Experimental and theoretical investigation of the frequency effect on low cycle fatigue of shape memory alloys

The effect of cyclic loading frequency on pseudoelastic behavior and low cycle fatigue of shape memory alloys (SMAs) is investigated. To this end, strain and stress controlled tensile fatigue tests on NiTi wires are performed. The material is first trained for a few cycles to stabilize the hysteresis loop and reach a shakedown state and then submitted to fatigue tests. An infrared camera is used to measure the surface temperature of the wire. Strain-controlled tests are performed at different total strain amplitudes, and stress-controlled ones are performed at different maximum stress values. As the frequency increases, the fatigue lifetime under strain controlled loading shows a decreasing trend for all the strain amplitudes tested. The situation in stress controlled tests is however more complicated because the fatigue lifetime shows a maximum stress-dependent trend: for different maximum stresses, the trend in variation of fatigue lifetime with frequency is different. Based on some considerations related to the thermomechanical coupling in SMAs, a theoretical explanation for the evolution of the mechanical response and fatigue lifetime with loading frequency is presented. A new strain-energy based fatigue model is proposed, and is validated by experiments performed under different load ratios.