BMFB 4283NDT & FAILURE ANALYSIS

Lectures for Week 1

Prof. Qumrul Ahsan, PhD Department of Engineering MaterialsFaculty of Manufacturing Engineering

1.0 Introduction to NDE

1.1 Definitions and Understanding of NDE1.2 Concept of Defects and Discontinuity1.3 Visual Inspection

Issues to address

NDE is the examination of an object with technology that does not affect the object’s future usefulness

i.e. refers to technology that allows a component to be inspected for serviceability, without impairing its usefulness

Non Destructive Evaluation (NDE)

Non destructive evaluation a term interchangeably with non destructive testing (NDT)

NDT means

The use of noninvasive techniques to determine the integrity of a material, component or structure

or quantitatively measure some characteristic of an object.

Methods of NDT

Visual

Liquid Penetrant

Magnetic Particle

Eddy Current

Ultrasonic

X-ray

Microwave

Acoustic Emission

Thermography

Laser Interferometry

Replication

Flux Leakage

Acoustic Microscopy

Magnetic Measurements

Tap Testing

Six Most Common NDT Methods• Visual

• Liquid Penetrant • Magnetic • Ultrasonic• Eddy Current• X-ray

What are Some Uses of NDE Methods?

• Flaw Detection and Evaluation• Leak Detection • Location Determination• Dimensional Measurements • Structure and Microstructure Characterization • Estimation of Mechanical and Physical Properties • Stress (Strain) and Dynamic Response

Measurements • Material Sorting and Chemical Composition

Determination

Fluorescent penetrant indication

NDE - A Full Spectrum Technology

NDE Technologies

Materials DevelopmentTo assist in product development

Design

ProcessingTo screen or sort incoming materials

ManufacturingTo verify proper assembly

In-Service MonitoringTo inspect for in-service damage

There are NDE application at almost any stage in the production or life cycle of a component

Common Application of NDT

• Inspection of Raw Products

• Inspection Following Secondary Processing

• In-Services Damage Inspection

Inspection of Raw Products• Forgings,• Castings,• Extrusions,• etc.

• Machining• Welding• Grinding• Heat treating• Plating• etc.

Inspection Following Secondary Processing

• Cracking• Corrosion• Erosion/Wear• Heat Damage• etc.

Inspection For In-Service Damage

How is NDE Applied ?• Process or quality control into feedback control

– Monitors the process– Feeds the sensor response back to the feedback controller– Controller controls the process variables

• Process or quality control into accept/reject criteria– Inspects the finished product– “go” - > accept the product to perform service– “no go” -> reject the product ; reprocessed, recycled or

scrapped

Understanding the NDE Choices

• Physical nature of the material property or discontinuity to be inspected

• Processes that govern NDE methods• Interaction of the probing field (or

material) with the test material• The potential and limitations of available

technology• Economic, environmental, regulatory, and

other factors

How much do we inspect ?• Statistics• Consequences of Part Failure• Larger Systems or Safety-Critical

parts• Retirement for cause• Risked-Informed Inspection

Definition of discontinuities• Discontinuity : An intentional or unintentional

interruption in the configuration of the part• Flaw : A detectable lack of continuity or a

detectable imperfection in a physical or dimensional attribute of a part

• Defect : One or several discontinuities that do not meet specifications

• Nonconforming : A part is deficient in one or more specified characteristics

Other important vocabularies• Indication : Observation of a discontinuity that requires

interpretation e.g. cracks, inclusions, gas pockets

• Interpretation : Determination whether an indication is relevant, nonrelevant or false

• False : Indication not due to presence of defects in the test material or due to test procedure

• Nonrelevant : An indication which has no relation to a discontinuity that is considered a defect in the part being tested

• Evaluation : Assessment of a relevant indication to determine wether specifications of the serviceability of the part are met

Discontinuities & Defects• NATURE OF DEFECTS

– Microscopic Defects– Macroscopic Defects

• ORIGIN OF DEFECTS– Inherent Defects– Processing Defects– Service Defects

• DETERIORATES PHYSICAL and MECHANICAL PROPERTIES of MATERIALS

• DETECTION of DEFECTS– Destructive Testing– Non-Destructive Testing

Destructive Test Nondestructive Test

Advantages • Measurements are direct and reliable• Quantitative measurements• Direct correlation between test measurements and material properties 

