13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Using Triaxiality Dependent Cohesive Zone Model in Low Cycle Fatigue Xuan Cao1,*, Michael Vormwald1 1 Material Mechanics Group, Technische Universität Darmstadt, 64287 Darmstadt, Germany * Corresponding author: cao@wm.tu-darmstadt.de Abstract: Cohesive zone models (CZM) are widely used for material fracture analysis. In monotonic loading process, the cohesive parameters are remarkably dependent on the specimens’ triaxiality conditions. A cyclic CZM is now introduced for crack initiation life and fatigue crack growth analysis. These two types of fatigue phenomena - crack initiation and fatigue crack growth - are to be described by the cyclic CZM with a unique set of parameters. Fatigue crack initiation is usually investigated using smooth specimen and fatigue crack growth is measured using compact tension specimen. The triaxiality condition in the CZM of these two standard specimens is strongly different. Realistic modelling of the phenomena therefore requires taking the triaxiality into account in the damage evolution law of the cyclic CZM. As a consequence in this paper a cyclic CZM combined with triaxiality dependent characteristic is proposed. This new model can be applied for specimens with different triaxiality conditions. The computation results including crack initiation life and fatigue crack growth simulation are compared with experimental data. In the very low cycle fatigue regime, a reasonable accordance of experimental and calculated results is achieved. Keywords: Cohesive zone model, Triaxiality dependence, Low cycle fatigue, Damage evolution 1. Introduction In actual engineering, fracture of the materials and structures is a very dangerous failure form and usually causes serious consequence. Thus, research on mechanisms of material fracture, controlling and reducing occurrence of fracture accidents are always important issues. Fracture mechanics provides the theoretical basis and application approaches for material fracture research. But sometimes the modelling expenses are very high for fracture mechanics concepts and other ideas should be introduced. Among all these methods, the cohesive zone model is very attractive for researchers. Cohesive zone model (CZM) is an interface damage model, it simplifies the fracture process zone into a narrow cohesive strip. Originally, the concept of CZM has been proposed by Dugdale [1] and Barenblatt [2]. In 1976, the CZM was firstly used by Hillerborg [3] in finite element calculation for concrete material. From then on, the CZM was applied widely as a numerical simulation model for material fracture. In the last 30 years, the CZM was developed by many investigators and scientific groups [4-10]. Now, it becomes a universal tool for material fracture analysis. Especially for material fracture under monotonic loading, the applications of the CZM are successful. However, using the CZM in material fatigue fracture is still at a starting stage. In the late 1990’s, the CZM was firstly extended by De-Andrés et al. [11] for fatigue crack growth simulation. A damage parameter was introduced to indicate the irreversible damage process. But in this model no description about the damage evolution is supplied. Soon afterwards, Yang et al. [12] developed a damage locus in the CZM to simulate fatigue crack growth. However, in this model the unloading and reloading stiffness have different definitions and the damage evolution is just dependent on the damage locus, these treatments are not convenient and accurate for application. Then the cyclic CZM proposed by Roe and Siegmund [13] explicitly introduced a damage variable and a damage evolution equation. The stiffness and traction of the cohesive zone degraded with the damage variable. The numerical calculation results of this model can reproduce many basic characteristics which are similar to the experimental phenomena in fatigue fracture. Nevertheless, there is no experimental verification for the damage evolution process and the other important factors for CZM
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