Fatigue Crack Growth Behavior Predicted By Plasticity-Corrected Stress Intensity Factor Range

Fatigue fracture is known as one of the main failure modes in metal components of engineering structures. In order to prevent fatigue failure, numerous empirical law and theoretical analysis methods have been proposed to predict the fatigue life since the middle of the 19 th century. Over the past few decades, continuing advances in modern machine, aerospace, ocean engineering have driven a substantial increase in the size of engineering structures as well as reduction in weight. Fatigue fracture of metal structures under high speed, high temperature and high loading conditions is a recurring challenge in modern industry. The investigations about the fatigue failure are always a great concern of scholars and engineers.The fatigue life of structures is predictable under give crack growth rates. Thus, most of the research attention about fatigue crack growth has been focused on the crack propagation speed before the crack reaches the critical failure length. The stress intensity factor ranges at a crack tip are proposed by Paris as governing parameters for fatigue crack growth rate in 1961. This landmark development established the theoretical basis for estimation of damage tolerance and fatigue life of engineering structures.However, series of experimental investigations in the past few decades have shown that the linear-elastic-fracture based Paris law is not capable of describing the fatigue crack propagating processes that dependents on the shape, size and the stress distributions of the plastic zone, such as the effects of load ratio, overload retardation, the transverse stress on biaxial fatigue as well as the propagation of physically short crack emanating from notch root. People have recognized that the crack-tip plastic zone plays an important role in fatigue crack growth, and devote tremendous effort to introduce plastic zone as a mechanical parameter of fatigue failure criterion. However, the variation of the shape, size and stress distributions in the crack-tip plastic zone, which are loading history dependent, are extremely complex under variable cyclic loading conditions. The analytic models and mechanical parameters developed during past few decades are not sufficient to describe the property of crack-tip plastic zone. These models are not capable of interpreting the intrinsic relationship between fatigue crack growth behavior and crack-tip plastic deformation. Therefore, how to describe the effect of the crack-tip plastic deformation on the fatigue crack growth behavior is one of the most concerned projects that have not been fully addressed yet.Recently, our research group has developed an approximate solution for the effect of crack-tip plastic deformation on stress intensity factor based on Eshelby equivalent inclusion theory and transformation toughening theory. The solution provides rigorous and effective analysis methods to reveal the effects of the crack-tip plastic zone for fatigue crack growth behavior in metal materials. In this thesis, a plasticity-corrected stress intensity factor(PC-SIF) range is developed for the fatigue crack growth behavior, and a series of important experimental phenomenon are considered, including the load ratio, overload retardation, transverse stress on biaxial fatigue and physically short crack emanating from notch root. The main outcomes are summarized as follows:(1) The expression of PC-SIF range pc?K and the detailed finite element analysis method for pc?K are established.(2) The PC-SIF range is introduced as a mechanical parameter in the Paris law, and is successfully implemented to describe the effects of the load ratio, overload retardation and transverse stress of biaxial fatigue on the fatigue crack growth rate. Comparisons with experimental results show that the PC-SIF range is an effective mechanical parameter capable of describing of the effects of the crack-tip plastic deformation.(3) The PC-SIF range is implemented as an effective mechanical parameter to reveal the effects of crack length, applied SIF range, overload ratio, consecutive overloads, load ratio and material properties on the overload retardation. The effects of static and cyclic transverse stress level, biaxial stress ratio, phase difference, cyclic stress ratio, and crack length on the biaxial fatigue crack growth are also been well described using the proposed PC-SIF range.(4) An approximate analytical solution for plasticity-corrected stress intensity factor range is developed for plane stress mode I and mode II crack under small scale yielding conditions based on Eshelby equivalent inclusion theory and transformation toughening theory.(5) An elastic-plastic stress intensity factor range is established as a general fatigue crack driving force according to Eshelby equivalent inclusion theory and transformation toughening theory. Preliminarily discussions for fatigue crack growth behavior of physically short crack emanating from notch root is carried on using the elastic-plastic stress intensity factor range.