Advantages • Tests are done directly on the object• 100% testing on actual

components is possible• Many NDT methods can be

applied on the same part hence many or all properties of interest can be measured.• In-service testing is possible• Repeated checks over a period of time are possible• Very little preparation is sufficient• Most test methods are rapid

Table 1.0 Comparison of Destructive and Non-Destructive Tests

Table 1.0 Comparison of Destructive and Non-Destructive Tests

Limitations • Tests are not made on the objects directly. Hence need to prove correlation between the sample specimen used and object• A single test may measure only one or few of the properties• In-service testing is not

possible• Measurement of properties

over a cumulative period of time cannot readily be possible• Preparation of the test

specimen is costly• Time requirements are

generally high

Limitations • Measurements are indirect• Reliability to be verified• Qualitative measurements• Measurements can also be done quantitatively• Skilled judgment and experience are required to interpret indications

Destructive Test Nondestructive Test

CASTING DEFECTS : Inclusions

• Inclusions are nonmetallic materials (oxides or sulphides)

• a lower melting point than the metal

• relative brittleness• these defects occur near the

surface as a “skin” effect. • occur at the centre of the

casting• they tend to be irregular in

shape, not spherical or oval.

CASTING DEFECTS : Porosity

• gas trapped in the molten metal

• formed by – release from the molten metal

itself– release from the green sand

mold, water vapour, or turbulence caused when pouring the metal

• form of small pockets or voids appears as round irregular or elongated shapes

CASTING DEFECTS : Shrinkage • Metal contracts or shrinks when changing from the liquid

to the solid state and defects will occur in a casting unless sufficient molten metal is available to “feed” it.

• Defects may take the form of cavities, branch-like tears • Shrinkage cavities occur usually at hot spots in the casting

CASTING DEFECTS• Piping : Central Cavity• As the casting solidifies,

the metal contracts if there is not an adequate supply of molten metal to the centre of the casting

• usually occurs in pure metals and alloys having narrow ranges of solidification temperature

• causes lamination

• Hot TearsIf a section begins to shrink while still hot and there is not a sufficient supply of liquid metal, the resulting internal stress will tear the metal. This is because while hot the metal has relatively low strength

Open Die Forging Defects

Fracture -– exhausted ductility– Intergranular fracture in

hot working

• Barreling - Friction• Solution -

– limited deformation per step

– Process anneal between steps

Closed Die Forging Defects

• Laps

Rolling Defects : Stringers

• Nonmetallic inclusions in slabs or billets, that are thinned and lengthened in the direction of rolling, by the rolling process, are called stringers.

Other Processing Defects

• Heat-Treated Cracks

• Grinding Cracks

undercut• Groove in the base material• Result of improper welding

technique (high travel speed, heat is too high)

underfill

Weld Defects

• Groove in the weld material• Result of improper welding

technique (inadequate filler material and high travel speed)

overlap • Excessive weld material (may not fuse to base)• Result of improper welding technique (welding

travel speed is too slow)

Incomplete root fusion or penetration

Excessively thick root face in a butt weld

Too small a root gapArc (heat) input too low

Weld Defects

• In MMA welding, the risk of incomplete root fusion can be increased by using the incorrect welding parameters and electrode size to give inadequate arc energy input and shallow penetration.

Large diameter electrode Small diameter electrode

Lack of side wall and inter-run fusion

Causes

• too narrow a joint preparation

• incorrect welding parameter settings

• poor welder technique

• magnetic arc blow

• Insufficient cleaning of oily or scaled surfaces

“These types of imperfection are

more likely to happen when welding

in the vertical position”

Lack of side wall fusion

Lack of inter-run fusion

• Porosity : Causes

• absorption of N, O, and H in the molten

weld pool which is then released on

solidification to become trapped in the

weld metal.

• N and O absorption in the weld pool

usually originates from poor gas

shielding.

• H can originate from from inadequately

dried electrodes, fluxes or the workpiece

surface. Grease and oil on the surface of

the workpiece or filler wire are also

common sources of hydrogen.

Distributed porosity

Surface breaking pores

Weld Defects

Slag inclusionsCauses

• Type of flux • Welder technique

Best practice

• Use welding techniques to produce smooth weld beads

and adequate inter-run fusion

• Use the correct current and travel speed to avoid

under-cutting the sidewall which will make the slag

difficult to remove

• Remove slag between runs paying particular attention

to removing any slag trapped in crevices

• Use grinding when welding difficult butt joints otherwise

wire brushing or light chipping may be sufficient to

remove the slag.

