Evaluation of Indications

Figure 2-34. Micrograph of a Cross-Section through a Stress Corrosion Crack

Table 2-7. Typical Photographic Exposure Settings for Fluorescent Indications (Film Speed: ASA 64; Filter: Wratten 2B)

TM-1-1500-335-23 Nondestructive Inspection Methods Manual
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T.O. 33B-1-1 2-88 2.7.6.3 Evaluation of Indications. Indications can be indistinct and blurred while still being highly visible. The following method may be used to verify and evaluate the type of indication. Carefully wipe the indication area once with a fast drying solvent such as light naphtha, xylene, methylene chloride or isopropyl alcohol. After the solvent has evaporated, spray a very light layer of solvent developer over the area and watch the indication as it begins and continues to develop. If it does not reappear, wipe again with solvent and examine the bare surface with a 3X to 5X magnifying glass. Evaluation of penetrant indications with a magnifying glass should be accomplished with the developer removed. Developer will blur and enlarge the indication. The initial evaluation should be done at low magnification (3X to 5X) with higher magnification (10X) used only after the indication has been located. If no penetrant bleed-out or surface imperfection can be seen, the original indication could have been non-relevant, possibly due to improper processing. 2.7.6.4 Photography of Indications. 2.7.6.4.1 General. Photography is a good method of producing a permanent record of penetrant indications. Photographs provide a very descriptive record since they show both the indication size and location on the part. They are permanent, reproducible to some extent, and the required equipment is available. Photographing penetrant indications is slightly different that normal photography and requires care, practice and a series of trial and error exposures to produce an optimum photograph. It is also very difficult to produce identical photographs when there is a time lapse between exposures. Photographs made at different times will vary due to a number of factors, such as changes in part position, camera position, black light intensity, or changes in film processing or development. 2.7.6.4.2 Camera Equipment. When photographing penetrant indications, which are generally very small, the camera must be held close to the object. This requires, at a minimum, a set of close-up lenses. Extension tubes or a bellows attachment also permits close-up photographs. Photographing fluorescent indications requires time exposures, a tripod or other means of holding the camera steady, and a cable release shutter. 2.7.6.4.3 Filters. Photographic emulsions or films have a higher response to ultraviolet than the human eye. When photographing fluorescent indications, the ultraviolet light must be removed or filtered to obtain a usable photograph. The basic filter used is a No. 2B. (The name Wratten is often associated with the filter numbers, after the man who devised the numbering system.) The 2B filter will absorb the invisible ultraviolet while passing the visible blue light. This approach, when used with color film, provides a photograph representative of what the eye sees. Color balance will be normal and the part will appear as a blue outline with the fluorescent indication appearing as bright yellow-green as normally seen. With black and white film, the part will be outlined and the indication will appear as a white line or dots. Some developers form a bright background that will decrease the contrast between the part and indication. This can be compensated for by using a 2E filter. The 2E filter reduces the background brightness without reducing the indication brightness. When using a 2E filter and color film, the color balance will shift and the photograph may be more yellow than desired. For black and white photography, Nos. 3, 4, 8 or 15 may be used to improve the contrast of the indication, but these filters will transmit only the light from the indication, and the part outline will not be visible. The white light can be flashed during the black light exposure to provide an outline of the part. Alternatively, to show the part, double expose the film using white or visible light for the second exposure. When using the double exposure procedure for black light photography, the white light exposure should be 1/3 or less of the normal exposure. This will make the part appear dark as it would in the normal inspection station. If a normal exposure were used, the contrast between the part and the indication would be largely lost. 2.7.6.4.4 Film Types. All types of color and black and white film Including Polaroid can be used. Slow film speed will increase contrast and decrease grain effects. 2.7.6.4.5 Camera Positioning. Penetrant indications are usually small. On large parts, it may not be possible to include the entire part in the photograph and still get acceptable detail on the indication. The camera must be moved in close to the indication,

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