Revealing the heterogeneity in weld microstructures using the thermomechanical dissipative heat source

Abstract

Mechanical deformation of a metal is accompanied by the dissipation of energy in the form of heat as a result of thermodynamically irreversible processes occurring at the microscale. This is applicable to deformation in both elastic and plastic regime so long as the thermodynamically irreversible processes are activated. It follows that there is a possibility of identifying the condition of the material microstructure by evaluating the heat dissipated during deformation. In the thesis, the continuous temperature rise due to the heat dissipation in a material under cyclic loading is obtained using an infrared (IR) detector. Most metals dissipate a very small amount of heat in their elastic range (few mK.s-1). As a consequence, the temperature change is usually below the thermal resolution of the infrared detector used. To enable an accurate measurement to be made, the experiments were conducted in a specially designed setup which eliminated parasitic heat sources.

Spatial averaging was used to improve the signal to noise ratio and the dissipative heat source was extracted from the thermal data using the thermomechanical heat diffusion equation. The spatial averaging technique successfully provided a consistent detection threshold of just under 1 mK.s-1. To demonstrate the effectiveness of the enhanced thermal resolution, the effect of material microstructure on the dissipative heat source was studied in 316L stainless steel. Different microstructures were produced by heat treating strip specimens to give a homogeneous field of observation over a large area of the IR detector. Monotonic tensile test and microhardness test were performed on each of the specimen to establish the change in properties resulting from the different microstructures. Micrographs were produced, which showed that the grain size only increased at the highest temperature, for the other heat treatment any difference in the dissipation would be mostly as a result of change in dislocation density. At equivalent stress levels, the microstructure had a significant effect on the dissipative heat source.

In the vicinity of welds, material microstructures are inhomogeneous over relatively small areas. To capture possible spatial heterogeneities in the heat source, a 3D (2D in space and time) least square estimation of the temperature evolution was performed in place of the spatial averaging technique. The temperature data were fitted over a small window which is swept throughout the entire data set resulting in a spatial map of the dissipative heat source. The method was verified using a ‘hole-inplate’ specimen under tensile cyclic loading which has an inhomogeneous stress field and hence dissipation. The approach was then used on the data collected from a welded 316L specimen exhibiting an inhomogeneous microstructure but tested in nominally uniaxial stress. It was shown that the region corresponding to the base metal had the highest dissipation followed by a gradual decrease in the heat source across the heat affected zone. The heat source increases subsequently as the centre of the fusion zone is approached. A modified procedure employing higher spatial resolution focusing on the fusion zone also revealed differences in the heat source between different weld passes. Strain measurements made on identical specimens using digital image correlation verified the material properties local to the weld. The work in the thesis clearly demonstrates that the dissipative heat sources associated with microstructural behaviour in welds can be identified successfully using IR thermography.

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Identifiers

ORCID for Fabrice Pierron: orcid.org/0000-0003-2813-4994

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