Cracks may be nucleated by inhomogeneous plastic flow in several different ways. This paper derives the appropriate nucleation criteria and compares the results with those obtained from earlier analyses, the limitations of which are examined in detail. In some circumstances nucleation occurs much more easily than previous theories suggest; the consequences of this conclusion are discussed, with particular reference to the formation of cleavage cracks in brittle materials and also to the formation of cracks by grain-boundary sliding during high-temperature deformation.
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