Estimation of loads causing fatigue failures in accident investigations

Abstract A procedure was developed by which the relationship between the load levels which led to fatigue failure of a part, and the crack depth and crack growth rate observed on the fracture surface, can be determined by combining basic materials data obtained from a compact-tension (CT) fatigue test with a finite element analysis of the part in question. The procedure, which permits the estimation of the load levels in the structure at any given crack depth and growth rate, is described, as well as the construction of the three-parameter plot which facilitates this determination. Two case studies are presented and discussed. The first concerns a cracked aluminum-alloy anchor fitting for a speed-brake actuator on the wing of an A-4 fighter aircraft. CT fatigue data showed that service loads experienced by the part were not excessive. However, deformation imbalance, perhaps due to wear in the actuator attachment bearing, may have been sufficient to cause an initiating flow in the flange, eventually resulting in a crack. The second case is a steel torsion-bar from a Subaru 600 automobile. Although the bar had been loaded in torsion, it developed the usual mode I fatigue crack which led to failure. Here material fatigue data led to the conclusion that the fatigue crack, which began at a mark probably left by the impact of a piece of gravel, could well have caused failure under normal loads between two consecutive inspection times.