Stored-energy-based fatigue criterion for shape memory alloys

Abstract

This paper presents a new stored-energy-based model for structural fatigue of shape memory alloys (SMAs) where the conversion of hysteresis work into dissipation and stored energy is discussed in detail. The results show that during cyclic pseudoelastic process, while part of the hysteresis work is dissipated into heat, the remainder is stored in dislocations and in residual martensite variants. At macroscopic scale, during first few cycles, the stored energy is large and strongly influences the thermomechanical behavior of the SMA, it then gradually decreases at each cycle and approaches zero at the shakedown state. At microscopic scale, however, the stored energy continues to accumulate in the micro-structure and when it reaches a critical value, fatigue cracks initiate and propagate. In this study, a stored-energy-based criterion is proposed and its validation with experimental data shows that the stored energy is the relevant parameter to predict the fatigue of SMAs.

Evolution of transformation characteristics of shape memory alloys during cyclic loading: transformation temperature hysteresis and residual martensite

“…The results are plotted in figure 11 where a small amount of residual martensite (3.094 × 10 −5 ) is introduced in the absence of plastic strain (ε p = 0), and this error is acceptable. The good linear character provides an experimental evidence of the prediction in the authors' previous work [22]: the variations in residual martensite fraction and plasticity are proportional in the sense that larger local plasticity will result in a larger residual stress field with more 'locked in' residual martensite variants.…”

“…In figure 2, the stress-induced martensite is completely oriented for any value of martensite fraction since the orientation finish stress is lower than forward transformation start stress. Consequently, the transformation strain, ε z , can be expressed as a linear function of martensite fraction z (in 1D) [22,29,30]:…”

“…Active transformation strain ε tr can be directly measured in figure 2 (the stabilized transformation strain; details can be found in authors previous work [22]). Finally, with equation ( 8) and ( 9), the pure plastic strain ε p can be calculated with to residual martensite fraction z RM .…”

“…An increasing number of works show that both functional and structural fatigue is initiated by transformation/orientation-induced plasticity [15,[17][18][19][20][21][22][23]. High local stress field resulting from the incompatible deformation at the austenite-martensite interfaces assists dislocation slip even when the macroscopic stress is below the yield strength.…”

“…Recent theoretical and experimental studies show that both local plasticity and low cycle fatigue of SMAs are sensitive to temperature, which indicates a strong coupling among temperature, thermal properties of SMAs (such as transformation temperatures, temperature hysteresis, and latent heat etc) and SMAs' fatigue behavior. Thermomechanical coupling in SMAs' fatigue behavior has been thoroughly investigated [13,22,23], however, the impact of this mechanical fatigue degradation on thermal properties of SMAs during temperature-induced transformation is still unclear, and in particular, the accumulation in residual martensite from fatigue cycling is rarely evidenced.…”