13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Stress State Dependent Failure Loci of a Talc-filled Polypropylene Material under Static Loading and Dynamic Loading Shaoting Lin1,*, Yong Xia1, Chin-Hsu Lin2, Jingyi Wang1, Gongyao Gu1 1 Department of Automotive Engineering, Tsinghua University, Beijing 100084, China 2 Vehicle Systems Research Laboratory, GM R&D, Michigan, USA * Corresponding author: linst06@gmail.com Abstract A set of stress state dependent failure loci of a talc-filled Polypropylene material under static loading and dynamic loading was obtained by using a combined experimental-numerical approach. Uniaxial tension, simple shear, notched tension and punching tests were carried out to identify fracture locus under nonnegative stress triaxiality. Corresponding finite element analysis was performed to obtain the evolution of stress triaxiality of each failure element in each type of test. Different fracture prediction techniques were applied in static loading and dynamic loading, respectively. Under dynamic loading, an average value of the stress triaxiality was identified to determine stress triaxiality of critical point during the whole loading process in each type of test. By comparing force-displacement curves from both test and modeling, equivalent plastic strain in identified stress triaxiality can be obtained. Under static loading, damage evolution rule was utilized to optimize failure locus, and quadratic function was selected to optimize fracture locus. Significant difference of fracture locus between static loading and dynamic loading was observed. Keywords Failure locus, Stress triaxiality 1. Introduction With the rising requirements for energy saving and environment protection, thermoplastics, as one of the lightweight materials, have been widely employed in many thin-walled components of intricate shape in the automotive industry such as in interior and exterior trims. This on the other hand drives the demand for the accurate characterization of mechanical properties like yielding criteria and failure locus for thermoplastics. Particularly, fracture of thermoplastic material is commonly observed in occupant and pedestrian impacts, and some of the purposely intended fractures are crucial for absorbing impact energy and reducing occupant injury. However, the fracture behavior of thermoplastics is multifaceted, and it is sensitive to loading speed, loading mode, temperature, etc. A number of commonly-used numerical ductile fracture models such as the constant equivalent strain criterion, the Johnson-Cook (J-C) fracture model, and the Wilkinson (W) failure mode [1] have been implemented in commercial codes such as ABAQUS, LS-DYNA and PAM-CRASH. The main drawback of these fracture models is that accurate predictions of failure can only be achieved for limited stress state and strain rate, which cannot be applied for thermoplastics. New failure model developed by Y. Bai [2] extends the Mohr-Coulomb (MC) criterion from 2D shell to 3D solid such that failure strain is a function of both stress triaxiality and Lode angle parameter. But none of these models consider the influence of strain rate on failure strain, which is crucial for capturing the fracture behavior of a thermoplastic. A general form of the strain based facture loci [3] can be written as follows. ̅ = ( ) = ( ℎ ̅ ) (1) where ̅ is effective plastic strain to fracture, is stress triaxiality defined by the ratio of hydrostatic stress ℎ to equivalent stress ̅. Considering the different failure behavior under static and dynamic loading for polymeric materials, we modify the Eq. (1) to the following equations. ̅ = ( )={ 1( ), > 0,static loading 2( ), > 0,dynamic loading (2)
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