Publications

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The 2018 Nonlinear Mechanics and Dynamics (NOMAD) Research Institute was successfully held from June 18 to August 2, 2018. NOMAD brings together participants with diverse technical backgrounds to work in small teams to cultivate new ideas and approaches in engineering mechanics and dynamics research. NOMAD provides an opportunity for researchers -- especially early career researchers - to develop lasting collaborations that go beyond what can be established from the limited interactions at their institutions or at annual conferences. A total of 17 students came to Albuquerque, New Mexico to participate in the seven-week long program held at the Mechanical Engineering building on the University of New Mexico campus. The students collaborated on one of six research projects that were developed by various mentors from Sandia National Laboratories, University of New Mexico, and academic institutions. In addition to the research activities, the students attended weekly technical seminars, various tours, and socialized at various off-hour events including an Albuquerque Isotopes baseball game. At the end of the summer, the students gave a final technical presentation on their research findings. Many of the research discoveries made at NOMAD are published as proceedings at technical conferences and have direct alignment with the critical mission work performed at Sandia.

Multiple fastener reduced-order models and fitting strategies are used on a multiaxial dataset and these models are further evaluated using a high-fidelity analysis model to demonstrate how well these strategies predict load-displacement behavior and failure. Two common reduced-order modeling approaches, the plug and spot weld, are calibrated, assessed, and compared to a more intensive approach – a “two-block” plug calibrated to multiple datasets. An optimization analysis workflow leveraging a genetic algorithm was exercised on a set of quasistatic test data where fasteners were pulled at angles from 0° to 90° in 15° increments to obtain material parameters for a fastener model that best capture the load-displacement behavior of the chosen datasets. The one-block plug is calibrated just to the tension data, the spot weld is calibrated to the tension (0°) and shear (90°), and the two-block plug is calibrated to all data available (0°-90°). These calibrations are further assessed by incorporating these models and modeling approaches into a high-fidelity analysis model of the test setup and comparing the load-displacement predictions to the raw test data.

Multiple fastener reduced-order models and fitting strategies are used on a multiaxial dataset and these models are further evaluated using a high-fidelity analysis model to demonstrate how well these strategies predict load-displacement behavior and failure. Two common reduced-order modeling approaches, the plug and spot weld, are calibrated, assessed, and compared to a more intensive approach – a “two-block” plug calibrated to multiple datasets. An optimization analysis workflow leveraging a genetic algorithm was exercised on a set of quasistatic test data where fasteners were pulled at angles from 0° to 90° in 15° increments to obtain material parameters for a fastener model that best capture the load-displacement behavior of the chosen datasets. The one-block plug is calibrated just to the tension data, the spot weld is calibrated to the tension (0°) and shear (90°), and the two-block plug is calibrated to all data available (0°-90°). These calibrations are further assessed by incorporating these models and modeling approaches into a high-fidelity analysis model of the test setup and comparing the load-displacement predictions to the raw test data.

A collaborative testing and analysis effort investigating the effects of threaded fastener size on load-displacement behavior and failure was conducted to inform the modeling of threaded connections. A series of quasistatic tension tests were performed on #00, #02, #04, #06 and #4 (1/4”) A286 stainless steel fasteners (NAS1351N00-4, NAS1352N02-6, NAS1352N04-8, NAS1352N06-10, and NAS1352N4-24, respectively) to provide calibration and validation data for the analysis portion of the study. The data obtained from the testing series reveals that the size of the fastener may influence the characteristic stress-strain response, as the failure strains and ultimate loads varied between the smaller (#00 and #02) and larger (#04, #06, and #4) fasteners. These results motivated the construction of high-fidelity finite element models to investigate the underlying mechanics of these responses. Two threaded fastener models, one with axisymmetric threads and the other with full 3D helical threads, were calibrated to subsets of the data to compare modeling approaches, analyze fastener material properties, and assess how well these calibrated properties extend to fasteners of varying sizes and if trends exist that can inform future best modeling practices. The modeling results are complemented with a microstructural analysis to further investigate the root cause of size effects observed in the experimentally obtained load-displacement curves. These analyses are intended to inform and guide reduced-order modeling approaches that can be incorporated in system level analyses of abnormal environments where modeling fidelity is limited and each component is not always testable, but models must still capture fastener behavior up to and including failure. This complimentary testing and analysis study identifies differences in the characteristic stress-strain response of varying sized fasteners, provides microstructural evidence to support these variations, evaluates our ability to extrapolate calibrated properties to different sized fasteners, and ultimately further educates the analysis community on the robustness of fastener modeling.

