Measurement of Deformation in Varying Stress Fields

Muyupa, Ellies (2020). Measurement of Deformation in Varying Stress Fields. PhD thesis The Open University.



Welding is one of the most widely used joining techniques for manufacturing complex engineering components. However, due to transient temperature cycles in the adjacent materials being joined, the local microstructure is altered and plastic strain in the surrounding materials is introduced resulting in a variation of mechanical properties across the weldment. When an extracted cross-weld test specimen is subjected to an external uniaxial load, there is interaction between the various constituents of the weld (such as the weld fused zone, heat affected zone and parent metal) resulting in development of constraint in the weaker material due to the adjacent stronger material. This affects the distribution of stress along and within the test specimen. The level of constraint developed is dependent on the strength mismatch between adjacent materials which may be graded across the heat affected zone and the specimen cross-sectional geometry.
With development of digital image correlation (DIC) and its application to measuring deformation of solids, it has become feasible to measure local stress-strain properties of cross-weld samples and also specimens of variable geometry at both room and high temperature. Local deformation measured by DIC is correlated with nominal stress based on an iso-stress assumption, where it is assumed that the various regions of the weld are arranged in series and that the stress is uniform over the cross-section area at any given location in the specimen. Thus, local mechanical properties are inferred by mapping the measured local strain against the global stress. In this context the effect of constraint is important because it introduces a local stress distribution that will differ from the global net section stress and can therefore introduce errors in determination of local stress-strain properties.
The influence of these factors raises questions concerning the interpretation of apparent local mechanical properties of cross-welds measured using DIC. Specifically, there is uncertainty as to whether the measured variation in local strain versus average cross-sectional stress (average global stress) represents the local mechanical properties of the material with negligible error. The severity of potential constraint errors will be influenced by the specimen geometry (particularly the tensile specimen thickness to width ratio for 2D DIC strain measurements) and the gradient of material property inhomogeneity along the specimen gauge length.
In the current study, measurement of deformation in varying stress fields using DIC was carried out. Two distinct methods of introducing varying stress fields were employed: in the first case, the stress field was varied by designing an hour-glass shaped specimen with the specimen width varying as a function of the gauge length. In the second case, stress fields were varied due to a change in the specimen thickness in mismatched dissimilar metal joints. To reduce the complexity in the developed stress fields in the region near the interface, two materials were joined using solid state diffusion bonding giving a step change in the material properties across a planar interface. Test specimens spanning the interface were studied by finite element modelling and experimental tests with DIC deformation monitoring.
Using DIC and a single hour-glass shaped specimen, multiple creep curves at different stress levels along the specimen gauge length have been successfully extracted for a test temperature of 525oC. The creep data obtained at 525oC from the hour-glass specimen test have been fitted to RCC-MR, Garofalo and Graham-Walles creep deformation models. A better correlation is achieved between the experimental data and the RCC-MR model predicted values when the local deformations from the single specimen are plotted as a function of time and true stress at the end of the specimen’s initial loading.
From both elastic-plastic FE models and experimental local stress-strain curves extracted from DIC measurements, it can be concluded that a thin (1mm thick or less) 6mm wide test specimen should be used to extract the local mechanical properties from tensile specimens that have varying material properties along the gauge length, such as found in cross-weld test specimens. This is because the thinner specimen reduces the size of the strain concentration region adjacent to the interface where errors in measured local mechanical properties can occur. As a ‘rule of thumb’, the intrinsic stress-strain properties of the material should not be inferred from regions within 1mm of the interface for these 1mm thick samples because of concentration errors.
The results obtained demonstrate the capability of DIC as a measurement technique for characterising the local deformation of specimens with varying stress fields. The findings also highlight some aspects of the technique which need to be considered when interpreting data from DIC measurements of local deformation from specimens with varying stress fields.

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