Fracture toughness, as commonly defined for unidirectional boron/ aluminum, was measured with center-notched, compact tension, and vari ous sized three-point bend specimens. All coupons were cut from the same plate of material. The values of fracture toughness determined were found to be specimen dependent, thus indicating that fracture toughness is not a material property for the size and type of specimens tested. The failure mode was also found to be specimen dependent. Crack growth colinear with the notch occurred in the center-notched and three-point bend speci mens, while the crack initiated and propagated perpendicular to the notch tip in compact tension specimens. The failure of linear elastic fracture mechanics to define a material property for these specimens was attributed to extensive matrix yielding in regions remote from the notch tip. The existence of widespread matrix yielding was confirmed with a strain gaged, center-notched tensile specimen.
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