AWS Certified Welding Inspector — FundamentalsNondestructive Examination (NDE)Hard
A CWI is overseeing a Radiographic Testing (RT) inspection of a stainless steel pipe weld. The radiographer is using an Ir-192 source. The specified source-to-film distance (SFD) is 30 inches (76.2 cm) and the part thickness is 1 inch (2.54 cm). What is the geometric unsharpness (Ug) if the effective focal spot size of the source is 0.15 inches (0.38 cm)?
- A0.007 inches (0.018 cm)
- B0.004 inches (0.010 cm)
- C0.005 inches (0.013 cm)
- D0.006 inches (0.015 cm)
Show answer & explanationAnswer & explanation
Correct answer: C. 0.005 inches (0.013 cm)
Geometric unsharpness (Ug) is calculated using the formula: Ug = (F * t) / (SFD - t), where F is the effective focal spot size, t is the part thickness, and SFD is the source-to-film distance. Plugging in the values: Ug = (0.15 inches * 1 inch) / (30 inches - 1 inch) = 0.15 / 29 = 0.00517 inches. Rounded to three decimal places, this is 0.005 inches.
Why the other options are wrong
- A. Incorrect calculation.
- B. Incorrect calculation.
- D. Incorrect calculation.
Geometric Unsharpness (Ug)
The blurring or lack of definition in a radiographic image caused by the finite size of the radiation source's focal spot.
- Formula: Ug = (F * t) / (SFD - t).
- F = effective focal spot size, t = part thickness, SFD = source-to-film distance.
- Minimizing Ug improves image quality and detectability of small discontinuities.
Memory trick: Sharp images need small spots, far sources, and thin parts.