Microwave nondestructive detection and quantitative evaluation of kissing defects in GFRP laminates

Kissing defects remain difficult to detect due to insufficient contrast. Herein we proposed a microwave equiphase frequency truncation method (EFTM) to detect and evaluate the thickness of kissing defects in GFRP laminates. It is found that the frequency difference varies linearly with the kissing defect thickness under the assumption of single reflection, and the linear slope is approximately in inverse proportion to the product of the thickness and permittivity term of the specimen. Theoretical calculation, simulation and experimental detection results of GFRP laminates and Teflon specimens all showed a fine linearity relationship between the frequency difference and the thickness of kissing defect. Besides, the experimental linear slopes were in good agreement with theoretical predictions for both GFRP and Teflon specimens, which indicates the applicability of this method for different dielectric materials. The experimental results clearly showed that kissing defects of 2.92μm was successfully detected and quantified. In summary, this method is feasible and effective in detecting and evaluating kissing defects in dielectric laminates.