Deciding Optimal Number of Fatigue Crack Growth Tests for Damage-Tolerant Design

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Deciding Optimal Number of Fatigue Crack Growth Tests for Damage-Tolerant Design

Bhachu, K. S. / Haftka, R. T. / Kim, N. H.

Fatigue crack growth coupon tests play an important role in designing safe and lightweight aircraft structures. How many fatigue crack growth coupon tests are enough is an important question to be answered. With few coupon tests, there will be a substantial uncertainty in design weight, with a significant chance of getting a heavier design, and a considerable chance of overestimating fatigue crack growth design life. A simulation study is presented to determine the optimal number of fatigue crack growth coupon tests by trading off testing cost against structural weight cost. It also gives a designer an idea on how increasing the number of tests reduces the risk of overestimating design life. The tradeoff is demonstrated for the damage-tolerant design of the lower spar caps of a business jet’s wing. With eight coupon tests, there is a 26% chance of incurring at least 2.4% weight penalty and 18% chance of overestimating the fatigue crack growth life by more than 10%. For 64 tests, the chances reduce to 3 and 0.5%, respectively.