Correlation between fatigue crack growth rate and striation spacing in XZM

Abstract

Much information can be learned from the fracture surface of a metallic component. At a microscopic level, the relationship between a fatigue fracture surface and the rate at which the crack propagated through the material is of particular interest. At certain growth rates, a fatigue fracture surface is comprised of striations which are the results of a cyclical loading being applied to a component. When evaluating a fatigue fracture surface to determine crack growth rates, a Transmission Electron Microscope (TEM) is employed to observe the striations. However, the following question arises, “Does one fatigue striation equal one load cycle”? In previously published literature, the relationship of one striation being equal to one load cycle has been confirmed for various aluminum alloys. The test data presented herein is for steel, i.e., a High Hot Hardness (HHH) gear material utilized in helicopter transmissions. The correlation of one striation being equal to one load cycle was obtained and documented during a fatigue crack growth test of X2M gear steel. A prime use of this data would be in the evaluation of a fatigue fracture surface to determine the initiation time and propagation rate for a service related failure.