Poor (convex) weld bead profile resulted in pockets of slag being trapped between the weld runs

Radiograph of slag inclusions

Weld Defects

Solidification crackingIdentification

Solidification cracks are normally readily distinguished from other types of cracks due to the following characteristic factors:

• they occur only in the weld

metal • they normally appear as

straight lines along the

centreline of the weld bead• as the cracks are 'open', they

are easily visible with the naked

eye

Causesweld bead during solidification has insufficient strength to withstand the contraction stresses generated as the weld pool solidifies. Factors which increase the risk include:

• insufficient weld bead size or shape • welding under high restraint • material properties such as a high

impurity content or a relatively large

amount of shrinkage on solidification

Weld Defects

Crack along the coarse grain structure in the HAZ

Hydrogen cracks in steels

CausesThere are three factors which combine to cause cracking:

• hydrogen generated by the welding process • a hard brittle structure which is susceptible to cracking • residual tensile stresses acting on the welded joint

The effects of specific factors on the risk of cracking are:• weld metal hydrogen • parent material composition • parent material thickness • stresses acting on the weld • heat input

Weld Defects

Visual Inspection

Outline

• Introduction• Basic principles • Manual Vision Inspection

– Human Vision – Common Inspection applications– Equipment

• Automated or Machine Vision Inspection– Machine Vision– Common Inspection Applications– Equipment

• Advantages and Limitations

Introduction• Visual inspection is commonly defined as “the examination

of a material, component, or product for conditions of nonconformance using light and the eyes, alone or in conjunction with various aids”.

• Visual inspection often also involves, shaking, listening, feeling, and sometimes even smelling the component being inspected.

• Visual inspection consists of at least two major processes. – The first is a search process. – The second is a process of combining relevant knowledge,

sensory input, and pertinent logical processes to provide an identification that some anomaly or pattern represents a flaw that poses a risk to the performance of the part.

• Visual inspection is commonly employed to support other NDT methods.

• Digital detectors and computer technology have made it possible to automate some visual inspections. This is known as “machine vision inspection.”

• The quality of an inspection are affected primarily by four factors.

– The quality of the detector (eye or camera).– The lighting conditions.– The capability to process the visual data.– The level of training and attention to detail.

Introduction

Introduction – Manual Versus Automated Inspection

• The majority of visual inspections are completed by an inspector, but machine vision is becoming more common.

• The primary advantage of an inspector is their ability to quickly adapt to a variety of lighting and other non-typical conditions, and their ability to use other senses.

• The primary advantage of a machine vision inspection system is their ability to make very consistent and rapid inspections of specific details of a component.

• Machine vision is primarily used in production applications where a large number of components require inspection and the inspection conditions can be closely controlled.

Basic Principles – Contrast Sensitivity

The graph to the right plots thevisibility of a spot as a function of theabove variables

• Contrast sensitivity is a measure of how faded or washed out an object 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 varies with– the size or spatial frequency

of a feature– The lighting conditions– Whether the object is lighter

or darker than the background

• Effective visual inspection requires adequate lighting.

• The type of inspection will dictate the lighting requirements. Inspection of components with fine detail and low contrast will require greater illumination than components with large details and high contrast.

• Light intensity may be measured with a suitable light meter. The unit of measure for white light is foot-candles (fc).

– A foot-candle is equal to the amount of direct light thrown by one standard candle at a distance of 1 foot.

• Inspection of components with fine detail and low contrast may require 100 foot-candles or more.

• Specification requirements for lighting should be reviewed prior to performing an inspection.

Basic Principles –Light Intensity Measurement

Basic Principles –Light Directionality

• The directionality of the light is a very important consideration.

• For some applications, flat, even lighting works well.

• For other applications, directional lighting is better because it produces shadows that are larger than the actual flaw and easier to detect.

Are the horizontal lines parallel or do they slope? How many black dots do you see?

Sometime the eye/mind has trouble correctly processing visual information.

Basic Principles –Optical Illusions

• For best results the inspector or machine vision operator must have:

– A basic knowledge of material processing, forming, machining and joining processes.

– A general understanding of design features, application and service requirements.

– Specific instructions on what to look for and specific accept/reject criteria.

