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The brittle fracture strength of a metal is described and discussed in terms of the following aspects: the origin of a metal's fracture strength; the mechanism of fracture initiation; the nature of the fracture strength of metals in simple tension as related to their resistance to plastic flow; the effect of various test conditions on the fracture strength of a given metal; the behavior of several metals under given severe test conditions; and the relationship of fracture behavior to conventional yield strength. It is demonstrated that while the theoretical strength of a metal is quite high, the presence of defects or imperfections results in failure at relatively low applied stresses. As measured in simple tension various metals or different lots of a given metal exhibit marked differences in fracture strength. These variations in fracture strength are related to differences in the resistance of metals to plastic flow. It is also shown that increasing the severity of the test conditions will cause brittle fractures to occur at higher temperatures and lower applied stresses. As tested under severe embrittling conditions various steels are found to exhibit consistent behavior trends, but their relative resistance to brittle fracture may be substantially different. For given test conditions the fracture resistance of several steels is shown to decrease markedly with increased yield strength. This correlation of yield strength with fracture resistance appears to be independent of such factors as chemistry, microstructure, tensile ductility, and conventional transition temperatures.
Wessel, E. T.
Westinghouse Research Laboratories, Pittsburgh, Pa