Demonstration of probabilistic-based durability analysis method for metallic airframes

Two different variations of a probabilistic-based durability analysis method are demonstrated and evaluated for countersunk fastener holes in the lower wing skin (7475-T7351 aluminum) of a fighter aircraft. The initial fatigue quality of the fastener holes is represented by an equivalent initial flaw size (EIFS) distribution/Probability of crack exceedance predictions at any service time are based on an EIFS distribution and two different service crack growth approaches. The service crack growth is divided into two segments. Segment 1 covers the small crack size region (e.g., 1.27 mm). Approach I treats the service crack growth as deterministic for both segments. In approach II, the service crack growth is treated as deterministic in segment 1 and stochastic in segment 2. Analytical predictions for the extent of damage (i.e., number of fastener holes exceeding specified sizes), based on approaches I and II, are compared and correlated with experimental results. Good correlations are obtained for both approaches. Approach II was found to be more conservative than approach I. Nomenclature #0 = reference crack size 0 (0) = EIFS at time t = 0 a(t) = crack size at time t AL, AU = lower and upper bound crack sizes, respectively, defining the range of fractographic data used for determining EIFS b\,b2 - crack-growth-rate exponents for crack growth segments 1 and 2, respectively (b\-bi=\\ used herein) fx(u) = probability density function of the lognormal random variable X