Publications

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Bolted joints often risk failure due to the loss of fastener preload when subjected to dynamic, multiaxial loads. This process is a complex problem that depends on multiple attributes such as loading direction, rate, contact within the threads and the interface, material properties, and many others. Current literature suggests that oscillatory shearing loads appear to be most detrimental to the loss of preload in threaded fasteners. To investigate the effect of less idealized loading conditions, an experimental setup employing a bolted c-beam structure is used to study loss of preload from various initial preloads during harmonic excitation near specific resonant frequencies of the structure. The preload force is measured using bolts equipped with internal strain gauges and the structure is excited at specific modes via sine dwell excitation with an electrodynamic shaker. The experiments were designed to measure loss of preload as a function of excitation duration and strength. A finite element model incorporating a fully-threaded joint is developed in parallel to investigate the effectiveness of each at measuring and predicting bolt loosening.

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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.

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Complementary gas-gun experiments and computational simulations have examined the time-resolved motion and post-mortem deformation of cylindrical metal samples subjected to impact loading. The effect of propagation distance on a compressive waveform generated in a sample by planar impact at one end was determined using a velocity interferometer to track the longitudinal motion at the center of the opposing rear (i.e., free) surface. Samples (25.4-mm diameter) were fabricated from aluminum (types 6061 and 7075), copper (OFHC = oxygen free, high conductivity), stainless steel (type 316), and cobalt alloy L-605 (AMS 5759; also referenced as Haynes®25 alloy). For each material, waveforms obtained for a 25.4-mm long cylinder corresponded to two-dimensional strain at the measurement point. The wave-profile data have been analyzed to (i) establish key dynamic material modeling parameters, (ii) assess the functionality of the Sierra Solid Mechanics-Presto (Sierra/SM) code, and (iii) identify the need for additional testing, material modeling, and/or code development. The results of subsequent simulations have been compared to benchmark recovery experiments that showed the residual plastic deformation incurred by cylinders following end, side, and corner impacts. ∗Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

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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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