Basic Principles

Inspection Applications

• Detection of surface anomalies such as scratches, excess surface roughness, and areas void of paint or plating.

• Crack, porosity, corrosion or other flaw detection.• Dimensional conformance.• Precision measurements.• Foreign object detection.• Component location.

Applications for visual inspection and many and range from looking a product over for obvious defect to performing detailed inspections. Some of the common applications include:

• Visual inspection of manufactured materials and components is a cost effective means of identifying flaws.

• Visual inspection of a casting reveals a crack between a threaded opening and a pressed fit.

• The aluminum sand casting has hot tears and shrinkage at the transition zones.

Inspection Applications –Flaw Detection

• In this example, visual inspection of a fire escape reveals a failure in a handrail tube.

• The failure is in the tube seam and is likely the result of ice expansion.

Inspection Applications – Flaw Detection

In-service inspections of existing components and structures is commonly accomplished visually.

Normal inspection practices for highway bridges rely almost entirely on visual inspection to evaluate the condition of the bridges.

Inspection Applications – Flaw Detection

Over 80 percent of all aircraft inspections are performed visually.

Inspection Applications – Flaw Detection

• Weld quality requirements are commonly determined through visual inspection.

• Many standards have established acceptance criteria for welds.

Slag rolled into toe of weld

Transverse weld crack

Inspection Applications – Flaw Detection

Dimensional Conformance

• Visual inspection is commonly employed for general dimensional conformance, assembly fit, and alignment between components

• Common applications include determining:– Weld size and tolerance.– Component dimensions.– Material alignment and allowable distortion.

Dimensional ConformanceWelds are commonly inspected for dimensional tolerance.• There are several types of gages used to inspect welding fit up and

finished weldments.• These gages are intended for general inspection where close tolerances

are not required.• The gage used is determined by the application.

Fillet gage set

Palmgren gage

VWAC gage

Cambridge gage

Dimensional Conformance

Visual inspection is commonly used to determine weld size and tolerances according to standards and engineering specifications

Throat measurement using a Palmgren gage.

Leg size determination with fillet gage.

Convexity measurement with VWAC gage.

Undercut in a weld is readily seen visually. In many cases its depth must be measured to determine if it exceeds code requirements.

Measurement of undercut depth with VWAC gage.

Dimensional Conformance

Dimensional Conformance Alignment/Distortion

• Visual inspection frequently involves checking materials and components for fit and alignment.

• Many standards establish allowable tolerances for fit and distortion.

• Structural fabrication requires dimensional inspection of finished components prior to shipment to the field site.

• Basic tools are used for the inspection. An inspector will set up string lines at known distances and plum them using a tape measure. Measurements are then taken at various locations and compared to code requirements. In this image a

fabricated girder is being inspected for distortion, sweep and web flatness.

Equipment

• Visual inspection equipment includes a variety of different tools. These may range from basic rulers, tape measures and spring type calipers to rigid or flexible borescopes and remote crawlers with cameras.

• Many tools have been designed for specific applications such as the various weld gauges.

• Some of the specialized tools such as crawlers have been designed to satisfy the inspection needs in applications where conventional techniques are not feasible.

• Sliding calipers are a precision refinement of the common rule, which results in greater accuracy of measurements.

• They may incorporate either a dial indicator or digital readout.

• Sliding-type calipers are commonly used to check dimensional tolerances of machined components, wear on components, and fit between components.

Equipment – Precision Measurements

Equipment – Precision Measurements

• Micrometers are precise measurement instruments used to make accurate direct readings in contact measurements.

• Micrometers are designed for inside, outside, and depth measurements, and are available in a wide variety of shapes and sizes.

• Micrometers may be either thousandth inch (.001”) or ten thousandth inch (.0001”) measurement capable.

Equipment – Transferring Gauges• Transfer instruments

are used to take measurements which are transferred to direct measurement devices.

• They consist of calipers, dividers, telescoping gages and small hole gages.

Equipment – Transferring Gauges• Spring type calipers are available

for contact measurements of inside and outside dimensions.

• They are useful for measuring distances between and over surfaces.

• They are commonly used to transfer dimensions or sizes between the work piece and standard measuring devices, such as graduated rules.

• The size of a linear or rounded indication of a discontinuity may be measured with dividers.

Equipment – Transferring Gauges• Small hole gages are a type of transfer instrument used to measure small

holes or slots.• They are generally supplied in sets with a range of 1/8” - 1/2”.• The actual measurement is determined by transferring a properly

adjusted gage to a micrometer.

