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http://ammtiac.alionscience.com The AMMTIAC Quarterly, Volume 1, Number 3

INTRODUCTIONVisual inspection is by far the most common nondestructivetesting (NDT) technique [1]. When attempting to determinethe soundness of any part or specimen for its intendedapplication, visual inspection is normally the first step in theexamination process. Generally, almost any specimen can bevisually examined to determine the accuracy of its fabrication.For example, visual inspection can be used to determine

whether the part was fabricated to the correct size, whetherthe part is complete, or whether all of the parts have beenappropriately incorporated into the device. [2]

While direct visual inspection is the most commonnondestructive testing technique (Figure 1), many other NDTmethods require visual intervention to interpret imagesobtained while carrying out the examination. For instance,penetrant inspection using visible red or fluorescent dye relieson the inspectors ability to visually identify surface indications.Magnetic particle inspection falls into the same category of

visible and fluorescent inspection techniques, and radiographyrelies on the interpreters visual judgement of the radiographicimage, which is either on film or on a video monitor. Theremainder of this article provides a summary of the visual test-ing method, which at the minimum requires visual contact

with the portion of the specimen that is being inspected.In arriving at a definition of visual inspection, it has been

noted in the literature that experience in visual inspection anddiscussion with experienced visual inspectors revealed that thisNDT method includes more than use of the eye, but alsoincludes other sensory and cognitive processes used by inspec-tors [3]. Thus, there is now an expanded definition of visual

inspection in the literature:Visual inspection is the process of examinationand evaluation of systems and components by useof human sensory systemsaided only by mechanicalenhancements to sensoryinput as magnifiers, dentalpicks, stethoscopes, and thelike. The inspection processmay be done using suchbehaviors as looking, listen-ing, feeling, smelling,shaking, and twisting. It included a cognitive

component wherein observations are correlatedwith knowledge of structure and with descriptionsand diagrams from service literature. [3]

PHYSICAL PRINCIPLESThe human eye is one of mankinds most fascinating tools. Ithas greater precision and accuracy than many of the mostsophisticated cameras. It has unique focusing capabilities andhas the ability to work in conjunction with the human brain sothat it can be trained to find specific details or characteristicsin a part or test piece. It has the ability to differentiate anddistinguish between colors and hues as well. The human eye iscapable of assessing many visual characteristics and identifying

various types of discontinuities*. The eye can perform accurateinspections to detect size, shape, color, depth, brightness,contrast, and texture. Visual testing is essentially used to detectany visible discontinuities, and in many cases, visual testingmay locate portions of a specimen that should be inspectedfurther by other NDT techniques.

Many inspection factors have been standardized so that cat-egorizing them as major and minor characteristics has becomecommon [4]. Surface finish verification of machined parts haseven been developed, and classification can be performed byvisual comparison to manufactured finish standards. In the fab-rication industry, weld size, contour, length, and inspection for

Selecting a Nondestructive Testing Method, Part II: Visual Inspection

This edition of TechSolutions is the second installment in a series dedicated to the subject of nondestructive testing. TechSolutions 1,published in Volume 1, Number 2 of theAMMTIAC Quarterly, introduced the concept of nondestructive testing and provided brief descriptions of the various nondestructive testing techniques currently available. This article continues the series and focuses in on the mostcommon nondestructive testing technique: Visual Inspection. The next TechSolutions article in this series will delve into detail aboutanother common nondestructive testing technique: Eddy Current Testing. Once the series on nondestructive methods is complete, we willcombine all of the articles into a valuable desk reference on nondestructive testing. - Editor

George A. MatzkaninTRI/AustinAustin, TX

Figure 1. Visual Inspection of a Torpedo Tube Aboard a Navy

Attack Submarine (Photo Courtesy of the Department of Defense;Photo Taken by JO3 Corwin Colbert, USN)

The human eye is one omankinds most fascinat

tools and is capable oassessing many visua

characteristics andidentifying various type

of discontinuities.

