Back to Basics
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The Basics of Visual Testing
by Frank A. Iddings*
Johnny Johnson was the editor of "Back to Basics" before I got the job. He put out an advertisement for his NDT classes that was a picture of the earth seen from space with the comment that God looked at his creation and pronounced it "good," making it the first visual test. Here is a very short article on the basics of visual testing. It notes the simplicity as well as some of the complexity of the technique. Mainly, you just need to be able to see it. Frank Iddings
Tutorial Projects Editor
Introduction
Visual testing would seem to many people to be so simple that there would not be any "basics" to consider. Simple? Yes. However, there are basics that should be considered if the test is to be successful and satisfactory. This article is just a short reminder of some of those basics.Visual testing is extremely important and often is the only form of nondestructive testing (NDT) used on some specimens. In some areas, such as aircraft NDT, it can account for over 70% of the testing done, so it really has to be done right. In any case, visual testing should be the first form of testing done and may be all that is required if unacceptable discontinuities are found. Performing visual testing of a specimen may make other NDT easier or better than if it is not done. For example, some bridge beams used for a bridge over the Mississippi River were found by visual testing to have cracked in shipment. Since specifications required radiography for proof of unacceptable discontinuities, the visual testing pinpointed the places for radiography to be done in order to obtain the necessary radiographs.
The main point of visual testing is that the inspector must be able to see the surface being tested.
The main point of visual testing is that the inspector must be able to see the surface being tested. That may seem simple, but it is not always so. Sometimes, the area to be tested is not adequately illuminated. A minimum of 160 lx (15 ftc) is necessary for general visual testing with 500 lx (46 ftc) being recommended for critical testing. The Illuminating Engineering Society suggests 1100 to 3200 lx (102 to 297 ftc) for critical work (ASNT, 1996). If the specimen is not well illuminated or is shadowed, a light meter should be used to determine whether additional light is needed. The specimen must also be clean.
The instrument being used for testing is the human eye. It needs to be up to the job being done - meaning it needs to be tested and corrected if necessary. Too often, we believe we can see better than we really can. I have a grandson who was certain he could see fine and he turned out to be almost legally blind in one eye and required correction for the other. Without a vision acuity test, he had no comparison as to what normal vision should be. In some cases, both vision acuity and color vision need to be checked. "Approximately 50% of Americans over the age of twenty need corrective eyeglasses" (ASNT, 1996). You might be one of them.
Also, the angle of viewing needs to be within 45 degrees of the normal. The ability to see well drops off quickly as we look at things from an angle other than straight on or normal to the specimen surface. Some tests require that specific lines on a card placed on the surface being tested be visible for the test to be performed (EPRI, 1982). Testing should not be done at an angle that brings intense reflected light into the eye. You have to be able to see what you are looking for. It may require more or even less light under some circumstances.
Sometimes, one only needs to take a simple look at something to determine that it has a problem. One engineer friend of mine only needed the ability to count to determine that a distillation column being delivered on a railcar was not correct. He knew that for every "in" penetration there had to be an "out" penetration; however, there were an odd number of penetrations on the column. It stayed on the railcar and went back to the manufacturer for the missing penetration. This was a much cheaper repair than if the column had been unloaded and erected at the site.
Other times, just seeing the specimen is not enough. The inspector may need some physical or mechanical aids to determine if the specimen is correct. Tape measures or rules, calipers, microthickness gages, squares or angle measurement devices, levels and plumb lines, thread gages and a variety of weld gages may be needed to help the inspector determine the "fitness for use" of a specimen. Without some of these simple mechanical aids, the test might be incomplete or incorrect.
Besides corrective eyeglasses and lamps for additional illumination, there are a variety of other aids that allow the inspector to see what would otherwise not be seen. In this case, we refer to the testing being done as visual and optical testing (VT). These optical aids include such items as mirrors, magnifying lenses, rigid and flexible borescopes, image enhancement technologies, video devices and machine vision instruments. Each of these aids assists the inspector in some way while extracting some disadvantage, such as reduced coverage, reduced ease of performance or higher cost. They all assist the inspector in being able to see surfaces for testing that would otherwise be difficult or impossible to see. The new video devices also allow permanent records of the test to be made, thereby allowing other inspectors to observe the test as it was performed.
Visual and optical testing has become much more complicated to solve the more difficult test problems of today. But the main basic remains: you need to be able to see it.
References
ASNT, Nondestructive Testing Handbook, second edition: Volume 10, Nondestructive Testing Overview, Columbus, Ohio, ASNT, 1996.EPRI NDE Center, Survey of NDE, Module 6, Charlotte, North Carolina, EPRI, 1982.
* 1635 Rob Roy Lane, San Antonio, TX 78251; (210) 647-7717; e-mail <profiddings@satx.rr.com>.
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