The stress-strain-temperature thermomechanical responses of NiTi shape memory alloys due to B2-B19′ martensitic transformation (MT) should ideally be phase and strain reversible in closed-loop thermomechanical load cycles, where the austenite and martensite phases do not undergo plastic deformation. However, this ideal behavior is only observed when MT occurs under zero or very low externally applied stresses. When MT occurs under higher externally applied stresses, it generates small plastic strains. These strains accumulate whenever MT proceeds under external stress, leading to the accumulation of residual plastic strains, internal stress, and lattice defects during cyclic thermomechanical loads. This accumulation results in the instability of cyclic thermomechanical responses, a phenomenon known as “functional fatigue.”