Improving the Spatial Resolution of the Contour Method

Naveed, Nida (2016). Improving the Spatial Resolution of the Contour Method. PhD thesis The Open University.



The contour method involves precise cutting of a body into two halves and provides a measured cross-sectional map of the residual stress acting normal to the cut surface. The method is being increasingly used to measure the residual stress field in critical engineering components that have high structural integrity requirements. Therefore, it is of paramount importance to improve the precision and accuracy of the method. The results of the method are critically affected by the cutting process, the deformation contour measurement accuracy and the data analysis procedure. The aim of the present thesis is to improve the reliability and accuracy of contour residual stress measurements by minimising errors and uncertainties that can be introduced during the cutting and data analysis steps of the technique. The first part of this thesis covers the design of a test specimen to benchmark the quality of cutting for contour method measurement. The design allows sequential trial cuts on a nominally stress-free specimen. Two different materials have been tested: a high yield strength mild steel (EN3B) and an austenitic stainless steel (304L). An accompanying characterisation record sheet for each trial cut has been devised to quantify the cut surface quality and features. The most important parameters that can be used to demonstrate the quality of the contour cut have also been identified in order to help optimise the cutting process. The second part of the thesis covers an investigation of important parameters that are used for contour stress measurements including surface deformation data collection spacing, deformation data smoothing and finite element mesh size. A simple approach for choosing initial parameters is developed based on an idealised cosine displacement function (giving a self-equilibrated one-dimensional residual stress profile). Guidelines are proposed to help the measurer select the most suitable choice of these parameters based on the estimated wavelength of the residual stress field. The outcomes of this research have successfully been applied to improve the spatial resolution and detailed characterisation of the residual stress field in three welded components.

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