Abstract
1. Electron microscopy by transillumination established that fatigue crack propagation in iron was accompanied by the development at the tip of the crack of a zone of intense plastic deformation, in which there was a substantial (three-to fivefold) increase in dislocation density and a corresponding reduction (1.8–3.0 times.) in cell size by comparison with those in the rest of the testpiece. 2. Comparison of the parameters of dislocational structure according to the results of electron microscopy by transillumination and the parameters of fatigue fracture surface relief in the crack propagation zone under conditions of plane strain according to the results of microfractographic research showed that stable fatigue crack growth at minimum speed persists until the size of the crack microstep exceeds the size of the secondary cellular structure subgrain. 3. The size of the secondary substructure cell DIIcell≈0.4 μ is practically independent of crack length in the zone of pure fatigue failure and determines the cyclic fracture toughness.
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