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 µm 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.