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Lesson 1 Fundamentals of Nondestructive Testing

ASM International, copyright 2012 Page 1

Fundamentals of Nondestructive Testing

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Table of Contents

Lesson 1: Introduction 4

Lesson 2: Liquid Penetrants 18

Lesson 3: Magnetic Particle Testing 50

Lesson 4: Magnetic Particle Inspection Applications 79

Lesson 5: Ultrasonic Testing Fundamentals 106

Lesson 6: Ultrasonic Testing Equipment 141

Lesson 7: Ultrasonic Testing Applications 169

Lesson 8: Radiographic Inspection 194

Lesson 9: Industrial Computed Tomography 270

Lesson 10: Eddy Current Fundamentals 304

Lesson 11: Eddy Current Applications 328

Lesson 12: Acoustic Emissions 356

Lesson 13: Thermal Inspection 378

Lesson 14: Microwave Optical Holography and Acoustic Holography 397

Lesson 15: Interpretation and Classification of Indications 441

Lesson 16: Nondestructive Testing Standards 466

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Fundamentals of Nondestructive Testing

Lesson 1: IntroductionNondestructive Testing comprises many terms used to describe various activities within the field. Some ofthese terms are nondestructive evaluation (NDE), and nondestructive inspection (NDI). These activitiesinclude testing, inspection, and examination, which are similar in that they primarily involve looking at (orthrough) or measuring something about an object to determine some characteristic of the object or todetermine whether the object contains irregularities, discontinuities, or flaws.

The terms irregularity, discontinuity, and flaw can be used interchangeably to mean something that isquestionable in the part or assembly, but specifications, codes, and local usage can result in differentdefinitions for these terms. Because these terms all describe what is being sought through testing,inspection, or examination, the term NDE (nondestructive evaluation) has come to include all the activitiesof NDT and NDI used to find, locate, size, or determine something about the object or flaws and allow the

investigator to decide whether or not the object or flaws are acceptable. A flaw that has been evaluatedas rejectable is usually termed a defect.

Selection of NDE Methods

Several different ways of comparing the selected NDE methods are presented in this article, but there isno completely acceptable system of comparison, because the results are highly dependent on theapplication. Therefore, it is recommended that a comparison be developed specifically for each NDE areaand application. The final validation of any NDE protocol depends on acceptance tests conducted usingappropriate calibration standards.

Upon completion of this course the learner can discuss nondestructive testing, which is convenientlydivided into nine distinct areas:

Flaw detection and evaluation

Leak detection and evaluation

Metrology (measurement of dimension) and evaluation

Location determination and evaluation

Structure or microstructure characterization

Estimation of mechanical and physical properties

Stress (strain) and dynamic response determination

Signature analysis

Chemical composition determination

The contents of this course are limited to flaw detection and evaluation. Information on the remaining

subjects can, however, is found in Volume 17of theASM Handbook on Nondestructive Evaluation andQuality Control

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Flaw Detection and Evaluation

Flaw detection is usually considered the most important aspect of NDE. There are many conceivableapproaches to selecting NDE methods. One approach is to consider that there are only six primaryfactors involved in selecting an NDE method(s):

Reasons for performing the NDEType(s) of flaws of interest in the object

Size and orientation of flaw that is rejectable

Location of the flaws of interest in the object

Size and shape of the object

Characteristics of the material to be evaluated

The most important question to be answered before an NDE method can be selected is: What is thereason(s) for choosing an NDE procedure? There are a number of possible reasons, such as:

Determining whether an object is acceptable after each fabrication step; this can be called in-process NDE or in-process inspection

Determining whether an object is acceptable for final use; this can be called final NDE or finalinspection

Determining whether an existing object already in use is acceptable for continued use; thiscan be called in-service NDE or in-service inspection

After the reasons for selecting NDE are established, one must specify which types of flaws are rejectable,the size and orientation of flaws that are rejectable, and the locations of flaws that can cause the object tobecome rejectable. The type, size, orientation, and location of flaws that can cause a rejection must bedetermined if possible, using stress analysis and/or fracture mechanics calculations. If definitivecalculations are not economically feasible, the type, size, and orientation of flaw that causes the object tobe rejected must be estimated with an appropriate safety factor.

