William Mark Scherzinger

A Method for Projecting Uncertainty from Sparse Samples of Discrete Random Functions - Example of Multiple Stress-Strain Curves.

This paper describes a practical method for representing, propagating, and aggregating aleatory and epistemic uncertainties associated with sparse samples of discrete random functions and processes. An example is material strength variability represented by multiple stress-strain curves from repeated material characterization tests. The functional relationship underlying the stress-strain curves is not known─no identifiable parametric relationship between the curves exists─so they are here treated as non-parametric or discrete glimpses of the material variability. Hence, representation and propagation of the material variability cannot be accomplished with standard parametric uncertainty approaches. Accordingly, a novel approach which also avoids underestimation of strength variability due to limited numbers of material tests (small numbers of samples of the variability) has been developed. A methodology for aggregation of non-parametric variability with parametric variability is described.

Temperature dependent ductile material failure constitutive modeling with validation experiments.

A unique quasi-static temperature dependent low strain rate constitutive finite element failure model is being developed at Sandia National Laboratories (Dempsey JF, Antoun B, Wellman G, Romero V, Scherzinger W (2010) Coupled thermal pressurization failure simulations with validation experiments. Presentation at ASME 2010 international mechanical engineering congress & exposition, Vancouver, British Columbia, 12–18 Nov 2010). The model is used to predict ductile tensile failure initiation using a tearing parameter methodology and assessed for accuracy against validation experiments. Experiments include temperature dependent tensile testing of 304L stainless steel and a variety of aluminum alloy round specimens to generate true-stress true-strain material property specifications. Two simple geometries including pressure loaded steel cylinders and thread shear mechanisms are modeled and assessed for accuracy by experiment using novel uncertainty quantification techniques.

Coupled Thermal-Mechanical Experiments for Validation of Pressurized, High Temperature Systems

High fidelity finite element modeling of coupled thermal-mechanical failure processes in complex systems requires, as a precursor, high quality experimentation on several levels. The materials must be characterized such that the entire range of loading parameters is encompassed. Meaningful validation experiments must be developed that allow for the steady, incremental ascension of validation towards system level complexity and, eventually, predictability. This paper describes a combined experimental/modeling effort towards validating failure in pressurized, high temperature systems.

Design and Implementation of Coupled Thermomechanical Failure Experiments

The importance of developing the capability to accurately and predictively model failure under combined thermal and mechanical loadings can not be overstated. Development of the necessary constitutive and failure models relies heavily on laboratory experiments that provide detailed information at several levels, from material characterization to laboratory scale validation experiments of increasing complexity, eventually leading up to full scale validation. This work is part of an interdisciplinary program that seeks to develop solutions to a large class of coupled thermomechanical failure problems. Coupled thermal-mechanical experiments with well-defined, controlled boundary conditions were designed and implemented through an iterative process involving a team of experimentalists, material modelers, computational developers and analysts.

Unified Creep Plasticity Damage (UCPD) Model for Rigid Polyurethane Foams

Experiments were performed to characterize the mechanical response of several different rigid polyurethane foams to large deformation. In these experiments, the effects of load path, loading rate, and temperature were investigated. Results from these experiments indicated that rigid polyurethane foams exhibit significant damage, volumetric and deviatoric plasticity when they are compressed. Rigid polyurethane foams were also found to be extremely strain-rate and temperature dependent. These foams are also rather brittle and crack when loaded to small strains in tension or to larger strains in compression. Thus, a phenomenological Unified Creep Plasticity Damage (UCPD) model was developed to describe the mechanical response of these foams to large deformation at a variety of temperatures and strain rates. This paper includes a description of recent experiments and experimental findings. Next, development of a UCPD model for rigid, polyurethane foams is described. Finite element simulations with the new UCPD model are compared with experimental results to show behavior that can be captured with this model.

Testing of constitutive models in LAME.

