NiTi wires and thin ribbons are used in shape memory actuators as active elements undergoing repeated thermal cycles. The NiTi actuators operate either under constant force (CF loading constraint) or they act against a variable force (VF loading constraint) from an elastic bias spring. Any stress riser represents a danger for potential actuator failure upon cycling. Although NiTi actuators always include stress risers either inherently in their design or as material flaws, their effect on actuation performance has, however, not been studied thoroughly yet. This issue is addressed in the present paper through numerical analysis of the effects of actuation load type (CF or VF loading constraint), maximum temperature, and stiffness of the bias springs (in VF loading constraint) on the local evolution of martensitic transformation (MT) and mechanical fields around the notch upon the cyclic thermomechanical loading of a thin NiTi shape memory notched ribbon. The analyses clearly show a strong amplification effect of the transformation strain on the stress concentration factor. The results reveal that the evolution of the forward MT upon cooling causes a sharp stress peak around the notch-tip the magnitude of which depends on the bias spring stiffness while upon heating, the stress at the notch-tip area relaxes due to the reverse MT of the surrounding bulk. Moreover, the simulations indicate that any overheating during the actuation is harmful to the notch-tip as the complete reverse MT at the notch-tip has stress rising effect while incomplete MT at the notch-tip has stress relaxing effect.

NiTi丝和薄带用于形状记忆执行器中，作为经过反复热循环的有源元件。NiTi执行器要么在恒定力（CF加载约束）下运行，要么抵抗来自弹性偏置弹簧的可变力（VF加载约束）。任何应力升高都可能导致执行器在循环时发生故障。尽管NiTi执行器始终在设计中固有地或在材料缺陷中都包含应力上升器，但是它们对执行器性能的影响尚未得到充分研究。本文通过对驱动负载类型（CF或VF负载约束），最高温度，NiF形状记忆带状薄带的循环热机械载荷时，偏压弹簧的刚度和刚度（在VF载荷约束下）对马氏体相变（MT）的局部演化和切口周围的机械场的影响。分析清楚地表明，转变应变对应力集中因子的强烈放大作用。结果表明，冷却时正向MT的演化会在缺口尖端附近产生一个尖锐的应力峰，应力峰的大小取决于弹簧的刚度，而加热时，由于反向MT导致缺口尖端区域的应力松弛周围的大部分。此外，