EXPERIMENTAL INVESTIGATIONS OF CRACK TRAPPING IN BRITTLE HETEROGENEOUS SOLIDS

Over the past two decades many mechanisms of toughening have been cons idered for brittle solids. Some of the most prominent ones, applicable to either monolithic materials or fiber-reinforced composites, includ e deformation-induced local transformations, microcracking, crack trap ping, crack bridging, and fiber pull-out. Few, if any, of these have b een studied in the past in a manner which permitted evaluation of the effects of individual mechanisms in the absence of other interacting m echanisms. Here, we present an experimental study of toughening by the process of crack trapping by second-phase particles (spheres and fibe rs) of such toughness that make them impenetrable by probing cracks, f orcing the cracks to bow around the obstacles with increasing applied load. The model fracture specimens employed here were wedge-loaded dou ble cantilever beams, cast of a brittle epoxy, containing macroscopic (3 mm diameter) inclusions of Nylon or polycarbonate, having elastic p roperties similar to the matrix. The tests were performed at -60-degre es-C to achieve controlled, stable crack propagation. Images of the cr ack fronts, advancing at velocities of about 10(-4) m/s, were recorded with good resolution, providing a continuous record of crack-front sh apes during the evolution of the crack-trapping process - from the ini tial pinning configuration through the transition to crack-flank bridg ing. Remarkable agreement between these images and crack-front shapes predicted by the numerical simulation of Bower and Ortiz is demonstrat ed. A parametric approach was adopted to study the influence of obstac le spacing, surface adhesion, and thermally induced residual stresses upon the observed crack-front behavior and enhanced stress intensity r equired to propagate the cracks past these obstacles. Analysis of the quantitative data has demonstrated that in brittle matrices containing particle volume fractions of approximately 0.2, toughness enhancement by over a factor of 2, relative to neat matrix values, may be achieve d through the crack-trapping mechanism alone, provided that a high lev el of adhesion can be maintained between the matrix and the tough rein forcing particles.