The type, size, orientation, and location of the rejectable flaw are often dictated by a code, standard, or

requirement, such as the American Society of Mechanical Engineers Pressure Vessel Code, a NuclearRegulatory Commission requirement, or the American Welding Society Structural Welding Code.

Flaw Location, Shape, and Size. After classification of the type of flaw to be detected, it is necessary toconsider the locations of the flaws in the object. Flaws can be conveniently classified as surface flaws oras interior flaws that do not intercept the surface. Two additional factors that affect NDE method selectionare the shape and size of the object to be evaluated. On the following pages, Tables 4 and 5 compareNDE techniques for varying size (thickness) and shape.

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Table 4 Comparison of NDE methods based on size of object to be evaluatedSurface only but independent of size

Visual

Liquid penetrant

Shallow depth or thin object (thickness approx. 1 mm, or 0.04 in.)

Eddy current

Magnetic methods

Increased thickness (thickness approx. 3 mm, or 0.12 in.)

Microwave

Optical holography

Acoustic methodsIncreased thickness (thickness approx. 100 mm, or 4 in.)

Pulsed methods

Computed tomography (CT)

Increased thickness (thickness approx. 250 mm, or 10 in.)

Radiography

Thickest (dimension approx. 10 m, or 33 ft)

Ultrasonic

Note: All NDE methods suitable for thick objects can be generally used on thin objects.

Table 5 Comparison of NDE techniques based on shape of object to be evaluated

Simplest shape

Optical holographyAcoustic methods

Thermal methods

Microwave

Eddy current

Magnetic methods

Radiography

Ultrasonic

Liquid penetrant

Visual

Computed tomography (CT)

Most complex shape

Table 1:2 lists information on the most common NDE methods. The suitable NDE method can beselected based on part geometry and material and expected size and location of the potential defects.Because each NDE method has a specific behavior, it is often desirable to select several NDE methodshaving complementary detection capabilities. For example, ultrasonic and radiographic methods can beused together to ensure the detection of both planar flaws (such as cracks) and volumetric flaws (such asporosity).

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Table 1:2

NDT Applications

Method Principle of Operation Application Limitations Advantages Materials Defects

LIQUIDPENETRANT

Liquid penetrants in cracks,on an otherwise cleansurface, attract particles ofapplied dye.

Locating fabricationdiscontinuities,stress cracks.

Will not find subsurfacedefects.

Permits one-operationinspection ofcomplex shapes

Non-magneticmetals and alloys;non-metals.

Surface cracks;laminations; poorbonding.

MAGNETICPARTICLE

Magnetic particles areattracted to breaks inmagnetic lines of force.

Surface and nearsubsurfaceinspection of partssensitive tomagnetization.

Not applicable to non-magnetic metals ormaterial.

Can defect flawsup to inchbelow surfaceunder goodconditions.

Iron and steel;nickel; cobalt.

Blow holes; cavities;forging cracks; laps;grinding cracks.

ULTRASONIC High frequency vibrationsare introduced into sample.Waves are reflected orscattered by discontinuities.

Quality control;Material inspection;Preventativemaintenance.

Sensitivity is reducedby rough-surface parts;odd-shaped pieces aredifficult to analyze; askilled operator isrequired.

Locates smallestdiscontinuities;can detectdefects withdimensions halfof the excitationwavelength;locates lack ofbonding.

All metals andalloys; sinteredcarbides; glass andceramics; rubber;structural plastics;concrete.

Rolling cracks, laps;segregated inclusions;poor brazing or otherbonds; forging cracks,laps; grinding cracks;improperly cured plasticlamination; insulatorcracks and voids.

Resonance systems:frequency is varied whennatural frequency of materialis hit and the amplitude ofvibration increases.

Thicknessmeasurement;discovering laminardefects andcorrosion wall-thinning.

Works best on partswith parallel sides.

Thicknesstesting from oneside; test resultsknownimmediately;portable.

