On the influence of fiber shape in bone-shaped short-fiber composites

Composite materials reinforced by bone-shaped short (BSS) fibers enlarged a t both ends are well-known to have significantly better strength and toughn ess than those reinforced by conventional, short, straight (CSS) fibers wit h the same aspect ratio. Comparing the fracture characteristics of double-c antilever-beam specimens made of BSS and CSS fiber composites reveals the d istinct mechanisms responsible for the toughness enhancement provided by th e BSS fiber reinforcement. Enlarged BSS fiber ends anchor the fiber in the matrix and lead to a significantly higher stress to pull out than that requ ired for CSS fibers, altering crack propagation characteristics. To study B SS fiber-bridging capability further, we examine the effects of increasing the size of the enlarged fiber end on the pull-out characteristics and iden tify the sequence of failure mechanisms involved in the pull-out process. H owever, large microcracks initiated at the enlarged ends can potentially ma sk the toughening enhancements provided by BSS fibers. To understand the in fluence of fiber-end geometry on debond initiation at the fiber ends, we an alyze the interfacial stresses around fiber ends varying in geometry using an elastic finite-element model. We note a bound to these in terms of the E shelby and Kelvin elastic solutions. (C) 2001 Elsevier Science Ltd. All rig hts reserved.