The effect of preliminary thermal cycling under load on the magnitude of the deformations driven by the shape memory effect

Results of the experimental study of the effect of preliminary thermomechanical impact (TMI) via thermal cycling under load on the deformations attributed to the shape memory effect (SME) are presented. Two modes of TMA were studied: i) thermal cycling under constant loads, the values of which were sequentially increased to the level of the maximum value; and ii) thermal cycling under constant loads, the values of which are successively reduced from the level of the maximum value to zero. Smooth cylindrical samples with a length and diameter of the working part 33 and 4 mm, respectively, made of TN-1 alloy with characteristic martensitic transition temperatures were used. The experiments were carried out in the mode of thermal cycling through the intervals of martensitic transitions at a constant value of torque. It is shown that thermocycling in conditions of mode (i) leads to a monotonic growth of the SME induced deformations, whereas subsequent thermocycling at the same load in conditions of mode (ii) leads to a noticeable increase in the SME induced deformations. A logistic model of the phenomena of martensite inelasticity observed in the materials with shape memory effect is proposed on the base of the Verhulst logistic equation which provides a fairly accurate description of the material deformation during all the stages of thermal cycling. A computational-experimental method has been developed to take into account the effect of a preliminary thermomechanical impact on SME induced deformations.