Fatigue Crack Growth Prediction under Spectrum Load Sequence in an Aluminum Alloy by K*-RMS Approach

Constant amplitude fatigue crack growth (FCG) tests were performed at various stress ratios in D16 (2024-T3 equivalent) aluminum alloy using single edge notched tension specimens. Empirical FCG law was derived in terms of crack driving force parameter, K*. The spectrum load sequence of a combat aircraft was approximated as an equivalent constant amplitude (CA) load sequence with maximum and minimum stresses as root-mean-square (RMS) maximum and minimum stresses of the spectrum, respectively. The fatigue crack growth behavior was then predicted under this apparent CA load sequence using the K*-RMS approach. For comparison, the conventional method of FCG prediction using crack closure concept was also performed. In this method, empirical FCG law in terms of effective stress intensity factor range, ▵K_eff was obtained from the constant amplitude FCGR data. Assuming a constant K _op level for spectrum sequence, FCG behavior was predicted through cycle-by-cycle method. Experimental FCG behavior under spectrum load sequence was determined and compared with predicted results. It was observed that the K*-RMS approach provided a fairly good correlation with results obtained from experimental as well as those predicted by the crack closure concept. The predicted fatigue life was conservative and the fatigue life ratio, N _pred/N_expt, was about 0.87. The simplicity of the proposed K*-RMS approach and the reasonable accuracy in fatigue life predictions are quite encouraging.