Fig. C-2. Comparison of Ductile and Brittle Failure Modes1
However, even the most brittle metal will exhibit some slight evidence of plastic deformation. Initiation of a crack normally occurs at a small flaw, such as a defect, notch or discontinuity, which acts as stress concentration, and rapidly propagates through the metal. Cracks resulting from machining, quenching, hydrogen embrittlement, or stress corrosion can cause brittle failures. Even normally ductile metals can fail in a brittle manner at low temperatures, in thick sections, at high strain rates such as impact loading, or when there are pre-existing flaws. Brittle failures normally initiate as a result of cleavage. Cleavage is a brittle failure mode that occurs by breaking of the atomic bonds. Brittle failures are characterized by rapid crack propagation with less energy expenditure than in ductile fractures.
Ductile fractures proceed only as long as the material is being strained, i.e. stop the deformation and crack propagation stops. At the other extreme, once a brittle crack is initiated, it propagates through the material at velocities approaching the speed of sound, with no possibility of arresting it. There is insufficient plastic deformation to blunt the crack. This makes brittle fractures extremely dangerous, i.e. there is usually no warning of impending fracture.
Some BCC and HCP metals, and steels in particular, exhibit a ductile-to-brittle transition, as depicted in Fig. C.3 when loaded under impact. At high temperatures, the impact energy is high and the failure modes are ductile, while at low temperatures, the impact energy absorbed is low and the failure mode changes to a brittle fracture. The transition temperature is sensitive to both alloy composition and microstructure. For example, reducing the grain size of
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