Ultrasonic Structural Healthmonitoring: a Probability of Detection Case Study

Ultrasonic interrogation of metal alloys has been demonstrated to be effective for monitoring fatigue-induced damage in a structural health monitoring (SHM) framework. Before such a method can be implemented, the probability of detection (POD) as a function of crack size must be quantified. POD curves are routinely generated for nondestructive evaluation (NDE) methods, typically by performing large numbers of measurements to capture the variability arising from variations in operators, probes, instruments and crack morphology. Such studies have not yet been carried out for many, if any, SHM methods, and thus identifying and quantifying relevant sources of variability have not generally been addressed. Considered here is monitoring of fastener holes for fatigue cracks, and POD curves are generated using essentially the same methods as are used for NDE but with differences in setting detection thresholds. Interpretation of the curves is discussed given that operator, sensor and instrumentation variability are no longer issues in the context of monitoring a specific structure.