Critical comments on the use of X-rays and Ir-192 for detecting linear discontinuities

Abstract

A notched IQI in nine variants was used to draw detectability curves (depth versus width) for steel specimens 6 to 48 mm in thickness, using X-ray and Ir-192 sources with appropriate focus sizes, and AGFA films D3, D4, D5 and D7. The crack detection index (CDI) of the slit IQI has a good correlation with the crack probability of detection (POD) for a CrMoV welded joint (28 mm thick) with induced thermo-mechanical cracks. POD depends on film type (± 10% class⁻¹) and film density (±50% for optical density variation between D = 1.5 – 3.5). The major influences of unsharpness and irradiation angle upon POD are also presented. The difference in sensitivity between X-rays and Ir-192 gamma rays can be explained by ‘energy-unsharpness’ factor variation with thickness. A correlation between slit IQI parameter (depth = width) and DIN (wire) IQI diameter was found. An experimental formula between CDI and total unsharpness (u_t) for X-rays and film density D = 2.5 – 3.0 was also found: Fu_t.CDI = 2.1, where F = 0.87–1.0 is a film factor.

Proposals for codes of good practice, to obtain good ‘crack sensitivity’ based on the presented data, are given.