Rainflow-on-the-Fly Methodology: Fatigue-Crack Growth under Aircraft Spectrum Loading

Crack-growth-rate tests were conducted on compact, C(T), specimens made of 7075-T7351 aluminum alloy over a wide range of constant-amplitude loading (R = P-min/P-max = 0.1 to 0.9) to establish the baseline crack-growth-rate curve for life-prediction analyses. Both compression precracking and load-reduction methods were used. A crack-closure analysis was used to collapse the Delta K-eff-rate data into a fairly narrow band over many orders of magnitude in rates using an appropriate plane-strain constraint factor. Life predictions were made on C(T) specimens using the FASTRAN Version 5.42 life-prediction code. Some improvements had been made in the code and the predictions were made under cycle-by-cycle simulations. Life predictions under Christmas-Treetype loading using the rainfiow-on-the-fly methodology were very good. And the predicted results on three different aircraft spectrum loading histories (a modified Falstaff, modified Mini-TWIST (Level BI), and a modified Wing-Gust-Maneuver spectrum), agreed to within 20% of the test data.