Assessment Method for the Effect of Strain Rate on Fracture Toughness

应变、应力、应变速率与温度之间的本构关系采用Bennet定义的应变速率温度参数，避免了准静态条件定义的模糊。为了获得裂纹尖端附近在动力条件下塑性功引起的局部温升，采用上述本构方程和迭代法，建立了动态热塑性弹塑性有限元方法。很明显，其中90%的塑性功转化为热量，其余的塑性功转化为另一种功，如在几种加载率下拉伸试验中形成的滑移带。在不同加载速率下进行了CT试验和用落锤试验机进行三点弯曲试验，测量了裂纹尖端附近的应变分布，结果表明，有限元计算结果与实测应变分布吻合较好。此外，利用Thermo Viewer对不同加载速率下CT试件裂纹尖端附近的塑性工作温升分布进行了测量，结果与实验结果吻合较好。结果表明，由于应变速率和温度对屈服应力的补偿作用，裂纹尖端附近的屈服应力保持一定的水平。这一结果支持了动态条件下的实验位移行为与静态条件下的实验位移行为相似。根据上述结果，提出了通过静态条件下断裂韧性试验获得任意应变速率下断裂韧性的评估方法。

Constitutive relation between strain, stress, strain rate and temperature is conducted by using the strain rate temperature parameter defined by Bennet for the purpose of avoiding the vagued definition of quasistatic condition. For the purpose of obtaining the local temperature rise due to plastic work in the vicinity of a crack tip under dynamical condition, the dynamic thermodplastic-elastic FEM, in which the above constitutive equation and iterative method are used, is developed. It became clear that the plastic work converts around 90 percent of this into the heat and the remaining plastic work converts into another works such as the formation of slip band from the tensile test under several loading rates.CT tests under several loading rates and 3-point bend tests by using a drop weight testing machine are carried out for the purpose of measuring the strain distribution in the vicinity of a crack tip and the FEM show to give the good agreement with the measuring strain distribution.Moreover the distributions of the temperature rise near the crack tip of CT specimen due to plastic work under several loading rates are measured by using Thermo Viewer and the FEM results also show the good agreement with the experimental ones. Moreover the FEM results show that the yield stress in the vicinity of a crack tip keeps some constant level as a result of compensating effect of strain rate and temperature on the yield stress. This result supports that the experimental displacement behavior in the dynamic condition is similar to that in the static condition.From above results, the assessment method for obtaining the fracture toughness under the arbitrary strain rate is proposed by the fracture toughness test under static condition.