Experimental Study On Fatigue Behavior Of NiTi Shape Memory Alloy Sheets And Stranded Wires

Vibration of engineering structures induced by earthquakes and typhoons may lead to structural damage and even disaster consequences.New engineering structures today should not only meet the functional and load bearing requirements under static loads,but also need to resist extreme actions such as seismic or typhoons with stochastic nature.Smart materials,such as Shape Memory Alloys(SMAs)emerge to meet the above requirement.SMAs has the advantages of pseudoelasticity,shape memory effect,high corrosion resistance and fatigue resistance,etc.It can absorb vibration energy,reduce the impact of vibration and prolong the service life of structures.This paper mainly studies the fatigue properties of shape memory alloys,and the author mainly studies on two aspects.The effects of temperature and strain amplitude on the fatigue properties of pseudoelastic SMA were investigated at first.Tensile fatigue tests were carried out with 4 test temperatures and 9 strain amplitudes under a total of 36 working conditions using commercially customized SMA dog bone specimens,then the relationship between fatigue properties of SMA under pseudoelastic condition and temperature and strain amplitude was tested.The fatigue fracture characteristics of SMA were observed by scanning electron microscopy after fatigue fracture.The research results show that,the higher the temperature,the shorter the fatigue life of SMA,the larger the strain amplitude,the shorter the fatigue life of SMA.The stress amplitude and elastic modulus increase with the increase of temperature,while the loss modulus decreases with the increase of temperature.The stress amplitude and loss modulus are positively correlated with the strain amplitude,while the elastic modulus is negatively correlated with the strain amplitude.The fatigue failure of SMA is a brittle fracture,and its failure characteristics are similar to that of ordinary metals.The larger the strain amplitude,the smaller the fatigue propagation region,and the higher the temperature will accelerate the crack nucleation and accelerate the destruction of SMA.On the other hand,the author study the fatigue performance of SMA stranded wires,a new physical form of SMAs.Firstly,the tensile response of SMA stranded wires is tested.Then 6 working conditions with different strain amplitudes were set and 2000 cycles of loading and unloading were conducted to test the functional fatigue degradation.Finally,in order to study the influence of strain amplitudes on the fatigue life of SMA stranded wires,tensile fatigue tests were carried out under 5 different strain amplitudes.The research results show that,SMA stranded wires has good pseudoelasticity and small residual strain in austenite state.Functional fatigue of SMA stranded wires occurs during loading and unloading cycles,and the larger the strain amplitude,the more obvious the functional degradation is.Functional fatigue related parameters reached stability after a certain period,and the number of cycles required for SMA stranded wires to reach stability was independent of the strain amplitude,and its functionality was not completely lost in the later stage of the test,but still remained at a certain level.The structural fatigue failure of SMA stranded wires does not occur suddenly,but gradually with the internal monofilament wire fracture.With the breaking of monofilament wire,the hysteretic curve of SMA stranded wires flattens,and the stress amplitude and energy dissipation decrease.