Repeatable bending actuation in polyurethanes using opposing embedded one-way shape memory alloy wires exhibiting large deformation recovery

Shape memory alloys (SMAs) are being used in adaptive structures for sensing and actuation because of the large amount of deformation and force that these materials can produce upon heating. Of particular interest, is the bending actuation of polymers, such as polyurethanes, by embedding opposing SMA wires. Previous investigations on these structures have focused primarily on embedding single or opposing SMA wires that have been trained to exhibit the two-way shape memory effect (SME) because of the simplicity with which the actuation behavior can be modeled and predicted. Alternatively, many potential adaptive structure applications can employ SMA wires exhibiting a one-way SME to eliminate the need for a priori two-way training of the SMA wires, and to increase the recoverable deformation in the structure. In this investigation, the repeatable bending actuation Of polyurethanes using opposing one-way SMA wires exhibiting large deformation recovery is characterized with a full-field deformation measurement technique known as digital image correlation. These results are analyzed with a one-dimensional actuation model previously proposed for predicting the bending actuation of elastomeric rods embedded with opposing two-way SMA wires. Using appropriate recovery strains and constitutive bending responses, this model was modified to characterize an 'equivalent two-way SME' recovery strain for the one-way SMA wires. A kinematic model of the bending actuation is also presented that attributes the origin of the equivalent two-way SME to the biasing forces acting on the SMA wires. Repeatable bending actuation was achieved in polyurethanes using one-way SMA wires prestrained 5% at an equivalent two-way SME recovery strain level of nearly 5%. These results indicate that repeatable bending actuation can be achieved in polyurethanes by using opposing SMA wires exhibiting the one-way SME instead of the two-way SME at a level 25% greater than reported for the two-way SME. The level of bending actuation will depend on the constraint of the SMA wires imposed by the stiffness of the polyurethane matrix. The symmetry of the bending actuation is affected by the symmetry of the local volume fraction of SMA wires and the symmetry of the prestrain levels in the wires. Furthermore, symmetric prestrain will enhance the level of equivalent two-way SME exhibited by the wires.