金属材料延性断裂韧度测试中的柔度方法研究

 对紧凑拉伸(CT)试样的柔度转动修正方法进行了研究,理清了现行柔度转动修正方法存在的问题,给出了新的转动修正方法,并提出了考虑转动效应的CT试样的裂纹嘴张开位移(CMOD,V₀)与加载线张开位移(V_LL)的转换公式。弹塑性有限元分析表明,CT试样产生刚性转动的中心并不在试样的剩余韧带中心,而是在偏靠裂纹尖端的位置;转动半径 R 基本不受材料本构关系的影响,仅与CT试样的裂纹长度a与宽度W之比(a/W)有关,从而提出了R与a/W之间的单调多项式;当裂纹尖端产生较大程度的塑性变形时,需要考虑转动效应对CT试样裂纹张开位移(COD)测量的影响, J积分的塑性功计算须采用经转动修正后的COD。采用两种延性材料Cr2Ni2MoV和16MnR对COD转换公式进行了实验验证，结果表明,基于转动分析的COD转换公式由于考虑了裂纹尖端附近区域的弹塑性变形行为，用于断裂韧性测试中的塑性功计算更加符合实际,且同实验结果符合良好;直通型紧凑拉伸(FFCT)试样的刚性转动对较大裂纹扩展情形下的J_R阻力曲线影响较大。

Investigation on Compliance Test Method for Ductile Fracture Toughness of Metallic Materials

Rotation correction method of unloading compliance for a compact tensile (CT) specimen is investigated, and a new reasonable rotation correction method for the CT specimen is proposed after making clear the problem of current method. Based on the new method, a transforming formula between crack mouth open displacement (CMOD,V₀) and crack open displacement (COD) along loading line (V_LL) for CT specimen in consideration of rotation effect is also proposed. The elastic-plastic finite element analyses show that the rotation center of the specimen is closed to the crack tip and does not locate at the center of the remaining ligament. Rotation radius R is little affected by the material constitutive relationship but it is relevant to the ratio a/W where the crack length a and width W are dimensions of the CT specimen, therefore, a monotonic polynomial formula to describe the relation between R and a/W is given. The rotation effect on COD measurement needs to be considered when larger plastic deformation occurs around the crack tip, and the evaluation of J-integral for CT specimen must use the corrected COD. Furthermore, two types of steels Cr2Ni2MoV and 16MnR are employed to verify the validity of the new rotation correction method experimentally. It shows that, the transforming formula between V₀ and V_LL with the rotation correction originating from plastic deformation near the crack tip is more reasonable for determining the plastic work for front force compact tension (FFCT) specimen, and the prediction results from the transforming formula are in agreement with the experimental results. The rigid rotation of the FFCT specimen has bigger influence on the estimation of J_R curves.