Probabilistic considerations are essential to qnde and shm

Probabilistic considerations play a dominant role in the four stages of the diagnostics and prognostics of fatigue damage in metals. Considerable attention has been given to the evolution and detection of pre-crack fatigue damage and the probabilistic aspects of subsequent macrocrack formation (Stage 1). For Stage 2 (macrocrack growth and detection), Paris law for crack growth under cyclic loading conditions can be useful, particularly if it is placed in a probabilistic context, and if the constants in the law are represented by probability distributions. By introducing the probability of detection concept, various probabilities related to the existence, after N cycles, of a crack larger than a critical size, can be determined in Stage 3 for purposes of prognostication. Paris law has its limitations and efforts to formulate other crack-growth laws will be mentioned. Finally, we will mention probabilistic aspects of optimizing the inspection schedule for Stage 4.