The Flexi-Rigid Transition: A Spiral Design Methodology for Shape Memory Alloy Adaptive Mechanical Systems

Shape memory alloys (SMAs) have been deployed in numerous successful applications, yet each has been developed as an isolated point solution exploiting a single SMA property. Existing design methodologies present linear procedures that describe what a designer should do, but not how design thinking actually occurs in SMA-based development. This paper contributes a unified spiral design methodology for SMA systems, rather than novel prototype devices. The Flexi-Rigid Transition (FRT) is organized as a three-starting-point, four-node spiral iterative process that recognizes three legitimate entry points: (A) mechanical primitive-driven, (B) context-resource-driven, and (C) function-requirement-driven. These converge through four iterative nodes — effect-function mapping, context-resource coupling, load-path-structure translation, and material-parameter convergence — forming a spiral that tightens toward a feasible design with each iteration. Five core design principles operationalize the methodology across force control, energy dissipation, kinematics, and interface domains. Liang-Rogers constitutive simulations, parameterized from published experimental data, validate the central physical law that multi-stage force output can be programmed through pure geometric cross-section variation. Four cross-domain conceptual demonstrations solely serve to illustrate the operability and cross-field transferability of the FRT framework; physical prototyping of these devices is beyond the scope of the present methodological study. The FRT methodology elevates SMA design from property exploitation to phase-transition orchestration, providing a systematic, mechanics-grounded framework for SMA-based adaptive device design.