Thermo-graphic Detection of Surface Cracks

Abstract

This paper describes a thermal gradient method for infrared thermography to detect surface crack in thin plate. Traditional thermal gradient method uses a deri-vative of thermal field, which may magnify a noise inevitable in experimental thermography induced mainly by emissivity variation of target surface. This study develops a thermal gradient method robust to the noise using a holomorphic function of temperature field in a thin plate under steady-state thermal condition. The holomorphic function of a given temperature field is derived for 2-D heat flow in the plate from Cauchy-Riemann conditions, and applied to define a contour integral that varies according to the existence and strength of a singularity in the domain of integration. The integral calcuated at each point of thermal image eliminates the temperature variation due to heat conduction & suppress the noise, so that its image emphasizes and highlights the singularity such as crack. This feature of holomorphic function is investigated numerically using a thermal field in thin plate that satisfies the Laplace equation representing steady-state heat flow. The simulation results show that the integral image selects and indicates asingularity like crack embedded artificially in the plate very well in a noisy environment.