IN-SITU CRACK-PROPAGATION IN PRESSURELESS SINTERED FIBER-REINFORCED COMPOSITES

Crack propagation in sintered SiC fiber reinforced alumina matrix comp osites was studied in situ inside a scanning electron microscope. Both crack front and crack wake toughening mechanisms were studied in isol ation. In the crack front study, the crack propagation behavior varied with fiber coatings. For the uncoated fibers, a three-fold load incre ase was necessary to advance the crack over the fibers. A gradual load increase was necessary to propagate the crack over the fibers that we re initially polymer coated. In samples where the fibers were either m etal or polymer/metal coated, the crack becomes unstable before reachi ng the fiber positions and the fibers had no effect in resisting the c rack propagation. However, the metal coated samples reached the highes t toughness values during the crack wake study, with an increase in to ughness of 75% over the monolithic samples. Crack profiles were measur ed and the crack opening displacements revealed direct evidence of the crack fiber interactions.