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The AMMTIAC Quarterly, Volume 1, Number 38

surface discontinuities are routinely specified. Many companieshave mandated the need for qualified and certified visual weldinspection. This is the case particularly in the power industry,

which requires documentation of training and qualification

of the inspector. Forgings and castings are normally inspectedfor surface indications such as laps, seams, and other varioussurface conditions.

INSPECTION REQUIREMENTSRequirements for visual inspection typically pertain to thevision of the inspector; the amount of light falling on the spec-imen, which can be measured with a light meter; and whetherthe area being inspected is in anyway obstructed from view.In many cases, each of these requirements is detailed inregulatory code or other inspection criteria [2].

Mechanical and/or optical aids may be necessary to performvisual testing. Because visual inspection is so frequently used,

several companies now manufacture gages to assist visualinspection examinations. Mechanical aids include: measuringrules and tapes, calipers and micrometers, squares and anglemeasuring devices, thread, pitch and thickness gages, levelgages, and plumb lines. Welding fabrication uses fillet gages todetermine the width of the weld fillet, undercut gages, anglegages, skew fillet weld gages, pit gages, contour gages, and ahost of other specialty items to ensure product quality.

At times direct observation is impossible and remote view-ing is necessary which requires the use of optical aids. Opticalaids for visual testing range from simple mirrors or magnifyingglasses to sophisticated devices, such as closed circuit televisionand coupled fiber optic scopes. The following list includes

most optical aids currently in use [2]:

Mirrors (especially small, angled mirrors) Magnifying glasses, eye loupes, multilens magnifiers,

measuring magnifiers Microscopes (optical and electron)

Optical flats (for surface flatness measurement) Borescopes and fiber optic borescopes Optical comparators Photographic records Closed circuit television (CCTV) systems (alone and

coupled to borescopes/microscopes) Machine vision systems Positioning and transport systems (often used with

CCTV systems) Image enhancement (computer analysis and enhancement)

Before any mechanical or optical aids are used, thespecimen should be well illuminated and have a clean surface.

After the eyeball examination, mechanical aids help to improvethe precision of an inspectors vision. As specifications andtolerances become closer, calipers and micrometers becomenecessary. The variety of gages available help to determinethread sizes, gap thicknesses, angles between parts, hole depths,and weld features.

As it becomes necessary to see smaller and smallerdiscontinuities, the human eyes require optical aids that enableinspectors to see these tiny discontinuities. However, theincreased magnification limits the area that can be seen at onetime, and also increases the amount of time it will take to lookat the entire specimen. Mirrors let the inspector see aroundcorners or past obstructions. Combined with lenses and placedin rigid tubes, borescopes enable the inspector to see inside

specimens such as jet engines, nuclear piping and fuelbundles, and complex machinery. When the rigidborescope cannot reach the desired area, flexiblebundles of optical fibers often are able to access thearea. Figure 2 shows visual inspection using a fiberoptic borescope. Some of the flexible borescopes havedevices that permit the observation end of the scopeto be moved around by a control at the eyepiece end.Some are also connected to CCTV systems so that alarge picture may be examined and the inspectionrecorded on videotape or digitally. When the video

systems are combined with computers, the images canbe improved which may allow details not observablein the original to be seen.

PRACTICAL CONSIDERATIONSVisual inspection is applicable to most surfaces, but ismost effective where the surfaces have been cleanedprior to examination, for example, any scale or loosepaint should be removed by wire brushing, etc. Visiontesting of an inspector often requires eye examinations

Figure 2. An Inspector at Tinker Air Force Base Gets a Magnified View of an

Engines High-Pressure Turbine Area with a New Digital Fiber-OpticBorescope. (Photo Courtesy of US Air Force; Photo taken by Margo Wright)




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with standard vision acuity cards such as Jaeger, Snellen, andcolor charts. Vision testing of inspectors has been in use forabout 40 years. Although many changes in NDT methods havetaken place over the years and new technologies have beendeveloped, vision testing has changed little over time. Also littlehas been done to standardize vision tests used in the industrialsector. For those seeking certification in the area of visual

testing, theASNT Level III Study Guide and Supplement onVisual and Optical Testingprovides a useful reference [5].