All metals andalloys; sinteredcarbides; glass andceramics; rubber;structural plastics;concrete; refractorymaterials.

PENETRATINGRADIATION

Penetrating rays (x-ray,gamma or neutron) cast

shadows on the other sideof solid objects; voidspermit more rays topenetrate; film radiographyrecords shadow onphotographic film; real-timemethods provide immediateresponse.

Manufacturing;quality control;

preventativemaintenance; weldinspection; findingforeign object inpackaged goods;geometric details ofenclosed surfaces;thickness gauging.

Economic limit to depthpenetration; hazardous

operation; not sensitiveto defects less than 1-2% thickness of totalmetal; complex shapesare difficult to analyze.

Permits visualanalysis of

buried defects orcomponents inassembly.

Metals; foods films;nonmetals;

assemblies.

Cavities, cracks,porosity, nonmetallic

inclusions, foreignparticles; incorrectthickness; incorrect ormissing components.

EDDY CURRENT Equipment constantlymeasures the impedance ofthe probe coil; when the coilis placed in contact with anyconductive material the coilimpedance changes in directrelationship with the specificmaterial properties and

Automatedproduction linetesting; identifyingwrong material.

Cannot give absolutemeasurement-onlyqualitative comparison;part edges complicateresults; a skilledoperator is required.

Can be adaptedto high-speedproduction lines;can be adaptedto manycomparativeanalyses.

Metals only. Surface finish,discontinuities, incorrectmaterial.

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constituent variations.

ACOUSTICEMISSION

Detects stress wavesemitted due to loading

Apply multiplesensors to anobject, then stressthe object; grid ofsensors locatesorigin of acousticemission

Sensitive to noise,vibration; identifieslocation of defect ratherthan type of defect.

Allows the wholevolume of thestructure to beinspected non-intrusively in asingle loadingoperation.

Aircraft, bridges,welds, metalforming operations,fiberglass andmetal pressurevessels, piping,storage tanks.

External and internalcorrosion, stress-corrosion cracking, weldcracking, lack of fusion,lack of penetration, andembrittled material.

THERMAL Surface temperatures can bevisualized by infraredsensors or coatings thatchange appearance with

temperature. Changes insurface temperature withtime are related to heat flowpatterns and can be used todetect flaws.

Locate near-subsurface defectsor damage thatalters heat transfer

through an object.

Requires precisecontrol of temperatureand heat input; difficultto inspect resin bonds.

Can be adaptedto productionline checking.

Circuit board solderjoints, solar cells,heat transferequipment,

honeycomb panels,nuclear fuelelements.

Delaminations, voids,liquid intrusion, clogs orleaks in heat transferequipment, fill level.

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Liquid Penetrant Testing

Liquid penetrant inspection is a method that is used to reveal surface breaking flaws (such as hairlinecracks) by bleed out of a colored or fluorescent dye from the flaw. The technique is based on the ability of

a liquid to be drawn into a "clean" surface breaking flaw by capillary action, as shown in Figure 1:1. Aftera period of time called the "dwell," excess surface penetrant is removed and a developer applied. Thisacts as a blotter. It draws the penetrant from the flaw to reveal its presence. Colored (contrast) penetrantsrequire good white light while fluorescent penetrants need to be used in darkened conditions with anultraviolet "black light".

Figure 1:1. Diagram illustrating the liquid penetrant inspection technique.

The advantage that a liquid penetrant inspection (LPI) offers over an unaided visual inspection is that itmakes defects easier to see for the inspector. There are basically two ways that a penetrant inspectionprocess makes flaws more easily seen. First, LPI produces a flaw indication that is much larger andeasier for the eye to detect than the flaw itself. Many flaws are so small or narrow that they areundetectable by the unaided eye. Due to the physical features of the eye, there is a threshold belowwhich objects cannot be resolved. This threshold of visual acuity is around 0.003 inch for a person with20/20 vision.

