A novel method using microwaves is proposed to detect and evaluate defects in glass fiber reinforced plastic (GFRP) laminates. A thin circular plastic film simulating delamination was inserted in GFRP laminates and was detected using microwave reflectometry. A focusing mirror sensor consisting of a horn antenna and two metal mirrors was used to improve the measurement resolution. In addition, the thickness of the film was calculated using a proposed model based on microwave propagation theory. Measuring the variation in the amplitude of the microwave reflectivity, a 7.5-μm-thick film was successfully detected in a 3-mm-thick GFRP laminate. Moreover, the calculated results of the inserted film thickness had a high degree of accuracy.
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