Q345R钢的Ⅰ-Ⅱ复合型疲劳裂纹扩展行为

Ⅰ-Ⅱ mixed-mode fatigue crack growth behavior for Q345R steel

复合型疲劳裂纹前沿多轴应力状态不断变化,裂纹扩展路径的走向难于预测;然而裂纹扩展路径直接影响裂纹扩展速率,从而影响裂纹体剩余寿命的准确评估和结构的安全性。在分步加载下,采用不同厚度的Q345R钢非对称紧凑拉剪(CTS)试样,进行Ⅰ-Ⅱ复合型疲劳裂纹扩展行为的实验研究。采用二维光学定位测量系统,实时跟踪和准确定位裂纹前沿,计算疲劳裂纹扩展长度;利用动态材料疲劳试验机,可以自动记录加载循环次数;根据裂纹扩展长度与循环次数的关系,采用七点递增多项式拟合方法,可以得到相对应的裂纹扩展速率。实验结束后,采用高倍电子显微镜拍照,可得到清晰的Ⅰ-Ⅱ复合型疲劳裂纹扩展路径。实验结果表明:非对称CTS试样一直经历Ⅰ-Ⅱ复合型疲劳裂纹扩展,裂纹扩展路径不断发生改变,偏斜程度主要受到加载角度的影响;加载方向改变后,裂纹扩展路径有向外载荷垂线方向靠近或跨越的趋势,裂纹扩展速率迅速下降,表现为过载延迟现象。

It was difficult to predict the law of crack growth path because of changing stantly on mixed-ode fatigue crack front. However,the crack growth path affected the the stress state con-fatigue crack growth rate directly,thus affecting the crack residual life evaluation and structure safety. The experiments of Ⅰ-Ⅱ mixed-mode fatigue crack growth were conducted with compact tension-shear （CTS） specimens made of Q345R steel under three step loading condition. Using a two-dimensional optical position measuring sys-tem, real-time tracking and accurate positioning of the crack front were conducted to calculate the fatigue crack length. The use of dynamic material testing machine may automatically calculate the loading cycle number. According to the relationship of the crack length and loading cycles, the corresponding crack growth rate could be obtained using seven point incremental polynomial fitting method. After the end of the experiment,the clear Ⅰ-Ⅱ mixed-mode fatigue crack growth path could be got by using high power elec-tron microscope. The results showed that asymmetric CTS specimens experienced the mixed Mode Ⅰ-Ⅱ loading condition ; crack growth direction was changed constantly,and was influenced by the loading angle mainly. After changing the loading direction, crack growth path was perpendicular to the loading direction approximately or crossing＇ crack growth rate decreased rapidly, presenting the overload retardation phe-nomenon.