The effect of flash power on the measurement of thermal effusivity using thermal wave imaging

Abstract

In aerospace applications, water or oil may ingress in the honeycomb structure, it is important to detect what kind of liquid ingression it is. In this study, a 20mm thick steel plate was milled eight circular holes (four 1.1mm depth and four 2mm depth) at the back side, each hole was filled with different materials: water, oil, air and wax. Thermal wave imaging technology was successfully used in many fields, such as aerospace, automobile, etc. quantitatively and qualitatively, it was used to measure the thermal effusivity of filled materials in this study. A special experimental setup was adopted that the steel sample was horizontally placed on a cover with holes faced above. The bottom surface of the detected sample is heated with a short pulse of light, the sample surface is instantaneously heated to a high temperature and captured by a high speed and high precision infrared camera. The generated heat at front surface propagates to the interior of the sample, and leads to a continuous decrease of the surface temperature. The theoretical model of temperature evolution with time was constructed, and the calculation procedure of embedded material filled in steel holes was deduced based on the theoretical model, and in which the air hole was used as the reference. In thermographic applications, different power supplies, detection distance and infrared camera, etc. may result different signal levels, and noise level may also vary which depends on the usage conditions of infrared camera. In this study, nine different flash power levels, which changed from full scale power level to one ninth linearly, were used to simulate different noise levels. The results of three different filled materials at nine different powers and the corresponding error among different powers were compared. The calculation results indicate that thermal wave imaging is a potential technology to test the thermal effusivity of an unknown material when it is embedded in a known material.