The relationship between constituent property and bending actuation of shape memory composites

Abstract

Shape memory composites (SMCs) made of shape memory alloy (SMA) wires embedded in a shape memory polymer (SMP) matrix show great promise in adaptive structures due to their ability to combine strong actuation force of SMAs with the large deformation of SMPs. However, in order to advance these SMCs past the proof-of-concept stage, methods for optimizing the material selection and volume fraction balance between the alloy and the polymer must be developed. Before such multi-objective optimization can take place, sensitivity studies must be conducted to determine which parameters are most impactful in determining the ultimate behavior of the SMC. The present study focuses on thermal and mechanical characterization of a group of SMCs composed of Nickel-Titanium SMA and a styrene based SMP in order to elucidate the critical material properties for a functioning SMC. Here, the SMA wires have been trained with a one-way shape memory effect (SME) and integrated on the surface of the SMP. A morphing system has been obtained by introducing SMA wires on the SMP system in the volume fraction range of 0.5% to 1.2%. The results show that the proper modulus balance between the SMP and SMA as well as the balance of activation forces are important features to consider when developing a SMC. Also important is a proper pairing of glass transition temperature for the SMP with the Austenite and Martensite transition temperatures for the SMA. In addition, the current study has shown that during the heating process of the SMC, the thermal expansion of the SMP appears to overcome the actuation forces of the SMA wires, showing that the thermal expansion is also a critical variable for the composite performance. Based on these results, both the SMA and the SMP constituent parameters of temperatures, elastic modulus, actuation force, and thermal expansion are considered with regards to future tailoring the SMC performance.