APTEK Mixed Mode Constitutive Driver (MMCD)

Mixed Mode Constitutive Driver (MMCD)

APTEK develops and licenses an interactive program for driving LS-DYNA material models called the Mixed Mode Constitutive Driver (MMCD).

The driver helps the analysts to:

Quickly evaluate the stress-strain behavior of LS-DYNA material models to gain an understanding of their applicability and limitations.
Develop and evaluate new or user-proprietary material models (called user defined material models).
Fit the material model parameters to test data. Parameter identification is accomplished either manually through visual comparison the model's stress-strain behavior with test data or automatically via use of the LS-OPT optimization program.
Prepare the material model portion of the LS-DYNA input file (*MAT).
Prepare report-quality graphs from stored and retrieved computed results and test data.

The driver complements the performance of finite element codes like LS-DYNA. Its intended use is to help analysts efficiently fit and evaluate material models prior to performing large-scale finite element analyses. Material response may include elastic, plastic, damage, and high strain rate behaviors.

Overview

The MMCD is a graphics-based and menu-driven program that interfaces with the LS-DYNA library of material models and the LS-OPT optimization code. The core of the MMCD is the driver, which calculates the stress-strain behavior of material models driven by combinations of strain increments and stress boundary conditions, i.e. pure shear stress, and combinations of uniaxial, biaxial, and triaxial compression and tension. MMCD input and output is accessed via pre- and post-processors; graphical user interfaces (GUIs) for easily selecting the material model parameters and load histories, and for plotting the output in both two (stress-strain curves) and three (yield surfaces) dimensions. The pre-processor, driver, and post-processor are combined into a web downloadable software package that operates seamlessly as a single code.

The MMCD may be used in conjunction with LS-OPT optimization software for automating the fitting procedure. The MMCD prepares the LS-OPT and MMCD input files from information supplied by the user via the MMCD GUI, executes LS-OPT and the model driver, and then gathers and plots the optimized results. Results include graphs of computed stress-strain curves fit to test data and values of fitted parameters versus LS-OPT iteration number.

Features

Web downloadable software package providing all functionality under a single, easy to use, user interface.
A select library of LS-DYNA material models including the user-defined material model. More models will be added as the MMCD continues to develop.
A database of experimental results for materials such as concrete, wood, and soil. More data will be added as the MMCD continues to develop. Users wishing to contribute additional data may contact the authors.
A read/write curve capability for incorporating and saving user-defined experimental data. This feature is useful for plotting new data or for data that is proprietary.
An automated procedure for fitting each material model by interfacing the MMCD with the LS-OPT code.
A library of predefined mixed-mode load histories that simulate common laboratory tests.
An optional method for inputting user-specified load histories.
The capability to plot 2D stress-strain (or stress-invariant) curves with and without experimental data.
The capability to plot 3D yield surfaces, and to rotate and translate those surfaces about various axes.
The capability to create report-quality graphics via a large selection of fonts, curve line types, symbol styles, and curve manipulations (scaling, swapping, integrating, cross-plotting, and shifting).

Advantages

Dedicated Software Package. The MMCD is dedicated to the efficient development, evaluation, and parameter identification (fitting) of material models used in finite element codes. It is not cluttered with extraneous features that the user has to learn and sort through. Although many software programs focus on helping the user develop geometric model input (mesh generators for nodes and elements), the MMCD's unique focus is on material model input via parameter identification and material model evaluation.
More Accurate and Consistent Fits to Data. The automated LS-OPT fitting procedure provides consistent fits from user to user that are less dependent upon the analysts experience and judgment than a manual fitting procedure.
Ease of Use. Pre- and post-processing and code execution are interactive and GUI-menu driven. The MMCD creates the input files, executes the driver and LS-OPT, gathers the stress-strain output, and then plots the data. Thus the MMCD is easy to use, even for a beginning analyst. Little knowledge of optimization theory or the LS-OPT code is required.
Variety of Loading Simulations. Unlike strain-based drivers, the MMCD will analyze both strain-based and mixed-mode behaviors. Fits to typical test data, like those conducted under uniaxial or biaxial stress, require mixed mode capabilities (see theory).
Fast Turn-Around Time. APTEK has determined that the time it takes an experienced analyst to set-up, run, and plot model output with the driver is approximately one-fifteenth that needed to perform a single element simulation. For less experienced analysts, the savings would be even greater. A number of features contribute to the fast turn-around time. First, driving a material model is quick computationally and requires less input (set-up) than a single element simulation that analysts typically use to evaluate material models. Second, the point and click features of the pre- and post-processors are extremely efficient. Additionally, the material models and experimental data are all incorporated into the same piece of software, allowing for rapid visualization. Having data readily available also saves the analyst time in searching for, and gathering experimental data. In addition, plotting yield surfaces in three dimensions is not available with most finite element post-processors. The MMCD saves time by eliminating the switch to 3D graphics plotting routines.

Benefits

Cost Effectiveness. All of the above advantages result in savings in cost (time) associated with selecting and fitting material models for use in finite element applications.
Better Finite Element Simulations. Dedicated material model evaluation and accurate and consistent fits to test data will result in improved finite element simulations.
Educated Analysts. Use of the MMCD will improve the analysts understanding of material model behavior.

Theory

Constitutive material models accept strain rate increments as input and output the resulting stresses. The MMCD driver passes strain rate increments directly to the material model, by-passing all the finite element coding associated with the nodal displacements, element forces, and the dynamic equations of motion. The MMCD calculates material response under both mixed-mode and strain-mode loading conditions. The term mixed mode indicates that the driver can load the material with a mixture of strain rate increments and stress-boundary conditions and then solve for the stresses through an iterative procedure. Examples of mixed-mode simulations are uniaxial and biaxial stress. Strain-mode simulations are those in which all strain components are specified, such that no free stress boundary conditions exist. In this case, no iteration is required, and the specified strain increments are the final strain increments. Examples of strain-controlled simulations are uniaxial strain and simple shear.

LS-DYNA and LS-OPT

LS-DYNA is a general purpose transient dynamic finite element program capable of simulating complex real world problems. It is developed and leased by the Livermore Software Technology Corporation (LSTC) (www.lstc.com). LS-OPT is LSTC's standalone design optimization and probabilistic analysis package. Click here for more info.