Abstract
The effect of plastic strain history on the stress-strain hysteresis curve in the initial stage of cyclic loading was analysed for commercially pure aluminum subjected to various strain controlled push-pull cyclic tests: constant strain cycling with or without a mean strain, and constant strain cycling after various prestraining. Each test was repeated until a stable stress-strain hysteresis loop was obtained. The stress magnitude for each half cycle was plotted against the cumulative plastic strain and compared with the monotonic tension curve. It was found that in the initial stage of cycling the stress-strain curves for different prestrains were not similar nor had such shapes that coincide with each other by a suitable translation. The stress-strain curve for each half cycle was expressed in terms of two quantities: (1) the value of permanent softening, which is the stress difference between the monotonic tension curve and the cyclic curve at the parallel point on the stress-cumulative plastic strain plot and (2) the transient hardening curve. The former depends strongly on the plastic strain range and cycle number but weakly on the prestain, while the latter depends only on the plastic strain occurred in the just preceding half cycle. A new parameter which is related to the monotonic tension curve was introduced to identify the stable behaviour under cyclic loading.
