Tech Briefs

Thermal Mechanical Fatigue Crack Growth Testing

Turbines in aircraft turbojet engines are being subject to increasingly higher temperatures to improve fuel efficiency. High turbine efficiency requires the ability of turbine blades to withstand gas temperature of the order of 1350 to 1550 °C. In polycrystalline materials, these increased temperatures would cause creep strains along grain boundaries that would be unacceptable. Even single-crystal materials must be pushed to their limits to insure that engine performance is maximized. Airfoils in modern gas turbine aircraft use a systems approach for cooling to achieve required component life. There are three basic components to these systems: a cast nickel single-crystal superalloy in combination with thermal barrier coatings, and a sophisticated cooling scheme consisting of intricately designed channels and holes through the core and surface of the airfoil.

Posted in: Briefs, Physical Sciences, Fatigue, Turbojet engines, Test procedures, Thermal testing

Evaluation of Three Extremity Armor Systems

The US Marine Corps and the US Army have been engaged in efforts to evaluate improved body armor, including armor to protect the extremities. These efforts are focused on both body armor performance (i.e., ballistic protection) and armor effects on the physical performance of personnel (e.g., body flexibility, mobility, and agility).

Posted in: Briefs, Physical Sciences, Kinematics, Defense industry, Protective clothing, Military vehicles and equipment

Low-Power Distributed Jammer Network

The Distributed Jammer Network (DJN) is composed of a large number of tiny, low-power jammers, which are distributed inside a target network and emit radio energy to disrupt its communications. Recent advancement in microelectromechanical system (MEMS) technology makes it possible to make jammers sufficiently small that a DJN can take the form of a dust suspending in the air, thus the name Jamming Dust. Miniaturization of jammers should be less challenging than that of wireless sensors since jammers just emit noise signal without requiring complex modulation, filtering, and other signal processing functions. Therefore, new miniature devices such as nanotube radio may find their first application in jamming dust.

Posted in: Briefs, Electronics & Computers, Microelectricmechanical device, Radio equipment, Telecommunications

Fundamentals of Metamaterials for High-Power RF Applications

Recent years have seen a burgeoning interest in the physics and applications of metamaterials. Briefly, metamaterials consist of a periodic array of subwavelength structures that produce an electromagnetic response not typically available in nature. Notably, composites of these structures in general exhibit both frequency and spatial dispersion, leading to a rich variety of physical phenomena such as backward wave propagation and a negative refractive index.

Posted in: Briefs, Electronics & Computers, Electromagnetic compatibility, Radio equipment, Composite materials, Materials properties

Thermal Analysis and Microhardness Mapping in Hybrid Laser Welds in a Structural Steel

The concept of combining a laser welding system with a conventional arc welding system (GTAW) was first proposed in order to improve the stability of the laser welding system and to allow greater flexibility in part fit-up. Prior research stated that using a YAG laser at a power of 3 kW, one was able to hybrid laser weld a 4-mm-thick aluminum alloy at a speed of 4 n/min. For a mild steel plate, butt welding at 1 m/min with 5 kW of 6-mm-thick plate was realized. Just as significantly as the weld speed was the ability to hybrid laser weld with gaps up to 1.5 mm in a plate 6 mm thick. The glass metal arc welding (GMAW) laser hybrid process can increase the gap bridging ability, i.e., it appreciably broadens the range of tolerances with regard to edge preparation quality. The arc’s energy input in the hybrid welding process also permits control of the cooling conditions. Via the keyhole, the laser beam brings about easier ignition of the arc, stabilization of the arc welding process, and penetration of the energy deep into the material. The improvement of the energy input leads to a greater welding depth and speed being achieved with the hybrid process compared with individual processes on their own.

Posted in: Briefs, Mechanical Components, Lasers, Welding, Steel, Thermal testing

Encapsulated Ball Bearings for Rotary Micro Machines

Microelectromechanical systems (MEMS)-fabricated silicon rotary elements for micro-motors, micro-generators, and micro-turbomachinery have received growing attention with applications in power conversion and actuation. Within these technologies, the bearing mechanism is the primary determinant of device performance and reliability. Active bearings, such as magnetic or electrostatic, have the advantage of being controlled during the operation, but at the cost of the accompanying circuitry. Passive bearings span a large range of velocities that include center-pin bushings with low revolution rates possible, and hydrostatic or hydrodynamic bearings with high revolution rates possible.

Posted in: Briefs, Mechanical Components, Microelectricmechanical device, Bearings

Development of High-Resolution Eddy Current Imaging Using an Electromechanical Sensor

An oscillating current flowing through a coil produces an oscillating magnetic field. When an electrically conducting material like a metal is brought close to the coil, the oscillating magnetic field produces eddy currents in the metallic material. The strength of the eddy current depends on the electrical conductivity of the material, the distance between the coil and the material, and the frequency of the excitation of the coil. The eddy currents in the electrical conductor produce a magnetic field opposing the magnetic field generated by the coil. The electrical impedance of the coil, placed in close proximity to a metal, is altered due to the eddy currents in the metal. Measurement of the change in the impedance is a method to determine the electrical conductivity of the metal.

Posted in: Briefs, Physical Sciences, Measurements, Sensors and actuators, Conductivity, Magnetic materials, Metals