United States Department of Transportation
i
The Influence of Strain Rate, Temperature Effects, and Instabilities in Failure Modeling for Metal Alloys
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2022-06-01
[PDF-9.37 MB]
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Abstract:To improve the survivability of structures it is important to understand the dynamic failure behavior under impact loading. Impact tests have revealed that mode of failure on metal superalloys thick plate at high-speed impact is what is known as Adiabatic shear band (ASB). The tabulated J-C material model is the current state of the art for FEM of high velocity impacts. The development of the tabulated J-C material model started from the consideration that materials under impact are affected by large deformations, high strain rates, temperature softening, and varying stress-states and that the failure is also changing as a function of the state of stress. Validated numerical 2D simulations revealed that the current J-C material model is successful in predicting this mode of failure only under the condition of using meshes composed of elements with a size that is of the same magnitude order of the ASB width. Because the ASB width of some high-performance metal alloys is in the order of 1μm, the material model cannot be use in practical real application to predict ASB. This thesis describes the upgrades implemented in the current Finite Element Model (FEM) of tabulated Johnson Cook (J-C) material model that allow the development of Adiabatic Shear Bands (ASBs) under the correct loading conditions in meshes with element size of practical use in current engineering applications. Ductile deformation and failure mechanism of Inconel 718 superalloy were investigated experimentally and numerically for quasi-static and dynamic conditions at various temperatures and stress states. Impact tests were used to derive high strain rate strain-stress characteristics, proven to be vital to correctly simulate ASBs, using hybrid explicit-implicit simulations. Tabulated inputs of characterized material tests results were directly used to describe both the constitutive and failure characteristics of the material model. Full scale impact tests were used to validate and show robustness, accuracy, and efficiency of the modified material model. It is shown that the modified J-C material model can predict ballistic limit and failure modes accurately for structures under impact, modeled with meshes composed of element of size compatible with modern commonly available computational resources even when the failure mode is ASB. The presented material model can be implemented into most available Finite Element software. As part of this research, it was implemented into the commercial Finite Element Solver LS-DYNA® as a modification of *MAT_TABULATED_JOHNSON_COOK (*MAT 224) for solid elements.
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