# Two-Way Shape Memory Effect of a Shape Memory Composite Strip
## 1 Introduction  
Shape memory alloy (SMA) is a kind of smart material with the advantages of high power density, large actuation force, and high fatigue resistance, with wide applications in the civil engineering, medicine, and aerospace fields [1; 2; 3]. Commonly, the SMA material can be fabricated into different shapes (e.g., wire, film, and sheets) and embedded into a soft polymer matrix to form a smart actuator with large deformation [4; 5; 6]. As to rubber-based actuator, the SMA is used as the active part to actuate the bending deformation by phase transformation from martensite to austenite during the heating process, and the rubber matrix is used as the passive part to recover the initial shape via high elastic recovery force [7]. Unfortunately, the total stiffness is low at room temperature with poor load-bearing ability. On the other hand, shape memory polymer (SMP) with the advantages of high deformation ability, good shape memory effect, tailored glass transition temperature, and variable stiffness properties can be used to replace the rubber matrix to improve the load-bearing ability at low temperatures [8; 9; 10]. To date, several studies have been carried out to investigate the two-way SME of rubber/SMP-based smart actuation devices. As to rubber-based smart actuation devices, [PERSON] et al. fabricated a miniaturized smart soft hinge composite actuator with pretensioned SMA wires at the eccentric position to grab and release different objects [11]. [PERSON] et al. developed a kind of SMA hybrid yarn (SMA-HY) with core-sheathe structures and characterized the reversible bending behavior for different structural styles [12; 13; 14; 15; 16]. [PERSON] et al. fabricated a soft hinge actuator by placing eccentrically the pretrained SMA wire into a flexible PDMS matrix to grab objects with a wide diameter range [17; 18]. Moreover, [PERSON] et al. presented a smart soft actuator by embedding fusible alloy, magnetic elements, and prestrained SMA wires into a flexible polymer matrix to significantly improve the structural stiffness under themartensite phase [19; 20; 21]. [PERSON] and [PERSON] et al. investigated the out-of-plane/in-plane bending deformation behavior of a composite plate [22; 23]. Although some work has been carried out about rubber-based smart actuation devices, the total stiffness is low at room temperature due to the low elastic modulus of rubber and SMA at martensite phase. SMP-based smart actuation devices are developed to improve the load-bearing ability at room temperature. [PERSON] et al. characterized the thermomechanical properties of SMP/SMA and developed a polyurethane-based (SMPU) composite plate by placing a NiTi SMA wire into the central part [24; 25; 26]. Moreover, [PERSON] et al. fabricated a novel kind of SMPU-based strip by embedding two types of heat-treated NiTi SMA tape facing in opposite directions to achieve the two-way reversible bending behavior [27]. Similarly, [PERSON] et al. demonstrated the two-way shape changing adaptation of SMP-based smart structures embedded with SMA strip in inner furniture and facade of architectural building with high structural stiffness at room temperature [28; 29]. [PERSON] et al. presented an idea for an SMA/SMP composite smart-bias strip to achieve two-way reversible bending deformation by placing a curved SMA wire into an SMP matrix [30]. [PERSON] et al. designed, fabricated, and characterized the two-way reversible bending behavior by embedding a NiTi SMA wire into an epoxy matrix [31]. Moreover, the 3D printing technology was introduced to fabricate the SMP-SMA composite parts with complex structure types [32; 33; 34]. [PERSON] et al. printed a PLA-based SMP strip embedded into SMA wire and demonstrated the reversible bending behavior of a crawling robot under Joule heating actuation [32]. [PERSON] et al. designed and fabricated two kinds of soft fingers by replacing the rubber matrix with SMP material to obviously improve the grasping ability at room temperature [33; 34].