Impact Response and Damage Tolerance of Composite Sandwich Structures

Abstract

The deformation and failure response of composite sandwich beams and panels under low velocity impact was reviewed and discussed. Sandwich facesheet materials discussed are unidirectional and woven carbon/epoxy, and woven glass/vinylester composite laminates; sandwich core materials investigated include four types of closed cell PVC foams of various densities, aluminum honeycomb, polyurethane foam, foam-filled honeycomb, and balsa wood. These materials were fully characterized under quasi-static loading and, in some cases, under high strain loading conditions as well. Sandwich beams were tested in an instrumented drop tower system under various energy levels, where load and strain histories and failure modes were recorded for the various types of beams. Peak loads predicted by spring-mass and energy balance models were in satisfactory agreement with experimental measurements. Failure patterns depend strongly on the impact energy levels and core properties. Failure modes observed include core indentation/cracking, facesheet buckling, delamination within the facesheet, and debonding between the facesheet and core. Sandwich beams with PVC cores tend to be more stable than beams with different cores. Although sandwich beams with balsa wood cores perform well under static loading, they fail catastrophically under impact loading due to the low fracture toughness along the grain direction. Sandwich beams with honeycomb cores show early catastrophic failure caused by the lack of continuity of the interface between the honeycomb core and the facesheet, resulting in debonding failure. In the case of sandwich panels, it was shown that static and impact loads of the same magnitude produce very similar far-field deformations. The induced damage is localized and is lower for impact loading than for an equivalent static loading. Static testing in general is more conservative regarding strain and damage levels. The load history, predicted by a model based on the sinusoidal shape of the impact load pulse, was in agreement with experimental results. A finite element model was implemented to capture the full response of the panel indentation. The investigation of post-impact behavior of sandwich structures shows that, although impact damage may not be readily visible, its effects on the residual mechanical properties of the structure can be quite detrimental.