UC San Diego

Advanced fiber reinforced polymer matrix composite aircraft are highly susceptible to impact damage. One source of impact is hail, which causes matrix cracking, delamination, and fiber breakage. Impacts near joints can cause disbond. Such damage decreases structural integrity, yet can be difficult to detect. Thus, it is important to gain an understanding of impact damage modes and the thresholds at which damage occurs. Many 61.0 mm diameter simulated hail ice (SHI) impacts at normal incidence angle were performed on full-scale stiffened carbon/epoxy panels to observe what damage resulted as a function of the impact location relative to the stiffeners. The study involved 1.42 m x 1.93 m curved panels made of Toray T800/ 3900-2 unidirectional carbon/epoxy with 16 ply quasi- isotropic skin, and hat shaped stringers. Impacts were applied at: locations away from the stringers (the middle of the bay), the middle of the stringers, on stringer flanges, and over shear ties. Failure threshold energies (FTE) and the progression of damage modes for each type of impact were established. Stringer flange impacts were found to be most critical, resulting in flange unzipping with a FTE range of 49-147 J at the free end of the flange, and a FTE of 183 J at the middle of the flange. The resulting damage states were found to be dependent on peripheral boundary conditions affecting local bending stiffness, such as where the skin and stringer stiffeners meet