Wright-Patterson Air Force Base

About: Wright-Patterson Air Force Base is a other organization based out in Wright-Patterson AFB, Ohio, United States. It is known for research contribution in the topics: Laser & Mach number. The organization has 5817 authors who have published 9157 publications receiving 292559 citations. The organization is also known as: Wright-Patterson AFB & FFO.

Frequency Control and Its Effect on the Dynamic Response of Flexible Structures

Abstract: The effect of structural optimization on optimal control design is studied in this paper. Structural optimiza- tion was treated as a problem of mass minimization with constraint on the open-loop frequency. The quadratic performance index, involving the state and control variables, was used in the design of the control system. A control system with only full-state feedback was considered. A procedure for generating the state and control weighting matrices by structural dynamics programs was outlined. By introducing simple scaling parameters, the weighting matrices were used effectively to achieve the desired control objectives. A number of case studies using a simple truss structure were made, in which vibration suppression with only initial disturbances was considered. The conclusion was that modification of the structural parameters (stiffness and structural mass) did not significantly alter the control design in this study. IBRATION control is an important consideration in the design of dynamic systems on the ground, in the air, and in space. The disturbances in ground and air vehicles are primarily caused by rough road (runway) profiles and airflow, such as gusts and powerplants. Similarly, in large space structures the disturbances are the result of slew- ing/pointing maneuvers, thermal transients, and mechanical machinery such as coolers, generators, etc. Control of the dynamic response is essential for maintaining the ride quality and performance requirements, as well as for the safety of the structure. The response of a structure is basically governed by three sets of parameters. The mass, damping, and stiffness repre- sent the structural parameters. The second set of parameters is due to the sources of external disturbances. These are generally external to the system and are considered as fixed inputs; thus their alteration is not within the realm of the structures/controls designer. The third set represents the control system, assuming that the structure is actively con- trolled. Control of the dynamic response by modification of the structural parameters alone is considered to be passive. Passive control is most appealing from both the reliability and maintainability points of view, if it can be achieved at all economically. Basically, the stiffness and mass modifica- tions result in frequency and mode changes, while the damp- ing affects the dissipation energy of the system. The damping can be significantly altered by either viscoelastic coatings (or constrained layer damping) or the provision of discrete dashpot mechanisms. The objective of vibration control is to design the structure and its controls either to eliminate vibration completely or to reduce the mean square response of the system to a desired level within a reasonable span of time. In addition, it is im- portant that this objective be achieved in some optimal way. For a structural designer, the optimal design represents an

Oxidation Resistance of Hafnium Diboride Ceramics with Additions of Silicon Carbide and Tungsten Boride or Tungsten Carbide

Abstract: Dense samples of HfB2–SiC, HfB2–SiC–WC, and HfB2–SiC–WB were prepared by field-assisted sintering. The WB and WC additives were incorporated by solid solution into the HfB2 and the HfC that formed during sintering. Oxidation of the samples was studied using isothermal furnace oxidation between 1600° and 2000°C. Sample microstructure and chemistry before and after oxidation were analyzed by scanning electron microscopy and X-ray diffraction. The addition of WC and WB did not alter oxidation kinetics of the baseline HfB2–SiC composition below 1800°C; however, at 2000°C, HfB2–SiC–WC and HfB2–SiC–WB had oxide scales that were 30% thinner than the oxide scale of HfB2–SiC. It is believed that WC and WB promoted liquid-phase densification of the HfO2 scale, thereby reducing the path of oxygen ingress, during oxidation.