Characterizing impact damage in GFRP/CFRP composites by determining thermal effusivity/diffusivity

Local variations of thermal effusivity and diffusivity are suggested as indicators of severity of impact damage in composites. A relative variation of diffusivity is a unique parameter of composite quality, while relative variations of apparent effusivity are time-dependent in non-adiabatic samples heated with a square pulse thus requiring a proper choice of optimum observation time. In this study, 36 samples made of glass-fiber-reinforced plastic and 54 samples made of carbon-fiber-reinforced plastic were fabricated. Each sample was subjected to a series of three impacts at the same location on each specimen. The energies of these impacts were 5, 10, and 20 J, applied in that or reverse order. Composite thermal properties were determined in a semi-automatic way with a small participation of the thermographer. This provided data spread from 0.6% to 2.4% for all evaluated parameters. The damage after each impact was evaluated by both one- and two-sided thermal nondestructive testing procedures on each impact site and on the surface opposite each impact site. The test results were statistically evaluated and revealed increasing values of effusivity/diffusivity as the impact energy increased and as second and third impacts were applied. However, if the order of impact energies was reversed, and changed to 20, 10, and 5 J, much smaller changes in damage were observed after each sequential impact. It appears that the most severe defect occurs after the first impact of 20 J, and further impacts of lower energies do not create much additional damage.