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Understanding Probability of Detection (POD) in Structure Health Monitoring Systems



Important criteria to qualify an on-board Structural Health Monitoring (SHM) system for aircraft flight demonstrations and fleet-wide usage include both an ability to detect damage of a specific size via the creation of a ‘Probability of Detection’ (POD) curve as well as an estimate of the associated false alarm rate. The POD curve provides the best estimate of crack size with 90% POD and 95% confidence (a90/95) and a basis for setting inspection intervals as part of maintaining aircraft structure integrity. MIL-HDBK-1823A [1] contains technical information related to generating a POD curve and is the guide used to quantify the detection capability of NDI systems used as part of the US Air Force’s Aircraft Structural Integrity Program (ASIP). While the intended outputs of SHM and NDI systems are very often the same, there are differences in parameters that affect their performance and implementation hat may require special attention. This paper will discuss using MIL-HDBK-1823A [1] techniques in the context of SHM and will provide valuable insight about how the characteristics of collected and processed SHM data affect the formulation of that system’s POD curve. Additional information covered includes discussions on controlling factors to be considered while collecting SHM test data and a degradation model that directly relates statistical characteristics of the data to simple and intuitive degradation coefficients.

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