Can metal plate specimens deform after removal of tensile loads?

We conduct experimental studies on fatigue of metals using an optical interferometric technique called the Electronic Speckle-Pattern Interferometry. The ESPI setup is configured to measure the in-plane displacement parallel to the tensile axis. The displacement is evaluated via analysis of interferometric fringes generated by subtracting the interferometric image taken at a time step from that taken at the initial time step; each fringe represents a contour of constant displacement occurring during the interval between the two time steps. We apply a tensile load to thin-plate specimens at a constant pulling rate to a pre-set stress substantially lower than the yield stress. In the course of these experiments, we have noticed that after the tensile machine stops pulling the specimens, the fringe pattern exhibits deformation. To investigate this mysterious but intriguing phenomenon, we have conducted a series of experiments on an aluminum alloy (A7075) specimen using the same experimental setup where we keep the interferometer running after the tensile machine stops pulling. Consequently, we have found that the specimen indeed experience deformation 10 - 15 sec after the grip stops.