Damage Studies in Composite Structures for Structural Health Monitoring using Strain Sensors

The advent of composite co-cured and co-bonded integrated construction in aircraft structures has lead to the replacement of fastened joints with bonded joints between the skins and the stiffeners. Skin-stiffener debonds could occur due to impact or other operational reasons and is usually internal. One of the aspects of structural health monitoring technologies is to be able to detect and assess skin-stiffener separation in composite structures. This article presents the results of tests conducted with composite testboxes to study the effect of skin-stiffener separation on the strain profiles. Different types of testboxes were fabricated and tested to compare bolted, co-bonded, and co-cured configurations. Bending loads were applied on the boxes, and the strains were measured near and away from the debonds using both strain gages and fiber Bragg grating sensors. The difference in the strains between healthy and unhealthy boxes are indicative of the presence of the debonds in the structure. These strain differences could be used in conjunction with suitable pattern recognition tools such as neural networks for damage detection and health monitoring.