Effect of Low-Velocity Impact Damage on the Compressive Strength of Thin and Thick Graphite-Epoxy Laminates

Composite Materials are being used increasingly in aircraft structures due to their high specific strength and stiffness, and the resultant weight savings. Traditionally, the use of graphite fiber-reinforced composites have been confined to secondary structures. However, requirements for reduced structural weight, improved aircraft performance, and efficiency are making the composite materials increasingly competitive for expanded usage in the primary, load carrying structures. In comparison to conventional metals, an understanding of the complex behavior of composites is still in its infant stage, and applications are based on knowledge gained through extensive experimental programs. Past experiences and experiments have confirmed that the graphite fiber-reinforced composite laminates have low ultimate strains, no plastic deformation range, and no usable strength in the thickness direction. These limitations become very obvious when laminates are subjected to impact loads. In this paper, the effect of low-velocity impact damage on the compressive strength of composite laminates is addressed. Due to the low-velocity impact loading laminates suffer an extensive internal damage such as delaminations and the damage on the back surface known as spalling. Therefore the study of impact damage susceptibility of these composite laminates is increasingly important. While considerable experimental and analytical studies have been made on the low velocity impact phenomenon, an experimental understanding of the progressive damage mechanics and its effect on the compressive strength is not fully known. In the experimental program, 16 ply, 32 ply and 48 ply laminated graphite/epoxy composite plates with quasi-isotropic lay up were tested for two different types of loading conditions, the first one was low velocity impact loads, and the second one was equivalent quasi-static loads. In both cases the laminates were held in a NASA designed steel fixture, which simulated the simply supported boundary conditions. The impactor tip area and weight of the impactor was kept constant for all the tests, and tests were performed using Dynatup low velocity impact testing facility. Preliminary impact tests were performed to establish the lower bound (incipient damage) and upper bound (visible back face damage, spalling) energy levels for each 16 ply, 32 ply and 48 ply laminates. Seven energy levels, first with 20 % below the lower bound, second and seventh lower and upper bounds, and third through sixth in between lower and upper bounds with an increment of 20 % were chosen to generate the progressive damage data for laminated composite plates of varying thickness subjected to low velocity impact loads. In addition to the impact tests, laminated plates were subjected to quasi-static loads which simulated low velocity impact loads. Both the static and impact tested laminates were C-Scanned to identify the internal damages.