A study on residual stresses in polymer composites using moiré interferometry

Moiré interferometry is applied along with the hole drilling technique to determine residual cure stresses in symmetric cross ply graphite epoxy laminates. Traditional moiré interferometry set-up using two collimated angle beams was employed to provide the virtual reference grating while a cross grating with a frequency of 1200 lines per mm was replicated on the specimen surface. Holes of different depths, each one penetrating one additional layer of the laminate, were drilled using a high speed air turbine drill to relieve the stresses in each layer sequentially. The strain distribution around each hole was computed from correlation of the undistorted carrier fringe pattern with the distorted fringe patterns around the holes. The measured strain distributions are compared to residual strain distributions by finite element analysis.