Fatigue Behavior Of NiTi Shape Memory Alloy Pipe Joints Under Random Vibration

Shape Memory Alloys(SMA)is a new type of intelligent metal material.It not only has good Shape Memory Effect(SME)and superelasticity(PE)properties,but also has high damping,high corrosion resistance and excellent biocompatibility,ect.Therefore,it has been widely used in many fields,such as construction,nuclear power,vehicles,aerospace,electronics,medicine,energy and so on.Among them,Ni-Ti shape memory alloy is a typical representative material of SMA,which is widely used in seismic components of civil buildings;connectors,fasteners and seals in mechanical engineering;some medical equipment in medical field,etc.At present,researchers have focused on Ni-Ti shape memory alloys mainly in constitutive models and microstructures.However,in practical engineering,due to the complexity of the structure of the Ni-Ti shape memory alloy components and the working environment,it is inevitably subject to the cyclical action of complex loads,Under the repeated action of external loads,the performance of Ni-Ti shape memory alloy may change,and even fatigue fracture may occur.Therefore,it is meaningful to carry out fatigue research on Ni-Ti shape memory alloy components.Aiming at the fatigue characteristics of Ni-Ti shape memory alloy under random vibration,the following research work has been carried out in this paper:1)The main characteristics of SMA and the constitutive model of SMA were discussed.Auricchio model based on generalized plasticity theory used in this paper was summarized.The parameters needed for the model were further given.The model was verified by finite element simulation,and the results show that the model can describe the mechanical behavior of SMA well.2)The analysis methods of random vibration fatigue life-time domain analysis method and frequency domain analysis method were discussed.Based on the fatigue test results of Ni-Ti alloy,the fatigue damage model of Ni-Ti alloy material was established.In frequency domain,a method for predicting the random vibration fatigue life of Ni-Ti alloys was proposed considering the effect of average stress,combined with the response stress probability density distribution function and the fatigue characteristics of SMA materials.3)In consideration of engineering practicality,the random vibration fatigue life prediction of the Ni-Ti alloy pipe joint system in a typical hydraulic pipeline was carried out.The finite element model of Ni-Ti pipe joint system was established byusing CAD software,which was imported into ANSYS finite element software for pretreatment.The modal analysis of Ni-Ti alloy pipe joint system was conducted based on equivalent mass method,and the natural frequencies and modes of the system are obtained.4)The frequency response characteristics of Ni-Ti alloy pipe joint were obtained by frequency response analysis.The response stress power spectral density curve is obtained by random vibration analysis under acceleration power spectral density excitation.Based on the S-N curve of SMA material and the theory of cumulative fatigue damage,the random vibration fatigue life of pipe joints was estimated by Steinberg method and the method proposed in this paper.The results show that the Ni-Ti alloy pipe joint can meet the flight requirements of the aircraft,and the life of Ni-Ti alloy pipe joints estimated by the method proposed in this paper is more accurate,can be used as a recommended method for estimating the fatigue life of structures under random vibration in the field of mechanical engineering.