Analysis of edge impacts on stiffened composite structures

A combined experimental and computational investigation was performed to determine the effects of low-energy impacts of stiffened composite structures on damage formation, peak impact force, and localized stresses. The investigation was limited to the cap region of composite stiffeners oriented on Light and Medium gage panels. Design variables investigated include stringer cap width, incident angle of impact, boundary conditions, panel length and panel gage. The experimental investigations included quasistatic indentation testing and instrumented drop-weight impact testing to characterize the response of the panels and to estimate the energy levels to cause Barely Visible Impact Damage (BVID). The BVID energy levels were determined to fall between 108-200 inlb. depending on panel gage and boundary conditions. Finite Element Analysis (FEA) was performed to compare to experiments, estimate peak forces and investigate localized stresses in the vicinity of the impact. The FEA included an explicit global dynamic analysis of the impact event to produce the force versus time response. The estimated peak force was then applied to an implicit quasi-static model to retrieve the stress state in the vicinity of impact. Results from the FEA suggest that the peak forces and stress states are highly dependent on the deformation behavior of the local region in the area of impact. The added area moment of inertia of the larger cap widths greatly increased the peak forces, stress areas and stress magnitudes. Therefore, the cap width was found to be of great significance in the production of impact damage. The incidence angle was also found to be of significance. As the impact angle from a normal impact was increased, the peak force decreased. Additionally, a decrease in impact energy of 10% using Potted boundary condition produced similar peak forces as the Free boundary conditions.