cyclic behavior

During a cyclic angle (shear strain) controlled experiment. A net increase in axial strain is observed. The increase in axial strain is
consistent with the number of cycles. I ran the experiment for almost 20 cycles and observed the similar behavior in case of pure Al. Also the axial load was maintained zero thought out the experiment.
I am looking for a possible reason (mechanics or microstructure) and reference related to it?
Thanks Amit
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