The effect of low-velocity impact damage on the residual compressive strength of graphite/epoxy laminates was investigated. The buckling behavior of delaminated composite laminates after low velocity impact was monitored by fiber optic sensors.
The impact tests were performed with a drop-weight machine. The ultrasonic C-scan was applied to determine multiple delaminations at various impact energy levels. Compression-After-Impact(CAI) tests were performed to determine the residual strength of the specimens. Visible cracks on the back surface were observed at impact energy level of 4.5J. Over 4.5J impact energy, the residual compressive strength rapidly decreased. In the case of 9J, the reduction of compressive strength was about 26%.
In order to monitor the buckling behavior of specimens with multiple delaminations, fiber optic sensors(FOS) were embedded in composite laminates. Strain gages were attached on the surface of specimens to compare with FOS in compression tests. As a result, strain gages only detected the surface strain but FOS monitored the deformation of the inside layers and the reduction of local bending stiffness. FOS were superior to strain gages in monitoring the buckling behavior in the region of multiple delaminations.
