【作者】 魏欣；

【导师】 王国珍；

【作者基本信息】 兰州理工大学 ， 材料加工工程， 2008， 硕士

【摘要】 本文通过有限元模拟和实验相结合的方法,主要研究了NiTi形状记忆合金在不同加载方式下缺口/裂纹试样的力学行为,即缺口/裂纹前端的应力-应变,马氏体相转变随外加载荷,试样缺口几何尺寸,应力状态,加载方式和加载历史的演化规律。对NiTi合金CT缺口试样的断裂机理及取样位向的影响也进行了研究。得到的主要结论如下:（1）NiTi合金缺口试样加载时,缺口尖端奥氏体向马氏体（A→M）转变开始和结束时的载荷以及卸载时马氏体向奥氏体（M→A）逆转变开始和结束时的载荷,即四个特征载荷,均随着缺口根半径的增加而增大。在特征载荷下,缺口前端的最大正应力σ_yy和等效应变ε以及相应于马氏体相变的应力平台的长度均随着缺口根半径的增加而增大。缺口前端的马氏体体积分数随着到缺口尖端距离的增加逐渐下降,在特征载荷下,随着根半径增加,马氏体体积分数增加,发生马氏体相转变的区域增大。在不同的特征载荷区间,应力σ_yy具有不同的分布特征。（2）在循环加载下,随着外加载荷的增加,裂纹尖端的最大正应力、应变和马氏体体积分数均增加,随离开裂纹尖端距离的增加逐渐减小。当载荷卸载到P_min=0时,仍残留有一些未转变的马氏体,类似于不能恢复的塑性区,在很靠近裂纹尖端形成压应力,随离开裂尖距离的增加,压应力逐渐减小到零。随循环次数N的增加,由于裂尖前累积的残余马氏体量越多,裂尖前的残余压应力和应变增加。裂纹尖端的正应力σ_yy、应变ε和马氏体体积分数f随应力场强度因子幅值△K和应力比R的增大而增大。（3）对于NiTi合金CT缺口试样,当载荷达到最大值P_max左右时,裂纹形成于缺口尖端的全马氏体区中。裂纹刚开裂时,主要为准解理和韧窝的混合断口,裂纹以准解理和微孔型延性断裂相混合的方式稳定扩展。当裂纹扩展超过150～200μm左右后,主要为准解理和解理的混合型断裂,以解理型断裂为主,裂纹扩展速度较快,为失稳扩展。裂纹由稳定扩展转变为失稳扩展的原因是:随裂纹扩展长度增加,正应力σ_yy和应变ε增加。失稳扩展的裂纹释放了弹性能,且载荷突然下降,裂尖前应力下降,因此失稳扩展的裂纹停止又转变为以准解理方式为主,并伴随着剪切撕裂的稳定扩展。不同取样位向的试样,其宏观断裂行为和细观断裂机理没有明显的不同。但在22.5°≤θ≤67.5°范围内,P_max较高,表明裂纹起裂和扩展的阻力要稍大一些,韧窝比例较多,此时断裂起裂以微孔型延性断裂为主,消耗能量高,因此起裂韧性高。（4）ModeⅠ加载,缺口前应力σ_e、应变ε和马氏体体积分数f的分布对称;ModeⅠ/Ⅱ复合型加载,缺口一侧钝化,一侧锐化,缺口前σ_e、ε和f的分布不对称,且σ_e、ε和f的最大值分布区域均出现在缺口钝化一侧;随偏心距S₀的减小（即ModeⅡ比例的增加）,σ_e、ε和f的最大值区域顺时针旋转的角度θ增大。平面应变状态下,缺口前端的三轴应力度σ_m/σ_e相对较大,对马氏体相转变的约束作用较大,P_s和P_f高于平面应力状态（S₀相同时）。当P大于P_f,缺口钝化侧尖端的σ_e、ε最高,裂纹将起裂于此尖端,并沿“塑性区”最小路径方向扩展于缺口前的全马氏体区中。随着S₀降低,特征载荷P_s、P_f和P_max以及开裂载荷P_i均增加,从而使韧性提高。在ModeⅠ比例较高时,裂纹倾向于失稳扩展,而ModeⅡ比例较高时,裂纹倾向于稳定扩展。同时马氏体相转变也有利于ModeⅠ/Ⅱ复合型加载时韧性的提高。更多还原

【Abstract】 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.更多还原