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Functional grading can be used for spatial control of local mechanical properties and control of the stiffness and energy absorption of a structure. Cellular contact-aided compliant mechanisms (C3M) are cellular structures with integrated self-contact mechanisms, i.e. the compliant segments experience self-contact during deformation. The contact changes the load path and the C3M cells can be tailored for a specific structural application, such as energy absorption. The focus of the paper is on C3M made of nickel titanium which exploit self contact, the superelastic effect, and functional grading to improve performance and increase energy absorption. Continuous and discrete functional grading models are implemented in finite element analyses of the C3M cells. It is found that there is a complex relationship among self contact, superelastic properties, and functional grading, which are tailored to improve the performance of C3M. The functionally graded cells can be realized through metal additive manufacturing of NiTi, where functional grading and superelasticity are achieved by varying the material composition locally.

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This article focuses on the design optimization of shape memory alloy compliant mechanisms with functionally graded properties to achieve a user-defined target shape. The functional grading is approximated by allowing the geometry and the modulus of elasticity of each zone to vary. The superelastic phenomenon has been taken into account using a standard nonlinear shape memory alloy material model with linear region of higher modulus of elasticity and a superelastic region with much lower modulus of elasticity. A large deflection beam model is integrated with a multi-objective evolutionary algorithm for constrained optimization of the structure’s mechanical properties and geometry. Examples illustrate the trade-offs between the objectives of minimizing shape error, maximum stress, and volume. It is observed that in the optimized designs, the elastic modulus and the geometry work together in regions where large flexibility is required to achieve the target shape.