A comparative evaluation of a deterministic and probabilistic approach for determining safety inspection intervals of airframe structures

This paper presents a comparative evaluation of a deterministic and a probabilistic approach applied to the analysis of two sets of test data obtained under laboratory conditions for the study of crack growth variability. The first set of data was from Virkler, et al., which used aluminium 2024-T3 mid-tension specimens subjected to constant amplitude loading. The second dataset was from experiments conducted by the authors on 7075-T7351 mid-tension specimens subjected to spectrum loading. Both the deterministic and probabilistic approaches were used to predict the safety inspection time for all coupons tested. For the probabilistic approach, a distribution of the initial flaw size and a master crack growth curve were derived from a number of randomly selected test results. The probability of failure is calculated based on the initial flaw size distribution, the master crack growth curve, the peak stress distribution and the material fracture toughness. A probability of failure of 10-7 is used to determine the time for inspection. For the deterministic analysis, the inspection time is determined by predicting the failure time then taking the inspection time as half of the predicted failure time. It was found that without the use of a factor of safety, probabilistic analysis is more conservative and that use of a factor of safety of 2 makes the deterministic prediction conservative but far from the exact values.