David Henann

A predictive, size-dependent continuum model for dense granular flows

Dense granular materials display a complicated set of flow properties, which differentiate them from ordinary fluids. Despite their ubiquity, no model has been developed that captures or predicts the complexities of granular flow, posing an obstacle in industrial and geophysical applications. Here we propose a 3D constitutive model for well-developed, dense granular flows aimed at filling this need. The key ingredient of the theory is a grain-size-dependent nonlocal rheology—inspired by efforts for emulsions—in which flow at a point is affected by the local stress as well as the flow in neighboring material. The microscopic physical basis for this approach borrows from recent principles in soft glassy rheology. The size-dependence is captured using a single material parameter, and the resulting model is able to quantitatively describe dense granular flows in an array of different geometries. Of particular importance, it passes the stringent test of capturing all aspects of the highly nontrivial flows observed in split-bottom cells—a geometry that has resisted modeling efforts for nearly a decade. A key benefit of the model is its simple-to-implement and highly predictive final form, as needed for many real-world applications.

Metallic glasses: viable tool materials for the production of surface microstructures in amorphous polymers by micro-hot-embossing

Metallic glasses possess unique mechanical properties which make them attractive materials for fabricating components for a variety of applications. For example, the commercial Zr-based metallic glasses possess high tensile strengths (?2.0 GPa), good fracture toughnesses (?10?50 MPa) and good wear and corrosion resistances. A particularly important characteristic of metallic glasses is their intrinsic homogeneity to the nanoscale because of the absence of grain boundaries. This characteristic, coupled with their unique mechanical properties, makes them ideal materials for fabricating micron-scale components, or high-aspect-ratio micro-patterned surfaces, which may in turn be used as dies for the hot-embossing of polymeric microfluidic devices. In this paper we consider a commercially available Zr-based metallic glass which has a glass transition temperature of Tg ? 350??C and describe the thermoplastic forming of a tool made from this material, which has the (negative) microchannel pattern for a simple microfluidic device. This tool was successfully used to produce the microchannel pattern by micro-hot-embossing of the amorphous polymers poly(methyl methacrylate) (Tg ? 115??C) and Zeonex-690R (Tg ? 136??C) above their glass transition temperatures. The metallic glass tool was found to be very robust, and it was used to produce hundreds of high-fidelity micron-scale embossed patterns without degradation or failure.

Theta-like specimens for measuring mechanical properties at the small-scale: effects of non-ideal loading

Abstract As material structures, and devices fabricated from them, shrink to the nano-scale, it is ever more difficult to measure accurately the mechanical properties needed for designing small-scale devices and for assessing reliability. A promising test configuration for localized, small-scale uniaxial tensile measurements is the theta specimen. Stress distributions in this test configuration are examined via finite element analysis. When loaded uniformly with a double-articulating loading platen, a uniform uniaxial stress state is realized in the test gauge. When alternative, more easily realized experimentally, loading configurations are used (i. e., line or point contact), large secondary stresses develop. For each loading configuration, influences of non-ideal load application are elucidated. Two design modifications are considered to mitigate undesirable stresses: a compliant layer and an extended upper loading tab. The latter yields the most favorable stress state and is the most tolerant to non-ideal loading.

Exploring phononic crystal tunability using dielectric elastomers

Tunable phononic crystals give rise to interesting opportunities such as variable-frequency vibration filters. By using soft dielectric elastomers, which undergo large deformations when acted upon by an external electric field, the frequency ranges of these band gaps may be adjusted, or new band gaps may be created through electrical stimuli. In this talk, we will discuss our finite-element-based numerical simulation capability for designing electrically-tunable, soft phononic crystals. The key ingredients of our finite-element tools are (i) the incorporation of electro-mechanical coupling, (ii) large-deformation capability, and (iii) an accounting for inertial effects. We present a demonstration of our simulation capability to the design of phononic crystals consisting of both square and hexagonal arrays of circular-cross-section threads embedded in a dielectric elastomeric matrix. Finally, we will consider electro-mechanical instabilities as alternative route for enhanced tunability. [This work was curr...

Microscale Thermoplastic Forming of Bulk Metallic Glasses: Numerical Simulations and Experiments

An extremely promising microscale processing method for bulk metallic glasses called thermoplastic forming has emerged in recent years. However, most of the recent experimental thermoplastic forming studies have been conducted by trial-and-error. In this paper, the large-deformation constitutive theory of Henann and Anand [1] is used as a numerical simulation tool for the design of micro-hot-embossing processes. This numerical simulation capability is used to determine appropriate processing parameters in order to carry out several successful micron-scale hot-embossing operation on the metallic glass Zr41.2 Ti13.8 Cu12.5 Ni10 Be22.5 (Vitreloy-1). By carrying out the corresponding physical experiments, it is demonstrated that microscale features in Vitreloy-1 may be accurately replicated under the processing conditions determined by use of the numerical simulation capability.© 2008 ASME

Microscale Thermoplastic Forming of Bulk Metallic Glasses: Numerical Simulation and Experiments

An extremely promising microscale processing method for bulk metallic glasses called thermoplastic forming has emerged in recent years. However, most of the recent experimental thermoplastic forming studies have been conducted by trial-and-error. In this paper, we use the large-deformation constitutive theory of Henann and Anand [1] as a numerical simulation tool for the design of a micro-hot-embossing process. This numerical simulation capability is used to determine appropriate processing parameters in order to carry out a successful micron-scale hot-embossing operation on the metallic glass Zr 41.2 Ti 13.8 Cu 12.5 Ni 10 Be 22.5 (Vitreloy-1). By carrying out a corresponding physical experiment, we demonstrate that micron-scale features in Vitreloy-1 may be accurately replicated under the processing conditions determined by use of the numerical simulation capability.