Takako Takei

Effects of dispersed microvoids on thermal expansion behavior of composite materials

Abstract In our earlier papers [T. Takei, H. Hatta, M. Taya, Mater. Sci. Eng. A131 (1991) 133; T. Takei, H. Hatta, M. Taya, Mater. Sci. Eng. A131 (1991) 145], we found that the coefficients of thermal expansion (CTEs) of particulate composites are lowered by dispersed microvoids in the matrix. In order to prove this void dispersion effect by an analytical model and to utilize this effect for wide control of composite CTE, an analytical parametric study is conducted in this paper. It is found in this study that the combination of fiber type reinforcement with high elastic modulus and low CTE and disk-shaped voids was identified to remarkably lower the composite CTE. This CTE control technique is based on the mechanism that microvoids in the matrix are compressed by compressive stress field developed around reinforcement when the composite is subjected to temperature rise. In order to confirm experimentally this void dispersion effect on CTEs, three types of composite materials with dispersed voids in their matrices were fabricated, particulate, short fiber, continuous fiber (3D) reinforced composites, and CTEs of these composites were measured. It was concluded from comparison between the predictions and experimentally observed CTEs that the CTE reduction by the void dispersion actually occurs and this effect is analytically predictable up to some limit volume fraction, which depends on a composite type.

Thermal Conductivity of Hybrid Short Fiber Composites

A combined analytical/experimental study has been undertaken to in vestigate the effective thermal conductivity of hybrid composite materials. The analysis utilizes the equivalent inclusion approach for steady state heat conduction (Hatta and Taya, 1986), through which the interaction between the various reinforcing phases at finite con centrations is approximated by the Mori-Tanaka (1973) mean field approach. The multiple reinforcing phases of the composite are modeled as ellipsoidal in shape and thus can simulate a wide range of microstructural geometry ranging from thin platelet to con tinuous fiber reinforcement. The case when one phase of the composite is penny-shaped microcracks is studied in detail. Multiphase composites consisting of a Kerimid matrix and Al 2O3 short fibers and Si3N4 whiskers were fabricated and after a careful study of their microstructure, their thermal conductivities were measured. Analytical predictions are shown to be in good agreement with experimental results obtained for the Al2O 3/Si3N4/ Kerimid short fiber composites.

Thermal expansion behavior of particulate-filled composites. I, Single reinforcing phase

Abstract The effect of reinforcement geometry on the thermal expansion behavior of particulate-filled bis-maleimide matrix composites has been examined both experimentally and theoretically. To clarify the geometrical effect, a wide range of reinforcement shapes, from short fiber to flake, was used. The comparison was made between experimentally obtained thermal expansion coefficients and those theoretically predicted by the use of Eshelbys equivalent inclusion method. The predicted values were shown to agree reasonably well with the experimental values. An attempt was then made to obtain the relation between the reinforcement geometry and processability condition. It was found that spherical-particle-filled composites exhibited superior thermal expansion behavior compared with short-fiber- or flake-reinforced composites from the viewpoint of processability, including void-free formation. In part II of this paper the effect of multi-reinforcing phases on the thermal expansion behavior of a composite is studied and the results of the following paper are compared with those of the present paper.

Interlaminar Reinforcement of Laminated Composites by Addition of Oriented Whiskers in the Matrix

To reinforce the interlaminar of laminated composites, a novel method was proposed by the authors [1]. In this method, whiskers were added in the matrix resin to reinforce the interlaminar, and the orientation of the whiskers were magnetically con trolled to improve the reinforcement efficiency. In this paper, mode I (cleavage) inter laminar fracture toughness was shown to be remarkably improved as a result of the addi tion of oriented whiskers to the direction of the plate thickness. Whereas, mode II (inplane shear) interlaminar fracture toughness and interlaminar shear strength (ILSS) were not raised. Parameters of interlaminar reinforcement such as volume fraction of whiskers and whisker dimension were also discussed for exploring the optimum condition.

Thermal expansion behavior of particulate-filled composites II: Multi-reinforcing phases (hybrid composites)

Abstract In part I of this paper, the effect of filler shape on the thermal expansion behavior of a composite with a single reinforcing phase was studied. In this paper emphasis is placed upon the effect of using multi-components as the reinforcement (hybrid composite) on the thermal expansion behavior of a short fiber-composite. The hybrid composite system examined consists of a Kerimid matrix and two kinds of reinforcement: Al 2 O 3 short fibers and Si 3 N 4 whiskers. The former reinforcement represents a large-sized fiber, while the latter plays the role of a small-sized fiber. This leads to two advantages: (1) an increase in the volume fraction of the reinforcement, thus resulting in a lower coefficient of thermal expansion (CTE) of the composite, and (2) control of the orientation of these fibers, thus leading to improvement in the thermal expansion behavior of the composite along the thickness direction.

Thermal expansion behavior of skeletonized 3D-composites

Abstract In a recent paper, the authors have pointed out that coefficients of thermal expansion (CTEs) of composites can be reduced by mixing microvoids in the matrix. In this paper, this principle was applied to 3D fabric composites (3DFCs). Three kinds of multiaxially reinforced 3DFC were formed into a porous (skeletonized) structure and the CTEs of these composites were measured. It was found that the CTEs of the composites were drastically reduced by introducing voids. Thus, by utilizing this technique, we can control the CTE of 3D composites in the range from the value of a usual 3D composite completely filled with the matrix, to nearly 0. These CTE values of skeletonized composites were shown to agreeareasonably well with the analytical predictions based on the Eshelbys equivalent inclusion method.