Frequency Domain Time Reversal Adaptive Focusing on Nondestructive Testing Imaging Method for Composite Materials

Frequency constraints limit traditional microwave nondestructive testing (MNDT) methods, while time-domain time-reversal approaches suffer from restricted bandwidth and sampling resolution. These limitations reduce the ability to detect profound or subtle defects in composite materials. To overcome this, we propose a novel frequency-domain time-reversal microwave nondestructive testing (FD-TRMNDT) method. This method scans the material under test (MUT) using an adaptive source positioning principle derived from time-reversal theory and identifies internal defects by tracking phase variations in the frequency-domain response. A time-reversal operator is constructed from multi-frequency S-parameters to enhance focusing and resolution. Experimental validation was performed on composite samples containing simulated air bubble defects. The proposed technique successfully detected defects as small as 0.027 \(\lambda \) (approximately 2 mm in diameter), with localization errors less than 0.1 \(\lambda \). The experimental results closely matched full-wave simulations, confirming the method’s accuracy and practical applicability.