The Mechanical Behavior Of Notch/crack Specimens Of NiTi Shape Memory Alloy At Different Load Modes

In this paper, a series of detailed FEM analysis and experiments are carried to study the mechanical behavior notch/crack specimens of NiTi Shape Memory Alloy tip .It includes the evolution of the tress-strain distribution and martensite transformation ahead of the notch/crack tips in NiTi SMA with the applied load, notch geometries, stress states, loading methods and loading history, The fracture mechanism and the effect of specimens orientation in CT notched specimens of the NiTi SMA are also investigated. The main results obtained are as follows:(1)The four characteristic loads, that is, starting and finishing loads of A→M transformation and the starting and finishing loads of converse M→A transformation increase with the increase of the notch root radius. The maximum normal stress and strain in front of notch and the stress plateau caused by the martensite transformation increase with the increase of the notch root radius at the characteristic loads. The martensite volume fraction in front of notch decreases gradually with the increase of the distance away from the notch tip. At the characteristic loads, the martensite volume fraction and the size of the martensite transformation zone increase with increasing the notch root radius .The stress distributions have different characteristics in the different load ranges.(2) Under the cyclic loading ,with increasing load, the maximum normal stress, strain and martensitic volume fraction ahead of the crack-tip increase, and it decreases decrease with increasing the distance from the crack- tip. When the load is unloaded to P_min=0, there is still some residual untransformed martensite in front of the crack tip, and the compressive stress is formed near the crack-tip. With increasing the distance from the crack tip, the compressive stress gradually decreases to zero. With increasing the cycle number N, the residual compressive stress and strain ahead of the crack-tip increase due to the more residual martensite ahead of the crack-tip. With increasing the stress intensity factor amplitude AK and load ratios R, the stressσ_yy, strain s and martensite volume fraction f ahead of the crack-tip also increase.(3) When the load reaches near the maximal loading P_max for the notched CT specimens of NiTi alloy, crack initiates in the full martensite transformation zone in front of the notch tips. When the crack starts to propagate from the notch tip, the basic fracture mechanism is the mixture of the quasi-cleavage and ductile fracture, and the crack propagates in a stable manner. When the crack length exceeds 150～200um, the crack propagates in an unstable manner due to the increase of theσ_yy andεat the crack-tip ,and the fracture mechanism is mainly cleavage with mixed quasi-cleavage .With releasing the elastic energy after the unstable crack-growth, the load decreases quickly, and the crack propagation changes into a stable manner due to the decrease of theσ_yy at the crack-tip.There are no significant differences in fracture behavior among the specimens with different orientation. But the P_max is higher in a range of 22.5°≤θ≤67.5°, which indicates that there exists higher resistance for crack initiation and propagation in thisθscale. There are a lot of dimples on the fracture surface, which means the fracture mechanism is mainly of ductile fracture, and it consumes much more energy, and has higher fracture toughness.(4) When the specimens are loaded only under Mode I load, the stressσ_e, strainεand martensite volume fraction f distribution are symmetric ahead of the notch tip. When the specimens are loaded under mixed Mode I / II load, one side of the notch is blunted, and the other is sharpen, and the stress-strain distribution and martensite volume fraction are asymmetric ahead of the notch. The area with maximumσ_e,εand f appears at the blunted side .With the decrease of the So (increasing the ratio of Mode II), the clock round angleθof the area with maximumσ_e, s and f increases. In plan strain state, theσ_m/σ_e are relatively large in front of the notch tip, so it can cause more constraint to martensite transformation. In the case of the same S₀, the characteristic loads P_s and P_f are larger in the plan strain state than that in the plan stress state.When the load is greater than the characteristic load P_f,σ_e andεat the blunted side of the notch tip are both highest, the microcracks will initiate at this side, and propagate in the full martensite zone in front of the notch tip. With the decrease of the S₀, not only the characteristic loads P_s, P_f and P_max ,but also the crack initiation load P_i increase, which increases the toughness. For the NiTi SMAs, when the ratio of Mode I is relatively higher, the crack tends to unstable growth; while the ratio of Mode II is relatively higher, the crack tends to stable growth. Martensite transformation in NiTi SMA can also help to enhance the toughness under the mixed Mode I/II loads.