Issues in estimating probability of detection of NDT techniques - A model assisted approach

In order to successfully implement Damage Tolerance (DT) methodology for aero-engines, Non-Destructive Testing (NDT) techniques are vital for assessing the remaining life of the component. Probability of Detection (POD), a standard measure of NDT reliability, is usually estimated as per MIL-HDBK-1823A standard. Estimation of POD of any NDT technique can be obtained by both experimental and model assisted methods. POD depends on many factors such as material, geometry, defect characteristics, inspection technique, etc. These requirements put enormous limitations on generating experimental POD curves and hence, Model Assisted Probability of Detection (MAPOD) curves are currently in vogue. In this study, MAPOD approaches were demonstrated by addressing various issues related to selection of crack sizes distribution, challenges involved in censoring and regression, estimation of distribution parameters, etc. Ultrasonic testing on volumetric defects has been identified as a platform to discuss the challenges involved. A COMSOL Multiphysics based FEM numerical model developed to simulate ultrasonic response from a Ti-6Al-4V cylindrical block has been validated experimentally. Further, the individual ultrasonic response from various Flat Bottom Hole (FBH) defects following lognormal distribution has been generated using the numerical model. a90/95 (detecting a flaw with 90% probability and 95% confidence) value obtained from POD curve showed that the POD value increased with an increase in decision threshold.