MODEL FOR PREDICTING ULTRASONIC PULSE-ECHO PROBABILITY OF DETECTION

This paper describes a mathematical model for predicting the theoretic al probability of detection (POD) of planar buried defects, for conven tional ultrasonic pulse-echo inspection. A model for the scattering of ultrasound by well-oriented planar defects is combined with noise the ory to produce a calculated capability of detection, based on the like lihood that the defect signal exceeds the specified threshold. The pro blem of false indications, the recording of a defect when none is pres ent, is also addressed by the model, showing how any improvement in PO D predictions must be considered in parallel with the associated chang e in the probability of false indication. We use examples to illustrat e how the model may be used to check on proposed inspections, showing how factors such as reporting threshold, probe scan pattern and the cr iterion for the number of probe positions at which an indication must be seen before a defect is recorded, all affect the probability of det ection and of false indication. The effects of defect roughness on det ection probabilities are studied. We also use the model to quantify th e uncertainties that result in POD predictions, when defect properties such as orientation, roughness, aspect ratio and depth within the spe cimen are themselves uncertain. The problems of equipment and human er ror are not addressed, although we discuss how, if these can be quanti fied, they may be incorporated into the model.