Incorporation of Inconel 718 material test data into material model input parameters for *MAT_224
A research team from George Mason University, Ohio State University, NASA and FAA has developed material data and analytical modeling that allows for precise input of material data into LS DYNA using tabulation and the *MAT_224 material model. The input parameters of this model are based on data from many experimental coupon tests including tension, compression, impact, shear and biaxial stress states. The material model also includes temperature and strain rate effects. This research effort involves the incorporation of the Inconel 718 material test data into the material model. This requires the development and validation of a set of material constants for this particular alloy, utilizing the tabulated input method of material model *MAT_224 in LS DYNA with consideration given to strain rate and temperature. Alloy Inconel 718 is a precipitation hardenable nickel based alloy designed to display exceptionally high yield, tensile and creep rupture properties at temperatures up to 1300degF. The sluggish agehardening response of alloy 718 permits annealing and welding without spontaneous hardening during heating and cooling. This alloy has been used for jet engine and high speed airframe parts such as wheels, buckets, spacers, and high temperature bolts and fasteners. *MAT_224 is an elastic plastic material with arbitrary stress versus strain curve(s) and arbitrary strain rate dependency, all of which can be defined by the user. Thermo mechanical and comprehensive plastic failure criterion can also be defined for the material. This requires a process of test data reduction, stability checks, and smoothness checks to insure the model input can reliably produce repeatable results. Desired curves are smooth and convex in the plastic region of the stress strain curves. Session: Aerospace 14 th International LS DYNA Users Conference 1 2 June 12 14, 2016 Introduction Inconel718 is an important engineering material widely used in aerospace applications. However, current constitutive models fall short in predicting the plastic and failure behavior of this material under various strain rates. One reason for this may be the high dependency of the properties of Inconel sheet on the manufacturing process and, hence, on the thickness of the sheet. This research effort, therefore, involves the development and validation of a set of material constants utilizing the tabulated input method of material model *MAT_224 in LS DYNA with consideration given to strain rate and temperature, specifically for a rolled 1/2 inch Inconel 718 sheet. *MAT_224 is an elastic plastic material model with arbitrary stress versus strain curve(s) and arbitrary strain rate dependency, all of which can be defined by the user. Thermo mechanical and comprehensive plastic failure criterion can also be defined for the material. [1] The deformation portion of the Inconel 718 *MAT_224 inputs are based on the results of uniaxial tensile testing conducted by Ohio State University [3][4][13] under plane stress conditions at varying strain rates and temperatures. Material model development and validation procedures are herein included as a reference for similar study. Methodology Many factors contribute to the variations of Inconel 718 measured material properties, including, but not limited to, the manufacturing and post manufacturing processes, the test specimen orientation in regards to plate grain direction, etc. To minimize some of these discrepancies, the material studied here is Inconel 718 1/2 inch plate manufactured by a sole company. The 1/2 inch rolled plate metal Inconel 718 material is modeled using *MAT_224 in LS DYNA. This is an isotropic elasto thermo visco plastic constitutive relationship which states that stress is dependent on strain, strain rate and temperature: T ij ij ij ij Equation 1 Where ij is stress, ij is strain, eij is the strain rate, and T is the temperature. In the elastic region, the Jaumann rate of the stress tensor, ij, is obtained as a linear function of elastic strain rates; this is a generalization of hypo elasticity: p ij ij ij p kk kk ij 2 Equation 2 Here e p ij are the components of the plastic strain rate tensor and and m are Lame constants. Young's modulus, E, and Poisson's ratio, may be converted by: 2 3 E Equation 3 2 Equation 4 14 th International LS DYNA Users Conference Session: Aerospace June 12 14, 2016 1 3 The material response in the plastic region is determined by a von Mises type yield surface in a six dimension stress space that can expand and/or contract due to strain hardening, rate effects and thermal softening: T T p eff p eff y p ij p ij y ij vm Equation 5 Where vm is the von Mises stress, e p eff is the equivalent plastic strain and e p eff is the equivalent plastic strain rate. As the material is assumed isotropic, the dependency of the yield surface upon plastic strain and plastic strain rate can be expressed purely as a function of the second invariant of each tensor. Note that states on the yield surface are plastic, whereas states below the yield surface are elastic. Mathematically this is expressed as follows: 0 0 0