Nondestructive Detection of Defects in GFRP Laminates by Microwaves

Abstract

The application of microwave reflectometry was studied for nondestructive detection of glass fiber reinforced plastic (GFRP) laminates. The specimens having different kinds of defects and dimensions were prepared. The change of the microwave signals was measured by scanning the specimens with a K band waveguide sensor which can propagate microwaves from 18 to 26 GHz. The influence of the frequency of microwaves and the thickness of the laminates on measurement sensitivity was investigated. As a result, it was shown that the defects of more than 0.2 mm thickness in the GFRP laminate can be detected. The stable measurement results were obtained at lower frequency though measurement sensitivity for small defects was better at the higher frequency. It is necessary to select the suitable frequency for the measurement objectives because the interference due to the reflection waves from the top and bottom surfaces of the measurement objectives is caused depending on the objective and defect dimensions. In addition, it was shown that the defect of an aluminum film can be detected within the GFRP laminates of 30 mm thickness.