Publications

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Glass forming materials like polymers exhibit a variety of complex, nonlinear, time-dependent relaxations in volume, enthalpy and stress, all of which affect material performance and aging. Durable product designs rely on the capability to predict accurately how these materials will respond to mechanical loading and temperature regimes over prolonged exposures to operating environments. This cannot be achieved by developing a constitutive framework to fit only one or two types of experiments. Rather, it requires a constitutive formalism that is quantitatively predictive to engineering accuracy for the broad range of observed relaxation behaviors. Moreover, all engineering analyses must be performed from a single set of material model parameters. The rigorous nonlinear viscoelastic Potential Energy Clock (PEC) model and its engineering phenomenological equivalent, the Simplified Potential Energy Clock (SPEC) model, were developed to fulfill such roles and have been applied successfully to thermoplastics and filled and unfilled thermosets. Recent work has provided an opportunity to assess the performance of the SPEC model in predicting the viscoelastic behavior of an inorganic sealing glass. This presentation will overview the history of PEC and SPEC and describe the material characterization, model calibration and validation associated with the high Tg (~460 °C) sealing glass.

To analyze the stresses and strains generated during the solidification of glass-forming materials, stress and volume relaxation must be predicted accurately. Although the modeling attributes required to depict physical aging in organic glassy thermosets strongly resemble the structural relaxation in inorganic glasses, the historical modeling approaches have been distinctly different. To determine whether a common constitutive framework can be applied to both classes of materials, the nonlinear viscoelastic simplified potential energy clock (SPEC) model, developed originally for glassy thermosets, was calibrated for the Schott 8061 inorganic glass and used to analyze a number of tests. A practical methodology for material characterization and model calibration is discussed, and the structural relaxation mechanism is interpreted in the context of SPEC model constitutive equations. SPEC predictions compared to inorganic glass data collected from thermal strain measurements and creep tests demonstrate the ability to achieve engineering accuracy and make the SPEC model feasible for engineering applications involving a much broader class of glassy materials.

The objective of this work was to understand the cracking of aluminum flame spray on an epoxy thermoset. In the experiments presented here, epoxy cylinders were uniformly coated with flame spray. The cylinders were put into a state of tensile stress by taking them to elevated temperatures and similarly put into a state of compression by taking them down to cold temperatures. Surface cracks on the outside of the cylinders were photographed and compared. The cylinders were cross-sectioned at room temperature to study how the aluminum surface cracks propagate into the epoxy. It was shown that thicker aluminum generates observable surface cracks at a lower temperature than a thinner coating does. The surface cracks cannot be seen at room temperature. However, some of the coating cracks propagate into the substrate and can be seen at room temperature when the cylinder is crosssectioned. The substrate cracks tend to be deeper with a larger coating thickness. Similarly, cracks are deeper when the substrate with a given thickness is taken to higher temperature. Supplementary examples that contain the addition of a hard inclusion (copper strip) between the aluminum and epoxy substrate at elevated temperatures were discussed, as well as delamination of the aluminum film at cold temperature.

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The performance and reliability of many mechanical and electrical components depend on the integrity of po lymer - to - solid interfaces . Such interfaces are found in adhesively bonded joints, encapsulated or underfilled electronic modules, protective coatings, and laminates. The work described herein was aimed at improving Sandia's finite element - based capability to predict interfacial crack growth by 1) using a high fidelity nonlinear viscoelastic material model for the adhesive in fracture simulations, and 2) developing and implementing a novel cohesive zone fracture model that generates a mode - mixity dependent toughness as a natural consequence of its formulation (i.e., generates the observed increase in interfacial toughness wi th increasing crack - tip interfacial shear). Furthermore, molecular dynamics simulations were used to study fundamental material/interfa cial physics so as to develop a fuller understanding of the connection between molecular structure and failure . Also reported are test results that quantify how joint strength and interfacial toughness vary with temperature.

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Multilayer coextrusion is applied to produce a tape containing layers of alternating electrical properties to demonstrate the potential for using coextrusion to manufacture capacitors. To obtain the desired properties, we develop two filled polymer systems, one for conductive layers and one for dielectric layers. We describe numerical models used to help determine the material and processing parameters that impact processing and layer stability. These models help quantify the critical ratios of densities and viscosities of the two layers to maintain stable layers, as well as the effect of increasing the flow rate of one of the two materials. The conducting polymer is based on polystyrene filled with a blend of low-melting-point eutectic metal and nickel particulate filler, as described by Mrozek et al. (2010). The appropriate concentrations of fillers are determined by balancing measured conductivity with processability in a twin screw extruder. Based on results of the numerical models and estimates of the viscosity of emulsions and suspensions, a dielectric layer composed of polystyrene filled with barium titanate is formulated. Despite the fact that the density of the dielectric filler is less than the metallic filler of the conductive phase, as well as rheological measurements that later showed that the dielectric formulation is not an ideal match to the viscosity of the conductive material, the two materials can be successfully coextruded if the flow rates of the two materials are not identical. A measurable capacitance of the layered structure is obtained.

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Internal residual stresses and overall mechanical properties of thermoset resins are largely dictated by the curing process. It is well understood that fiber Bragg grating (FBG) sensors can be used to evaluate temperature and cure induced strain while embedded during curing. Herein, is an extension of this work whereby we use FBGs as a probe for minimizing the internal residual stress of an unfilled and filled Epon 828/DEA resin. Variables affecting stress including cure cycle, mold (release), and adhesion promoting additives will be discussed and stress measurements from a strain gauge pop-off test will be used as comparison. 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.