Equipment – Transferring Gauges

• Telescoping gages make inside measurements such as hole diameter and slot width.

• They are designed to be measured by a micrometer after being set to the hole or slot size.

• To make accurate measurements it is important to make sure the telescoping gage is aligned properly in the measuring faces of the micrometer.

Direct and Remote Visual Inspection

• Many codes refer to direct visual examination as a visual inspection which requires that access to the area is sufficient to place the eye within 24 inches of the surface to be examined and at an angle of not less than 30o to that surface

• If these requirements cannot be met, then remote visual inspection may be used.

• Remote visual inspection may be accomplished with the use of a number of optical aids such as, mirrors, magnifiers, and rigid or flexible borescopes.

Optical Aids• Mirrors are valuable aids in visual

inspection, they allow the inspection of threaded and bored holes, inside surfaces of pipes and fittings, as well as many others.

• Magnifiers assist the visual inspector by enlarging the size of the object being examined.

• Comparators are a magnifier with a measuring capability. The comparator has interchangeable reticles which provide measurements for threads, angles, linear measurement, diameters and radii.

Optical Aids• Borescopes are visual aids used for the inspection of

internal surface areas.• They are designed for remote viewing in difficult to reach

areas such as jet engines, cylinders, tanks, and various enclosed chambers.

• Borescopes are available in many different diameters and lengths, and are classified as rigid or flexible.

Visual Inspection With A Borescope

Clean Surface Corrosion Damage

Optical Aids• Advances in technology has allowed video equipment

to be adapted to portable and robotic devices.• Portable video probes allow inspectors to remotely

perform examinations in closed chambers which are inaccessible by convention inspection means.

• Robotics have been developed whereby cameras can be affixed to crawlers and submersibles.– Retrieval tools can be affixed to robotics to remove

foreign objects.• Conventional recording techniques may

be employed for image capture and storage with many of the remote video inspection methods.

• Machine vision technology uses an imaging system and a computer to analyze an image and to make decisions based on that analysis.

• In inspection applications, the machine vision optics and imaging system enable the processor to "see" objects precisely and thus make decisions about which component meet a specific inspection criteria.

• Machine vision can eliminate human factor error that might result from doing difficult, tedious, or boring tasks. It also allows process equipment to be utilized 24 hours a day.

Machine Vision – Basic Principles

The following process steps are common to all machine vision applications:

• Image acquisition: An optical system gathers an image, which is then converted to a digital format and stored into computer memory.

• Image processing: A computer processor uses various algorithms to enhance elements of the image that are of specific importance to the process.

• Feature extraction: The processor identifies and quantifies critical features in the image (e.g., the position of holes on a printed circuit board, the number of pins in a connector, the orientation of a component on a conveyor) and sends the data to a control program.

• Decision and control: The processor's control program makes decisions based upon the data. Are the holes within specification? Is a pin missing?

Machine Vision – Basic Principles

Key System Elements• A variety of components are

included in a machine vision system, which depend on the environment, the application, and the budget. However, the following components are common to all vision systems : – Front-end optics: this includes the lighting, the lens, and the camera. – Frame grabber: this is a computer processor board that accepts the

video input from the camera, digitizes it, and stores it for analysis. – Processor: A computer processor is required to control the vision

application. – Control Software: Computer software is used for controlling and

executing vision tasks.

Machine Vision - Equipment

Advantages of Visual Inspection

• Readily used on almost all materials.• Simple to perform.• Low in cost, (application dependent).• Relatively quick.• Results may be permanently recorded.• Can be automated.

Limitations of Visual Inspection

• Direct inspections are limited to surfaces only.• Indirect inspections require greater inspector

knowledge and training.• Inspector dependent, knowledge of materials and

processing, eye sight.• Standards (workmanship) may be difficult to obtain.

Elements in Visual Inspection

• Test Object• Inspector• Optical Instrument• Illumination• Recording

Evaluation in Visual Inspection

• All visual tests are to be evaluated in terms of the acceptance criteria specified in the appropriate product standard

• The results of this test then need to be recorded for future reference

Conclusions on Visual Inspection

• Visual Inspections are simple quick and widely used NDT techniques to examine the surface (exterior or interior) for both qualitative and quantitative assessment

• Automated visual inspection for faster mode and required more skill.