Assessment of the heterogeneous microstructure in the vicinity of a weld using thermographic measurements of the full-field dissipative heat source

During a material deformation process, part of the mechanical energy is dissipated as heat due to thermodynamically irreversible processes occurring at the microscale of the material. In particular, part of the plastic deformation energy is transformed into heat and is referred to as 'intrinsic dissipation' as it is intrinsic to the material behaviour. The intrinsic dissipation is a heat source that is sensitive to microstructural states which can be used to identify different microstructural regions resulting from material processing such as welding. To determine the heat source in a full-field manner, it is necessary to use an infrared camera to measure any temperature rise in a specimen undergoing elastic cyclic loading. Unlike the intrinsic dissipative heat source, the temperature change is sensitive to thermal exchanges with the surroundings. Hence, the thermomechanical heat diffusion equation is used to determine the full-field dissipative heat from the thermographic temperature measurement by implementing an image processing procedure based on least squares fitting enabled by specially devised experimental approach. The procedure is verified by deriving both the thermoelastic and dissipative heat sources from a 'hole-in-plate' specimen manufactured from 316L stainless steel, that is, a specimen with a known stress distribution. The approach is then applied to a 316L laser welded specimen, and it is demonstrated that the different microstructures resulting from the welding process can be identified with the procedure. The heterogeneous microstructure is confirmed using micrographs and further verified by the different stress-strain behaviour obtained for each microstructural region using digital image correlation (DIC).