Full-field deformation and failure analysis for compression after impact of carbon fibre reinforced polymer laminates

Residual compressive strength after impact is a critical design factor in assessing damage tolerance of carbon fibre-reinforced polymers (CFRP) for structural applications in aircraft. This study investigated compression after impact (CAI) performance of quasi-isotropic CFRP laminates by measuring 3D deformation and strain using three-dimensional digital image correlation (3D DIC). Before the CAI, these test panels were subjected to dropweight impact testing with energy levels representing visible impact damage (VID). During the CAI testing, deformation behaviours throughout the full loading sequence were captured by twin-camera imaging systems at the front and back faces of panels at standard speed. A single-camera high-speed imaging system captured ultimate failure behaviours at the sample front face. The single-camera high-speed system used four planar mirrors to achieve 3D DIC measurements. This study investigated the effect of boundary conditions on the compressive behaviours of the test panels, where the effect of additional support plates at the front and back surfaces to reduce the unsupported area was compared with the boundary condition of standard anti-buckling guides. The panels with added supports were found to experience deformations dominated by local buckling and final out-ofplane displacements in an S-shaped profile. These panels also exhibited residual strength reductions correlating to impact damage levels and ultimate failure initiating at the damaged region and propagating rapidly along the transverse axis. In comparison, the panels without added supports exhibited failure dominated by global buckling as manifested by large out-of-plane displacements, which had an arc-shaped profile. The DIC measurements were compared against those of mounted biaxial strain gauges. The single-camera high-speed system was shown not only to be feasible for high strain rate testing applications; it also offered a significant cost saving compared to an equivalent twin-camera high-speed system.