Some aspects of variability in fatigue crack initiation and propagation in thick aluminium alloy sections

As part of an investigation to extend the lives of wings of the Mirage IIIO aircraft, a fatigue testing programme and extensive fractographic examination was undertaken on thick specimens representing the critical section of the spar to provide information on variability in fatigue crack initiation and propagation. A good linear relationship was found between the log life and log crack depth for individual specimens. At the smallest crack depth used for analysis (0.3mm) no significant differences were found between the standard deviations of log life to crack initiation, for propagation or to final failure, nor in the corresponding standard deviations of arithmetic lives. However, in considering the more general question of whether the greater contribution to variability in total life comes from that in initiation life or that in propagation life, it is shown that account must be taken of the analytical basis of the assessment — whether arithmetic or logarithmic.