On the effect of residual stresses upon the fatigue behavior of advanced airframe alloys

Aluminum-lithium alloys are currently being considered as a replacement for conventional aluminum alloys in the aerospace industry. The obvious attractions of these alloys are significant reduction in density, increase in elastic modulus, and compatibility with existing metal forming techniques.;This investigation examines the beneficial effects of compressive residual stresses upon the fatigue fracture behavior of high strength aluminum-lithium alloys and the targeted conventional alloys. Specifically, attention is devoted to the characterization of shot-peening induced residual stresses, their effect on fatigue crack initiation and propagation, and the development of a model which is capable of predicting the fatigue crack growth rate in the presence of residual stresses.;A modified hole-drilling technique, based on finite element calibration, was developed to measure the non-uniform profile of the resulting residual stress field. Room temperature uniaxial fatigue tests were conducted on unpeened and peened specimens using digitally controlled electro-hydraulic testing apparatus. Fatigue crack initiation, short crack growth, long crack growth and crack closure were all examined in the study. Crack initiation and growth were monitored using the replica technique. Relaxation of residual stresses was also monitored by measuring residual stresses at various stages of the fatigue test sequence.;The effect of residual stresses was accounted for in a theoretical model through the superposition of applied and residual stress intensity factors. In view of the three-dimensional nature of the encountered cracks, three-dimensional stress intensity factor solutions under uniaxial loading are developed. A new method for obtaining 'discretized' weight functions of cracked geometries, along with the slice synthesis technique are developed in this thesis and used to obtain the applied and residual stress intensity factors.;The results reveal that the fatigue life of the specimens is improved by peening. This improvement is governed by the peening conditions, the applied loads and the tested materials. The higher fatigue fracture performance of peened specimens is a direct result of slower crack propagation caused by the reduction of the crack driving force. Furthermore, the results reveal that the crack growth rates in the peened specimens are reduced within and even beyond the compressive layer of the residual stress field.