Publications

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Traditionally, material identification is performed using global load and displacement data from simple boundary-value problems such as uni-axial tensile and simple shear tests. More recently, however, inverse techniques such as the Virtual Fields Method (VFM) that capitalize on heterogeneous, full-field deformation data have gained popularity. In this work, we have written a VFM code in a finite-deformation framework for calibration of a viscoplastic (i.e. strain-rate dependent) material model for 304L stainless steel. Using simulated experimental data generated via finite-element analysis (FEA), we verified our VFM code and compared the identified parameters with the reference parameters input into the FEA. The identified material model parameters had surprisingly large error compared to the reference parameters, which was traced to parameter covariance and the existence of many essentially equivalent parameter sets. This parameter non-uniqueness and its implications for FEA predictions is discussed in detail. Lastly, we present two strategies to reduce parameter covariance – reduced parametrization of the material model and increased richness of the calibration data – which allow for the recovery of a unique solution.

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This document outlines the preliminary analysis of the 2D Challenge 2.0 images. They currently consist of a new Star pattern series of images created by Benoît Blaysat. Another image set may be created by Phillip with an unknown displacement field based on Sample 14 from the 2D Challenge 1.0.

This document outlines the preliminary analysis of the Stereo-DIC Challenge Plate translation test images.

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This document will outline the test plans for the Hill AFB Mk 84 aging studies. The goal of the test series is to measure early case expansion velocities, sample the fragment field at various locations, and measure the overall shockwave and large fragment trajectories. This will be accomplished with 3 imaging systems as outlined in the sections below.

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With the rapid spread in use of Digital Image Correlation (DIC) globally, it is important there be some standard methods of verifying and validating DIC codes. To this end, the DIC Challenge board was formed and is maintained under the auspices of the Society for Experimental Mechanics (SEM) and the international DIC society (iDICs). The goal of the DIC Board and the 2D–DIC Challenge is to supply a set of well-vetted sample images and a set of analysis guidelines for standardized reporting of 2D–DIC results from these sample images, as well as for comparing the inherent accuracy of different approaches and for providing users with a means of assessing their proper implementation. This document will outline the goals of the challenge, describe the image sets that are available, and give a comparison between 12 commercial and academic 2D–DIC codes using two of the challenge image sets.

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Our results for the two sets of impact experiments are reported here. In order to assist with model development using the impact data reported, the materials are mechanically characterized using a series of standard experiments. The first set of impact data comes from a series of coefficient of restitution experiments, in which a 2 meter long pendulum is used to study "in context" measurements of the coefficient of restitution for eight different materials (6061-T6 Aluminum, Phosphor Bronze alloy 510, Hiperco, Nitronic 60A, Stainless Steel 304, Titanium, Copper, and Annealed Copper). The coefficient of restitution is measured via two different techniques: digital image correlation and laser Doppler vibrometry. Due to the strong agreement of the two different methods, only results from the digital image correlation are reported. The coefficient of restitution experiments are "in context" as the scales of the geometry and impact velocities are representative of common features in the motivating application for this research. Finally, a series of compliance measurements are detailed for the same set of materials. Furthermore, the compliance measurements are conducted using both nano-indentation and micro-indentation machines, providing sub-nm displacement resolution and uN force resolution. Good agreement is seen for load levels spanned by both machines. As the transition from elastic to plastic behavior occurs at contact displacements on the order of 30 nm, this data set provides a unique insight into the transitionary region.

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A “good” speckle pattern enables DIC to make its full-field measurements, but oftentimes this artistic part of the DIC setup takes a considerable amount of time to develop and evaluate for a given optical configuration. A catalog of well-quantified speckle patterns for various fields of view would greatly decrease the time it would take to start making DIC measurements. The purpose of this speckle patterning study is to evaluate various speckling techniques we had readily available in our laboratories for fields of view from around 100 mm down to 5 mm that are common for laboratory-scale experiments. The list of speckling techniques is not exhaustive: spray painting, UV-printing of computer-designed speckle patterns, airbrushing, and particle dispersion. First, we quantified the resolution of our optical configurations for each of the fields of view to determine the smallest speckle we could resolve. Second, we imaged several speckle patterns at each field of view. Third, we quantified the average and standard deviation of the speckle size, speckle contrast, and density to characterize the quality of the speckle pattern. Finally, we performed computer-aided sub-pixel translation of the speckle patterns and ran correlations to examine how well DIC tracked the pattern translations. We discuss our metrics for a “good” speckle pattern and outline how others may perform similar studies for their desired optical configurations.

One nearly ubiquitous, but often overlooked, source of measurement error in Digital Image Correlation (DIC) arises from imaging through heat waves. “Heat waves” is a colloquial term describing a heterogeneous refractive index field caused by temperature (and thus density) gradients in air. Many sources of heat waves exist in a typical DIC experiment, including hot lights, a heated sample, sunlight, or even a hot camera. This paper presents a detailed description of the error introduced to DIC measurements as a result of heat sources being present in the system. We present characteristic spatial and temporal frequencies of heat waves, and explore the relationships between the location of the heat source, the focal length of the lens, and the stand-off distance between the camera and the imaged object. Finally, we conclude with suggested methods of mitigating the effects of heat waves first by careful design of the experiment and second through data processing. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under contract No. DE-AC04-94AL85000.

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A novel, experimental method is presented for measuring the coefficient of restitution during impact events. These measurements are used to indirectly validate a new model of elastic-plastic contact. The experimental setup consists of a stainless steel sphere that is attached at the bottom of a 2.2 m long pendulum. The test materials are of the form of 1 inch diameter pucks that the sphere strikes over a range of velocities. Digital image correlation is used to measure the displacement and velocity of the ball. From this data the coefficient of restitution is calculated as a function of velocity. This report details the experimental setup, experimental process, the results acquired, as well as the future work.