Brittle fracture: Weakest link or process zone control?

Abstract: Does brittle fracture instability start from a single event such as a cracked carbide or sulfide or does it start from a series of discontinuous cleavage events, which coalesce together? In fact, either of these may occur depending on the test temperature (low or high), the amount of stored elastic energy at a crack tip (blunt or sharp), and the microstructure of brittle second phases (coarse or fine). The present study was to sort out which of these processes occurs in a coarse-grained high strength, low alloy (HSLA) steel tested at—80 C (193 K). Statistical distributions of grains and fracture origins were identified in this 108 m grain size steel. This was accomplished with compact tension specimens equipped with plezoceramic transducers to evaluate microcleavage onset. Termination of the test at various stress intensity levels, K_I, followed by fatigue cracking at ΔK << K,_I, allowed isolation of multiple cleavage origins. Both the amount of cleavage fracture and a preliminary estimate of fracture toughness could be interpreted in terms of process zone concepts. These rely on a local cleavage fracture stress controlled by microgeometry and a quasi-static equilibrium governed by the process zone size and strength. The conclusion for the steel in the present study is that a discontinuous process zone approach is most descriptive of the stochastic events leading up to brittle fracture.