Distributed Strain Monitoring for Damage Evolution in CFRP Bolted Structures with Embedded Optical Fibers

Abstract

The authors proposed fiber-optic-based damage monitoring of carbon fiber reinforced plastic (CFRP) bolted joints. Optical fibers were embedded along bolt holes and strain change along the optical fiber induced by internal damage was measured by a Brillouin Optical Correlation Domain Analysis (BOCDA), which is a high spatial resolution distributed strain sensing system. This study began by investigating damage modes of CFRP bolted joints after bearing failure. Effective embedding positions of optical fibers were then proposed and their feasibility was evaluated by finite element analysis simulating the damage propagation in the bolted joint and consequent strain change. Finally, verification tests were conducted using specimens with embedded optical fibers at various positions. It was clearly shown that damage could be detected using residual strain due to fiber-microbuckling (kinking) damage or permanent deformation of neighboring plies. Furthermore, damage size and direction could be estimated from the change in the strain distribution. The system developed is quite useful for a first inspection of large-scale composite structures in aerospace applications.