Practical application of damage tolerance to the assessment of rotorcraft structural integrity

The crack growth behavior in typical metallic material used in naval helicopters under actual load spectra was studied. The effect of the fatigue crack-growth threshold, Delta K-th on crack growth from a corner crack under the helicopter load spectra, the sensitivity of the crack-growth threshold to the initial crack length and the effect of the NASGRO crack growth model parameters c, n, p, q to the crack growth life were evaluated. The results showed that for the. typical material a stress reduction of about 23% from the baseline stress value was required to achieve an acceptable minimum crack growth time to failure of 1000 flight hours. The fracture toughness K, and the three parameters c, p, q have little effect on the crack growth life but the parameter n has a significant effect that a 0.01 fraction increase leads to crack growth life more than 0.19 decrease.