Gary L. Gray

Nonlinear Spacecraft Dynamics with a Flexible Appendage, Damping, and Moving Internal Submasses

We study the attitude dynamics of a single-body spacecraft that is perturbed by the motion of small oscillating submasses, a small e exible appendage constrained to undergo only torsional vibration, and a rotor immersed in a viscous e uid. We are interested in the chaotic dynamics that can occur for certain sets of the physical parameter values of the spacecraft when energy dissipation acts to drive the body from minor to major axis spin. Energy dissipation, which is present in all spacecraft systems and is the mechanism that drives the minor to major axis transition, is implemented via the rotor. We not only obtain an analytical test for chaos in terms of satellite parametersusing Melnikov’ s method, but wealso use extensivenumericalsimulation to check the rangeof validity of the Melnikov result.

A classical mechanics approach to the determination of the stress–strain response of particle systems

This paper presents the numerical implementation of a Lagrangian-based approach for the determination of the stress–strain behaviour of solids via molecular dynamics (MD). This approach is based on continuum homogenization and it offers a framework in which the notions of effective stress and effective deformation for a particle system can be said to have the same meaning that they have in a continuum context. Since the effective stress response of the system is not based on the notion of virial stress, the paper presents three MD calculations to demonstrate how the continuum-based notion of effective stress differs from that of virial stress.

On the notion of average mechanical properties in MD simulation via homogenization

This paper reviews the continuum notions of average mechanical properties such as stress and strain and the meaning of such notions when molecular dynamics (MD) is used to compute them. The focus of this paper is a discussion of how boundary conditions are actually enforced in MD and what corresponding notion of averages is therefore appropriate. Finally, we discuss the concepts as they relate to the Parrinello–Rahman approach as a specific example.

Shape memory alloy coils optimized for electrical connectors

In an effort to reduce costs associated with automotive electrical connectors, auto manufacturers have looked to tin-coated terminals as an effective alternative to more expensive gold-coated terminals. Tin, however, is highly subject to a wear phenomenon known as fretting corrosion, which increases the contact resistance and renders the terminals useless. One solution to minimize fretting corrosion is to increase the terminal normal force. In ordinary electrical terminals, an increased normal force leads to other problems such as high insertion and removal forces. This work is a continuation of previous research in which a Ni-Ti shape memory alloy (SMA) coil was developed in order to increase the high temperature normal force of tin-coated terminals while maintaining moderate room temperature insertion forces. In particular, this work addresses the cyclic stability of the SMA coil when subjected to repeated temperature cycles over extended periods, the maximization of normal force provided by the coil for the amount of SMA used, and the long-term high-temperature performance of the SMA. This research has resulted in the reduction of the SMA wire diameter by nearly 30%, an increase in high temperature removal force of the terminal from approximately 20 N without the SMA to 60 N with it, and an associated reduction in cost by nearly 50% over the previous design. To accomplish these improvements, the SMA composition was changed from 55.1 wt% Ni to 49.7 wt% Ni, and the optimum training temperature for the new composition was found to be 400/spl deg/C.

Performance of a shape memory alloy coil-shaped clamp for enhanced normal force in pin-and-receptacle electrical connectors

Electrical connectors can experience a significant degradation in performance due to fretting corrosion, which is particularly problematic when the connector undergoes vibration. Fretting corrosion can be reduced or eliminated if the relative motion between the pin and receptacle is reduced or eliminated. One practical means of reducing the relative motion is to increase the normal force between the pin and receptacle. Unfortunately, increasing the normal force has traditionally implied increasing the insertion and removal forces required for mating and separation of the pin and receptacle. In this paper we describe a connector in which the normal force between the pin and receptacle is increased after the connector is mated and decreased before separation. We implement our approach for connectors that are put in service in elevated temperature environments by placing a coiled shape memory alloy (SMA) wire around the receptacle part of a standard pin-and-receptacle connector. We experimentally demonstrate the efficacy of this approach by quantifying the increase in normal force between the pin and receptacle and by showing a substantial increase in connector lifetime in a vibratory environment.

Shape memory alloy coil-shaped clamp for enhanced normal force in electrical connectors

Many electrical connectors experience a significant degradation in performance due to fretting corrosion, which is particularly problematic when the connector undergoes vibration. Fretting corrosion can be reduced or eliminated if the relative motion between the pin and receptacle is reduced or eliminated. One practical means of reducing the relative motion is to increase the normal force between the pin and receptacle. Unfortunately, increasing the normal force has traditionally implied increasing the insertion and removal forces required for mating and separation of the pin and receptacle. In this paper we propose a unique solution to the linked problems of fretting corrosion and insertion/removal forces. Using our approach, the normal force between the pin and receptacle is increased after the connector is mated and decreased before separation. We implement our approach for connectors that are put in service in elevated temperature environments by placing a coiled shape memory alloy (SMA) wire around the receptacle part of the connector. We will describe our concept and discuss our early experiments in which we demonstrate the effect of the SMA on the normal force under realistic conditions. In addition, we will describe future experiments designed to simulate lifetime tests on these connectors undergoing vibrations and to ascertain the effects of a large number of temperature cycles on the properties of the SMA.

Estimation of intrinsic stresses and elastic moduli in thin films

An approach is presented for the determination of the residual stresses and elastic moduli of particle systems resulting from computer simulations of particle or atomic deposition. The proposed technique is based on fundamental concepts of elasticity and is capable of capturing the variation of stresses and moduli as functions of position within the system. Application to a perfect FCC crystal and a simple particle system is demonstrated.

A Micromechanics-Based Notion of Stress for Use in the Determination of Continuum-Level Mechanical Properties via Molecular Dynamics

By formulating a continuum homogenization problem that includes inertia effects, a link is established between continuum homogenization and the estimation of effective mechanical properties for particle ensembles whose interactions are governed by potentials (e.g., as is seen in molecular dynamics). The focus of this chapter is on showing that there is a fundamental consistency of ideas between continuum mechanics and the study of discrete particle systems, and that it is possible to define a notion of effective stress applicable to discrete systems that can be claimed to have the same meaning as it has in continuum mechanics.

Optimization of shape memory alloys for use in electrical connectors

In an effort to reduce costs associated with automotive electrical connectors, auto manufacturers have looked to tin-coated terminals as an effective alternative to more expensive gold-coated terminals. Tin, however, is highly subject to a wear phenomenon known as fretting corrosion, which increases the contact resistance and renders the terminals useless. One solution to minimize fretting corrosion is to increase the terminal normal force. In ordinary electrical terminals, an increased normal force leads to other problems, such as high insertion and removal forces. This work is a continuation of previous research in which a Ni-Ti shape memory alloy (SMA) coil was developed in order to increase the high temperature normal force of tin-coated terminals while maintaining moderate room temperature insertion forces. In particular, this work addresses the cyclic stability of the SMA coil when subjected to repeated temperature cycles over extended periods, the maximization of normal force provided by the coil for the amount of SMA used, and the long-term high-temperature performance of the SMA. This research has resulted in the reduction of the SMA wire diameter by nearly 30%, an increase in high temperature removal force of the terminal from approximately 20 N without the SMA to 60 N with it, and an associated reduction in cost by nearly 50% over the previous design. To accomplish these improvements, the SMA composition was changed from 55.1 wt% Ni to 49.7 wt% Ni, and the optimum training temperature for the new composition was found to be 400/spl deg/C.