The formation of sub-grain structure by alternating plastic strain

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Abstract
A micro-beam x-ray technique has been applied to the fracture surfaces produced by slow-growing fatigue cracks. The cracks were propagated in metal sheets subjected to pulsating tension such that the rate of growth of the crack was proportional to its instantaneous length. The fracture surfaces showed a highly developed sub-grain structure with large misorientations > 13°. The sub-grain size was found to be independent of the range of cyclic stress used to propagate the crack and to be characteristic of the metal. The progressive development of the sub-grain structure was followed in torsion specimens subjected to large ranges of plastic-strain ∼10−2. If the process of sub-grain formation is regarded as an essential feature of the mechanism of fatigue crack propagation then the order of susceptibility of metals to fatigue crack growth and the phenomenon of non-propagating surface cracks can be interpreted.