Abstract Shape memory alloys (SMAs) have been deployed in numerous successful applications — vascular stents, orthodontic archwires, aerospace release mechanisms — yet each application has been developed as an isolated point solution exploiting a single SMA property for a single function. No systematic methodology exists that treats the reversible martensite–austenite phase transformation as a generalizable mechanical design resource. Here we propose the Flexi-Rigid Transition (FRT) — a unified design methodology that formalizes SMA phase-transition-based device design into a four-step procedure delivering four categories of mechanical functional output: (a) adaptive constant-force conformity, (b) dynamic impact energy dissipation, (c) multi-stage programmable force control, and (d) intracavitary adaptive support. The methodology is operationalized through five core design principles spanning four fundamental mechanical domains — force control, energy dissipation, kinematics, and interfaces — each defined with its governing physical mechanism and quantitative design equation. The methodology is illustrated through four design demonstrations spanning aerospace, impact protection, and medical domains: a fully passive spacecraft sun-tracking mechanism (~12 parts replacing 50+ electronic components), an SMA superelastic honeycomb with three-tier graded energy absorption (reusable, in contrast to single-use aluminum honeycomb), a passive Parkinsonian tremor-damping glove exploiting superelastic hysteresis at the joint level, and a graded-force orthopedic external fixation splint with physician-adjustable compression via wire count. Liang–Rogers constitutive simulations, parameterized from published experimental data on geometrically graded NiTi, serve as the quantitative validation anchor: variable cross-section SMA elements produce discrete multi-stage force plateaus with plateau force ratios precisely matching cross-sectional area ratios, a prediction independently confirmed by the experiments of Chen et al. (2023) and Meng et al. (2016). The FRT methodology elevates SMA design from property exploitation to phase-transition orchestration, providing a systematic, mechanics-grounded starting point for SMA-based adaptive device design across biomedical, aerospace, and protective engineering domains.
hanbin zhao (Sat,) studied this question.