QUANTITATIVE FRACTOGRAPHIC ANALYSES OF FATIGUE-CRACK GROWTH IN LONGERONS OF IN-SERVICE HELICOPTER ROTOR-BLADES

An analysis is made of striation spacing measurements as observed on f atigue crack surfaces of specimens and longerons of helicopter rotor-b lades made from the aluminium alloy AVT. It is shown that. for the com bined cyclic tensile stresses at various R-ratios and static rotation angles, alpha, striation spacings were controlled by an equivalent int ensity factor K-e=KIF(alpha,R). Values of the F(alpha) for R(i) = cons t were computed. The crack growth life in terms of the number of fligh ts and stress equivalents were computed for various longeron sections of the helicopter rotor-blades which had failed in service.