J. Franklin Dempsey

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.

Modeling and Simulation in Validation Assessment of Failure Predictions for High Temperature Pressurized Pipes

A unique quasi-static temperature dependent low strain rate finite element constitutive failure model has been 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, 12–18 Nov 2010) and is being to be used to predict failure initiation of pressurized components at high temperature. In order to assess the accuracy of this constitutive model, validation experiments of a cylindrical stainless steel pipe, heated and pressurized to failure is performed. This “pipe bomb” is instrumented with thermocouples and a pressure sensor whereby temperatures and pressure are recorded with time until failure occurs. The pressure and thermocouple temperatures are then mapped to a finite element model of this pipe bomb. Mesh refinement and temperature mapping impacts on failure pressure prediction in support of the model validation assessment is discussed.

Experiments and Predictions of Large Deformation and Failure in Thermomechanical Loading Environments

The response of 304L stainless steel to combined mechanical and thermal loadings is studied to enable the development of validated computational simulation methods for predicting deformation and failure in coupled thermomechanical environments. Experimental coupling was accomplished on axisymmetric tubular specimens that were mechanically loaded by internal pressurization and thermally loaded asymmetrically by side radiant heating. Mechanical characterization experiments of the 304L stainless steel tube material was completed for development of a thermal elastic-plastic material constitutive model used in the finite element simulations of the validation experiments. The design and implementation of the experimental methodology and results of preliminary experiments were presented at 2010 SEM Annual Conference [1, 2].

Analysis of Laser Weld Induced Stress in a Hermetic Seal

Laser welding of glass-to-metal electrical connectors is a common manufacturing method for creating a hermetically sealed device. The materials in these connectors, in particular the organic glass, are sensitive to thermal induced residual stress and localized heating. An analytical laser weld model is developed that provides simulation and analysis of both thermal and mechanical effects of the welding process. Experimental studies were conducted to measure the temperature at various locations on the connector. The laser weld is modeled using both surface and volumetric heating directed along the weld path to capture the thermal and mechanical response. The weld region is modeled using an elastic-plastic weld material model, which allows for compliance before welding and stiffening after the weld cools. Results from a finite element model of the glass-to-metal seal are presented and compared with experimental results. The residual compressive stress in the glass is reduced due to the welding process but hermeticity is maintained.

UQ and V&V techniques applied to experiments and simulations of heated pipes pressurized to failure

This report demonstrates versatile and practical model validation and uncertainty quantification techniques applied to the accuracy assessment of a computational model of heated steel pipes pressurized to failure. The Real Space validation methodology segregates aleatory and epistemic uncertainties to form straightforward model validation metrics especially suited for assessing models to be used in the analysis of performance and safety margins. The methodology handles difficulties associated with representing and propagating interval and/or probabilistic uncertainties from multiple correlated and uncorrelated sources in the experiments and simulations including: material variability characterized by non-parametric random functions (discrete temperature dependent stress-strain curves); very limited (sparse) experimental data at the coupon testing level for material characterization and at the pipe-test validation level; boundary condition reconstruction uncertainties from spatially sparse sensor data; normalization of pipe experimental responses for measured input-condition differences among tests and for random and systematic uncertainties in measurement/processing/inference of experimental inputs and outputs; numerical solution uncertainty from model discretization and solver effects.

Detailed Measurements of Thread Deformation and Failure in Thin Walled Aluminum Alloy Joints

This paper describes the development and implementation of the experimental design, apparatus and measurement methods for quantifying the deformation of threads during loading to failure. A linear thread geometry is used to allow direct optical and contacting measurements of key displacements along the loading axis and across the threaded engagement section. Full field optical measurements of thread pairs are collected for post-processing using digital image correlation methods. Thread geometry parameters and material pairings are studied.

Characterization of Liquefied Natural Gas Tanker Steel from Cryogenic to Fire Temperatures

The increased demand for Liquefied Natural Gas (LNG) as a fuel source in the U.S. has prompted a study to improve our capability to predict cascading damage to LNG tankers from cryogenic spills and subsequent fire. To support this large modeling and simulation effort, a suite of experiments were conducted on two tanker steels, ABS Grade A steel and ABS Grade EH steel. A thorough and complete understanding of the mechanical behavior of the tanker steels was developed that was heretofore unavailable for the span of temperatures of interest encompassing cryogenic to fire temperatures. This was accomplished by conducting several types of experiments, including tension, notched tension and Charpy impact tests at fourteen temperatures over the range of -191 oC to 800 oC.