| Due to the unique shape memory effect(SME)and super-elasticity(SE),excellent corrosion resistance and biocompatibility,NiTi shape memory alloys(SMAs)have been widely used in biomedicine,industrial engineering,aerospace and other fields.The NiTiSMAs are subjected to a thermal-mechanical coupling cyclic loading in service,therefore,fatigue failure is a key problem to be solved urgently.However,the existing researches about fatigue mainly aimed at the SE of NiTi-SMAs.For the shape memory NiTi alloys,due to the incompleteness of experimental conditions such as temperature control device,the existing studies mainly focused on the two-way shape memory effect(TWSME)without considering the accurate controlling of temperature change rate,and rarely involved the cyclic deformation and fatigue failure of NiTi-SMAs by addressing the one-way shape memory effect(OWSME)related to the processes of repeated mechanical loading,mechanical unloading,heating and cooling.Therefore,on the basis of systematic experimental research,it is a key scientific problem to establish a thermal-mechanical coupling fatigue life-prediction model considering the cyclic degradation of OWSME.To this end,systematic thermal-mechanical coupling fatigue experiments of NiTi-SMA micro-tubes by addressing the OWSME are firstly carried out under different stress levels,stress rates and stress paths;then,a damage-based fatigue life prediction model is established in this work.The main contents include the following aspects:(1)Based on the method of current heating,an experimental device which can quickly change the temperature and accurately control the thermal loading rate in a certain temperature range is designed.The device adopts Lab VIEW software for serial port control and data acquisition.A power supply and a cooling component are used to change the temperature.The device can achieve a good temperature control during the testing.Then,combined with the mechanical experimental equipment,an experimental platform of thermal-mechanical coupling cyclic loading for the NiTi-SMA micro-tubes is established.(2)The OWSME fatigue experiments of NiTi-SMAs micro-tubes under different stress levels,stress rates and loading paths are carried out through the self-designed thermalmechanical coupling experimental device.The effects of stress levels,stress rates and loading paths on the OWSME cyclic degradation arere analyzed,and the mechanism of OWSME cyclic degradation is revealed.Meanwhile,the fatigue lives of NiTi-SMAs under different stress levels,stress rates and loading paths are analyzed,the effects of stress levels and stress rates on the fatigue life are revealed,the difference of fatigue lives under uniaxial and nonproportional multiaxial loading is compared,and the damage evolution features under different loading conditions are summarized,which provide a foundation for the establishment of related fatigue lie-prediction model.(3)Based on the analysis of the damage evolution features for the NiTi-SMAs,a uniaxial damage evolution model considering the mechanical damage and thermal damage is established by choosing the dissipation energy as the damage variable.The damage evolutions under different stress rates and stress levels are predicted.Meanwhile,combined with a given failure criterion,a damage-based fatigue life prediction model is established,and the uniaxial fatigue lives of NiTi-SMA micro-tubes are predicted accurately.(4)By considering the influence of multiaxial loading paths on the martensitic reorientation,the uniaxial damage evolution model is extended to a multiaxial one.Meanwhile,a damage-based multiaxial fatigue life prediction model is established by incorporating a factor of non-proportionality,and the prediction results are within twice scatter band,and most of them are located within a scatter band of 1.5 times,which shows a good prediction ability of the proposed life-precition model.The established life-prediction model provides a solid theoretical basis for the engineering application of NiTi-SMAs. |
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