Local-stress-induced thermal conductivity anisotropy analysis using non-destructive photo-thermo-mechanical lock-in thermography (PTM-LIT) imaging

When tensile loading is exerted on a sample, internal stress will be generated to counterbalance the external force. The accumulation of local stress results in the change of thermal properties and the sample becomes locally anisotropic, wherein lies the significance of correlation between mechanical properties and thermal parameters. In this research, anisotropic thermal conductivity was considered as the direct result of tensile loading and was interpreted by a theoretical three-dimensional anisotropic diffusion-wave model. Photo-thermo-mechanical lock in thermography (PTM-LIT) was introduced and used with a focused laser beam and a mid-infrared camera viewing an aluminum alloy sample fixed on a dynamic home-made tensile rig. A numerical two-dimensional Fourier transform was used to compute the thermal-wave field and the effects of several important factors were investigated. Both theoretical and experimental images were analyzed with an isothermal conductivity anisotropy contour fitting approach. It was demonstrated that PTM-LIT can qualitatively and quantitatively reveal otherwise hard-to-measure mechanical property behavior of materials subjected to tensile loading from the stress-free state to its ultimate level of mechanical strength before fracture.