Gerald W. Wellman

Capturing the influence of surface constraints in small and thin samples using polycrystalline plasticity theory

A rate-dependent, single-crystal plasticity model for face-centred cubic crystal structures has been implemented into a large strain elastic - plastic, finite-element code to examine the mechanical influence of the reduced surface constraints of relatively small polycrystalline aggregates. The implemented model simulates deformation of a polycrystal composed of cubic grains where each grain is a single finite element. Mechanical constraint is varied by changing (a) specimen thickness and (b) specimen volume, relative to grain size. Numerical uniaxial tensile tests have been performed to a strain level of 0.01. Direct and statistical examination of the model results revealed the reduced flow stress of grains at specimen surfaces, edges and corners. The results of these simulations are in good agreement with previous experimental studies which suggest that 5 - 10 grains across the minimum dimension of a structure are necessary to approximate true continuum polycrystalline response.

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.

Coupled Simulations of Mechanical Deformation and Microstructural Evolution Using Polycrystal Plasticity and Monte Carlo Potts Models

The microstructural evolution of heavily deformed polycrystalline Cu is simulated by coupling a constitutive model for polycrystal plasticity with the Monte Carlo Potts model for grain growth. The effects of deformation on boundary topology and grain growth kinetics are presented. Heavy deformation leads to dramatic strain-induced boundary migration and subsequent grain fragmentation. Grain growth is accelerated in heavily deformed microstructures. The implications of these results for the thermomechanical fatigue failure of eutectic solder joints are 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].

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.

Crack Tip Growth Measurement Using Digital Image Correlation

We present the results of sub-surface crack-length measurements in an aluminium sample that are inferred from surface strains calculated using digital image correlation (DIC). DIC is a photometric technique that uses a calibrated stereo camera system to calculate the 3D shape and motion of an imaged scene [1,2]. Image correlation has already proven itself to be useful for crack tip investigation due to the ability to obtain a relatively dense set of strain measurements in a region of interest. Traditionally, this has been accomplished using 2D-DIC techniques that require only a single camera [3], however, the more complex geometry of the current experiment required 3D information to correctly map the crack-tip strains onto the curved surface. This complex sample geometry was carefully constructed to maintain a stable strain field in order to produce crack growth at rates easily captured with high-resolution digital cameras operating with a 4-Hz frame rate (see Fig. 1). A preliminary experiment was conducted where the crack growth was halted and x-ray methods were used to find the sub-surface crack-tip location. The sub-surface crack location corresponded to a surface strain of 10%. This value was then used for tracking the crack extension as the specimen failed. The three samples tested showed excellent agreement. The crack length and applied load versus sample extension (measured with a linear variable differential transformer) for a single sample is shown in Fig. 2. Also shown are images of the strain field at assorted times during the test. The theoretical “peanut” shaped crack-tip strain predicted by the plane stress Tresca criterion is evident in the measured strain field.