Ultrasonic Temperature Compensating Method for Tracking Decomposition Front in Silica-Phenolic Thermal Protection Material

Abstract

This paper presents an ultrasonic method for tracking the decomposition front in ablating silica-phenolic thermal protection system material. The method exploits diffuse ultrasonic backscatter from the material microstructure to account for the temperature-dependent change of the speed of sound. The method is demonstrated in a series of ablation tests performed at a heat flux of provided by an oxy-acetylene torch. Validation of the method is performed using machining, ultrasound, and micro–computed tomography scanning of the ablated samples. The measurement error in all tests does not exceed 1 mm. Analysis of the data suggests that, during the active ablation phase, temperature rise in the virgin material zone is low and the speed of sound correction is not required, whereas the main advantage of the method comes during the heat soak-back phase. Performed thermo-acoustic characterization of silica phenolic shows a linear dependence between the speed of sound and temperature up to 210°C. This finding opens a way to use the backscatter for estimation of temperature distribution in the virgin material. Finally, the study presents the measurement of acoustic attenuation to provide a reference for the application of the method to additional thermal protection materials, except silica phenolic.