tailieunhanh - Encyclopedia of Smart Materials (Vols 1 and 2) - M. Schwartz (2002) Episode 11

Tham khảo tài liệu 'encyclopedia of smart materials (vols 1 and 2) - m. schwartz (2002) episode 11', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | D B Figure 1. Typical SMA behavior in tensile tests and bending applications a stress-strain curve of shape memory martensite material b schematic of a shape memory application c stressstrain curve of superelastic austenite in tension d superelastic behavior in a bending application. Ơ o Ơ o Initial Removal Position of Load Applied Loading Shape-memory alloys may also be trained to exhibit a two-way shape memory effect. Similar to the thermal shape-memory effect two-way shape memory TWSM requires special thermomechanical processing to impart shape memory in both martensitic and austenitic phases. A trained shape in the austenitic phase reverts to a second trained shape upon cooling allowing the material to cycle between two different shapes. This TWSM is theoretically ideal for many shape-memory applications however practical uses are limited due to behavior instability and complex processing requirements. Superelastic Effect. This effect known also as pseudoelastic describes material strains that are recovered isothermally to yield mechanical shape-memory behavior. The phenomenon is essentially the same as the thermal shape-memory effect although the phase transformation to austenite Af occurs at temperatures below the expected operating temperature. If the austenitic phase is strained by an applied load a martensitic phase is induced by stress and the twinning process occurs as if the material had been cooled to its martensitic temperature. When the applied load is removed the material inherently prefers the austenitic phase at the operating temperature and its strain is instantly recovered. A typical stress-strain curve is depicted in Fig. 1c and a schemati superelastic application is shown in Fig. 1 strain curve indicates a difference in stre ing loading and unloading that is known a stress-strain hysteresis. Alloys Several alloys have been developed that ing degrees and types of shape-memory most commercially successful have been N and Cu-based alloys although