Experimental and numerical analysis of elastic-plastic strains and stresses ahead of a growing fatigue crack

Reliability and fatigue life estimation of structural and mechanical components subjected to cyclic loads require predictions of stresses and strains at critical locations such as notches and cracks. Such analyses are concerned with solving complex elastic-plastic boundary problems involving varying geometrical boundaries and cyclic loading. The objective of the paper is an analysis of elastic-plastic strains and stresses arising in the plastic zone adjacent to the tip of a propagating fatigue crack. The analyzed below boundary value problem concerns Compact Tension specimen with growing fatigue crack subjected to non-symmetric constant amplitude cyclic loading history. A special finite element code has been developed to calculate the elastic-plastic stress-strain response ahead of a growing Mode I fatigue crack. The code consists of the cyclic plasticity Mroz-Garud model, contact elements and a module simulating the growth of the fatigue crack. The simulation was carried out for the SAE4140 steel material. The aim of the analysis was to study both the crack opening displacement field behind the moving crack tip and fluctuations of the elastic-plastic strains and stresses ahead of the crack. It has been found that both the crack opening displacement and the crack tip stress-strain fields are related to each other and occasional contacts between crack surfaces do not have as strong effect on the crack tip stress-strain affairs as it is often claimed. It was also found that the crack region adjacent to the crack tip often stays open regardless of optically visible presence of the contact between crack surfaces away from the crack tip. The effect of the plasticity induced residual stress field on the crack opening displacement field at the maximum and minimum load levels has been illustrated as well. The studies indicate that the simulation of the load history effect based only on the contact between crack surfaces, known as the crack tip closure, and manipulation of the minimum stress intensity factor is insufficient for appropriate modeling of the spectrum loading effects on the stressstrain history ahead of the crack tip and subsequently on the fatigue crack propagation process. Limited experimental measurements of near the crack tip strains influenced by an overload were also presented and discussed in terms of its effect on the crack tip strain field at the maximum and minimum load level.