Jeffrey W. Simons

Shape control of large lightweight mirrors with dielectric elastomer actuation

Space-based astronomy and remote sensing systems would benefit from extremely large aperture mirrors that can permit greater-resolution images. To be cost effective and practical, such optical systems must be lightweight and capable of deployment from highly compacted stowed configurations. Such gossamer mirror structures are likely to be very flexible and therefore present challenges in achieving and maintaining the required optically precise shape. Active control based on dielectric elastomers was evaluated in order to address these challenges. Dielectric elastomers offer potential advantages over other candidate actuation technologies including high elastic strain, low power dissipation, tolerance of the space environment, and ease of commercial fabrication into large sheets. The basic functional element of dielectric elastomer actuation is a thin polymer film coated on both sides by a compliant electrode material. When voltage is applied between electrodes, a compressive force squeezes the film, causing it to expand in area. We have explored both material survivability issues and candidate designs of adaptive structures that incorporate dielectric elastomer actuation. Experimental testing has shown the operation of silicone-based actuator layers over a temperature range of -100 °C to 260 °C, suitable for most earth orbits. Analytical (finite element) and experimental methods suggested that dielectric elastomers can produce the necessary shape change when laminated to the back of a flexible mirror or incorporated into an inflatable mirror. Interferometric measurements verified the ability to effect controllable shape changes less than the wavelength of light. In an alternative design, discrete polymer actuators were shown to be able to control the position of a rigid mirror segment with a sensitivity of 1800 nm/V, suggesting that sub-wavelength position control is feasible. While initial results are promising, numerous technical challenges remain to be addressed, including the development of shape control algorithms, the fabrication of optically smooth reflective coatings, consideration of dynamic effects such as vibration, methods of addressing large-numbers of active areas, and stowability and deployment schemes.

Cut Resistance of High-strength Yarns

The cut resistance of three high-strength yarns under tension-shear loading conditions was measured by pressing a knife blade transversely at a constant rate against a yarn gripped at its ends. The load-deflection relation and the energy required to cut through the yarn were determined for Kevlar (aramid), Spectra (polyethylene), and Zylon (polybenzobisoxazole, or PBO). The cut energy and strain to initiate cutting were highest for Zylon, least for Kevlar, and depended on the slicing angle, the sharpness of the blade, and the pre-tension in the yarn. The dependencies are explained by changes in failure mode of the fibers within the yarn. The test provides input needed for computational simulations of ballistic response of fabrics to sharp fragments and should be useful in designing slash-resistant gloves and clothing.

Slow Penetration of Ballistic Fabrics

A quasistatic penetration test is designed and implemented to provide insight into the evolution and phenomenology of fabric deformation and failure during penetration. The results are needed to guide development of a physics-based computational model of fabric response to projectile impact. The test involves slowly pushing a rigidly held fragment simulator into and through a single ply fabric specimen. The stroke and load on the penetrator are recorded during the test, and a videocamera and microphone enable the fabric failure phenomena to be time-correlated with the load-stroke history. The three distinct modes of fabric failure observed in these tests—local yarn rupture, remote yarn failure, and yarn pullout—are the same modes observed in impact tests, although the extent to which each mode occurs is different in static and dynamic tests. The conditions under which the different failure modes occur and the effects on the load-stroke curve and the energy absorbed are determined.

Thermal fatigue behavior of J-lead solder joints

Thermal cycling environments were applied to test specimens with J-lead eutectic Sn-Pb solder joints to assess the effect of lead and package material selection. Metallographic examination and finite element analysis were used to evaluate the effect of thermal cycling environments on solder joints. Matching the coefficient of thermal expansion (CTE) of the lead and solder was found to be crucial to reducing thermal cycling damage for the J-lead solder joints examined. Matching the CTE of the package and the board was substantially less important. The number of thermal cycles 1.9 nucleate cracks in the J-lead solder joints correlated reasonably well with the number of cycles to produce a 50% drop in the load range in isothermal strain controlled mechanical fatigue tests on Sn/Pb solder. >

Cause of the Port Neal ammonium nitrate plant explosion

Abstract In 1994, an explosion in an ammonium nitrate neutralizer of a chemical processing plant in Port Neal, Iowa, resulted in fatalities, injuries, and widespread property damage. A lawsuit was brought by the plant operator against the neutralizer designer, and each party retained a team of scientists and engineers to determine the cause. The key question was whether the explosion initiated within or outside of the sparger that injected nitric acid into the ammonium nitrate solution. After extensive investigations and months of debate, an agreement could not be reached. To resolve this impasse, an independent expert was appointed by the ruling US District Court judge to review the evidence provided by the investigative teams, examine the recovered sparger fragments and test articles, and provide an opinion. This article presents the findings and conclusions from this investigation.

Finite-Element Analysis of Fatigue Lifetime in Pavements

A computational model was developed to predict fatigue life of pavements under repeated loading and implemented into the three-dimensional finite-element code DYNA3D and the two-dimensional finite-element code NIKE2D. The model simulates the cracking response of flexible or rigid pavements under fatigue. An equation for fatigue crack growth was developed, which grows cracks under single cycles of loading at stresses well below yield. The cracks are incorporated into the material response and result in anisotropic behavior and decreased stiffness for cracked pavements. A procedure for estimating fatigue lifetimes by performing a limited series of calculations was developed. For each calculation, crack growth rate for a single loading cycle is calculated, crack extension is extrapolated to many cycles, and the cracking in the pavement is updated. The next cycle is calculated for the damaged pavement. The procedure is repeated until full damage is reached. Well-controlled laboratory bending fatigue test results generated at the University of California at Berkeley (UCB) for asphalt pavement were used to verify that the model assumptions are appropriate for modeling fatigue damage growth in asphalt pavement. The UCB bending fatigue tests were simulated using the repeat loading algorithm in DYNA3D. Calculations of crack growth for a given load cycle were compared in the NIKE2D and DYNA3D implementations.

Dynamic Stress Behavior in Catalytic Combustors

Dynamic stress behavior during catalytic combustion of methane has been simulated under transient warm-up, cool-down, and cyclic conditions. The numerical model combines a two-dimensional solution to the transport equations, solution of an energy balance on the monolith wall, and the NIKE3D structural analysis code to predict thermal stresses. The model also includes a detailed heterogeneous kinetics model for a proprietary palladium oxide (PdO) catalyst, but the model ignores gas-phase reactions. Results illustrate that thermal stresses as high as 630 MPa can form during transient operating modes, which risks structural failure of the ceramic monolith. The maximum computed thermal stress concentrations occur near the inlet of the monolith. Peak transverse stresses (which act to form axial cracks) typically form near the inlet and centerline of the monolith structure, while peak axial stresses form near the edges of the flat plate that represents the monolith structure. Increasing the preheat temperature of the incoming fuel and air mixture lessens the peak thermal stress. To a first approximation, the magnitude of the peak transverse stress during any transient cycle considered with our model can be estimated from the maximum value of the gradient in the computed temperature profiles.

Modeling the Flow of Fragmented, Brittle Material

In prior papers [1,2], we presented a granulated material model called FRAGBED for use in finite element “hydrocodes” applied to penetration of ceramic armors. It is a nonlocal, multiplane plasticity model (see, for example, Batdorf and Budianski [3], Curran et al [4], Bazant et al [5,6]). FRAGBED proved to be useful in computational simulations and associated interpretations of penetration experiments in which ceramic armors were attacked by long rod penetrators [2].