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This study details a complimentary testing and finite element analysis effort to model threaded fasteners subjected to multiple loadings and loading rates while identifying modeling sensitivities that impact this process. NAS1352-06-6P fasteners were tested in tension at quasistatic loading rates and tension and shear at dynamic loading rates. The quasistatic tension tests provided calibration and validation data for constitutive model fitting, but this process was complicated by the difference in the conventional (global) and novel (local) displacement measurements. The consequences of these differences are investigated in detail by obtaining calibrated models from both displacement measurements and assessing their performance when extended to the dynamic tension and shear applications. Common quantities of interest are explored, including failure load, time-to-failure, and displacement-at-failure. Finally, the mesh sensitivities of both dynamic analysis models are investigated to assess robustness and inform modeling fidelity. This study is performed in the context of applying these fastener models into large-scale, full system finite element analyses of complex structures, and therefore the models chosen are relatively basic to accommodate this desire and reflect typical modeling approaches. The quasistatic tension results reveal the sensitivity and importance of displacement measurement techniques in the testing procedure, especially when performing experiments involving multiple components that inhibit local specimen measurements. Additional compliance from test fixturing and load frames have an increasingly significant effect on displacement data as the measurement becomes more global, and models must necessarily capture these effects to accurately reproduce the test data. Analysis difficulties were also discovered in the modeling of shear loadings, as the results were very sensitive to mesh discretization, further complicating the ability to analyze joints subjected to diverse loadings. These variables can significantly contribute to the error and uncertainty associated with the model, and this study begins to quantify this behavior and provide guidance on mitigating these effects. When attempting to capture multiple loadings and loading rates in fasteners through simulation, it becomes necessary to thoroughly exercise and explore test and analysis procedures to ensure the final model is appropriate for the desired application.

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A series of tests on NAS1352-06-6P threaded fasteners were coupled with analysis to fit constitutive models, evaluate multiple modeling approaches, and ultimately predict failure. Experiments loading the fasteners in tension at both quasistatic and dynamic loading rates were performed to obtain calibration and validation data for the analysis. The fastener was modeled with two low-fidelity approaches - a "plug" of hex elements retaining the nominal fastener geometry (without threads) and a "spot weld", which incorporates similar geometry but the fastener is sliced near its mid-plane to define a tensile loaddisplacement relationship between the two exposed surfaces - to accommodate the use of these modeling methods in a larger, more detailed finite element analysis. Both modeling approaches were calibrated using quasistatic test data and then extended to the dynamic analyses to compare with the analogous test results. The analysis accurately reproduces most acceleration time-histories observed in the dynamic testing but under predicts failure, indicating the possible presence of strain rate effects that have been neglected in the constitutive models.

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Mechanical modeling was undertaken to support the Waste Isolation Pilot Plant (WIPP) technical assessment team (TAT) investigating the February 14th 2014 event where there was a radiological release at the WIPP. The initial goal of the modeling was to examine if a mechanical model could inform the team about the event. The intention was to have a model that could test scenarios with respect to the rate of pressurization. It was expected that the deformation and failure (inability of the drum to contain any pressure) would vary according to the pressurization rate. As the work progressed there was also interest in using the mechanical analysis of the drum to investigate what would happen if a drum pressurized when it was located under a standard waste package. Specifically, would the deformation be detectable from camera views within the room. A finite element model of a WIPP 55-gallon drum was developed that used all hex elements. Analyses were conducted using the explicit transient dynamics module of Sierra/SM to explore potential pressurization scenarios of the drum. Theses analysis show similar deformation patterns to documented pressurization tests of drums in the literature. The calculated failure pressures from previous tests documented in the literature vary from as little as 16 psi to 320 psi. In addition, previous testing documented in the literature shows drums bulging but not failing at pressures ranging from 69 to 138 psi. The analyses performed for this study found the drums failing at pressures ranging from 35 psi to 75 psi. When the drums are pressurized quickly (in 0.01 seconds) there is significant deformation to the lid. At lower pressurization rates the deformation of the lid is considerably less, yet the lids will still open from the pressure. The analyses demonstrate the influence of pressurization rate on deformation and opening pressure of the drums. Analyses conducted with a substantial mass on top of the closed drum demonstrate that the drums will still open provided the pressure is high enough. Investigation teams should look for displaced drum lids when searching for drums that have pressurized and failed. The mechanical modeling study for this program is summarized in the following memo. Following a brief introduction, there is a summary of a brief literature review of previous pressure testing of drums, an explanation of the model, presentation of the key results, some discussion, and concluding with a summary and key points.