Speaker
Description
This study investigates the propagation velocity of thermal waves, which is fundamental knowledge for improving the efficiency of active thermography inspections. The velocity of the thermal waves has been studied and described in many publications; the velocity vp is obtained as the square root of 2αω (α and ω are thermal diffusivity and angular frequency, respectively). However, this velocity is considered as the phase velocity in general wave theory, and the thermal energy (or wave packet) should propagate at the group velocity vg. Based on the wave theory of group velocity, the authors calculate the group velocity of thermal waves and estimated that the vg equals 2vp. To verify this estimation, experiments to observe the group velocities of thermal waves were performed. In the experiments, bar-shaped specimens made from three different materials (aluminum, stainless steel, and acrylic resin) were used. One end of the specimens was periodically heated using a halogen lamp to generate thermal waves propagating along the bar-shaped specimen one-dimensionally, and temperature change in the side surface of the specimen was observed by an infrared camera. The temperature–time relationships on the side surface were observed at multiple points, and the propagation velocities of the thermal waves were obtained from the relationship between the time delay in temperature change and the distance between the measurement points. In order to observe the propagation velocity of input thermal wave packets, which should be the group velocity vg, several post-processing steps (such as bandpass filtering and cross-correlation processing) were applied to the temperature data. The experimental results on aluminum specimens showed that the observed the propagation velocities obtained after the post-processing procedures agreed with the theoretically estimated group velocity (vg = 2vp). This demonstrates the validity of the theoretical estimation and implies the existence of group velocity in thermal waves. On the other hand, observation of the group velocities in the stainless steels and acrylic resins was difficult. This should be caused by the difference in amplitude attenuation; the attenuation of thermal waves propagating in stainless steel and acrylic is much greater than that in aluminum. Based on these results, the conditions under which group velocity is observable (which should be determined by the thermal properties of the material and temperature resolution of the infrared camera used) will be discussed in the conference.
