Ping-Hua Lin

Rotating-bending fatigue of a TiNi shape-memory alloy wire

The rotating-bending fatigue of a TiNi shape-memory alloy wire was investigated. The influence of air and water atmospheres, temperature, strain amplitude and rotational speed on the fatigue life was discussed. In the case of the strain amplitude due to the rhombohedral-phase transformation, the fatigue life lengthened above 107 cycles. In the case of the strain amplitude due to the martensitic transformation, the fatigue life in air was shorter than that in water.

Pseudoelastic Behaviour of TiNi Shape Memory Alloy Subjected to Strain Variations

The pseudoelastic behaviour associated with the martensitic transformation (MT) of TiNi shape memory alloys subjected to strain variations has been investigated experimentally. The results are summarized as follows. Firstly, with respect to strain variations within the MT region: (1) The reverse transformation stress does not depend on the maximum strain. (2) The MT stress and reverse transformation stress decrease with an increase in the number of deformation cycles. (3) The rate of variation in both transformation stresses with cycling decreases with the increase in the number of cycles. Secondly, with respect to strain variations beyond the completion points of MT and reverse transformation: (1) At the start points of MT and of the reverse transformation, over shooting of both transformation stresses appears. (2) The higher the temperature and the larger the maximum strain, the larger the overshoot of both transformation stresses and the larger the decrease of the reverse transformation stress.

Cyclic Deformation of TiNi Shape Memory Alloy.

An experiment with cyclic loading and unloading and an experiment with heating and cooling under the constant stress of TiNi shape memory alloys were carried out. The starting condition and the completing one of the martensitic transformation and the reverse transformation, the cyclic behavior of recovery strain and the conditions for application of recovery deformation were discussed. The results are summarized as follows. During the thermomechanical cycles, the yield stresses of the martensitic transformation and the reverse one decrease, but the transformation temperatures increase. The starting condition and the completing one of both transformations under the cyclic deformation are described by the behavior of the transformation lines in the stresstemperature plane. During the thermal cycles under constant stress, the recovery strain may appear or diminish depending on the relation among the initial strain, the heating temperature and the cooling temperature, thus showing complex cyclic deformation behavior. The cyclic recovery strain is represented by the relation among the heating temperature, the cooling temperature, the transformation lines and the transformation strain. It is important for the shape memory alloy elements to determine the conditions for application based on the cyclic characteristics of the transformation stress, the transformation strain and the transformation temperature.