Annealed metals under alternating plastic strain

Abstract
Annealed metals (copper, nickel, aluminium and $\alpha $-brass were subjected to cycles of small alternating torsion in which the amplitude of plastic strain was held constant; measurements were made of the proof stresses required to impose the successive reversals of strain. Variations in this proof stress, which shows how the metal strain-hardens at each amplitude, are correlated with structural changes shown by X-ray diffraction and metallographic examination. The results show that the rate of increase of the strain-hardening which occurs during the early cycles decreases during subsequent cycles, indicating that the plastic strain in cyclic deformation may be non-hardening. Further, when hardening thus ceases the deformation still proceeds by slip, indicating that slip movements in cyclic deformation may also be non-hardening. Again, fragmentation of the grains which increases in the initial hardening stage, depending on amplitude, also ceases during the subsequent non-hardening stage, confirming the modified mode of deformation implied by the two foregoing observations. Finally, the total plastic strain, the sum of the reversals added irrespective of sign, obtainable in the non-hardening stage without fracture, may be many orders higher than the strain obtainable in static deformation. The observations are interpreted on the view that to-and-fro slip in cyclic straining may build up in the slip bands surface disturbances that relax excessive internal stresses and start fatigue cracks by the geometry of their formation.

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