Evaluation of Errors Associated with Cutting-Induced Plasticity in Residual Stress Measurements Using the Contour Method

Experimental Mechanics, Feb 2017

Cutting-induced plasticity can lead to elevated uncertainties in residual stress measurements made by the contour method. In this study plasticity-induced stress errors are numerically evaluated for a benchmark edge-welded beam to understand the underlying mechanism. Welding and cutting are sequentially simulated by finite element models which have been validated by previous experimental results. It is found that a cutting direction normal to the symmetry plane of the residual stress distribution can lead to a substantially asymmetrical back-calculated stress distribution, owing to cutting-induced plasticity. In general, the stresses at sample edges are most susceptible to error, particularly when the sample is restrained during cutting. Inadequate clamping (far from the plane of cut) can lead to highly concentrated plastic deformation in local regions, and consequently the back-calculated stresses have exceptionally high values and gradients at these locations. Furthermore, the overall stress distribution is skewed towards the end-of-cut side. Adequate clamping (close to the plane of cut) minimises errors in back-calculated stress which becomes insensitive to the cutting direction. For minimal constraint (i.e. solely preventing rigid body motion), the plastic deformation is relatively smoothly distributed, and an optimal cutting direction (i.e. cutting from the base material towards the weld region in a direction that falls within the residual stress symmetry plane) is identified by evaluating the magnitude of stress errors. These findings suggest that cutting process information is important for the evaluation of potential plasticity-induced errors in contour method results, and that the cutting direction and clamping strategy can be optimised with an understanding of their effects on plasticity and hence the back-calculated stresses.

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Evaluation of Errors Associated with Cutting-Induced Plasticity in Residual Stress Measurements Using the Contour Method

Evaluation of Errors Associated with Cutting-Induced Plasticity in Residual Stress Measurements Using the Contour Method Y.L. Sun 0 1 M.J. Roy 0 1 A.N. Vasileiou 0 1 M.C. Smith 0 1 J.A. Francis 0 1 F. Hosseinzadeh 0 1 0 School of Mechanical, Aerospace and Civil Engineering, The University of Manchester , Sackville Street, Manchester M13 9PL , UK 1 Engineering and Innovation Department, The Open University , Walton Hall, Milton Keynes MK7 6AA , UK Cutting-induced plasticity can lead to elevated uncertainties in residual stress measurements made by the contour method. In this study plasticity-induced stress errors are numerically evaluated for a benchmark edge-welded beam to understand the underlying mechanism. Welding and cutting are sequentially simulated by finite element models which have been validated by previous experimental results. It is found that a cutting direction normal to the symmetry plane of the residual stress distribution can lead to a substantially asymmetrical back-calculated stress distribution, owing to cutting-induced plasticity. In general, the stresses at sample edges are most susceptible to error, particularly when the sample is restrained during cutting. Inadequate clamping (far from the plane of cut) can lead to highly concentrated plastic deformation in local regions, and consequently the back-calculated stresses have exceptionally high values and gradients at these locations. Furthermore, the overall stress distribution is skewed towards the end-of-cut side. Adequate clamping (close to the plane of cut) minimises errors in back-calculated stress which becomes insensitive to the cutting direction. For minimal constraint (i.e. solely preventing rigid body motion), the plastic deformation is relatively smoothly distributed, and an optimal cutting direction (i.e. cutting from the base material towards the weld region in a direction that falls within the residual stress symmetry plane) is identified by evaluating the magnitude of stress errors. These findings suggest that cutting process information is important for the evaluation of potential plasticity-induced errors in contour method results, and that the cutting direction and clamping strategy can be optimised with an understanding of their effects on plasticity and hence the back-calculated stresses. EDM cutting; Finite element analysis; Stress redistribution; Uncertainty; Welding - Residual stresses are self-equilibrating stresses in a stationary material or structure which is free of external loads. They can be either detrimental or beneficial, depending on their role in interacting with in-service loading [1, 2]. From a design perspective, it is important to quantify the residual stresses developed via the manufacturing of engineering components as the residual stresses can be a key factor influencing damage and failure. A variety of techniques have been developed to measure residual stresses in metallic components [1, 3], among which the contour method [4] is relatively new. In comparison with some of the more commonly used techniques (e.g. X-ray/ neutron diffraction and hole drilling), the contour method has several advantages, such as insensitivity to microstructural variation (grain/texture distribution), the capability of mapping stresses across an entire cross-section, and accessibility using relatively available machine tools and metrology equipment. The contour method is a destructive strain release technique based on Bueckner’s principle of elastic superposition [5]. The standard contour method is implemented in the following steps [4, 5]: (1) experimentally cutting the sample, typically with wire electrical discharge machining (WEDM); (2) measuring the out-of-plane displacements (i.e. contour) of the created cut surfaces; (3) processing the measured data; and (4) back-calculating the residual stresses using finite element (FE) analysis for the majority of cases. In the final step of the contour method, normally, a 3D finite element model (FEM) of one of the cut parts is created and the negative values of the out-of-plane displacements (after data processing) are applied on the cut surface as nodal boundary conditions. A linear elastic stress analysis is then performed to reconstruct the residual stresses. Alternatively, Kartal [6] has developed analytical solutions for samples with simple geometry, which have been used to analyse residual stresses developed during welding [7]. Further details of the contour method are described elsewhere [5, 8, 9]. Cutting is the first and most critical step in the contour method. Several sources of errors in the cutting step could lead to significant uncertainties in the measured results. These include cutting artefacts, bulge errors and plastic deformation [5, 8, 10]. The errors (e.g. cutting artefacts), which are entirely associated with the nature of WEDM cutting and are independent of the stress state in the sample, can be avoided in most cases [8, 10]. Also, they cou (...truncated)


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Y. L. Sun, M. J. Roy, A. N. Vasileiou, M. C. Smith, J. A. Francis, F. Hosseinzadeh. Evaluation of Errors Associated with Cutting-Induced Plasticity in Residual Stress Measurements Using the Contour Method, Experimental Mechanics, 2017, pp. 719-734, Volume 57, Issue 5, DOI: 10.1007/s11340-017-0255-5