SELECTED EXAMPLESTwo major studies of visual inspection thathave been carried out in recent years providea great deal of insight into the reliabilityof visual inspection. Since visual inspection isthe predominant NDT technique used forbridge inspection, the Federal Highway

Administration (FHWA) NondestructiveEvaluation Validation Center (NDEVC)conducted a comprehensive study to examine

the reliability of the visual inspection methodfor highway bridges [6]. Performance trials

were conducted using 49 state bridge inspec-tors to provide overall measures of the accura-cy and reliability of routine and in-depthinspections. One of the objectives was tostudy the influence of several key factors inorder to provide a qualitative measure of theirinfluence on the reliability of routine andin-depth inspections. Figures 3 and 4 showroutine and in-depth inspections at a SafetyTesting and Research (STAR) facility.

Among the findings is that vision testing

of inspectors is almost nonexistent, with onlytwo state respondents indicating that theirinspectors had their vision tested. From theroutine inspections it was found that theinspections were completed with significantvariability in the results. This variability

was most prominent in the assignment ofcondition ratings, but was also present inexamination documentation. From thein-depth inspection tasks it was observedthat in-depth inspections were unlikely tocorrectly identify many types of specific discontinuities for

which this inspection is frequently prescribed. As an example,only 3.9 percent of weld inspections correctly identified thepresence of crack indications.

As a result of this study, it was recommended that theaccuracy and reliability of both routine and in-depthinspections could be improved through increased trainingof inspectors in the types of discontinuities that should beidentified and the methods that would frequently allowidentification to be possible. Also, additional research is neededto determine whether ensuring minimum vision standardsthrough vision testing programs (with corrective lenses, if

necessary) would benefit bridge inspection. Additional detailsare fully documented in the two-volume final report [6].

In the second comprehensive study of visual inspection,experiments were performed at the Federal Aviation Adminis-trations (FAAs) Aging Aircraft Nondestructive InspectionValidation Center (AANC) to provide a benchmark measure ofthe capability for visual inspection performed under conditions

that are realistically similar to those usually found in majorairline maintenance facilities [3]. More than80 percent of inspections on large transportcategory aircraft are visual inspections.Small transport and general aviation aircraftrely on visual inspection techniques evenmore heavily than do large transport aircraft.Visual inspection, then, is the first line ofdefense for safety-related failures on aircraftand provides the least expensive and quickestmethod for assessing the condition of anaircraft and its parts [3]. Therefore, accurateand proficient visual inspection is crucial

to the continued safe operation of theair fleet and it is important that its reliabilityshould be high and well-characterized. Theexperiments at the AANC were conducted ona Boeing 737 aircraft test bed, as well as ona sample library of well-characterized flawsin aircraft components or simulated compo-nents. Figure 5 shows visual inspection insidean aircraft.

Results showed substantial inspector-to-inspector variation. For example, on aspecific task of looking for cracks frombeneath rivet heads, the 90 percent pro-

bability of detection (percentage of cracksexpected to be detected) crack length for11 inspectors ranged from 0.16 to 0.36 inch,

with the 90 percent probability of detectioncrack length for a twelfth inspector being0.91 inch. Also noted was a high variabilityfrom one inspection task to another asperformed by the same inspector. Results ofthese experiments have paved the way forother organizations to better understand theintricacies of visual inspection in developing

laboratory and field visual inspection protocol.

