Azar Parvizi-Majidi

Cured Shape of Cross-Ply Composite Thin Shells

For a composite laminated plate, it has been found that classical laminate theory (CLT) can not always predict the final cured shape correctly and geometric nonlinearity must be considered. For composite laminated shells, experiments show that the cured shape depends on stacking sequence, radius, thickness, and size. This paper investigates the cured shape of several cross-ply composite shells. The cured shape of a cross-ply shell is generally cylindrical. A model is established to predict the cured shape. The model is based on the Rayleigh-Ritz energy method and considers geometric nonlinearity. Modeling and experiment show that, for certain stacking sequences, the generator of the cured shape may be orthogonal to the original generator, while other stacking sequences can lead to deeper or shallower cured shapes. The predictions of the model are very close to the results of FEM analysis and experiment. This model can be used as a guide in the manufacture of unsymmetric cross-ply laminates.

High temperature soft magnetic materials: feco alloys and composites

We have systematically investigated the microstructural effects including grain size, precipitation, and structural order parameter on the high temperature magnetic and mechanical properties of FeCo-based commercial alloys. At high temperatures the equilibrium nonmagnetic precipitates significantly deteriorate the soft magnetic properties. Poor mechanical properties are mainly due to the nature of the ordered structure of FeCo alloys. Based on this understanding we have designed new magnetic composites by reinforcing FeCo alloys with high strength fibers. The magnetic and mechanical properties can thus be improved independently through optimizing the magnetic matrix and fiber network, respectively. These new magnetic composites show excellent soft magnetic and mechanical properties. In particularly, negligible creep has been observed at 600/spl deg/C.

Pinning effect of the grain boundaries on magnetic domain wall in FeCo-based magnetic alloys

We have studied the dynamics of grain growth and the pinning effect of grain boundaries on magnetic domain walls in FeCo soft magnetic alloys. It has been found that grain growth takes place at temperatures above 600 °C. The activation energy for grain growth in a disordered state at 820 °C is about 57.4±0.5 kcal/mole. The effect of grain size on magnetic properties has been singled out by keeping the same ordering parameter (S=0 and 0.88) for all samples studied. Microstructural characterization and magnetic measurements indicate that the grain size significantly affects the magnetic coercivity. A linear relationship between the coercivity and the reciprocal of the grain size has been universally found regardless of the heat-treatment histories. Lorenz microscopic observation demonstrates that grain boundaries act as pinning sites for the magnetic domain wall movement.

Damage characterization of two-dimensional woven and three-dimensional braided SiC-SiC composites

A comprehensive investigation of the room temperature behaviour of two-dimensional woven and three-dimensional braided SiC-SiC composites fabricated by the chemical vapour infiltration route has been conducted. A morphological study of the residual porosity in the composites revealed the existence of primarily two populations of pores: small intrayarn pores and larger interyarn pores. The sizes and the shapes of the two types of pores depended largely on the fibre architecture; the two step braided composite in which the majority of the fibre yarns were orientated along the axial direction exhibited the smallest pore size. The pore size and shapes in turn influenced the onset of damage in the composites under tensile loading. Damage was found to be initially matrix dominated, thus being essentially independent of the fibre architecture. At higher stress levels, however, fibre dominated damage prevailed. Unlike the tensile behaviour, where damage led to non-linearity in the stress-strain curve, the compressive behaviour of the composites was linear elastic almost up to failure. The off-axis tensile properties as well as compression after tension behaviour of the two-dimensional woven composites were also investigated. The information obtained from these tests provides the basis for the modelling of damage in these materials.

Novel soft magnetic composites fabricated by electrodeposition

Soft magnetic composites have been fabricated by electrodepositing FeNi and FeCo onto W fibers with a diameter of 20 and 100 μm. Structural and compositional characterizations indicate that FeNi and FeCo-based composites are of fcc and bcc structure, respectively. The mechanical strengths are significantly improved depending on the volume fraction of W fibers. To further improve the mechanical properties of these composites, we have codeposited soft magnets and Al2O3 powders, resulting in an increase in Vickers hardness of more than 100%. Magnetic measurements show that as-deposited fibers are not magnetically soft. After proper thermal annealing, the samples exhibit excellent soft magnetic properties.

Mechanical properties of Fe-Co soft magnets

The tensile behavior of two magnetically soft alloys, Fe-49Co-2V and Fe-27Co, has been characterized as a function of testing temperature, grain size, and degree of long-range order. Several trends in the yield strength of the two alloys have been noted and possible mechanisms for their occurrence discussed. Ordering is found to markedly lower the yield strength of the Fe-49Co-2V. Both alloys exhibit three distinct regions in their yield strength vs. temperature curves. At lower and higher temperatures, i.e. regions I and III, the yield strength shows the normal drop with increasing temperature. In the intermediate temperature range (region II), however, Fe-49Co-2V with a B2 ordered structure demonstrates an anomalous strengthening with increasing temperature while the yield strength remains constant in the disordered Fe-27Co. Both alloys exhibit a Hall-Petch type relationship in their yield strength as a function of grain size and show a decrease in the strain hardening coefficient with increasing grain size.

