are glued in place to ensure reproducible results as fatigue testing proceeded. The in-service monitoring of fatigue cracking of machine components is often accomplished without removing the component from its assembly.

For example, 150 mm (6 in.) diam, 8100 mm (320 in.) long shafts used in pressure rolls in papermaking machinery developed fatigue cracks in their 500 mm (20 in.) long threaded end sections after long and severe service. These cracks were detected and measured at 3-month intervals, using a contact-type straight-beam search unit placed on the end of each shaft, without removing the shaft from the machine.

When the cracks were found to cover over 25% of the cross section of a shaft, the shaft was removed and replaced. In another case, fatigue cracking in a weld joining components of the shell of a ball mill 4.3 m (14 ft) in diameter by 9.1 m (30 ft) long was monitored using contact type angle-beam search units. The testing was done at 3-month intervals until a crack was detected; then it was monitored more frequently. When a crack reached a length of 150 mm (6 in.), milling was halted and the crack repaired.

6.15: Stress Measurements

Stress Measurements

With ultrasonic techniques, the velocity of ultrasonic waves in materials can be measured and related to stress (Ref 16). These techniques rely on the small velocity changes caused by the presence of stress, which is known as an acousto-elastic effect. The technique is difficult to apply because of the very small changes in velocity with changes in stress and because of the difficulty in distinguishing stress effects from material variations (such as texture; see Ref 17). However, with the increased ability to time the arrival of ultrasonic pulses accurately (±1 ns), the technique has become feasible for a few practical applications, such as the measurement of axial loads in steel bolts and the measurement of residual stress (Ref 5)..

The real limitation of this technique is that in many materials the ultrasonic pulse becomes distorted, which can reduce the accuracy of the measurement. One way to avoid this problem is to measure the phase difference between two-tone bursts by changing the frequency to keep the phase difference constant (Ref 5). Small specimens are used in a water bath, and the pulses received from the front and back surfaces overlap. The presence of stress also rotates the plane of polarization of polarized shear waves, and there is some correlation between the angle of rotation and the magnitude of the stress. Measurement of this rotation can be used to measure the internal stress averaged over the volume of material traversed by the ultrasonic beam.

6.16: Bond Testing

The real limitation of

6.App-1: TOFD IntroductionNOTE: Not in the exam syllabus or BOK

6.App-1.1 TOFD Basic Theory

TOFD is usually performed using longitudinal waves as the primary detection method. Ultrasonic sensors are placed on each side of the weld. One sensor sends the ultrasonic beam into the material and the other sensor receives reflected and diffracted ultrasound from anomalies and geometric reflectors.

TOFD provides a wide area of coverage with a single beam by exploiting ultrasonic beam spread theory inside the wedge and the inspected material. When the beam comes in contact with the tip of a flaw, or crack, diffracted energy is cast in all directions. Measuring the time of flight of the diffracted beams enables accurate and reliable flaw detection and sizing, even if the crack is off-oriented to the initial beam direction.

During typical TOFD inspections, A-scans are collected and used to create B-scan (side view) images of the weld. Analysis is done on the acquisition unit or in post-analysis software, positioning cursors to measure the length andthrough-wall height of flaws.

Keywords:

■ Tip Diffraction■ Off-oriented to the initial beam direction■ Time of Flight■ A-scan / B-scan■ Post analysis software

6.App-1.2 Main Benefits of TOFD for Weld Inspection

Based on diffraction, so relatively indifferent to weld bevel angles and flaw orientation

Uses time of arrival of signals received from crack tips for accurate defect positioning and sizing

Precise sizing capability makes it an ideal flaw monitoring method Quick to set up and perform an inspection, as a single beam offers a large

area of coverage Rapid scanning with imaging and full data recording Can also be used for corrosion inspections Required equipment is more economical than phased array, due to

conventional nature (single pulser and receiver) and use of conventional probes

Highly sensitive to all weld flaw types

TOFD offers rapid weld inspection with excellent flaw detection and sizing capacities. The diffraction technique provides critical sizing capability with relative indifference to bevel angle or flaw orientation. TOFD can be utilized on its own or in conjunction with other NDT techniques.

