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Session 10

Investigation of Failure Criterion in Dynamic Torsion Tests with Solid Cylindrical Specimens
When investigating the limiting states of materials under dynamic loading conditions, it’s important to specify the dependency of plastic failure strain on the stress state. Usually, such dependence is build upon the experimental data obtained from dynamic tests in tension and compression of solid cylindrical specimens with different working part geometry, followed by a monotonic extrapolation. In the recent studies [1] the existence of complex, non-monotonic dependence of failure strain on the stress state parameters is shown for a number of materials. In these cases, a mentioned set of tests is not enough to construct a reliable criterion relations. In statics, one of the most informative experiments for the failure criterion construction is a torsion test on solid or thick-walled cylindrical specimens. Although a nonuniform stress state arises in the sample in this case, effective methods of its interpretation are developed [2,3]. The theory of this experiment conformably to the dynamic processes at large plastic strains has not yet been developed. Using the LS-DYNA implemented virtual test bench, the experimental setup for the solid cylinder torsion test with high strain rates and methods of its stress state identification are discussed. It is shown that for the strain rate range of 102-104 1/s the kinematic hypotheses that are taken in the quasi-static torsion are valid, that allows the effective use of known methods of the sample’s stress state decoding.
A novel transversely-isotropic 3D elastic-viscoplastic constitutive law for modeling fiber matrix composites
A transversely isotropic elastic-viscoplastic constitutive law with a novel 3D failure crite- rion is presented, addressing high pressure effects, strain rate sensitivity in yielding and failure and volumetric plastic strain. The constitutive equations are derived in the frame- work of transversely-isotropic invariants, which allow for a coordinate system independent formulation and an easy parameter identification. Triaxiality dependent non-linearities are taken into account and entirely different yielding behavior under uniaxial/biaxial compression, uniaxial/biaxial tension and under in-plane/transverse shear stress states is addressed. Hardening curves for each loading state can easily be input either via tabu- lated data or optionally by use of a three parameter power law. Lateral plastic straining due to volumetric plastic compression and dilatation is load path dependent as well. In order to control the lateral plastic straining in each stress state, a non-associated flow rule, assuming a plastic potential which gives the direction of the plastic flow is introduced. The applicability of this novel material law is shown by two examples.The first one is a short fiber reinforced thermoplastic PA6GF60, the second one adresses off-axis tensile and compression tests of a unidirectional carbon-epoxy IM7-8552, which is widely used in aircraft industry. For PA6GF60, a complete test setup for characterizing the novel transversely isotropic yield surface is used for validation. All test cases are simulated and compared with these experiments. The sensitivity of the plastic Poisson coefficient and the influence on the simulated load displacement curves are discussed. Strain rate effects are obtained from dynamic uniaxial tensile tests and are considered by a viscoplas- tic approach. Unidirectional carbon-epoxy IM7-8552 reveal pronounced yielding under combined shear- compression loadings as it is observed in off-axis compression tests. Fur- thermore, the glass transition temperature of epoxy resin drops from above 200◦ C to operating temperature in the presence of high pressures. This results in a change of me- chanical properties, effecting the elastic parameters as well as the yielding behavior.This change of mechanical properties and the pronounced non-linear behavior in the presence of high pressure due to matrix yielding can be modeled properly with this new approach.
On Fracture Criterion of Titanium Alloy under Dynamic Loading Conditions
One of the most important factors in ensuring the adequacy of the mathematical modeling of limiting states of structures is the choice of the material local fracture criterion and accurate determination of its parameters. The paper discusses some traditional approaches to the construction of local failure criteria of metals under dynamic loading, methods of their parameters identification, as well as the development of these approaches on the example of impact penetration problem. The work focuses on the possibility of modeling viscous and brittle types of fracture within a single deformation type criterion, while the dependence of fracture strain on the stress triaxiality ratio can became complicated and nonmonotonic. The quality of the considered criteria is determined by comparing the results of virtual simulation with the data of full-scale experiments that implement various types of stress state and failure mechanisms. The results of full-scale and virtual compression, tension and penetration dynamic tests of the titanium alloy samples are given. Virtual experiments were conducted using nonlinear LS-DYNA code.
Recent Enhancements to the GISSMO Failure Model in LS-DYNA
Development and verification of a material model for prediction of containment safety of exhaust turbochargers
For predicting the containment safety of turbochargers against failure of rotors at elevated temperatures and dynamic loading the complex deformation and damage behaviour of the respective materials has to be determined by appropriate experiments and on the other hand the temperature and strain rate dependency has to be described by a material model to simulate the component behaviour under these complex loading conditions. The investigations focus on the cast iron alloy EN-GJS-400 with nodular graphite. Its mechanical behaviour under uniaxial and multiaxial tension as well as under compression and shear loading has been investigated for a variety of loading rates and temperatures. For the numerical modelling of the containment safety of turbochargers a material model has been developed with the capability to describe the specific deformation behaviour of casting materials, e.g. different properties under tension and compression, temperature and strain rate dependence. The deformation behaviour was described with a model for thermally activated flow and the damage behaviour with a Johnson-Cook type model and an extended failure model (bi-failure model) respectively. The material model has been verified by numerical simulations of penetration tests under highly dynamic impact loading conditions. Also a containment test on a turbocharger was simulated.