A SIMPLIFIED LABORATORY APPROACH FOR THE PREDICTION OF SHORT CRACK BEHAVIOR IN ENGINEERING STRUCTURES

Abstract

Abstract— Conventionally determined fatigue threshold information (ASTM E647) can lead to non‐conservative estimates of fatigue lifetimes when these data are utilized in damage tolerant design assessments. The non‐conservative nature of such data can be attributed primarily to the development of excessively large amounts of crack closure at low R‐ratios, particularly at near threshold stress intensity factor levels. These high closure levels attenuate the effective stress intensity condition prevailing at the crack tip and confound attempts to predict the behavior of short cracks that exhibit limited crack closure. A modified test procedure, involving constant maximum stress intensity factor (Kcmax) test conditions, is described which identifies fatigue crack propagation (FCP) threshold behavior in the absence of detectable amounts of crack closure. These data have been generated with conventional long crack specimens for several aluminum, iron, and nickel‐based alloys and which are shown to closely simulate the FCP response of short cracks in these engineering materials. As such, the modified threshold test procedure, incorporating constant Kmax loading conditions, represents a valuable tool in the prediction of the cyclic lifetime of engineering components. The stress‐cyclic lifetime (S‐N) curve for aluminum butt‐welded beams was computed based on Kcmax data and found to be in excellent agreement with actual test results.