6.App-1.3 More Reading on Time of Flight Diffraction (TOFD)

6.App-1.3.1 The TheoryTime of flight diffraction (TOFD) detects flaws using the signals diffracted from the flaw’s extremities. Two angled compression wave probes are used in transmit-receive mode, one each side of the weld. The beam divergence is such that the majority of the thickness is inspected, although, for thicker components, more than one probe separation may be required. When the sound strikes the tip of a crack, this acts as a secondary emitter which scatters sound out in all directions, some in the direction of the receiving probe. A ‘lateral wave’ travelling at the same velocity as the compression waves, travels directly from the transmitter to the receiver. The time difference between the lateral wave and the diffracted signal from the flawprovides a measure of its distance from the scanned surface. If the flaw is large enough in the through wall dimension, it may be possible to resolve the tip diffracted signals from its top and bottom, thereby allowing the through wall height of the flaw to be measured.http://www.iteglobal.com/services/advanced-ndt/time-of-flight-diffraction-tofd/

Due to the low amplitude of the diffracted signals, TOFD is usually carried out using a preamplifier and hardware designed to improve signal-to-noise performance. As the probes are scanned along the weld, the RF A-Scan signals are digitised and displayed in the form of a grey-scale image showing flaws as alternating white and black fringes.

Depending on which direction the probes are moved over the component surface, it is possible to construct ‘end-view’; (B-scan TOFD) or ‘side-view’(D-scan TOFD) cross-sectional slices. TOFD can also utilise Synthetic Aperture focusing or beam modelling software to minimise the effects of beam divergence, thereby providing more accurate location and sizing information.

TOFD is generally recognised as the most accurate ultrasonic technique for measuring the through-wall height of planar flaws that lie perpendicular to the surface and as a method for detecting and quantifying crevice corrosion at the weld root. At present, national standards for the application of TOFD exist, however, no acceptance criteria have been agreed upon.

The TOFD technique is suited for the detection and sizing of all types of embedded flaws, especially those planar in nature. However, the detection of small near the scan surface flaws can be more difficult due to the presence of the lateral wave response which often occupies several millimeters of the depth axis on images.

Tips Diffractions

TOFD

Diffracted wave from upper end of crackDiffracted wave from lower end of crack

Crack

Back-wall echo

ReceiverTransmitter

Diffracted wave from lower end of crack

Diffracted wave from upper end of crack

Lateral wave

Crack height can be calculate by measuring propagation delayed time of diffraction wave

6.App-1.2 Application Examples

■ TOFD for Weld Root Corrosion and Erosion

For piping and other flow systems, certain conditions exist that lead to corrosion and erosion in the weld root and the heat-affected zone (HAZ) of the weld. The contributing factors are often metallurgical, chemical, or flow related, and the resulting metal loss can lead to failure of the weld/base metal. The shape of the corroded or eroded weld or base metal can make ultrasonic inspection extremely difficult to apply, thus impeding accurate detection and measurement of anomalies.

The time-of-flight diffraction (TOFD) technique proves to be a valid option for evaluating weld root corrosion and erosion, as well as similar conditions such as FAC (flow-accelerated corrosion). The goal of any of these inspections is to accurately measure the wall thickness, the weld, and the HAZ. The unpredictable shape of the remaining material often makes pulse-echo ultrasonic inspection ineffective.

http://www.olympus-ims.com/en/applications/tofd-for-weld-root-corrosion-and-erosion/

TOFD has been used for some time for general weld inspections. It has proven to be a rapid and easily deployable method with an excellent capacity for sizing. One of the inherent strengths of TOFD for detection and sizing purposes is its relative indifference to the orientation of defects because of its primary use of diffracted versus reflected energy.

The TOFD technique utilizes two transducers: a transmitter transducer floods the inspected region with sound in the forward direction; on the opposite side of the weld, a receiver transducer is positioned to receive diffracted and reflected energy from the back wall or from anomalies present in the region. Common pulse-echo techniques can be misdirected by the shape of the region, resulting in imprecise measurement and assessment.

Figure 5-3 – Preferential weld corrosion in lean amine (Reference 5)

Figure 5-2 – Hot Lean Amine Corrosion of Carbon Steel:

Weld Root Corrosion and Erosion

Pulse-echo shear wave beam being reflected at an off angle.

Illustration of diffracted energy reflecting off weld root/HAZ in all directions.

For these types of weld inspections, TOFD is typically performed from three positions for each weld: (1) centered on the weld, (2) offset to the left, and (3) offset to the right.

Scanning from these particular positions helps to achieve the best results. This method ensures detection of the highest point of material loss, determines from which side of the weld the erosion/corrosion indications are originating, and eliminates any masking caused by the back wall signal. Depending on the instrument, these scans can be run concurrently or in separate acquisitions.

