Brittleness of coated tungsten wire

The fracture characteristics of 300- and 500- mu tungsten wires coated with brittle tungsten carbide and recrystallized nickel layers of various thickness were studied experimentally. A fracture mechanism is proposed. As a result of bending strains, a narrow crack forms at the surface of the wire and propagated toward its center. Because of the brittleness of the surface layer, the crack propagates at a high rate, generating an elastic strain wave at its front. When the crack reaches the interface between the brittle layer and the plastic matrix, the stress intensity increases and the shock imparted to the wire leads to fracture. (IAA)