Design method of the hottest shape memory alloy co

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Design method of shape memory alloy metallic spring Jia Bao Xian (Petroleum University) abstract: this paper introduced the identifying principle, method and present situation of development of shape memory alloy metallic spring A discussion has been carried out on three kinds of the commonly used designing method.

Key words: Shape momory alloy,Helical spring, Design method.

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at present, shape memory alloys (SMA) are increasingly widely used, involving electrical appliances, machinery, transportation, chemical industry, medical treatment, energy, daily necessities and other fields, especially in robots and manipulators, and the form of application is mainly SMA coil springs

sma is characterized by shape memory effect. After it is shaped at high temperature, it is cooled to low temperature (or room temperature) and causes residual deformation. If it is reheated to phase transition temperature, the residual deformation will disappear and return to the inherent shape at high temperature. Then, it is cooled or heated again, and the shape at high temperature will continue to remain unchanged. The above process can be carried out repeatedly, As if the alloy remembers the shape given by the high-temperature state, this phenomenon is called one-way memory; If the material is specially treated, it can repeatedly remember the two shapes of high temperature and low temperature in the subsequent heating and cooling cycle, which is called two-way memory; Some shape memory alloys continue to cool to a lower temperature while realizing two-way memory, and they can have shapes completely opposite to those at high temperature, which is called omni-directional memory (also known as full memory). As shown in Figure 1

Figure 1 Schematic diagram of three types of shape memory effect coil springs mostly use one-way or two-way memory effect, especially the two-way memory effect. With the rise and fall of temperature, it can realize the action in two directions, which is very convenient to use

based on the phase transition mechanism of SMA, this paper describes the stress-strain relationship of SMA, discusses the design of SMA coil spring, and introduces three design methods of SMA coil spring. 2 thermoelastic martensitic transformation

sma's shape memory effect comes from thermoelastic martensitic transformation. Generally, the higher the hardness, once the horse's martensite is formed, it will continue to grow with the decrease of temperature. If the temperature rises, it will decrease, and disappear in a completely opposite process. The difference between the two free energies is used as the driving force of phase transition. The temperature t0 at which the two free energies are equal is called the equilibrium temperature. Martensitic transformation occurs only when the temperature is lower than the equilibrium temperature t0. On the contrary, reverse transformation occurs only when the temperature is higher than the equilibrium temperature t0

Figure 2 shows the change relationship between martensite content and temperature, in which there are four important parameters:

ms - martensitic transformation starting temperature

mf -- end temperature of martensitic transformation

as -- starting temperature of reverse (austenite) transformation

af -- end temperature of reverse (austenitic) transformation

Figure 2 Relationship between martensite content and temperature in SMA, martensitic transformation is not only caused by temperature, but also by stress. This kind of martensitic transformation caused by the pressure drop rate at room temperature when cement is loaded into such plastic bags is called stress-induced martensitic transformation, and the transformation temperature has a linear relationship with stress, as shown in Figure 3

Fig. 3 Relationship between transformation temperature and stress when t AF, martensitic transformation occurs under the action of external force, resulting in residual deformation. If it is heated above AF, reverse transformation will occur, making the strain disappear and return to the original state (as shown in Fig. 4), which shows its shape memory effect. In the process of shape recovery, if constrained by external force, a large restoring force will be generated in the material, and the magnitude of the restoring force is the same as that of the strain γ、 Temperature T and martensite content ξ Closely related. Using this feature, SMA can be used as a driving element

Figure 4 shape memory effect 3 characteristics of SMA stress-strain relationship

the biggest feature of shape memory effect is that it integrates temperature sensing and driving functions. As a driving element, it is similar to ordinary coil spring without moisture, but it is different. It is mainly manifested in:

(1) the stress-strain curve of ordinary coil spring is linear, while the stress-strain curve of SMA coil spring is nonlinear

(2) the stress-strain characteristic of ordinary coil spring is independent of temperature, while this characteristic of SMA coil spring is closely related to temperature

(3) the characteristic curve of ordinary spring in the process of loading and unloading coincides, while the characteristic curve of SMA has temperature hysteresis or strain hysteresis

to sum up, the stress-strain curve of SMA is nonlinear, and the mechanical and mechanical properties such as shear elastic modulus are not constant, but change with temperature. Therefore, the design formula of ordinary coil spring cannot be simply used. 4 mechanical model and characteristic quantity of SMA

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