Influence of material thickness on the response of carbon-fabric/epoxy panels to low velocity impact

Low velocity impact tests were carried out on graphite fabric/epoxy laminates of different thicknesses, using a hemispherical impactor. The force and absorbed energy at the point of delamination initiation, the maximum force and related energy, and penetration energy were evaluated. From the experimental results, all these quantities, except the energy for delamination initiation, followed the same trend, increasing to the power of approximately 1.5 with increasing plate thickness. For what concerns the force at delamination initiation, it is shown that its trend agrees with the assumption of a Hertzian contact law, coupled with the hypothesis that only the shear stress is responsible for delamination. It is also demonstrated that the force-displacement curves recorded for the different thicknesses sensibly superpose with each other, when the forces are scaled to the power 1.5 and the displacements are held unchanged. This explains the observed dependence of the maximum force, energy at maximum force, and penetration energy on the thickness. Finally, the energy at delamination initiation is calculated by an analytical model, assuming that the total energy is shared in two parts, one of which stored in flexure, and the other in the material volume close to the contact zone.