Deployable morphing structure using multi-phase alloy
Reconfigurable structures achieve different shapes by multistable metamaterials or material graded morphing sheets. However, the multistable materials have limited numbers of deformable shapes and morphing sheets require external power supply to maintain deformed shape. Versatile soft machines simultaneously require complex configurations with limited power supply, load bearing capacity, and transition between multiple states reversibly. We have introduced a self sufficient shape morphing metamaterial that possess reversibly polymorphic three-dimensional reconfigurability. The metamaterial consists of a rigidity tuning kirigami endoskeleton made of a Field’s metal (melting point of 62 oC ) which is low morphing point alloy (LMPA). The LMPA of multilayered kirigami composite is embedded with heating layer in a complete elastomeric encapsulation. The rapid morphing, locking, and reversibility is demonstrated by a tri-axial kirigami composite in the solid state. The sheet is deformed by pneumatic pressurized membrane in which state the free-standing kirigami can perform load bearing capacity function. To recover the initial shape, the embedded heater is activated to cause phase transition of LMPA kirigami (IR mage of inset). The liquefied kirigami returns to a relaxed flat configuration and become available for new morphing shape.
The rapid morphing is solid phase of LMPA is a result of plastic deformation of kirigami joints. A linear kirigami pattern is tested in solid phase by applying a strain of εappl = 100%. The structure shows linear stiffness until 10% strain which is followed by a flat force-strain profile which suggests plasticization. The underlying elasto-plastic morphing mechanism of the endoskeleton in the solid-state is studied by finite element analysis (FEA). FE simulation at εappl = 100% show the plastic hinge formation at the beam joints. This elasto-plastic behavior allows the composite to be morphed into different stable configurations. Note that, the phase transition of LMPA by joule heating redistribute the stress. A temperature dependent modulus is modeled in FE platform to explain the two stepped loading process. In first step a deformation is applied at the stiff state of the LMPA endoskeleton which is followed by a thermal step to soften the endoskeleton. We find the stress concentration abound the joint in first step which gets distributed by thermal rigidity tuning at 60 oC. Note that the thermal treatment can heal the plastic zone to reinstate the elastic behavior of LMPA for repeated usage.
