Impact and post-impact response of a composite material to multiple non-coincident impacts
Date
2010
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Publisher
University of Delaware
Abstract
It is well known that laminated composite materials are susceptible to damage by
out-of-plane impact, often associated with a corresponding reduction of mechanical properties. Much research has been conducted to characterize the impact and post-impact response of various composite systems, but little attention has been given to the proximity effects of repeated impacts. Structural composite panels developed for military vehicles must survive numerous impacts of various energies with distinct damage characteristics. The size, number, and proximity of these events may have a unique influence on the structure‟s residual performance.
A series of low-velocity drop-weight impact tests were conducted on S-2 glass/epoxy samples simply-supported along two edges. Three single-impact energies were used to determine the effect of damage size on the residual performance. Two of those energies were used to study the effect of impact proximity; 0.5in and 2in distances separated the two impacts. Along with damage size, the residual compressive strength and flexural properties were measured. An elastic finite element model was developed to approximate the impact damage as an elliptical inclusion. The damaged modulus and inclusion dimensions were used to uniquely determine the inclusion stiffness. The corresponding stress concentration was then used to predict the experimental loss of strength. Increasing incident impact energies exhibited greater absorbed energy, associated with larger damage dimensions. Post-impact tests and stress analyses revealed that both damage size and inclusion stiffness are related to the extent of overall structural degradation. Thus, simply considering the damage to be a hole may be overly conservative. Multiple impact tests showed that when separated by 2in, the initial impact damage does not influence the material‟s response during a second impact. At 0.5in, however, the damage areas significantly overlap, increasing the absorbed energy and degree of damage. This correlated to a significant loss of flexural modulus and strength as well as compressive strength. With these results it seems that a multiple non-coincident impact method could potentially gauge the damage tolerance of various composite systems. Also, it is feasible to model the damage dimensions and elastic flexural modulus as a nondestructive means to reasonably predict the residual flexural and compressive strength.