The Effects of Elasto-Plastic Behavior at the Crack Tip on Fatigue Crack Propagation

Abstract

Emphasis on physical meanings of the hysteresis loop denoting the relationship between loads and strains near the crack tip, variations of the loading tensile plastic load zone and the unloading compressive plastic load zone are studied respectively under various fatigue test conditions, and the effects on acceleration, deceleration and non-propagation during fatigue crack propagation are also studied. Two types of crack closure are obtained from tests, and the influence of each load zone of the hysteresis loop on fatigue crack propagation is evaluated. Result shows that load zones P RPG~Pmax and P RCPG~PCF play an important role in the crack opening/closure behavior, and the fatigue crack propagates difficultly if the compressive plastic zone at the crack tip is not enough. Keywords:Fatigue crack propagation, Crack tip, Opening/closure, Hysteresis loop, compressive plastic zone, Variable amplitude loading. INTRODUCTIONThe effect of elasto-plastic behavior at the crack tip on fatigue crack propagation has been known since Elber [1] affirmed crack closure phenomenon during fatigue crack propagation by experiment. An effective stress intensity factor range ΔK eff (=K max -K op ) was defined by an opening stress intensity factor K op , and the opening load P op was often measured by ASTM 2% offset method or curve fitting method [2], as well as an unloading elastic compliance method brought by Kikukawa et al. [3]. Otherwise, Nisitani and Chen [4] defined an effective stress intensity factor range ΔK eff (=K max -K cl ) by means of a closure stress intensity factor K cl , and the closure load P cl was determined based on the inflexion point on the unloading compliance. But, loads P op and P cl were dependent on geometric criterion of the compliance curve obtained by global measurement techniques, and the method neglects the physical meaning of compliance change caused by plastic deformation at the crack tip. Recently, Toyosada and Niwa [5] investigated the physical change of the compliance during a load cycle by local measurement techniques, presented a concept of Re-tensile plastic zone's generated load P RPG and established ΔK RPG (=K max -K RPG ). He also presented a method for determining the special loads such as opening/closure loads through overlapping the original hysteresis loop near the crack tip on the loop which rotates at the coordinate origin by 180º. However, the above parameters cannot evaluate the effect of the compressive plastic zone at the crack tip on fatigue crack propagation, and there was an inaccuracy caused by the manual operation after experiment.In this paper, by using global measurement techniques, the hysteresis loop of strains via loads denoting elasto-plastic behavior near the crack tip is obtained. Loads at inflexions on the hysteresis loop are calculated by differential and the hysteresis loop is divided into several physical load zones. Fatigue tests of CT specimen are carried out under variable amplitude loading. Emphasis on variations of the loading tensile and the unlo...