TOFD is deployed by scanning the weld with a semiautomatic or fully automatic scanner. Scan settings are set to determine scan resolution. The resulting data file can be saved indefinitely for review and comparison to future scans. After data is acquired, it is analyzed to identify any areas of concern, either directly on the instrument or in post-analysis software. Shifts in data (time/depth) are measured in order to assess the severity of metal loss. The cursors can then be positioned to define areas for depth or thickness measurement readings. Weld defects such as porosity, lack of fusion, and cracking can also be detected when scanning for corrosion and erosion.

Scan of weld with cursor positioned on an uncorroded area; A-scan shows good lateral wave and back wall signal with no indications in between.

Scan of weld with cursor positioned on a corroded area; A-scan shows shift in time of back wall signal from material loss.

Measurement of good area shows thickness as 7.39 mm; TOFD (m-r) reading shows the distance between the positioned cursors.

Measurement of corroded area shows thickness as 5.28 mm; cursors are positioned at top of plate (0) and highest point of material loss. In this example, there is 2.11 mm of material loss due to corrosion.

6.11.3.3TOFD for Corrosion Measurement Equipment (Typical)

OmniScan SX or MX2 (PA or UT models, depending on the number of channels desired and if phased array capability is needed).

TOFD circumferential scanner (HST-Lite or similar, depending on the desired number of probe holders and other application specifics; for example, pipe versus plate).

TOFD probe and wedges (various frequencies, angles, and materials). Couplant delivery system, WTR-SPRAYER-8L or similar. TomoView Analysis or OmniPC post-analysis software (optional).

6.App-1.3.4 TOFD Benefits for Corrosion/Erosion Measurement

Rapid scanning. Cost effective. Auditable and retrievable permanent data sets. Accurate sizing capability. Excellent detection, even on irregularly shaped areas of metal loss. Fast post-acquisition analysis results. Portable and user-friendly TOFD scanning packages.

TOFD for Weld- TOFD Parallel Scanning

6.App-1.3.5 Overview on Scanning Direction

Most typical TOFD inspections are performed with the send and receive transducers on opposite sides of the weld and scanning movement parallel to the weld axis. The main purpose of this “perpendicular” (defined by beam to weld relationship) scanning is to quickly perform weld inspection with the weld cap or re-enforcement in place. This technique can give location in the scan axis, the indication length, height of indication and flaw characterization information. One of the weaknesses of this technique is the lack of index positioning (or where between the probes) the indication is located. This information is usually obtained with complimentary pulse echo ultrasonics when the weld is left in place.

? Carriage movement direction

■ Perpendicular Scanning

Scanning direction “parallel” to the weld axis. Beam direction “perpendicular”to the weld axis.

One of the weaknesses of this technique is the lack of index positioning (or where between the probes) the indication is located.

■ Parallel TOFD scanning:

Where the scan direction and beam direction are the same is less used, for obvious reasons of not being able to cover the entire length of weld rapidly, more complex movement pattern required of scanner mechanisms, and complexity of the data output of an entire weld inspected. This technique does have advantages when it is possible to be performed.

Typical “Perpendicular” Weld Scanning Setup and Data Collected. Data is side view of weld from scan start to scan finish down the weld. Position of encoder and scanning direction are highlighted.

Typical “Parallel” Weld Scanning Setup and Data Collected. Data is side view of weld from scan start to scan finish across the weld. Position of encoder and scanning direction are highlighted.

■ Benefit of TOFD Parallel Scanning

Although perpendicular TOFD scanning down the weld can give highly accurate depth measurement, generally speaking a parallel scan will give more accurate depth information as well as flaw information, and location in the index position in the weld. With perpendicular scanning, no index position is possible without multiple offset scans being performed or complimentary NDT techniques to position the flaw. In parallel scanning Index position is ascertained by locating the minimum time peak, which corresponds to when the indication is centered between the two probes. For these reasons this technique is often used in critical crack sizing inspections, as well as change monitoring, in other words, monitoring a crack or other defect for growth until it reaches a critical level at which time it is repaired or replaced. For these reasons the technique is often performed on critical components that are costly to shut down for repair, often in the Power Generation industry. More information is often gathered from the flaw as diffraction occurs across the flaw instead of just down the flaw.

6.App-1.3.6 Further Reading- Introduction to Phased Array

http://www.olympus-ims.com/en/ndt-tutorials/intro/ut/

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ut testing section 6 selected applications & techniques

Engineering

gate of casting

thicker section of casting

blow hole porosity

molten metal

solidified metal

shrinkage stress

small hole

complexshape forging