Thermal protection structure (TPS) plays a key role in protecting hypersonic flight vehicles from harsh service environment and has been widely used. As a common ablative thermal protection material, quartz ceramic TPS realizes thermal insulation by generating ablation during the reentry phases. The structural state of quartz ceramic TPS therefore is of great importance to the service performance and safety of hypersonic flight vehicles, and shows an urgent need for structural health monitoring (SHM). However, the monitoring of real ablation of quartz ceramic structure is barely considered in the current study, let alone online monitoring under harsh service environment, which brings a great challenge to the implementation of reliable ablation monitoring. Aiming at online ablation monitoring of quartz ceramic structure, this paper performs experimental research by taking advantage of guided wave (GW)-based monitoring method. An ablation experiment and monitoring system is built to generate real ablation under high temperature and high speed airflow and perform GW monitoring. The maximal temperature of the system can reach about 2100 °C. The feasibility of ablation monitoring is verified by analyzing ablation-induced influence on GW signals. Gaussian mixture model (GMM) is adopted to combine with GW method to suppress the high temperature variation caused by uncertainty influence on GW signals during the ablation process, so that reliable monitoring of ablation can be realized. Experimental results show that online monitoring of ablation propagation of quartz ceramic structure under high temperature variation is achieved, which can be further used for ablation quantification evaluation.

Lamb Wave (LW)-based structural health monitoring method is promising, but its main obstacle is damage assessment in varying environments. LW simulation based on piezoelectric transducers (referred to as PZTs) is an efficient and low-cost method. This paper proposes a multiphysics simulation method of LW propagation with the PZTs under temperature effect. The effect of temperature on LW propagation is considered from two aspects. On the one hand, temperature affects the material parameters of the structure, the adhesive layers and the PZTs. On the other hand, it is considered that the thermal stress caused by the inconsistency of thermal expansion coefficients among the structure, the adhesive layers, and the PZTs affect the piezoelectric constant of the PZTs. Based on the COMSOL Multiphysics, the mechanic–electric–thermal directly coupling simulation model under temperature effect is established. The simulation model consists of two steps. In the first step, the thermal-mechanic coupling is carried out to calculate the thermal stress, and the thermal stress effect is introduced into the piezoelectric constant model. In the second step, mechanic–electric coupling is carried out to simulate LW propagation, which considers the piezoelectric effect of the PZTs for the LW excitation and reception. The simulation results at −20 °C to 60 °C are obtained and compared to the experiment. The results show that the A0 and S0 mode of simulation signals match well with the experimental measurements. Additionally, the effect of temperature on LW propagation is consistent between simulation and experiment; that is, the amplitude increases, and the phase velocity decreases with the increment of temperature.

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In recent years, the importance of Structural health monitoring (SHM) has been widely recognized. Among all the SHM methods, Guide Wave (GW) based SHM method using bonded piezoelectric transducers proves promising for its sensitivity to minor damage and minor amplitude loss in the large monitoring area. The thermal protection structure (TPS) is the most fundamental guarantee to ensure the safety of aerospace vehicles, and its structural health monitoring has great safety significance and economic value; however, it is often used in high-temperature environments. Conventional GW experimental methods fail in high-temperature environments due to the effects of high-temperature environments, so experimental methods for stable and reliable acquisition of GW signals at high temperatures need to be considered. In this paper, an experimental method for regular purchase of GW signals in a high-temperature environment, which uses a high-temperature sensor, a high-temperature resistant glue, and a high-temperature integration process, is proposed. The experimental results show that the practical method can obtain stable and reliable GW signals at 200°C, beneficial for damage monitoring in high-temperature environments.

Thermal protection structures (TPS) are the key component in the design and manufacture of the reusable launch vehicle. The probability of failure of TPS in an aerodynamic thermal environment during reentry is much higher than that of ordinary structures. There is an urgent need to develop a structural health monitoring (SHM) method that can achieve rapid diagnostic for TPS to realize the high reliability and maintainability of the reusable launch vehicle. Hence, this paper aims at the typical TPS made of C/C composite material, the feasibility of SHM methods based on the active piezoelectric guided wave was studied and verified. And the propagation characteristics of guided wave in the TPS such as the multi-mode, the signal frequencies of monitoring and the effects of material anisotropy on the propagation characteristics of the guided wave were analyzed. Then, a guided wave monitoring experiment of C/C TPS with simulation damage was carried out, and the variation of a guided wave under different damage levels was analyzed from the aspects of amplitude, phase, damage scattering and damage index. The results show that the SHM methods based on active piezoelectric guided wave can effectively monitor the damage and damage expansion on the surface of C/C TPS.