CONCLUSIONSDespite advances in other NDT technologies, visualinspection will likely remain the first inspection method usedin many field applications. As new mechanical and opticalaids become available, the reliability of visual inspection willincrease to more acceptable levels. It is expected that additionalvisual inspection standards will be developed to provideguidance in applying visual inspection for nondestructivetesting. Visual inspection will continue to be an importantNDE inspection approach that will often identify areas

http://ammtiac.alionscience.com The AMMTIAC Quarterly, Volume 1, Number 3

A D V A N C E D M A T E R I A L S , M A N U F A C T U R I N G A N D T E S T I N GAMMTIAC

Figure 3. Part of a Routine

Bridge Visual Inspection [6].

Figure 4. Part of an In-DepthBridge Visual Inspection Using

a Man-Lift [6].

Figure 5. Visual InspectionExperiment inside a Boeing 737.

(Photo Courtesy of the Federal

Aviation Administration)




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of structures or components where more advanced NDEmethods need to be applied.

REFERENCES[1] Nondestructive Testing Handbook, Volume 8: Visual and OpticalTesting, Technical Editors M.W. Allgaier and S. Ness, AmericanSociety for Nondestructive Testing, Columbus, OH, 1993.[2] F.A. Iddings, Visual Inspection, Materials Evaluation, Vol. 62,No. 5, May 2004, pp. 500-501.[3] F.W. Spencer, Visual Inspection Research Project Report onBenchmark Inspections, U.S. Department of Transportation, Federal

Aviation Administration, Washington, DC, 1996.[4] S. Kleven and L. Hyvarinen, Vision Testing Requirements for Indus-try, Materials Evaluation, Vol. 57, No. 8, August 1999, pp. 797-803.[5] ASNT Level III Study Guide and Supplement on Visual andOptical Testing, American Society for Nondestructive Testing,Columbus, OH, 2005.[6] Reliability of Visual Inspection for Highway Bridges, PublicationNos. FHWA-RD-01-020 and FHWA-RD-01-021, June 2001.

NOTE & GENERAL REFERENCES* In general, as described in the literature of the American Societyfor Nondestructive Testing, inspectors determine the presence orabsence of indications of discontinuities. Whether or not adiscontinuity is a defect is based upon the design criteria, and isgenerally not up to the inspector.[1] S. Endoh, H. Tomita, H. Asada, and T. Sotozaki, PracticalEvaluation of Crack Detection Capability for Visual Inspection in

Japan, Durability and Structural Integrity of Airframes: Proceedings ofthe 17th Symposium of the International Committee on AeronauticalFatigue, June 9-11, 1993.[2] J.W. Shoonard, J.D. Gould, and LA. Miller, Studies of VisualInspection, Ergonomics, Volume 16, No. 4, 1993, pp. 365-379.[3] E.D. Megaw, Factors Affecting Visual Inspection Accuracy,

Applied Ergonomics, March 1979, pp. 27-32.[4] J.N. Riley, E.P. Papadakis, and S.J. Gorton, Availability ofTraining in Visual Inspection for the Air Transport Industry,

Materials Evaluation, 1996, pp.1368-1375.

VisualInspectionSummary

Discontinuity types Cracks(e.g., what types can the method detect) Holes

Corrosion Blisters Impact damage

I.e., most discontinuities that are surface breaking or result in adeformation at the surface

Size of discontinuities Approximately 0.25 inches and larger

Size of system that can be inspected Any, i.e., Large (e.g., aircraft skin), Medium (e.g., structuralsupport), Small (e.g., electrical circuitry)

Field portable method? Yes

Inspection restrictions Surface areas only Some internal and inaccessible areas cant be inspected visually.

Inspector training (level and/or Recommendedavailability e.g., who provides training) Commercial training schools available

Equipment None Optical aids optional except where internal surfaces require

borescopes or small television cameras, etc.

Certification required Depends on application Certification available through the American Societyfor Nondestructive Testing (ASNT)

Relative cost of inspection Low(i.e., compared to other methods) Equipment cost can range from nothing to a modest price,

but this method can be labor intensive

Probability ofDetection for NDE

NDE CapabilitiesDatabook



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