A two-stage analytical model and damage mechanisms in GFRP laminates subjected to normal impact by cubic fragments

Glass fiber-reinforced polymer (GFRP) composites are widely utilized as external protective structures in the aerospace field. To elucidate the mechanical response and damage mechanisms of GFRP laminates subjected to impact, a two-stage analytical model encompassing localized small-deformation crushing and global deformation progressive damage was established based on stress wave theory and energy conservation principles. The model incorporates energy absorption mechanisms such as crushing, shearing, delamination, and back-face stretching, and introduces an effective thickness to describe the propagation of various damage modes. It effectively predicts the time-history evolution of the fragment velocity, the proportion of different energy dissipation modes within the laminates, and the ballistic limit. Furthermore, the critical transition boundary for the damage mechanisms was determined based on the analytical results. The analytical model was validated against numerical simulations and experimental data from the literature. The results indicate excellent agreement between the theoretical predictions and the experimental results, demonstrating that the model effectively describes the fragment penetration process. The impact damage process in the laminates is primarily governed by both through-thickness and in-plane stress waves. Specifically, through-thickness stress waves dictate the initiation of damage modes, whereas in-plane stress waves determine the extent of damage propagation. As the influence of the local crushing stage increases, the damage mechanism of laminates with varying thicknesses shifts from global deformation to shear opening, and the dominant energy dissipation mode transitions from tensile energy to crushing energy.