Two-way effect

It is not unusual to want to investigate 10 factors, but it is unusual, and not necessary, to do 1024 experiments to discover 10 main effects and 45 two-way effects. However, care must be taken when deciding not to perform... 

This two-way effect is labeled mutually constitutive by constructivists. See Emmanuel Adler, Seizing the Middle Ground Constructivism in World Politics, European Journal of International Relations 3(3) (1997), pp. 319-63. However, I would not go so far here as some constructivists who argue that domestic ideology or self-identity drive changing perceptions of national interest. See the articles in Katzenstein, ed., Culture of National Security. 

Different from most normal materials, besides the shape changes resulted from elastic, plastic, or thermal contributions, the SMA has three additional shape changes a one-way effect shape change, a two-way effect shape change, and a pseudoelasticity shape change [6]. 

Two-way effect the effect that the material remembers two different shapes one at low temperatures, and the other at high temperatures. In this situation, external loads are not necessary (intrinsic two-way effect). Normally, an SMA remembers its high-temperature shape and immediately forgets the low-temperature shape upon heating to recover the high-temperature shape. However, it can be trained to remember to leave some reminders of the deformed low-temperature condition in the high-temperature phase. 

Shape memory is the ability of a material to remember its original shape, either after mechanical deformation, which is a one-way effect, or by cooling and heating, which is a two-way effect. This phenomenon is based on a structural phase transformation. 

Some of the energy conversion effects can be two-way effects. For example, the piezoelectric effect was defined based on the crystals ability to convert stress into electric charge. 

Shape memory elements with a two-way effect will remember a high-temperature shape as well as a low-temperature shape. The element flips between both shapes depending on the temperature. 

If a SM coil spring with the two-way effect is heated, it will return to its predefined high-temperature shape - in Fig. 6.52 this is the compressed form. Upon cooling, the spring stretches to reach its low-temperature... 

Fig. 6.52. Intrinsic two-way effect of a shape memory alloy spring...

A shape memory element must be specially trained to display the two-way effect. Training t3rpically consists of several cycles of deformation to the desired low-temperature shape and subsequent shape recovery by heating [73]. Only very low forces can be exerted when the SM element changes from its high-temperature shape to its low-temperature shape for this reason, steel springs or other elastic elements are added to the system to guarantee a full return to the low-temperature shape. 

Shape memory alloys with the two-way effect can be utilized as actuators to generate repeated movements. To activate the shape memory effect and the corresponding movement, the SM element must be heated above the transition temperature A. Heating may be accomplished in different ways ... 

The shape that the SM actuator recovers to when heated is imprinted into the alloy by an annealing process. For instance, to fabricate a coil spring a SM wire is wound around a mandrel and annealed for 1... 2 hours at 350. .. 500 °C. Annealing temperature and duration have a strong influence on the actuators properties, such as the trainable two-way effect, the effect stability, and the hysteresis behavior. 

The two-way effect will be stabihzed after 20... 100 thermal and mechanical cycles. Due to the ability of the martensite (low-temperature phase) to form a twinned crystalline structure, different areas of the actuator element may be strained in different ways extension, compression, or shear are deformations that will be reverted to by heating. This variety offers the interesting opportunity to adapt the actuators shape change to the special needs of the actuating task. By this means, transmission links or gears may be eliminated, which helps reduce the size and price of a system. 

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