The primary result of the sponsored research is the development and application of the boundary element method for two- and three-dimensional fatigue crack growth analysis. The two-dimensional formulation developed previously (AFOSR Contract No. F49620-84-C-0042) was extended to investigate the crack tip behavior of long and short cracks under cyclic loading. The influence of residual plasticity on stress intensity factor was used to obtain an unambiguous estimate of the plastic zone size. It was demonstrated that the effect of the plastic wake on the stress intensity factor for crack opening (closure) and the effect of the residual stress on the retardation are identical manifestations of the same plasticity process. The boundary integral equations also provide insight to the mathematical equivalence of these two effects. Keywords: Fracture mechanics, Fatigue crack growth, Crack retardation, Crack closure, Crack opening displacement, Boundary element method. (jes).

What can be added to the fracture mechanics of metal fatigue that has not already been said since the 1900s? From the view point of the material and structure engineer, there are many aspects of failure by fatigue that are in need of attention, particularly when the size and time of the working components are changed by orders of magnitude from those considered by st traditional means. The 21 century marks an era of technology transition where structures are made larger and devices are made smaller, rendering the method of destructive testing unpractical. While health monitoring entered the field of science and engineering, the practitioners are discovering that the correlation between the signal and the location of interest depends on a priori knowledge of where failure may initiate. This information is not easy to find because the integrity of the physical system will change with time. Required is software that can self-adjust in time according to the monitored data. In this connection, effective application of health monitoring can use a predictive model of fatigue crack growth. Earlier fatigue crack growth models assumed functional dependence on the maximum stress and the size of the pre-existing crack or defect. Various possibilities were examined in the hope that the data could be grouped such that linear interpolation would apply.

As surface flaws grow in high-strength nickel-base superalloys, the growth along the free surface (dc/dN) can be much slower than growth in the depth direction (da/dN), even after accounting for variations in K along the crack front. This behavior reduces the crack aspect ratio (c/a) and can result in the formation of stable shear lips at the free surface during fatigue crack growth. Trantina et al. recently performed elastic-plastic finite-element analyses of surface cracks which showed that as the applied stress approached the elastic limit, a relatively large region of constraint loss was observed where the crack front intersects the free surface. This concept was used to develop a constraint-loss modification to the Newman-Raju K solution for the surface flaw problem. This model has been applied to physically small surface crack growth in the advanced nickel-base superalloy René 95. The predicted results match the dc/dN data, which are slower than and not parallel to the corresponding da/dN-K data. This model has been combined with a dual-Walker exponent approach to model nonzero stress (R) ratios. This methodology has been used to predict the residual lives of surface flaws growing in both uniform and nonuniform stress fields within a factor of two of the experimentally observed lives. This model has also been successfully applied to small surface flaws and can predict the lives of cracks with depths (a) as small as 65 ?m (0.0025 in.).

This book offers a concise introduction to fatigue crack growth, based on practical examples. It discusses the essential concepts of fracture mechanics, fatigue crack growth under constant and variable amplitude loading and the determination of the fracture-mechanical material parameters. The book also introduces the analytical and numerical simulation of fatigue crack growth as well as crack initiation. It concludes with a detailed description of several practical case studies and some exercises. The target group includes graduate students, researchers at universities and practicing engineers.