Articles

• Controlling plasticity in the contour method of residual stress measurement (PhD Thesis),

1. Traoré (2013) – British Library-UK

http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.590800

Abstract:

The contour method has emerged as a promising technique for residual stress measurement in relatively large, thick and complex engineering components. The method involves making a cut in the sample of interest, measuring the subsequent relaxed deformation profile of the cut surface and using this profile to back-calculate the original out-of-plane residual stress field by finite element modelling. The method is based on the theory of elasticity in that the stress relaxation during test specimen cutting is assumed to be entirely elastic. However, when measuring residual stresses of magnitude approaching the material yield strength, plasticity can occur which introduces errors in the measured stress profile. The main aim of this thesis was to develop methods of mitigating and estimating plasticity •induced errors in contour method residual stress measurements. Based on the principles of fracture mechanics, an analogy was made between the stress relaxation process and a cracked plate to investigate the origin of plasticity in the contour method. It was demonstrated that that the cut tip stress intensity factor (KT) and the corresponding plastic zone parameters are the most important parameters for characterising plasticity-induced errors in the contour method. Extensive finite element analyses were carried out to understand and control the errors associated with plasticity with a view of improving the accuracy and reliability of the method. The outcomes of this research provide a valuable insight into how accumulation of plasticity for different restraining conditions affects the performance of the contour method. A novel cutting strategy that aims at mitigating plasticity-induced error by controlling the severity of the cut tip stress concentration (i.e. stress intensity factor) during the cutting process has been developed. Furthermore, procedures (correlations) are developed to estimate the plasticity-induced errors in the results of the contour method. Finally, guidelines are proposed and applied to a case study for mitigating the errors associated with plasticity in a contour method residual stress measurement.

• Crack path selection at the interface of wrought and wire + arc additive manufactured Ti–6Al–4V,

1. Zhang, X. Zhang, X. Wang, J. Ding, Y. Traoré, S. Paddea, S. Williams (2016)

Materials & Design, Volume 104, 15 August 2016, Pages 365-375

https://doi.org/10.1016/j.matdes.2016.05.027

Abstract:

Crack propagation deviation tendency in specimens containing an interface between wrought alloy substrate and Wire + Arc Additive Manufacture (WAAM) built Ti–6Al–4V is investigated from the viewpoints of microstructure, residual stress and bi-material system. It is found that a crack initiated at the interface tends to grow into the substrate that has equiaxed microstructure and lower resistance to fatigue crack propagation. Experimental observations are interpreted by finite element modelling of the effects of residual stress and mechanical property mismatch between the WAAM and wrought alloy. Residual stresses retained in the compact tension specimens are evaluated based on measured residual stress in the initial WAAM built wall. Cracks perpendicular to the interface kept a straight path owing to the symmetrical residual stress distribution. In this case, the tangential stress in bi-material model is also symmetric and has the maximum value at the initial crack plane. In contrast, cracks parallel to the interface are inclined to grow towards the substrate due to the mode II (or sliding mode) stress intensity factor caused by the asymmetric residual stress field. Asymmetric tangential stress in the bi-material model also contributes to the observed crack deviation trend according to the maximum tangential stress criterion.