RESEARCH ON LOAD ENHANCEMENT METHOD FOR FATIGUE CRACK GROWTH TEST

To explore approaches of shortening fatigue crack growth (FCG) test period by load spectrum simplification and enhancement, various FCG models were reviewed, and a generalized expression was acquired. A load enhancement method magnifying all the peak and valley loads simultaneously was proposed, and the relationship between FCG lives under baseline and enhanced spectra was derived. Crack propagation tests were conducted using middle-crack tension (M (T)) specimens made of AA LY12-CZ and typical fuselage panel specimens made of AA 2524-T3 and AA 7075-T62. M (T) specimens were tested respectively under one random spectrum, two block spectra and corresponding enhanced spectra with four enhancement factors. Fuselage panel specimens were tested respectively under constant amplitude spectrum and 1.1 times enhanced spectrum. The experimental results demonstrate that the proposed load enhancement method can shorten FCG test periods effectively without changing the failure mode. The analysis result indicates that the difference between predicted FCG lives and experimental data is less than 15%, which satisfies engineering accuracy requirement.