They believe the concept could have several applications. One example is inflatable space structures, which are being evaluated for use with the International Space Station. Researchers could reinforce these structures with flexible composite webbing containing redundant load paths to help them better withstand an accidental internal pressure pulse. Armanios and Dancila also see potential applications in the reinforcement of cables, tethers, mountain climbing ropes, parachutes and possibly crashworthy helicopter seat restraints.
The researchers conceived the redundant load path concept from observing the tough tearing response of a mesh bag, the kind that is often used to package fruit. When subject to load, it tears at one location and then another as applied force is increased. Finally, it gives in and fails completely, Dancila explains. In their experiments testing the concept, the researchers are using glass-fiber-reinforced packaging tape to create a tailored structure with redundant load paths. They apply varying amounts of load to it and observe (with a slow motion camera) a succession of partial failures along the redundant load paths before overall structural failure. The redundant load paths keep the load almost constant and require more energy to break the entire structure.
In other work, Dancila and Armanios have been awarded a patent for an efficient concept using a piezoelectric actuator to control airflow over aircraft wings and other lifting surfaces. The actuator is made from piezoelectric material, which deforms in response to electrical stimulation. This deformation modulates a jet of air in intensity and direction, complementing traditional methods of controlling airflow.
Armanios and former Ph.D. student David Andrew Hooke also hold two patents for a method of and device for testing elastically tailored composite structures that both extend and twist in response to force. These structures could improve the performance of helicopters. Other potential applications are for spoilers in race and sports cars, and in sporting goods that are tailored to the user's abilities.
Investigations of structural materials for automobiles — specifically cast aluminum alloys — are the focus of Georgia Tech research by materials scientists/engineers Dr. Arun Gokhale and Dr. David McDowell. Because they cannot stretch much without breaking, cast aluminum alloys traditionally have not been used in automotive frame assemblies, despite the potential for substantial vehicle weight savings and enhanced fuel economy. Their objective is to establish quantitative relations between the cast aluminum microstructure and mechanical properties, such as strength, toughness and fatigue resistance. Knowledge about such relationships could improve foundry practice, enhance design methods and allow use of these alloys in automobile frames within the next decade.
photo by Gary Meek
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Materials researcher Dr. Joe Cochran and his colleagues are using conventional powder processing techniques to form linear cellular structures from non-metallic powder precursors. The research holds promise for the creation of high-strength, lightweight structures, which can be manufactured in high volume at a low cost. One initial use for the materials is in aircraft heat exchangers.
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McDowell — who holds the Carter N. Paden Jr. Distinguished Chair in Metals Processing — is conducting finite element simulations at Georgia Tech and in collaboration with Sandia National Laboratories on representative microstructures. This work includes analyses of actual digitized microstructure images obtained in Gokhale's laboratory. From these analyses, researchers can build engineering models for the response of cast aluminum alloys at microstructural-length scales. The simulations divide the material into small pieces, known as microstructural volume elements, and assign linear or nonlinear phase properties to each element. This procedure helps researchers understand the way failure processes originate and develop under applied loading conditions.
With this data, Gokhale and McDowell are developing next-generation models for microstructure-fracture toughness-fatigue property relations. These models reveal useful information about the role of particles with a diameter much less than that of a human hair, for example. This information, in turn, explains much about the formation and growth of fatigue cracks in cast aluminum structures. With models that incorporate such microstructural details, foundries that cast aluminum parts can improve the performance of these alloys and understand cost-performance tradeoffs.
A new generation of structural materials is also being developed for use in aircraft heat exchangers and in electronic devices, among other applications. A team of materials scientists/engineers and mechanical engineers led by Dr. Joe Cochran is developing linear metal structures with a cellular, or honeycomb-like, interior. The researchers are using conventional powder processing techniques to form the structures from non-metallic powder precursors, principally oxides. The structures are then fired in hydrogen to convert them to metals.
