Simulations and experimental study on imaging of thick defect in reusable thermal protective system using microwave NDT

Reusable thermal protective system (TPS) plays a key role for protecting space vehicles against the aerothermal heating. Thus the characterization of TPS performance becomes an important issue in their production, processing and applications. This paper demonstrates the detection and imaging of inner disbond and thick defect, which considerably compromises the performance of TPS materials within a two-layer TPS configuration (a special designed thick heat insulation tile backed by an aluminum alloy plate). The methodology employed for this investigation is a near-field microwave non-destructive testing (NDT), encompassing both simulations and experimental study. A 3D microwave model for the two-layer TPS structure containing the specified defects was established, and the magnitude and phase of the waveguide input impedance (i.e. S11 parameters) for each node within the X-band (8.2 ∼ 12.4 GHz) frequency range were obtained in the work. The simulation results reveal distinct featured amplitude and phase images of defects at various frequencies. Correlation processing algorithms were employed to make the images dimensionally reduction and featured enhancement. Finally, the results were verified by experimental measurements using an open-ended rectangular waveguide. Interestingly, the disbond defect between the two layers could be clearly identified by the phase information after correlation processing algorithms. The proposed method appears to offer a contactless, intuitive, and effective approach for characterizing defects within TPS materials.