Effect of specimen thickness on the fracture surface energy of <100> axis tungsten single crystals

Abstract The fracture surface energy has been determined on sheet crystals with a single-edge crack normal to the tensile axis. The crystals were pre-cracked at 77 K using a spark discharge technique and tested immediately at the same temperature. The thickness of the crystals was varied between 94 and 670 μm. The shape of the spark crack varied with specimen thickness and crack length. The fracture surface energy was approximately constant at about 1.7 J/m2 for specimen thicknesses above 400 μm. For thicknesses below this value the fracture surface energy increased rapidly to 30 J/m2 for a specimen thickness of about 200 μm. This increase is attributed to a changeover from plane strain to plane stress fracture. The specimen thickness also affected the plane on which crack propagation occurred. In thick specimens the initial (100) spark crack continued to propagate on the (100) plane. As the specimen thickness decreased there was an increasing tendency for initial propagation to occur on planes close to {110}.