Effect of multiple non-coincident impacts on residual properties of glass/epoxy laminates

A series of low-velocity drop-weight tests were conducted on glass/epoxy laminates to study the effect of multiple non-coincident impacts on residual compression and flexural properties. The impact characteristics and residual properties were recorded to determine the damage tolerance to various impact energies and impact separation distances. A flexure after impact method was also developed and compared to the compression after impact test. Experimental results were combined with a finite element model of the test. The damaged region was modeled as an elastic inclusion of reduced stiffness resulting in a stress concentration that was used to predict residual strength. Post-impact tests and stress analyses revealed that damage size, inclusion stiffness, and strength reduction were sensitive to impact energy and impact separation distances. Multiple impacts at larger distances create distinct, non-interacting damage regions that cause no more reduction in residual properties than the damage regions from a single impact. However, with a small separation distance, the damage regions from multiple impacts overlap and display increased energy absorption and reduction in the residual mechanical properties.