The second way that LPI improves the detectability of a flaw is that it produces a flaw indication with ahigh level of contrast between the indication and the background also helping to make the indication moreeasily seen. When a visible dye penetrant inspection is performed, the penetrant materials are formulatedusing a bright red dye that provides for a high level of contrast between the white developer. In otherwords, the developer serves as a high contrast background as well as a blotter to pull the trappedpenetrant from the flaw. When a fluorescent penetrant inspection is performed, the penetrant materialsare formulated to glow brightly and to give off light at a wavelength that the eye is most sensitive to underdim lighting conditions. More details regarding the liquid penetrant technique are provided in Lesson 2.

Magnetic Particle Testing

Magnetic particle inspection is a method of locating surface and subsurface discontinuities inferromagnetic materials. The presence of a discontinuity is detected by the use of finely dividedferromagnetic particles applied over the surface, with some of the particles being gathered and held by amagnetic leakage field that forms over the discontinuity. This magnetically held collection of particlesforms an outline of the discontinuity and generally indicates its location, size, shape, and extent. Magneticparticles are applied over a surface as dry particles, or as wet particles in a liquid carrier such as water oroil.

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The principal industrial uses of magnetic particle inspection are final inspection, receiving inspection, in-process inspection and quality control, maintenance and overhaul in the transportation industries, plantand machinery maintenance, and inspection of large components. During receiving inspection, semi-finished purchased parts and raw materials are inspected to detect any initially defective material.Magnetic particle inspection is extensively used on incoming rod and bar stock, forging blanks, and roughcastings.

Fig. 1:2. Electromagnetic yoke showing position and magnetic field for the detection of discontinuities parallel to a

weld bead.

Some of the advantages of magnetic particle testing include:

Sensitive means of locating small and shallow surface cracks in ferromagnetic materials

No electronic readout is necessary; indications are produced directly on the part surface

Little or no limitation on the size or shape of the part being inspected

Some of the limitations of magnetic particle analysis include:

The method can be used only on ferromagnetic materials and can be adversely affected bynon-magnetic coatings (e.g. paint)

Multiple sequential inspections with different magnetization is sometimes required

Demagnetization following inspection is often necessary

Care is necessary to avoid local heating and burning of finished parts or surfaces at thepoints of electrical contact

More details regarding the magnetic particle testing technique are provided in Lessons 3-4

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Ultrasonic Testing

Ultrasonic Testing (UT) uses high frequency sound energy to conduct examinations and makemeasurements. Ultrasonic inspection can be used for flaw detection/evaluation, dimensionalmeasurements, and material characterization.

A typical UT inspection system consists of several functional units, such as the pulser/receiver,transducer, and display devices. A pulser/receiver is an electronic device that can produce high voltageelectrical pulses. Driven by the pulser, the transducer generates high frequency ultrasonic energy. Thesound energy is introduced and propagates through the materials in the form of waves. When there is adiscontinuity (such as a crack) in the wave path, part of the energy will be reflected back from the flawsurface. The reflected wave signal is transformed into an electrical signal by the transducer and isdisplayed on a screen. Signal travel time can be directly related to the distance that the signal traveled.Information about the defect location, size, and orientation can sometimes be obtained.

Ultrasonic Inspection is a very useful and versatile NDT method. Some of the advantages of ultrasonicinspection that are often cited include:

The depth of penetration for flaw detection or measurement is superior to other NDT methods.

It is sensitive to both surface and subsurface discontinuities.

Only single-sided access is needed when the pulse-echo technique is used.

It is highly accurate in determining defect position and estimating size and shape.

Minimal part preparation is required.

It has other uses, such as thickness measurement, in addition to flaw detection.

As with all NDT methods, ultrasonic inspection also has its limitations, which include:

Surface must be accessible to transmit ultrasound.

Skill and training is more extensive than with some other methods.

It normally requires a coupling medium to promote the transfer of sound energy into the testspecimen.

Materials that are rough, irregular in shape, very small, exceptionally thin, or not homogeneousare difficult to inspect.

Linear defects oriented parallel to the sound beam may go undetected.

More details regarding the ultrasonic testing technique are provided in Lessons 5-7.

