Publications

Abstract not provided.

Biological tissues are uniquely structured materials with technologically appealing properties. Soft tissues such as skin, are constructed from a composite of strong fibrils and fluid-like matrix components. This was the first coordinated experimental/modeling project at Sandia or in the open literature to consider the mechanics of micromechanically-based anisotropy and viscoelasticity of soft biological tissues. We have exploited and applied Sandia's expertise in experimentation and mechanics modeling to better elucidate the behavior of collagen fibril-reinforced soft tissues. The purpose of this project was to provide a detailed understanding of the deformation of ocular tissues, specifically the highly structured skin-like tissue in the cornea. This discovery improved our knowledge of soft/complex materials testing and modeling. It also provided insight into the way that cornea tissue is bio-engineered such that under physiologically-relevant conditions it has a unique set of properties which enhance functionality. These results also provide insight into how non-physiologic loading conditions, such as corrective surgeries, may push the cornea outside of its natural design window, resulting in unexpected non-linear responses. Furthermore, this project created a clearer understanding of the mechanics of soft tissues that could lead to bio-inspired materials, such as highly supple and impact resistant body armor, and improve our design of human-machine interfaces, such as micro-electrical-mechanical (MEMS) based prosthetics.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

This paper presents constitutive models for the anisotropic, finite-deformation viscoelastic behavior of soft fiber-reinforced composites. An essential assumption of the models is that both the fiber reinforcements and matrix can exhibit distinct time-dependent behavior. As such, the constitutive formulation attributes a different viscous stretch measure and free energy density to the matrix and fiber phases. Separate flow rules are specified for the matrix and the individual fiber families. The flow rules for the fiber families then are combined to give an anisotropic flow rule for the fiber phase. This is in contrast to many current inelastic models for soft fiber-reinforced composites which specify evolution equations directly at the composite level. The approach presented here allows key model parameters of the composite to be related to the properties of the matrix and fiber constituents and to the fiber arrangement. An efficient algorithm is developed for the implementation of the constitutive models in a finite-element framework, and examples are presented examining the effects of the viscoelastic behavior of the matrix and fiber phases on the time-dependent response of the composite.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Long-term reliability testing of Micro-Electro-Mechanical Systems (MEMS) is important to the acceptance of these devices for critical and high-impact applications. In order to make predictions on aging mechanisms, these validation experiments must be performed in controlled environments. Additionally, because the aging acceleration factors are not understood, the experiments can last for months. This paper describes the design and implementation of a long-term MEMS reliability test bed for accelerated life testing. The system is comprised of a small environmental chamber mounted on an electrodynamic shaker with a laser Doppler vibrometer (LDV) and digital camera for data collection. The humidity and temperature controlled chamber has capacity for 16 MEMS components in a 4×4 array. The shaker is used to dynamically excite the devices using broadband noise, chirp or any other programmed signal via the control software. Driving amplitudes can be varied to maintain the actuation of the test units at the desired level. The actuation is monitored optically via the LDV which can report the displacement or velocity information of the surface. A springmass accelerated aging experiment was started using a controlled environment of 5000 ppmv humidity (roughly 13% at room temperature), temperature of 29 °C, and ±80μm maximum displacement of the mass. During the first phase of the experiment, the resonant frequency was measured every 2 hours. From 114.5 to 450 hours under stress, measurements were taken every 12 hours and after that every 24 hours. Resonant frequency tracking indicates no changes in the structures for 4200 hours of testing.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

The present study examines the strain-rate sensitivity of four high strength, high-toughness alloys at strain rates ranging from 0.0002 s-1 to 200 s-1: Aermet 100, a modified 4340, modified HP9-4-20, and a recently developed Eglin AFB steel alloy, ES-1c. A refined dynamic servohydraulic method was used to perform tensile tests over this entire range. Each of these alloys exhibit only modest strain-rate sensitivity. Specifically, the strain-rate sensitivity exponent m, is found to be in the range of 0.004-0.007 depending on the alloy. This corresponds to a {approx}10% increase in the yield strength over the 7-orders of magnitude change in strain-rate. Interestingly, while three of the alloys showed a concominant {approx}3-10% drop in their ductility with increasing strain-rate, the ES1-c alloy actually exhibited a 25% increase in ductility with increasing strain-rate. Fractography suggests the possibility that at higher strain-rates ES-1c evolves towards a more ductile dimple fracture mode associated with microvoid coalescence.

Abstract not provided.

Abstract not provided.