Constitutive models for computational solid mechanics codes are in LAME--the Library of Advanced Materials for Engineering. These models describe complex material behavior and are used in our finite deformation solid mechanics codes. To ensure the correct implementation of these models, regression tests have been created for constitutive models in LAME. A selection of these tests is documented here. Constitutive models are an important part of any solid mechanics code. If an analysis code is meant to provide accurate results, the constitutive models that describe the material behavior need to be implemented correctly. Ensuring the correct implementation of constitutive models is the goal of a testing procedure that is used with the Library of Advanced Materials for Engineering (LAME) (see [1] and [2]). A test suite for constitutive models can serve three purposes. First, the test problems provide the constitutive model developer a means to test the model implementation. This is an activity that is always done by any responsible constitutive model developer. Retaining the test problem in a repository where the problem can be run periodically is an excellent means of ensuring that the model continues to behave correctly. A second purpose of a test suite for constitutive models is that it gives application code developers confidence that the constitutive models work correctly. This is extremely important since any analyst that uses an application code for an engineering analysis will associate a constitutive model in LAME with the application code, not LAME. Therefore, ensuring the correct implementation of constitutive models is essential for application code teams. A third purpose of a constitutive model test suite is that it provides analysts with example problems that they can look at to understand the behavior of a specific model. Since the choice of a constitutive model, and the properties that are used in that model, have an enormous effect on the results of an analysis, providing problems that highlight the behavior of various constitutive models to the engineer can be of great benefit. LAME is currently implemented in the Sierra based solid mechanics codes Adagio [3] and Presto [4]. The constitutive models in LAME are available in both codes. Due to the nature of a transient dynamics code--e.g. Presto--it is difficult to test a constitutive model due to inertia effects that show up in the solution. Therefore the testing of constitutive models is primarily done in Adagio. All of the test problems detailed in this report are run in Adagio. It is the goal of the constitutive model test suite to provide a useful service for the constitutive model developer, application code developer and engineer that uses the application code. Due to the conflicting needs and tight time constraints on solid mechanics code development, no requirements exist for implementing test problems for constitutive models. Model developers are strongly encouraged to provide test problems and document those problems, but given the choice of having a model without a test problem or no model at all, certain requirements must be kept loose. A flexible code development environment, especially with regards to research and development in constitutive modeling, is essential to the success of such an environment. This report provides documentation of a number of tests for the constitutive models in LAME. Each section documents a separate test with a brief description of the model, the test problem and the results. This report is meant to be updated periodically as more test problems are created and put into the test suite.

Library of Advanced Materials for Engineering - LAME

Constitutive modeling is an important aspect of computational solid mechanics. Sandia National Laboratories has always had a considerable effort in the development of constitutive models for complex material behavior. However, for this development to be of use the models need to be implemented in our solid mechanics application codes. In support of this important role, the Library of Advanced Materials for Engineering (LAME) has been developed in Engineering Sciences. The library allows for simple implementation of constitutive models by model developers and access to these models by application codes. The library is written in C++ and has a very simple object oriented programming structure. This report summarizes the current status of LAME.

Validation of a New Aluminum Honeycomb Crush Model with Dynamic Impact Tests.

This paper describes a process for validating a new constitutive model for large, high strain-rate deformation of aluminum honeycomb, called the Honeycomb Crush Model (HCM). This model has 6 yield surfaces that are coupled to account for the orthotropic behavior of the cellular honeycomb being crushed on-axis and off-axis. The HCM has been implemented in the transient dynamic Presto finite element code for dynamic impact simulations. The HCM constitutive parameters were identified based on Presto finite element models that were used to simulate uniaxial and biaxial crush tests of 38 lb/ft3 aluminum honeycomb and reported in an earlier paper. This paper focuses on validating the HCM in the Presto code for application to impact situations that have honeycomb crush velocities up to 85 ft/sec. Also, a new approach for incorporating rate sensitivity into the model is described. A two-stage energy absorber with integrated aluminum honeycomb is described as the configuration for dynamic impact validation experiments. The test parameters and finite element model will be described along with the uncertainty quantification that was done and propagated through the model. Finally, correlation of model predictions and test results will be presented using an energy based validation metric.

Characterization of aluminum honeycomb and experimentation for model development and validation :volume II, honeycomb experimentation for model development and validation.

The crush of aluminum honeycomb is a very attractive shock mitigation concept for dissipating large amounts of kinetic energy in laydown weapon systems such as the B61-7 and for shipping container applications. This report is the second of a three-volume set describing aluminum honeycomb crush behavior and model validation. Volume I documents an experimental study of the crush behavior of high-density aluminum honeycombs. Volume III is yet to be published. It will cover the execution of the validation plan described in Volume II. This report, Volume II, describes the need for an improved constitutive model for the large deformation of aluminum honeycomb and is intended to document the procedure that was followed to provide data to calibrate and validate a new constitutive model for large deformation of aluminum honeycomb. The emphasis is on the experimental procedures, but sufficient model description is given to motivate the experiments that were documented herein. The model is first discussed along with the metric, or measuring stick, that will be used to quantify the model’s fit with test data. Next, a description of the necessary constitutive tests and the associated test data are shown that are being used to calibrate the model parameters for the new Honeycomb Crush Model. Parameters for the linear elastic portion of the model are described first, followed by the nonlinear crush parameters. Next, a description of the dynamic experiments used to quantify strain rate sensitivity of the honeycomb are given. The final three chapters cover the basic model (single physics or Tier 1) validation and the combined physics or Tier II model validation steps. Finally, all the calibration and validation data are presented.