Damage assessment in hybrid laminates using an array of embedded fibre optic sensors

Hybrid laminates typically consist of alternate layers of fibre-reinforced polymer and aluminium alloy. Developed primarily for fatigue critical aerospace applications, the hybrid laminates are orthotropic materials with lower density and higher strength compared to the aluminium alloy monolith. One of the damage mechanisms of particular interest is that of fatigue crack growth, which for hybrid laminates is a relatively complex process that includes a combination of delamination and fibre bridging. To facilitate the development of a unified model for both crack and damage growth processes, a remote sensing system, reliant upon fibre optic sensor technology, has been utilised to monitor strain within the composite layer. The fibre optic system, with capacity for sub microstrain resolution, combines time domain multiplexing with line switching to monitor continuously an array of Bragg grating sensors. Herein are detailed the findings from a study performed using an array of 40 sensors distributed across a small area of a test piece containing a fatigue crack initiated at a through-thickness fastener hole. Together with details of system operation, sensor measurements of the strain profiles associated with the developing delamination zone are reported.