Speaker
Description
Polymethacrylimide (PMI) foam is widely used in aerospace applications due to its lightweight, high strength, and excellent thermal protection performance. However, its low dielectric constant and closed-cell microstructure make internal debonding defects difficult to detect using conventional nondestructive testing methods. To address the need for defect characterization in thermal protection composites, infrared thermography was first employed to evaluate its response capability and applicability for internal defects. The results indicate that infrared thermography exhibits limited sensitivity to small debonding defects with heights ranging from 0.1 to 0.4 mm, making reliable internal localization challenging. To overcome this limitation, a microwave near-field reflection method based on an open-ended rectangular waveguide is proposed. A multilayer dielectric electromagnetic model is established to analyze the defect response mechanism, and key parameters, including probe distance, operating frequency, and scanning step size, are optimized through numerical simulations and experiments, enabling high-sensitivity and high-resolution detection of internal defects. In terms of image processing, anisotropic diffusion filtering is introduced to effectively suppress Rayleigh speckle noise while preserving defect edge features. Under optimized conditions, the developed automated inspection system is capable of stably imaging 16 artificial defects, and signal-to-noise ratio analysis demonstrates that the image quality is significantly superior to that obtained using mean and Gaussian filtering methods. The results demonstrate that the proposed microwave near-field method effectively compensates for the limitations of infrared thermography in detecting small internal defects, enabling comprehensive defect characterization of thermal protection composites and providing a potential approach for integrating rapid surface inspection with high-resolution internal imaging in hybrid nondestructive evaluation.