A model for shape control of cross-ply laminated shells using a piezoelectric actuator

This article investigates the technique of using a piezoelectric (P) material as the actuator to control the deformation of thin unsymmetric cross-ply laminated shells. A theoretical model is developed using the Rayleigh–Ritz principle to predict the effect of piezoelectric layer on laminated shell deformation. Several stacking sequences are investigated. Owing to the curvature of the shell, the position of the piezoelectric layer has a great effect on the deformation. The effect of the P-layer as the outer surface or inner surface is investigated. It is found that to achieve the same amount of deformation, different levels of electric fields have to be applied depending on the P-layer configuration. The efficiency of the P-layer on shells with various stacking sequences is also studied and it is observed that the efficiency is determined by the stacking sequence of the composite layers. This model can be used as a guide to design the piezocomposite laminated shell.

Silicon carbide (NicalonTM) fibre-reinforced borosilicate glass composites: mechanical properties

The objective of this study was to assess the applicability of an extrinsic carbon coating to tailor the interface in a unidirectional NicalonTM–borosilicate glass composite for maximum strength. Three unidirectional NicalonTM fibre-reinforced borosilicate glass composites were fabricated with different interfaces by using (1) uncoated (2) 25 nm thick carbon-coated and (3) 140 nm thick carbon coated Nicalon fibres. The tensile behaviours of the three systems differed significantly. Damage developments during tensile loading were recorded by a replica technique. Fibre–matrix interfacial frictional stresses were measured. A shear lag model was used to quantitatively relate the interfacial properties, damage and elastic modulus. Tensile specimen design was varied to obtain desirable failure mode. Tensile strengths of NicalonTM fibres in all three types of composites were measured by the fracture mirror method. Weibull analysis of the fibre strength data was performed. Fibre strength data obtained from the fracture mirror method were compared with strength data obtained by single fibre tensile testing of as-received fibres and fibres extracted from the composites. The fibre strength data were used in various composite strength models to predict strengths. Nicalon–borosilicate glass composites with ultimate tensile strength values as high as 585 MPa were produced using extrinsic carbon coatings on the fibres. Fibre strength measurements indicated fibre strength degradation during processing. Fracture mirror analysis gave higher fibre strengths than extracted single fibre tensile testing for all three types of composites. The fibre bundle model gave reasonable composite ultimate tensile strength predictions using fracture mirror based fibre strength data. Characterization and analysis suggest that the full reinforcing potential of the fibres was not realized and the composite strength can be further increased by optimizing the fibre coating thickness and processing parameters. The use of microcrack density measurements, indentation–frictional stress measurements and shear lag modelling have been demonstrated for assessing whether the full reinforcing and toughening potential of the fibres has been realized.

Directional dependence of fracture toughness in hot-pressed SiC-whisker reinforced alumina at room and elevated temperatures

The directional dependence of fracture toughness was investigated in commercially available hot-pressed SiC-whisker reinforced alumina composites at room and elevated temperatures. Whisker orientation was transversely isotropic with respect to the pressing axis in these composites. Composite bend bars were chevron-notched and loaded in four-point bending to induce stable crack propagation in two different crack systems: defined here as a combination of crack plane and direction with reference to the pressing axis. The average fracture toughness, KIc , was found to be approximately 30% less for a crack system with its crack plane parallel and crack direction perpendicular to the pressing axis compared to a crack system with both its crack plane and direction parallel to the pressing axis through 1200°C. The crack systems with the lower fracture toughness exhibited a characteristic high concentration of whiskers aligned parallel to the crack direction within the crack plane; this appeared to he associated with decreased or unpromoted crack deflection. This fracture mechanism evidently dominated the fracture process through 1200°C because measured fracture toughnesses were independent of temperature; crack-wake toughening mechanisms, whose efficiency is a function of the residual thermal stresses, are expected to be temperature-dependent.

Tensile Behavior of Weft Knitted Glass/Epoxy Composites with Laid-In Yarns

Tensile behavior of mono-layer and multi-layer composites of weft knitted glass fabric in an epoxy matrix has been studied. The knitted fabric had a knit/knit stitch structure. The effect of laid-in yarns on the composite properties was examined by utilizing knitted fabrics with 0% (i.e., no laid-in yarns), 2.2% and 4.4% by volume of laid-in yarns in the weft direction. It was observed that the laid-in-yarns, even at the small volume fractions used in this study, significantly improved the weft-direction strength and stiffness of the composites without having any adverse effect on the warp-direction properties. Increasing the number of fabric layers from one to three led to major enhancements in the weft-direction strength and fracture strain of the composite with 4.4% laid-inyarns, due to the larger number of bridging laid-in yarns.