Modelling the radiography of thick-section welds

This paper describes the experimental validation of a simple model of radiography (Pollitt, 1962), which treats flaws as smooth, parallel-sided slots. Six thick-section specimens were manufactured to represent welds in Magnox pressure vessels. The welds contain 16 large, realistic, planar defects. They were radiographed under various exposure conditions and evaluated by two interpreters. The specimens were then sectioned to determine defect size, orientation, gape and roughness. The experimental data show variations in detectability that are strongly correlated with theoretical predictions. In almost all cases, Pollitt theory is either accurate or pessimistic. We have used the experimental data to derive statistical models for the reliability of radiography. Such models can be used to estimate the probability of detecting a defect (with associated confidence limits), from knowledge of its size, orientation and other relevant parameters