Composite materials reinforced by bone-shaped short (BSS) fibers enlarged at both ends are well-known to have significantly better strength and toughness than those reinforced by conventional, short, straight (CSS) fibers with the same aspect ratio. Comparing the fracture characteristics of double-cantilever-beam specimens made of BSS fiber composites reveals the distinct mechanisms responsible for the toughness enhancement provided by the 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 required for CSS fibers, altering crack propagation characteristics. To study BSS fiber-bridging capability further, we examine the effects of increasing the size of the enlarged fiber end on the pull-out characteristics and identify the sequence of failure mechanisms involved in the pull-out process. However, large microcracks initiated at the enlarged ends can potentially mask the toughening enhancements provided by BSS fibers. To understand the influence of fiber-end geometry on debond initiation at the fiber ends, we analyze 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 Eshelby and Kelvin elastic solutions. © 2001 Elsevier Science Ltd. All rights reserved.