Application of Laser-Ultrasound to Non-contact Temperature Sensing of Heated Materials

Abstract

In this paper we present a new non-contact method with a laser ultrasonic technique for measuring both surface and internal temperature distributions of a heated material. The principle of the temperature measurement is based on temperature dependence of the velocity of the surface acoustic wave (SAW) or longitudinal wave (LW) propagating through a material. An effective method consisting of ultrasonic pulse-echo measurements and an inverse analysis is developed. In the inverse analysis, ultrasonic data are coupled with a one-dimensional finite difference calculation to realize robust and fast estimation of temperature distributions. In addition, a laser-ultrasound system is employed to perform non-contact ultrasonic measurements. To demonstrate the practical feasibility of the method, experiments with an aluminum plate being heated up to 110 oC are carried out. At first, SAWs propagating on the surface of the plate are measured using a laser interferometer based on a photorefractive two-wave mixing. The transit time of the SAW is measured every 0.3 s during heating and used for the inverse analysis to determine the surface temperature distribution of the plate. It is verified that the results determined by the ultrasonic method almost agree with those measured using an infrared radiation camera. Secondly, LWs propagating through inside the heated plate are measured using the laser interferometer. The internal temperature distribution in the plate is then determined from the transit time of the LW. Thus, it has been demonstrated that the proposed method can provide effective non-contact measurements of the transient variation in the temperature distribution of a heated material.