Ductile and brittle crack growth: Fractography, mechanisms and criteria

Metallographic and fractographic observations of crack growth in various materials suggest that atomically brittle behavior is less common than generally supposed, and that transitions from ductile to brittle behavior produced by changing the environment, temperature, and microstructure usually involve a decrease in the scale of the microvoid-coalescence crack growth process. The observations show that the microvoid-coalescence process can not only vary enormously in scale but can produce a wide variety of fracture modes and appearances, including fractures with a macroscopic cleavage-like appearance. Mechanisms of ductile and brittle fractures and criteria for ductile versus brittle behavior are discussed in the light of these observations. It is concluded that the proportion of dislocations injected from crack tips relative to those egressing around crack tips from near-crack-tip sources has a major influence in determining whether ductile or brittle behavior occurs.