Penumbra Four Times the Calculated Value on a 25 mm Weld
A 25 mm carbon steel butt weld is radiographed with a 3.0 mm focal-spot X-ray unit at 700 mm source-to-film distance. The film is in direct contact with the plate, and the source-side surface of the weld is 20 mm from the film. Using D7 film at density 2.4, the calculated geometric unsharpness is 0.09 mm. The radiograph shows several dark, crack-like indications along the weld toe. When the Level II measures the penumbra at a machined edge on the same film, it is 0.35 mm, about four times the calculated value. The film is from a new box but the same darkroom and processor. The Level II must decide whether the toe indications are genuine cracks, whether the technique is acceptable, and what to do before signing the report.
1. Calculate the geometric unsharpness and explain what the 0.35 mm measurement implies. Rank at least three plausible causes and state how you would test each.
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Geometric unsharpness is Ug = F × t / d, where F is the effective focal-spot size, t is the object-to-film distance and d is the source-to-object distance. Here d = 700 − 20 = 680 mm, so Ug = 3.0 × 20 / 680 = 0.088 mm, rounding to 0.09 mm. A measured edge penumbra of 0.35 mm cannot be explained by the stated geometry. The effective focal spot would have to be F = Ug × d / t = 0.35 × 680 / 20 ≈ 11.9 mm, or the source-to-object distance would have to be much shorter. Ranked causes: (1) the actual focal spot is larger than 3.0 mm, or off-focus radiation is significant - check with a pinhole camera or star pattern; (2) the SFD is not 700 mm - measure it physically rather than trusting the scale or tape; (3) poor film–screen contact or film movement - run a contact test and repeat the exposure; (4) the edge used for measurement is not sharp, or the reader is measuring a processing artifact. Until the cause is identified, the film is not valid for sizing crack-like indications.
2. A second exposure made at 20° with a smaller focal spot shows the same indication at the same weld location but shorter. What is your disposition, and what does the negative magnetic-particle test mean?
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Persistence is the key. A film artifact such as a scratch, lint, or screen mark usually stays fixed to the film or cassette; a true weld defect stays fixed to the weld. Changing the beam angle to 20° changes the projection of the weld and the film position. If the indication remains at the same weld location and changes shape only as expected from the new angle, it is in the weld. The sharp upper end, alignment with the fusion line and dark, straight body favor lack of fusion or a toe crack rather than rounded slag. A negative surface magnetic-particle test does not clear it, because lack of fusion and cracks can be subsurface or oriented unfavorably for MT. Disposition: hold or reject the weld, report the indication as planar and suspect, and request UT or a dedicated RT re-shoot with a smaller focal spot and higher SFD. Do not sign acceptance based on a film whose geometric unsharpness was four times the calculated value.