Marthinus C. Van Schoor

Material Damping in Aluminum and Metal Matrix Composites

The material damping in beam-like specimens of aluminum and metal matrix com posites was measured. A unique apparatus to determine damping by free decay while the specimens are in free fall in a vacuum was used. The specimens tested include 2024-T3 and 6061-T4 aluminum, and unidirectional graphite/metal matrix specimens with P55 and P100 fibers and 6061 Aluminum and AZ91C Magnesium as matrix materials. Tests were conducted to determine the dependence of damping on frequency and stress level. For the aluminum specimens, the material damping followed the Zener model at very low stress levels. Below the Zener relaxation frequency, a strong dependence of damping on stress was found for even moderate stress levels. Damping for the aluminum matrix materials was slightly above that predicted by the Zener model for a homogeneous bar of the matrix aluminum. For the magnesium matrix specimens, damping significantly above the Zener prediction for the homogeneous matrix material was observed.

Measurement of the modal parameters of a space structure in zero gravity

An analytic and experimental study of the changes in the modal parameters of space structural test articles from 1 to 0 g is presented. Deployable, erectable, and rotary modules were assembled to form three one- and two-dimensional structures in which variations in bracing wire and rotary joint preload could be introduced. The structures were modeled as if hanging from a suspension system in 1 g, and unconstrained, as if free floating in 0 g. The analysis is compared with ground experimental measurements made on a spring/wire suspension system with a nominal plunge frequency of 1 Hz and with measurements made on the Shuttle middeck. The degree of change in linear modal parameters, as well as the change in nonlinear nature of the response, is examined. Trends in modal parameters are presented as a function of force amplitude, joint preload, and ambient gravity level.

Hybrid Scaled Structural Dynamic Models and Their Use in Damping Prediction

Analytical and experimental techniques for the prediction and ground verification of the damped structural dynamics of space structures are developed. The options available for similarity-scaled model testing, including replica and multiple scale approaches, are reviewed. For the case when the distortion of potentially dissipative or nonlinear joints, which would be required in multiple-scale modeling, is impractical, a new type of modeling is introduced, which uses a hybrid of joints at replica scale and connecting elements at a modified multiple scale. The model design requirements for replica, multiple-scale, and hybrid models are developed, and the expected scaling of nonlinear dissipation in joints is derived. A damping prediction scheme is developed that relies on a finite element model of the undamped structure and measurements of the individual joint properties to predict the modal damping of the truss attributable to the joints. A hybrid-scaled model of a segment of the space station was built and dynamically tested. The predicted and measured truss damping compared favorably.

Latching valve control using ferromagnetic shape memory alloy actuators

Ferromagnetic Shape Memory Alloys (FSMA) are a class of active materials based on nickel alloys which offer controllable, large mechanical straining based on applied mechanical stress and magnetic fields. Actuation is based on crystallographic switching between meta-stable martensite phases. The high speed, binary switching, and no power hold behavior of the FSMA material are particularly well suited for latching valve applications such as hydraulic valves, optical switches, and electromechanical relays. This paper describes the design, development, and testing of a FSMA based actuator system to drive an industrial, hydraulic, latching valve. An opposing actuator configuration is used to switch the valve spool between spool positions, as well as to reset the opposing actuator element. Specific issues addressed in the design include FSMA material strain modeling, spool movement modeling, magnetic driving coil design and circuit controller design. A prototype system, based on a commercially available latching valve platform, was constructed and tested. Size of the complete system, including two FSMA actuators, valve body, and spool is 19 × 19 × 89 mm. Maximum valve actuation frequency of the prototype system is 133Hz in a 1000 psi hydraulic test bed.

Global structure modeling using force-state component identification

The nonlinear-dissipative modal behavior of a structure is predicted from measured component characteristics and compared with modal test data. The force-state component identification methodology is applied to multiaxis testing of deployable, erectable, and rotary joint components of the middeck 0-gravity dynamics experiment structure. Analytic models are are used to approximate the force-state data and are assembled into a global model. A linearization algorithm is used to predict frequency response of the model. Predictions correlate well with 0-gravity and ground test modal data that are shown to depend on joint preload, force amplitude, and gravity conditions. Moderately nonlinear behavior is both predicted and observed in the configuration containing the rotary joint.

Piezoelectric structural acoustic leak detection for pressurized pipelines

This paper describes the development and testing of a structural-acoustic sensing and alert system that continuously monitors a pipeline without the need of an external power source. This system is based on Mides patented PowerAct conformable packaged piezoelectric actuator and sensor. These sensors are extremely sensitive with very high gain and can detect the most minute and high frequency strains. Since leaks in high-pressure gas pipes fit this description, there is currently no better sensor to apply to the specific problem. The results of this effort led to the design of a system that can sense, locate and report leaks and impacts in a buried pipeline system. This system is estimated to cost less than

Borehole discontinuities for enhanced power generation

400 / km of pipe. In conclusion, the results of this paper are encouraging and indicate that it is feasible to use the PowerAct sensors in a cost effective system to locate impacts and leaks in pipelines.