Annealed metals under alternating plastic strain

Abstract

Annealed metals (copper, nickel, aluminium and α -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 m ay build up in the slip bands surface disturbances that relax excessive internal stresses and start fatigue cracks by the geometry of their formation.