Study On the Crack Plastic Zone of Bridge Structure Steel and Its Application
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摘要: 为了研究裂纹塑性对裂纹扩展的影响，利用工程简化算法、应力函数法、扩展有限元法对桥梁钢裂尖塑性区的尺寸和形状分别进行了计算；由于平面应力和平面应变情况下尾迹场循环塑性的特性不同，利用不连续扩展有限元对两种情况下尾迹场的循环塑性和塑性累积进行了模拟分析，探讨了裂尖塑性区、循环塑性区的形成和尾迹场产生压应力的机理.研究结果表明：裂尖塑性区尺寸与应力水平（名义应力与屈服极限的比值）的平方成正比，当应力水平大于0.4时，裂尖塑性区尺寸需要考虑应力水平的影响；裂尖塑性区的形状以蝶形向前伸展，使裂纹尾迹场免受裂尖高应力场的拉伸作用，有利于裂纹闭合；裂尖塑性区存在材料的逆向流动，在循环塑性区裂纹表面的塑性累积产生压应力效应有利于裂纹提前闭合，这种塑性诱发的裂纹提前闭合对研究变幅加载、过载引起的裂纹扩展滞后有重要意义.Abstract: In order to study the influence of crack plasticity on crack growth, the shape and size of crack tip plastic zone of bridge steel were calculated by using engineering simplification algorithm, the stress function method and extended finite element method respectively. As a result of the cyclic plasticity of wake field under plane stress and plane strain being different, cyclic plasticity and plastic accumulation of crack tip and wakes were simulated and analyzed by using the discontinuous extended finite element method. The formation of crack tip plastic zone, cyclic plastic zone and the mechanism of compressive stress of wake field were studied. The results showed that the appearances and sizes of plastic zone at the crack tip were proportional to the square of the stress level (the ratio of nominal stress to the yield limit). When the stress level was greater than 0.4, the size of the crack tip plastic zone had to be taken into account the effect of the stress level. The butterfly-shaped crack tip plastic zone symmetrically extended forward, shielding the tensile force of the crack tip high stress field on the wake field, which was beneficial to crack closure. There exists reverse flow of material in plastic zone of crack tip. The compressive stress effect induced by the plastic accumulation on the crack surface was beneficial to the crack closure in advance. Crack closure produced by plasticity had an important influence on the retardation of crack extension in the case of variation load and overload.
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图 2 由应力函数计算的Ⅰ型裂纹裂尖塑性区结果
Figure 2. Estimated resultsfrom stress function for the mode Ⅰ
图 3 Ⅰ型裂纹裂尖塑性区平衡校正前后的结果比较
Figure 3. Comparison of crack tip plastic zone before and after equilibrium correction for mode Ⅰ crack
图 4 不同应力比情况下中心裂纹塑性区
Figure 4. Central crack plastic zone under different stress ratios
表 1 各种计算方法的塑性半径比较
Table 1. Comparison of plastic radii of calculation methods
mm 计算方法 修正前后 σn/Sy 0.8 0.6 0.4 KI+T 前 3.20 1.80 0.80 后 6.40 3.60 1.60 Westergoard函数 前 4.03 2.27 1.01 后 8.06 4.54 2.02 XFEM — 4.28 1.72 0.70 
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