Effect of load sequences on crack propagation under random and program loading

Crack propagation was studied in 2024-T3 Alclad sheet specimens under two types of random loading and under program loading with a very short period (40 cycles) and program loading with a longer period (40,000 cycles). In the program tests Lo-Hi, Lo-Hi-Lo and Hi-Lo sequences were employed. The loads were based on a gust spectrum. The crack rates were about the same under random loading and program loading with the short period. Under program loading with the longer period the crack rates were 2.5 times slower on the average, while a significant sequence effect was observed in these tests. Fractographic observations indicated different cracking mechanisms for the random tests and program tests with a short period on the one hand and the program tests with the longer period on the other hand. Implications for fatigue tests in practice are discussed.