Composite materials are widely applied in aerospace, civil engineering, and sports equipment. Various damages produced during fabrication and long-term use can destroy its original mechanical properties, which brings safety and structural healthy concerns. Microwave imaging based on time reversal (TR) is one of the most promising nondestructive testing methods for portable, low-cost, and accurate testing with the advantages of auto-focus and super-resolution. This paper applied microwave TR for the detection of buried small damage in composites backed by metal plates. Strong reflection from composite–metal interfaces brings challenges in successfully achieving time-reversal auto-focusing on small and weak-scattering damages in composites. Traditional target localization methods, including the entropy regularization method and time-integrated energy method, may result in the wrong localization of small damages. The main contribution of this paper is that the localization problem caused by the strong reflection from metal plates is revealed first, and the target initial reflection method from through-wall-radar imaging is introduced to solve it. The performance of three target localization methods is investigated, and the physical reasons for failure or successful localization are discussed in detail. Some performance influence factors, such as the arrangement of receivers or the total time step of received signals, are also discussed. Good performance for the detection of a single small damage with a weak scattered signal is achieved, and the performance for detecting multiple damages is studied. All time-reversal simulations are carried out based on the finite-difference time-domain method.

中文翻译：

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微波时间反转对复合材料中埋藏小损伤的无损检测

复合材料广泛应用于航空航天、土木工程、体育器材等领域。制造和长期使用过程中产生的各种损伤会破坏其原有的力学性能，带来安全和结构健康问题。基于时间反转（TR）的微波成像是最有前途的无损检测方法之一，具有自动聚焦和超分辨率的优点，可实现便携式、低成本、精确测试。本文应用微波 TR 来检测金属板背衬复合材料中埋藏的小损伤。复合材料-金属界面的强反射给成功实现复合材料中小散射损伤和弱散射损伤的时间反转自动聚焦带来了挑战。传统的目标定位方法，包括熵正则化法和时间积分能量法，可能会导致小损伤的错误定位。本文的主要贡献在于首次揭示了金属板强反射引起的定位问题，并引入穿墙雷达成像的目标初始反射方法来解决该问题。研究了三种目标定位方法的性能，并详细讨论了定位失败或成功的物理原因。还讨论了一些性能影响因素，例如接收器的布置或接收信号的总时间步长。取得了良好的检测弱散射信号的单个小损伤的性能，并研究了检测多个损伤的性能。所有时间反演模拟均基于时域有限差分法进行。