Abstract
This article presents an experimental study on the fatigue behaviour of cracks emanating from cold-expanded holes utilizing thermoelastic stress analysis (TSA) and synchrotron X-ray diffraction (SXRD) techniques with the aim of resolving the long-standing ambiguity in the literature regarding potential relaxation, or modification, of beneficial compressive residual stresses as a result of fatigue crack propagation. The crack growth rates are found to be substantially lower as the crack tip moved through the residual stress zone induced by cold expansion. The TSA results demonstrated that the crack tip plastic zones were reduced in size by the presence of the residual compressive stresses induced by cold expansion. The crack tip plastic zones were found to be insignificant in size in comparison to the residual stress zone resulting from cold expansion, which implied that they were unlikely to have had a notable impact on the surrounding residual stresses induced by cold expansion. The residual stress distributions measured along the direction of crack growth, using SXRD, showed no signs of any significant stress relaxation or redistribution, which validates the conclusions drawn from the TSA data. Fractographic analysis qualitatively confirmed the influence on crack initiation of the residual stresses induced by the cold expansion. It was found that the application of single compressive overload caused a relaxation, or reduction in the residual stresses, which has wider implications for improving the fatigue life.
Highlights
The manufacturing of aerospace structures requires thousands of fastener holes to be drilled for the purpose of assembly
The study presented in this paper has used thermoelastic stress analysis (TSA) and synchrotron X-ray diffraction (SXRD) techniques to analyse the behaviour of fatigue cracks emanating from cold-expanded mandrel exit side mandrel entry side
A characteristic trend was observed for cracks initiating from cold-expanded holes: namely that, the crack growth rate decreased to a minimum at approximately 3 mm from the hole edge beyond which it increased as the crack grew out of the influence of the residual compressive stresses associated with cold expansion
Summary
The manufacturing of aerospace structures requires thousands of fastener holes to be drilled for the purpose of assembly. Stefanescu et al [11] later reaffirmed their conclusions by determining the residual stress relaxation close to the hole edge, resulting from compressive overloads, using a laboratory X-ray diffraction technique It is, well understood that the fatigue loads, causing large-scale plastic deformation at the hole edge, could significantly alter the initial residual stress distribution. Özdemir & Edwards [13] used a similar Sachs technique to show pronounced stress relaxation in 7075 aluminium specimens The cause of this relaxation was reported to be the growth of short fatigue cracks from the hole edge, which were arrested as the residual stress distribution stabilized. The experimental study presented in this article investigates the propagation of fatigue cracks emanating from cold-expanded holes and their influence on the surrounding residual stresses, with the aim of resolving the above-mentioned differences found in the literature by establishing the reasons for the potential redistribution of these residual stresses
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