DETERMINATION OF FRACTURE-TOUGHNESS AND BRIDGING TRACTIONS FROM CRACK-OPENING DISPLACEMENT MEASUREMENTS IN PARTICULATE COMPOSITES OF DIAMOND IN ZINC-SULFIDE

The fracture toughness of zinc sulfide ceramic and a series of ZnS diamond particle composites was studied through measurements of crack opening displacement profiles. Five composites were fabricated using weight fractions of 10, 20 and 30% of 0-1 mum diamond particles, and 10 and 20% of 1-3 mum particles in a ZnS matrix. The cracks were grown using a novel specimen geometry and a loading technique that permitted stable crack growth even in small specimens. The fracture toughness of each material was calculated on the basis of the displacement profiles and the material properties, as opposed to the applied loads and the specimen geometry. The pure zinc sulfide material exhibited nearly ideal brittle behavior, and the toughness measurements agreed closely with other methods. The greatest toughening occurred in the 1-3 mum particle size composites, in which weak bridging tractions (less-than-or-equal-to 100 MPa) over a short distance along the crack (50-100 mum) could explain the reduced displacements near the crack tip. Even smaller size bridging zones (5-10 mum) may have been present in the 0-1 mum particle size composites, but elastic shielding alone can explain the observed toughening. The specimen geometry used here permitted toughness measurements using small specimens (< 5 mm) but is limited to materials having bridging zones that are less than about 250 mum.