Fatigue Hardening of Polycrystalline Copper, Nickel, and Aluminum

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Abstract
Fatigue hardening of polycrystalline copper, nickel, and aluminum has been determined by measuring the changes in the area of the hysteresis loop. It can be described as a superposition of two types of hardening, called VI and VII. These two types can far better be resolved by the specific irreversible work of deformation than by the increment of subsequent stress amplitudes. Hardening VI occurs mainly at high strain amplitudes and is not peculiar to fatigue. Saturation (I) is reached between N=10 and N=104, depending on strain amplitude and stacking fault energy. Hardening VII is effective only at low and medium strain amplitudes. It is characterized by a very low hardening coefficient. Saturation (II) is reached between N=104 and N=106. Intermediate overloading during saturation (II) results in a pronounced softening, indicating that hardening VII is caused by the production of small‐scale obstacles.

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