Incorporation of flexural hinge fatigue-life cycle criteria into the topological design of compliant small-scale devices

The design synthesis of compliant mechanisms yields optimized topologies that combine several stiff parts with highly elastic flexural hinges. The hinges are often represented in a finite element analysis by a single node (one-node hinge), which leaves the actual physical meaning of the hinge (to be fabricated) ambiguous. In order to circumvent this problem, in this work, one-noded hinges have the fatigue-life incorporated into them during the design synthesis by embedding analytical expressions accounting for stress concentration, surface finish, non-zero mean stresses and superposed multiple loading conditions into the formulation. Various flexural hinges with rectangular, circular and parabolic profile geometries are investigated. By incorporating the hinge geometry and fatigue behavior into the design process, unclear interpretation issues that would be encountered during any later manufacturing stage of a compliant mechanism design are removed. Examples are provided to illustrate the overall process.