Porosity Evaluation of Composites Using the Encapsulated Cantilever Fiber Bragg Grating Based Acousto-Ultrasonic Method

The potential of encapsulated fiber Bragg grating (FBG) has not been fully explored although it has been widely used to detect temperature. In this study, an encapsulated FBG with the cantilever structure was demonstrated to have high ultrasonic sensing performance and was then used in an acousto-ultrasonic detection for porosity evaluation of composite. The cantilever structure of the encapsulated FBG embodied the resonance phenomenon for receiving an ultrasonic signal with a high signal-to-noise ratio of 38 dB. It thereby ensured the accuracy of the subsequent acousto-ultrasonic sensing on a carbon fiber reinforced plastic (CFRP) laminate. The polynomial function between the porosity and attenuation coefficient detected from the acousto-ultrasonic method was constructed from three repeated experiments, which can be used to evaluate the porosity of the composite. The accuracy of the polynomial function and the reliability of the proposed method was further experimentally validated. In addition, the temperature sensing capability of the sensor retained as it successfully monitored the forming temperature with an average difference below
$0.7~^{\circ }\text{C}$
. These results indicated that the presented method can precisely and effectively evaluate porosity, and the encapsulated cantilever FBG sensor was versatile in the field of composite health monitoring.