Analyses of Fatigue Crack Growth and Closure Near Threshold Conditions for Large-Crack Behavior

A plasticity-induced crack-closure model was used to study fatigue crack growth and closure in thin 2024-T3 aluminum alloy under constant-R and constant-Kmax threshold testing procedures. Two methods of calculating crack-opening stresses were compared. One method was based on contact-K analyses and the other on contact crack-openingdisplacement (COD) analyses. These methods gave nearly identical results under constant-amplitude loading but under load-reduction (threshold) simulations the contact-K analyses gave lower crack-opening stresses than the contact-COD method. Crackopening stresses determined from the compliance-offset method agreed with results from the COD analyses. But crack-growth predictions tend to support the use of contact-K analyses. Crack-growth simulations showed that remote closure can cause a rapid rise in opening stresses in the near threshold regime for low-constraint (plane-stress) and high applied stress levels for both low and high stress ratios. Under low applied stress levels and high constraint (near plane-strain), a rise in opening stresses was not observed near threshold conditions. But crack-tip-opening displacements (CTOD) were of the order of measured oxide thicknesses in the 2024 alloy and these oxide layers may have an influence on opening stresses. In contrast, under constant-Kmax testing the CTOD near threshold conditions were an order-of-magnitude larger than measured oxide thicknesses. Residualplastic deformations under both constant-R and constant-Kmax threshold simulations were several times larger than the expected oxide thicknesses. Thus, residual-plastic deformations, in addition to oxide and roughness, play an integral part in threshold development. INTRODUCTION In 1970, Paris [1] proposed a method to determine very slow fatigue-crack-growth rates and showed the development of a fatigue-crack-growth threshold, ∆Kth. Later, Saxena et.al [2] and an ASTM Task Group developed a standard test method for nearthreshold crack-growth-rate measurements. The use of thresholds in the design of structures subjected to cyclic loading has greatly increased over the past 20 years.