Laser-based, non-invasive monitoring and exponential analysis of the mechanical behaviour of materials with structural inhomogeneities in heat transfer, towards thermal equilibrium

The mechanical response of an inhomogeneous sample, simulating wooden marquetry, is studied after the application of thermal load and increased initial temperature of the object. The presence of defects in the inner parts of the sample creates areas of diverse material properties, which respond in a different manner and rate during relaxation towards equilibrium. Energy is locally transferred, resulting in a spatially non-uniform deactivation process and deformations among the different parts of the object. With the performed experiments monitoring of the real-time mechanical response of the full surface of the target after thermal excitation towards equilibrium is achieved, within a completely non-destructive optical scheme. The experimental set-up used is a Digital Holographic Speckle Pattern Interferometry optical scheme. The set-up provides high-resolution, full-field dynamic information on the differential mechanical displacements, through the wrapped sequentially recorded interferometry fringes. Through a one-dimensional investigation and analysis of the temporal evolution of the displacement, the evaluation of the relaxation process of complex targets becomes feasible. The results are discussed in the context of the structural condition of the targets, of certain thermal material properties. After thermal loading, an initial fast energy redistribution dominates. On longer time intervals, long after the equilibrium temperature is reached, a slower mechanical relaxation takes place. The local rise in the number of interference fringes brings to light an ongoing transfer of kinetic energy values between areas in the interior of the object. This observation indicates a mechanical effect lasting long after the total volume of the sample reaches initial mean temperature values.