Recent advances in manufacturing of multifunctional materials have provided opportunities to develop structures that possess superior mechanical properties with other concurrent capabilities such as sensing, self-healing, electromagnetic and heat functionality. The idea is to fabricate components that can integrate multiple capabilities in order to develop lighter and more efficient structures. In this regard, due to their combined structural and electrical functionalities, electrically conductive carbon fiber reinforced polymer (CFRP) matrix composites have been used in a wide variety of applications in most of which they are exposed to unwanted impact-like mechanical loads. Experimental data have suggested that the application of an electromagnetic field at the moment of the impact can significantly reduce the damage in CFRP composites. However, the observations still need to be investigated carefully for practical applications. Furthermore, as the nature of the interactions between the electro-magneto-thermo-mechanical fields is very complicated, no analytical solutions can be found in the literature for the problem. In the present thesis, the effects of coupling between the electromagnetic and mechanical fields in electrically conductive anisotropic composite plates are studied. In particular, carbon fiber polymer matrix (CFRP) composites subjected to an impact-like mechanical load, pulsed electric current, and immersed in the magnetic field of constant magnitude are considered. The analysis is based on simultaneous solving of the system of nonlinear partial differential equations, including equations of motion and Maxwell's equations. Physics-based hypotheses for electro-magneto-mechanical coupling in transversely isotropic composite plates and dimension reduction solution procedures for the nonlinear system of the governing equations have been used to reduce the three-dimensional system to a two-dimensional (2D) form. A numerical solution procedure for the resulting 2D nonlinear mixed system of hyperbolic and parabolic partial differential equations has been developed, which consists of a sequential application of time and spatial integrations and quasilinearization. Extensive computational analysis of the response of the CFRP composite plates subjected to concurrent applications of different electromagnetic and mechanical loads has been conducted. The results of this work verify the results of the previous experimental studies on the subject and yield some suggestions for the characteristics of the electromagnetic load to create an optimum impact response of the composite.

The densification mechanisms of polycrystalline α-alumina by spark plasma sintering are highly contradictory, with different research groups suggesting diffusion to dislocation controlled mechanisms to be rate controlling. The specific objective of this work was to investigate densification mechanisms of α-alumina during the intermediate and final stages of sintering by SPS, analyze the microstructural development and establish sintering trajectories. In addition, zirconia and yttria were added in different weight percentages to study the effect of solute concentration on the densification kinetics of spark plasma sintered alumina. The present work adopts a different approach from the classical method adopted previously to analyze the sintering kinetics and densification mechanisms of alumina in SPS, although existing models for hot pressing were adopted for the basic analysis. The densification behavior was investigated in the temperature range 1223-1573 K under applied stresses of 25, 50 and 100 MPa and grain sizes between 100 and 250 nm. The SEM micrographs reveal equiaxed grains with no abnormal grain growth in the dense samples. The ‘master sintering curve’ shows grain size to be primarily dependent on density, irrespective of the applied stresses or temperature. The stress exponent of 1 along with an inverse grain size exponent of 3 and activation energy of 320-550 kJ mol-1 suggests Al3+ grain boundary diffusion as the rate controlling densification mechanism in alumina. The densification rates are marginally slower in compositions with 0.1% Y2O3 and ZrO2 content possibly due to the smaller grain sizes used in this study which leads to faster rates compared to earlier reports. However, higher Y2O3 and ZrO2 content led to decrease in densification rate by more than an order of magnitude possibly due to presence of a second phase which increases the effective path length for diffusion, thereby reducing the densification rates. Presence of Y2O3 and ZrO2 in the compositions with 0.1% Y2O3 and ZrO2 were confirmed by TEM studies. The Y3Al5O12 (YAG) phase developed between 1223 and 1273 K and suppressed densification and grain growth in alumina. In spite of higher temperatures required for alumina-YAG and alumina-zirconia composites to attain density ~99%, the alumina grain size in the composites was smaller than that in pure alumina due to the Zener drag effect. The stress exponents obtained for Y2O3 and ZrO2 composites at both the concentrations yield a value of n~ 2, which indicates a change in densification mechanism from pure alumina. The higher stress dependence of these composites could be due to presence of solute and second phase formation, both of which retard densification rates. The inverse grain size exponents obtained are between 1 and 2; both stress exponent and grain size exponent values suggest an interface reaction controlled diffusion mechanism occurring in these composites, independent of the Y2O3 and ZrO2 content. Higher activation energies are obtained with the Y2O3 and ZrO2 composites of higher content, respectively, due to presence of second phase particles at grain boundaries. The presence of solutes at grain boundaries hinders grain boundary diffusion of alumina, leading to interface reaction controlled process; this is confirmed by superimposing standard aluminum grain boundary and lattice diffusion data on to stress-densification rate data obtained in this work. A comparison of stress exponents using current experimental data adopting the present and the classical approaches show a wide difference in their values indicating a change in the rate controlling diffusion path, necessitating a review of the assumptions made on the basic equations used in previous SPS studies.