| The pre-crash technology combines the environmental sensing function of the active safety technology with the protection function of the passive safety system.When a potential collision is detected,the pre-crash control unit sends a signal to optimize the passive safety equipment quickly before the collision to minimize the injuries of occupants or pedestrians during the collision process.Shape Memory Alloy(SMA)has much higher recoverable deformation than ordinary metal materials,and can absorb a large amount of energy during deformation.At the same time,the mechanical properties of SMA such as stiffness and energy absorption density are closely related to its temperature.Based on the characteristics of SMA and the requirements of a pre-crash system,this paper proposes that SMA can be introduced as energy-absorbing material into the design of passive safety devices,which can also be an execution device for a pre-crash system.Taking the pre-crash technology as the starting point and SMA wires as the specific research object,this paper explores the energy absorption and protection mechanism of SMA Material,mainly from two perspectives:reversible recovery and controllable energy absorption,by means of modeling,simulation,experiment and theoretical deduction.The influence of various factors including temperature on the energy absorption characteristics of SMA as well as the heating response speed of SMA is discussed,providing a basis for the application of SMA in the automotive pre-crash system.Firstly,the basic characteristics of SMA are analyzed from the point of view of phase transformation.The existing static and dynamic constitutive models of SMA are also introduced.The energy absorption characteristics of superelastic SMA and martensite SMA are qualitatively analyzed using stress-strain curves,and it is found that the difference of unloading stage leads to the difference of energy absorption between superelastic SMA and martensite SMA during a complete loading and unloading process.Secondly,the effects of temperature,strain amplitude and loading rate on the mechanical properties of SMA are investigated by quasi-static tensile tests.Meanwhile,piecewise linear models of superelastic SMA and martensite SMA are established respectively,based on Brinson constitutive model,and the energy absorption of SMA is calculated quantitatively.Based on the model,the effects of temperature and strain amplitude on the energy absorption characteristics of SMA are simulated.The simulation results are in good agreement with the experimental results.Furthermore,models considering the effect of strain rate are established.The numerical simulation results show that the simulation results considering strain rate are closer to the experimental results than those without considering strain rate.Then the impact-testing device is designed and constructed,and a series of impact tests are carried out.The impact energy absorption characteristics of martensitic SMA wires and superelastic SMA wires are investigated with impact velocity as experimental variables.The results show that the energy absorption capacity of martensitic SMA wires is better than that of superelastic SMA wires.The effect of temperature on the impact energy absorption performance of martensitic SMA wires is also studied.The results show that the shape of the impact force-displacement curves change obviously with the temperature,and the energy absorption decreases with the increase of the temperature until the temperature rises to A_f,after which the energy absorption changes little with the temperature.The energy absorption and protection performance of both martensitic SMA wires and superelastic SMA wires in the case of impact failure are also studied.The results show that the energy absorption of martensitic SMA wires increases steadily with the increase of impact velocity,while the energy absorption of superelastic SMA wires increases greatly when plastic deformation occurs.At the same time,the following conclusions are drawn:the strength limit of superelastic SMA wires is much higher than that of martensitic wires,but martensitic wires have better plasticity and impact toughness.In particular,the effect of temperature on the energy absorption and protection performance of superelastic SMA in the case of impact fracture is discussed.The results show that the energy absorption of superelastic SMA decreases with the increase of temperature.What's more,the energy absorption and protection performance of martensitic SMA wires after repeated impact are studied.The results show that the martensitic SMA wires after 20 impacts still have the energy absorption characteristics of their initial state,but when they are heated above A_f,the fracture occurs earlier,which means that the impact toughness decreases.Finally,the heating response speed of SMA is studied.Based on the SMA resistance model,the heat balance equation considering SMA resistance characteristics is established.The response speed of SMA to electrified heating is obtained by numerical simulation and verified by experiments.The results show that the error between theoretical and experimental values is small,and the response speed of SMA to electrified heating meets the requirements of pre-crash system. |
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