ASTM和ISO标准断裂韧度测试方法比较研究

Study and Comparison of Test Method for Measurement of Fracture Toughness between ASTM and ISO Standards

ASTM E1820-11和ISO 12135-2002标准是测试断裂韧度的主要标准,然而这两个测试标准在钝化线、有效数据区间和阻力曲线拟合等方面存在较大的差异,导致断裂韧度测试结果的较大不同。研究这两个测试标准钝化线的依据,比较它们的差异。ASTM标准采用理想弹塑性材料假设,ISO标准基于材料的真应力应变关系满足幂次定律,ISO标准钝化线斜率比ASTM标准钝化线斜率大;ASTM标准采用两参数方程阻力曲线,而ISO标准采用三参数方程阻力曲线。提出根据材料应力应变特性来选择合适的试验标准,对于带屈服平台的材料可选用ASTM标准,不带屈服平台的材料可选用ISO标准。选取Q345R钢进行断裂韧度试验,用ASTM和ISO两个标准对试验结果进行评估,试验结果表明:Q345R的应力应变曲线在屈服阶段存在屈服平台,ASTM钝化线能更好反映裂纹尖端钝化。ASTM和ISO两个标准测得的断裂韧度分别为235.29 kJ/m^2和179.37 kJ/m^2,两者相差24%。

ASTM E1820-11 and ISO 12135-2002 are the main two testing standards of fracture toughness. There are some obvious differences between these two standards, such as blunting line, effective data and J -R curve equation, which cause discrepancy of the fracture toughness results. The bases of the blunting line of the two standards are studied and the differences of the two standards are compared. The ASTM blunting line is based on elastic-perfectly plastic model of material, and the ISO blunting line is from the supposition of power law true stress strain relation. The ISO blunting line slope is more than that of the ASTM. The resistance curve of two parameter equation is used in the ASTM standard, while the curve of three parameter equation is adopted in the ISO standard. It is suggested that the test standard can be selected according to stress strain characteristics of the material. ASTM standard can be adopted for the materials with yield platform, while ISO standard can be used for the materials without yield platform. Fracture toughness tests of Q345R steel are carried out, and the ASTM and ISO standards are assessed by the test results. The results show that the stress strain curve of Q345R exist the yield platform. ASTM blunting line can better reflect crack tip bunting. The measured fracture toughness is 235.29 kJ/m2 by the ASTM standard and 179.37 kJ/m2 by the ISO standard, respectively, and there is difference of about 24%.