金属材料三维断裂韧度厚度效应的有限元模拟

随着金属材料大壁厚结构件在工程中的广泛应用，对其断裂韧度的厚度效应研究具有重要的科学意义和工程价值。本研究基于有限元和实验相结合的方法，对金属材料断裂韧度的厚度效应进行预测。首先，通过一组薄壁厚金属材料标准三点弯曲试验得到试样失效时的临界载荷值，并利用内聚力模型与基于虚拟裂纹闭合技术的裂纹扩展模拟方法得到裂纹扩展时的单元临界能量释放率。随后，以此临界能量释放率作为裂纹扩展的启裂准则门槛值，通过有限元计算得到不同试样厚度下裂纹启裂时的裂尖断裂参数随着厚度的变化规律。最后，为了验证有限元模拟结果的准确性，本研究进行了另外两组不同厚度下三点弯曲试样的断裂韧度试验，并将试验结果与有限元结果进行了对比，验证了有限元所模拟的断裂韧度厚度效应的准确性。本研究旨在，通过薄壁厚三点弯曲试样的实验结果结合有限元模拟工作，即可实现金属材料断裂韧度的整个厚度效应曲线，为任意厚度下金属材料断裂韧度预测提供一种可靠的研究方法，有益于缩减试验成本，为大壁厚工程结构件的失效预测提供依据。

The Elucidation of Thickness Effect of Three-dimensional Fracture Toughness by Using the Finite Element Method

Due to the extensive applications of metal structures with large thickness in character, the corresponding fracture toughness of structure is needed for the accurate assessment of structural failure. However, the dimension of cracked components along the direction of three-dimensional crack-tip line has significant impact on the fracture toughness of material, which is generally known as the “thickness effect”. The present research focuses on predicting the fracture toughness of material with arbitrary thickness by combining the finite element method (FEM) calculations with experimental data. First, the critical loads of a group of specimens of thin thickness at fracture are recorded by the three-point bending tests performed on single-edge notched beam SENB specimens. The critical energy release rate (ERR) of material is achieved by using the cohesive zone model (CZM) and virtual crack closure technique (VCCT). Second, the critical ERR is applied as a material constant in the FEM models. The maximum ERR criterion is applied to predict the critical load while crack initiates. The variations of several representative crack-tip fracture parameters (K, J-integral, T-stress and the out-of-plane constraint factor Tz) with respect to the thickness of specimen are calculated using the FEM, and the corresponding analysis is addressed subsequently. Finally, another three groups of X70 SEB specimens are tested to verify accuracy of the FEM results through comparison. The present work provides a reliable method to study the thickness effect on the fracture toughness of material, based on which, fracture toughness of metal material with arbitrary thickness can be predicted. Furthermore, the thickness and fracture toughness of material can be correlated through several three-point bending tests on thin SENB specimens, and the corresponding results can be depicted by a curve or mathematical expression. The present work will be beneficial to the reduction in experimental cost and structure integrity assessment.