The research, funded by the U.S. Navy and Defense Advanced Research Projects Agency (DARPA), holds promise for the creation of high-strength, lightweight structures, which can be manufactured in high volume at a low cost. These structures, which can be converted to a number of different metals — including stainless steel and copper — offer three to four times greater strength than similar current lightweight structures, the researchers report. These linear cellular materials also provide excellent heat transfer at low weight with low pressure drop — making them good heat exchangers. And the materials are quite durable and adaptable in shape. One other advantage of linear cellular materials is their ability to absorb energy.
courtesy Dr. Christopher Lynch
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A process called optical birefringence yielded these color fringes. Researchers use the fringes to visualize stress and electric field concentrations associated with flaws and cracks in a transparent piezoelectric material, which deforms in response to electrical stimulation. In this experiment, mechanical engineering Assistant Professor Dr. Christopher Lynch applied electric field to a notched specimen. The resulting contours of field concentration are shown.
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An additional application for the new materials could be in the next-generation space shuttle, where the researchers' triangular-celled tiles might serve as the thermal protection system to replace ceramic tiles that line the current shuttle's exterior. NASA's proposal calls for hexagonal-cell honeycomb core tiles where cells run perpendicular to shuttle face sheets. But the Tech researchers' triangular-cell sheets would run the cells parallel to the face sheets. Potential advantages include increased strength and impact resistance, lighter weight and substantially reduced manufacturing cost.
In other research that may be applied to aircraft and automotive structures, mechanical engineer Dr. Christopher Lynch is characterizing, modeling and testing the reliability of piezoelectric materials. These materials change shape in response to an applied voltage and develop a voltage in response to applied mechanical loads. They are being used in active and adaptive structures, such as sensors and actuators for active damping of vibration (in aircraft, automobiles and sports equipment, for example) and for electronically controlled shape changes.
In research funded by DARPA through Northrop Grumman's Smart Wing Program, Lynch is using piezoelectric and shape memory materials to change the shape of a wing without using a hinged control surface. The hinge point in conventional control surfaces induces flow separation and increases drag. Preliminary wind tunnel testing indicates that elimination of the hinge significantly reduces drag.
Placing piezoelectric materials in these long-lifetime, high-cycle applications places stringent reliability requirements on the materials. With a grant from the National Science Foundation, Lynch is studying the materials' constitutive behavior (stress/strain/electric field relations) and fracture behavior. Lynch has detailed the material's characterization and developed finite element models to analyze stress and electric field concentrations that lead to dielectric breakdown and cracking.
Recent successful commercial applications of piezoelectric materials include vibration suppression in high-end K2 skis and active damping control in shocks for mountain bikes. As this research continues its transition to the commercial market, piezoelectric materials will be used to reduce noise in automobiles and aircraft, to reduce vibration in trains and to increase the efficiency of internal combustion engines by accurately controlling fuel injection.
— Jane M. Sanders
For more information, contact:
(1) Dr. Erian Armanios, School of Aerospace Engineering, Georgia Tech, Atlanta, GA 30332-0150. (Telephone: 404-894-8202) (E-mail: erian.armanios@aerospace.gatech.edu);
(2) Dr. David McDowell, School of Mechanical Engineering, Georgia Tech, Atlanta, GA 30332-0405. (Telephone: 404-894-5128) (E-mail: david.mcdowell@me.gatech.edu);
(3) Dr. Arun Gokhale, School of Materials Science and Engineering, Georgia Tech, Atlanta GA 30332-0245. (Telephone: 404-894-2887) (E-mail: arun.gokhale@mse.gatech.edu);
(4) Dr. Joseph Cochran, School of Materials Science and Engineering, Georgia Tech, Atlanta, GA 30332-0245. (Telephone: 404-894-6104) (E-mail: joe.cochran@mse.gatech.edu);
(5) Dr. Christopher Lynch, School of Mechanical Engineering, Georgia Tech, Atlanta, GA 30332-0405. (Telephone: 404-894-6871) (E-mail: christopher.lynch@me.gatech.edu);
(6) Dr. Stefan Dancila, School of Aerospace Engineering, Georgia Institute of
Technology, Atlanta, GA 30332-0150. (Telephone: 404-894-8197) (E-mail:
stefan.dancila@aerospace.gatech.edu)
