In-Situ Curing and On-Line Consolidation of Polymer Composites
An efficient method for manufacturing large, high-performance composite shells and structures is a process called filament winding. The purpose of this project is to investigate the use of local infrared heating to continuously-cure filament-wound composite structures. Fundamental to proving the viability of this novel process is understanding the radiative heat transfer upon the winding's surface and its affect upon the resin's cure kinetics, optical characteristics, and mechanical properties. In order to predict the thermophysical properties of the composite tape, a generic model, based in electromagnetic theory, of radiative heat transfer through a material in which dependent scattering occurs (as in a composite due to its aligned microstructure) will be developed. A thermomechanical model, which was developed for hoop winding and which can be used to predict the process window as a function of winding speed and heat deposition rate, will be extended to include realistic winding patterns. "Proof-of-principle" experiments will be conducted using an existing pilot-scale infrared oven and a bench-scale filament-winder currently being designed and built. Comparisons will be made as to the efficiency of continuous curing versus batch curing.
This work will done in collaboration with Dr. John R. Howell, a radiation heat transfer expert at UT-Austin.
Process-Induced Fiber- and Ply-Waviness in Composites
Current fabrication techniques for producing composite cylinders and shells frequently introduce fiber- and ply-waviness, a local microbuckling phenomenon, in the structure. This phenomenon and the attendant complex residual stress distributions have been blamed for drastically reduced structural strengths in thick-section composite cylinders subjected to compressive loading; in fact, the residual stresses can be high enough to fail a part prior to any in-service loading.
The primary objective of this project is to theoretically and experimentally describe the development of fiber- and ply-waviness within continuous-fiber polymeric composites during processing, such as by filament- and tape-winding. The effects of cure and/or thermal shrinkage and cumulative tow tension upon the process-induced waviness in composite laminates will be investigated. The local "microbuckling" can be described in terms of a characteristic amplitude and wavelength . A two-dimensional, linear viscoelastic, micromechanical model will be developed in order to predict ( , ) for a given resin system. For verification/refinement of the model, -ply and -ply/-ply test panels of a prototypic thermoplastic and a thermoset will be stage cured/cooled and then subjected to creep tests (using a range of compressive stresses simulating those imposed during processing) and the resulting ( , ) measured. Results will be further compared with wavelengths observed in thick-section, filament-wound cylinders which have been produced using either batch or in-situ IR curing (see In-Situ Curing above). This work will ultimately lead to the development of criteria for the mechanical properties of the resin system and recommended cool-down cycles in order to maximize the compressive strength of composite structures.
Mass and Heat Transfer during Gas-Metal-Arc Welding
In gas-metal-arc welding (GMAW), the weld filler metal is obtained by melting a bare wire electrode with heat generated in an electric arc, established by the power supply, between the bare wire electrode and the workpiece. Molten droplets of metal detach from the electrode tip, traverse the narrow arc gap and collect in the weld pool.
The primary objective of the research is to construct a geometrically realistic, internally-consistent model which describes the mass and heat transfer due to the combined electrical, mechanical, and thermal effects at the electrode tip of GMAW. It will depict the drop formation process as a function of the welding current, arc length, weld metal, wire diameter, and burn-off rate. As a paradigm, argon-shielded droplet formation and detachment from a typical mild-steel, positive- (or reverse-) polarity electrode carrying a direct electric current corresponding to spray transfer (250-300 A) will be considered. An estimate of the electrode shape will be obtained using conservation of energy subject to prescribed heat fluxes due to electron condensation, radiation, convection, and conduction. The drop transfer frequency will be predicted using a time-marching, prediction-correction scheme in which the drop grows according to the prescribed burn-off rate. Liquid-metal motion and pressure within the droplet will be determined from the magnetohydrodynamic equations, which will be solved using a Svanberg vorticity-streamfunction formulation. Subsequent to validation with experimental results, this model will guide the development of simpler, although congruent process models for sensing and control of GMAW systems (see Process Control Models below).
Process Control Models for Automation of Gas-Metal-Arc Welding Systems
Mechanization of the GMAW process requires sophisticated control systems. Integral to these control systems are process models which, using feedback from the sensors, calculate the appropriate manipulated variables for the welding parameters such as electrode feed rate, welding current, arc length, etc. These process models must define how variations in the individual welding parameters (such as electrode polarity, electrode extension, electrode composition, shielding-gas chemical activity) alters the fluid flow and heat transfer in the weld area. A direct extension of that described above, this project will involve developing simplified process models which capture the physics of the full model and can be used for real-time control.
This work will done in collaboration with Dr. Glenn Y. Masada, a control systems expert at UT-Austin.
Production of Metal Powders using Applied Magnetic Fields
Fine and ultrafine powders are increasingly important to the metals industry for applications in powder metallurgy and plasma spraying. Current atomization techniques, which make use of jets of inert gases (or simply water) to break-up a stream of molten metal, produce a distribution of particle sizes, not all of which are useful. Sifting is almost always necessary in order to stratify the powder into size categories. If a narrow size range of powder is needed, yields can be low (10%) and much material must be reworked (if it can be at all). A significant cost savings could be achieved even if the yields could be just doubled.
Ideally, powder producers would like a process for controlling the size of particles produced. One method may be to capitalize on the fact that liquid metal streams carrying axial electric currents are intrinsically unstable. The objective of this project is to analytically and experimentally investigate the effect of imposing a strong magnetic field on the molten stream in order to preferentially atomize powders. A thermomechanical model will be developed, guided by experiments, to predict the particle size and cooling rate as a function of the combined mechanical, thermal, and electrical effects. Particle microstructures will positively confirm the cooling rate predictions.
This work has involved duct flows through strong, uniform magnetic fields and linear stability analyses of high-velocity boundary layers. I have received funding from Argonne National Laboratory in support of some of this work. A large portion of this work was completed as a graduate student in the Theoretical and Applied Mechanics Department of the University of Illinois at Urbana under the direction of Professor John S. Walker. My dissertation research considered the large-radius elbow problem and then advanced to the sharp elbow problem, which became the bulk of my dissertation. I developed a novel hybrid numerical-analytical scheme for modeling liquid metal flow through a series of rectangular, thin walled ducts. This scheme has been subsequently used successfully to model a variety of liquid-metal duct flows, including a backward elbow, a simple manifold, an electromagnetic pump, and to study electromagnetic flow control between parallel ducts. Finally, the dissertation research provided the basis for the linear stability analyses for high-velocity boundary layers in thin-walled duct flows.
STRUCTURED MODELING--EXTENDED BOND GRAPHS
Due to my background in MHD, interest in electromechanical systems, and familiarity with bond graphs, I have become involved in an effort to develop extended bond graphs (EBG) for electromagnetic and electromechanical systems. In this endeavor, I have co-supervised a Ph.D. student to develop extended bond graphs for electromagnetic and electromechanical continua. This work was further extended to consider piezoelectric, thermopiezoelectric, and magnetostrictive materials.
STRUCTURED MODELING--EXTENDED BOND GRAPHS