Penetrating Radiation and Gamma Ray Testing

Like light rays on photographic film, x-rays and gamma rays can be used to produce images onradiographic film. X-rays have more energy than light-rays so they can penetrate and travel throughmaterials that light-rays cannot. X-rays that are sent through an object can produce an image on a pieceof film placed on the opposite side of the object. Because x-rays pass through the object, and becausematerials can absorb x-rays to varying degrees depending on their chemical composition, density, andthickness, internal features or defects can be seen on the radiograph in Figure 1:3.

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Figure 1:3. Exemplar radiograph; Contrast due to internal density differences provides details on the internal structure

of the test object.

Film is not always used. Specialized pieces of equipment have been developed that are sensitive to X-rays and produce an image that can be viewed on a video monitor. Examples include the instrumentsused at the airport to X-ray passengers' baggage, and a CT (CAT ) scan or MRI image produced as amedical diagnostic.

In industrial settings, X-rays are used to produce radiographs of materials and objects to locate internalcracks and other defects. Many production materials, parts and devices are radiographed to ensure

quality, and welders often radiograph their welds to make sure the welds are free of defects. In thiscourse, the focus is on industrial applications of penetrating radiation, including film radiography, digitalradiography, and computed tomography (CT) imaging.

More details regarding X-ray and gamma-ray radiography, including real-time radiography, are providedin Lesson 8. The foundation established in Lesson 8 is built upon in Lesson 9, which discusses ComputedTomography (CT) imaging and digital radiography.

Eddy Current Testing

Eddy current inspection is one of several NDT methods that use the principal of electromagnetism asthe basis for conducting examinations. Several other methods such as Remote Field Testing (RFT), andFlux Leakage, also use this principle.

Eddy currents are created through a process called electromagnetic induction. When alternating currentis applied to a conductor, such as copper wire, a magnetic field develops in and around the conductor asshown in Figure 1:4. This magnetic field expands as the alternating current rises to maximum andcollapses as the current is reduced to zero. If another electrical conductor is brought into close proximityto this changing magnetic field, current is induced in this second conductor. Eddy currents are inducedelectrical currents that flow in a circular path. They get their name from eddies that are formed when aliquid or gas flows in a circular path around obstacles when conditions are right.

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Figure 1:4 Eddy Current Induction

One of the major advantages of eddy current as an NDT tool is the variety of inspections andmeasurements that can be performed. In the proper circumstances, eddy currents can be used for crackdetection, material and coating thickness measurements, and conductivity measurements (materialsorting, detection of heat monitoring and heat damage, and case depth determination).

Some of the advantages of eddy current inspection include:

Sensitive to small cracks and other defects

Detects surface and near-surface defects

Minimum part preparation is required and immediate results

Test probe does not need to contact the part

Inspects complex shapes and sizes of conductive materials

Portable, non-contact, and versatile method

Some of the limitations of eddy current inspection include:

Only conductive materials can be inspected

Surface must be accessible to the probe

Skill and training required is more extensive than other techniques

Surface finish and roughness may interfere

Limited penetration depth and difficulty in detecting flaws that are parallel to probe direction

More details regarding the eddy current testing technique are provided in Lessons 10-11.

Acoustic Emission

For many centuries, people have been keenly aware that stressed materials can emit noises. The

creaking of weakened timbers or "tin cry" are familiar examples indicating that materials audibly object tobeing stressed. The noises emitted frequently serve as a precursory warning that something is beginningto fail structurally.

This phenomenon, now known as acoustic emission, is caused by microscopic and macroscopic failuresin a material under stress. In much of the original research, monitoring was performed at audiblefrequencies. However, for a variety of reasons, virtually all monitoring now takes place at ultrasonicfrequencies.

Acoustic emission is the spontaneous generation of elastic waves created by and during localized but real

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movements in a material under stress. The stress causing the emission can be of either an externallyapplied or residual (locked in) nature. Static or dynamic loading conditions can initiate the emission. Shearmovements or crack growth will generate acoustic emission. Examples can include slip in a ductile metalor a brittle fracture in a ceramic. For metals and other applicable materials, the pile up and breakaway ofdislocations appears to be the fundamental microscopic mechanism.

