Study on Simple Method to Measure the Dynamic Fracture Toughness of Ductile Material

在确定冲击断裂韧性时，如果塑性区与裂纹和试样尺寸相比尺寸有限，则计算动态应力强度因子。另一方面，如果试样在断裂前表现出相当大的塑性，则必须评估j积分，即Rice j积分的惯性增强版本。冲击载荷作用下动态断裂力学参数K_I(t)或j积分的时变可以用有限元法计算。然而，这种方法需要大量的计算机时间，并且不方便在材料测试中使用，本项目的目的是开发一种简单的方法来确定韧性材料的冲击断裂韧性。主要结果如下：(1)在不考虑面积积分奇异性的情况下，用传统有限元法计算j积分，误差在5%左右。用可变换为线积分的函数拟合近尖场，可以解析地求出奇异面积积分。(2)通过对动应力强度因子公式进行塑性区修正，推导出三点和一点弯曲试件的简单j积分公式。与弹/粘塑性有限元计算相比，该方法获得了合理的精度，cpu时间在有限元方法的1%以内。(3)进行了冲击断裂韧性试验。研究了检测裂纹起裂的方法。对于比较脆的材料，电势法是有用的。然而，如果试样断裂具有相当大的塑性变形，则不可能通过这种方法检测裂纹的起裂。这种延展性材料需要进一步的研究。

In determining the impact fracture toughness, the dynamic stress intensity factor is computed if the plastic zone is limited in size compared with crack and specimen dimensions. on the other hand, the J-integral which is the inertia enhanced version of Rice's J-integral has to be evaluated if the specimen exhibits considerable plasticity prior to fracture. The time variation of the dynamic fracture mechanics parameter K_I(t) or J-integral under impact loading may be computed by the finite element method. However, this method requires a large computer time and is not convenient to use in material testing, The purpose of the present project is to develop a simple method to determine the impact fracture toughness of ductile materials. The main results are summarized as follows:(1) The J-integral can be computed by the conventional finite element method with about 5% errors without considering the singularity of the area integral. The singular area integral can be evaluated analytically by fitting the near-tip field with a function which is transformable into a line integral.(2) Simple formulae of J-integral for the three-point and one-point bend specimens have been derived by applying a plastic zone correction to the formulae of the dynamic stress intensity factors. A reasonable accuracy is obtained by the present method as compared with the elastic/viscoplastic finite element computation and the CPU-time is within 1% of the finite element method.(3) The impact fracture toughness test has been conducted. Methods to detect the initiation of crack have been examined. For relatively brittle materials the electric potential method is useful. However, it is impossible to detect crack initiation by this method if the specimen was fractured with a considerable amount of plastic deformation. Further investigation is needed for such ductile materials.