By mounting multiple sensors on the object or structure to be tested, the location of acoustic emission can

be determined. Once located, other non-destructive evaluation techniques may be used to identify thespecific nature and severity of the defect.

More details regarding the acoustic emission testing technique are provided in Lesson 12.

Thermal Testing

Thermal NDE (also known as Thermal Inspection, Thermography, Thermal Imaging, Thermal WaveImaging and Infrared Testing) methods involve the measurement or mapping of surface temperatures asheat flows to, from and/or through an object. The simplest thermal measurements involve making pointmeasurements with a thermocouple. This type of measurement might be useful in locating hot spots, forexample in a worn out bearing that is heating up due to frictional forces.

In its more advanced form, the use of thermal imaging systems allow thermal information to be rapidlycollected over a wide area and in a non-contact mode. Thermal imaging systems are instruments thatcreate pictures of heat flow rather than of light. Thermal imaging is a fast, cost effective way to performdetailed thermal analysis. Figure 1:5 shows a thermal image of the space shuttle as it lands.

Fig 1:5. Representative thermal image of the space shuttle upon landing. (Photo courtesy of NASA/JPLCaltech/IPAC)

The thermal measurement method has many applications, including heat flow analysis, leak detection,material debonding, material thinning, and electrical shorting. They are used by the police and military for

night vision, surveillance, and navigation aid; by firemen and emergency rescue personnel for fireassessment, and for search and rescue; by the medical profession as a diagnostic tool; and by industryfor energy audits, preventative maintenance, and processes control. The basic premise of thermographicNDT is that the flow of heat from the surface of a solid is affected by internal flaws such as debonding,voids or inclusions. The use of thermal imaging systems for industrial NDT applications is the focus of thismaterial.

More details regarding thermal testing are provided in Lessons 12 and 13.

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Additional Inspection Techniques

Microwave Inspection Testing

Microwaves are a form of electromagnetic radiation located between radio broadcast frequencies andinfrared in the electromagnetic spectrum. This electromagnetic energy is attenuated when it is transmittedthrough a material, allowing for a variety of features to be detected by measuring the reflected andtransmitted waves. These include delaminations, voids and porosity, inclusions, and layer thicknesses.

Some of the advantages of microwave inspection include:

High penetration (deep inspection thickness) in nonmetallic materials

Contactless monitoring of mechanical quantities (e.g. displacement, velocity, acceleration,vibration, level)

Polymeric process properties such as crosslink density and degree of cure

Detection and monitoring of moisture content

Some of the limitations of microwave inspection include:

Microwaves totally reflect from metallic surfaces and cannot penetrate into metals

Lower resolution for void detection compared to acoustical techniques

High sensitivity can complicate interpretation of microwave inspection results

More details regarding microwave inspection are provided in Lesson 14.

Holography Testing

Holographic nondestructive testing is a precision optical technique combining laser, microwave, infrared,or acoustic holography with interferometry. A hologram is initially made of the material under test using

coherent wave energy. The material is then gently stressed and its new wave front (image) opticallycompared with the initial hologram. This procedure creates a holographic interferogram which revealsanomalous regions as localized interference fringe patterns superimposed on the material's surface, asshown in Figure 1:6.

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Figure 1:6. Double-exposure time-lapse interferogram of a defective pneumatic tire under stress. The contouredfringe patterns (arrows) indicate regions of the sidewall and tread where there is no bond between layers of the curedtire.

Some of the advantages of holography include:

Good sensitivity for detection of honeycomb defects such as debonds, crushed core, node

separations, and splice line defects

Monitoring of plastic creep of metals under static load

Examination of the behavior of complex vibrating materials and assemblies

Detection of deep flaws possible in metal structures using acoustical holography

Some of the limitations of holography include:

High initial cost of equipment

Relatively large equipment size

Typically require controlled test environments

An internal flaw can be detected only if its presence is manifested by strain on the material

surface

Generally, the deeper a flaw is located beneath the surface, the larger it must be for detection

More details regarding holographic inspection are provided in Lesson 14.

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