Unidirectional composites, thermal expansion

Figure 5.88 Predicted thermal expansion coefficients in the axial and transverse direction for continuous, unidirectional glass-fiber-reinforced epoxy matrix composite. Reprinted, by permission, from N. G. McCrum, C. P. Buckley, and C. B. Bucknall, Principles of Polymer Engineering, 2nd ed., p. 265. Copyright 1997 by Oxford University Press.

It is worth pointing out that carbon fibre itself has anisotropic thermal expansion properties, and therefore this mismatch between the carbon fibres and the a-sialon matrix should be considered in both the radial and axial directions when carbon fibres are unidirectionally aligned in the composite. The thermal stress caused by thermal expansion differences between the carbon fibres and the matrix in the radial (cr) and axial (oa) directions can be estimated from the formulae (Chawla, 1993 Kerans and Parthasarathy, 1991) ... 

The above discussion pertains to unidirectional composites that are initially free of matrix cracking examples would include Nicalon SiCf/CAS, Nicalon SiQ/1723 glass, Nicalon SiQ/LAS, and SCS-6 SiQ/HPSN. For composites such as Cf/borosilicate, where the thermal expansion coefficient of the matrix is substantially greater than that of the fiber, microcracks can develop in the matrix during fabrication. These composites do not exhibit a linear stress-strain response (Stage I), even for small applied loads. 

The effects of heat treatment temperatures on thermal conductivity, thermal conductivity at high temperatures and thermal expansion behavior have been studied. At room temperature, the value of thermal conductivity for unidirectional (UD) carbon-carbon composites is 700 W/m K. In the case of three-dimensional (3D) carbon-carbon composites, this value is determined by the volume of the fiber arrangements. On the other hand, the thermal expansion of carbon-carbon composites in the fiber axial direction is chiefly governed by the thermal expansion of the fiber. 

Naim [178] performed linear elastic stress analysis of residual stresses in unidirectional high-performance composites containing high-modulus fibers and an interphase region. Naim and Zoller [179] provided data for composites with thermoset and thermoplastic matrices, and showed by linear elastic stress analysis how the buildup of residual thermal stresses during fabrication is related to the disparate thermal expansion properties of the fibers and matrices. 

II. 1.2b. 1.Thermal Expansion Thermal expansion of unidirectional SiC/RBSN composite is mainly a function of constituents volume fractions and measurement direction relative to the fiber, and is not affected by constituents porosity. Measurement of linear thermal expansion with temperature in nitrogen for the 1-D SiC/RBSN composites parallel and perpendicular to the fibers indicates a small amount of anisotropy (Fig. 5). This is attributed to small difference in thermal expansion coefficients of SiC fibers (4.2 x 10 ) and RBSN matrix (3.8 x 10 ) as well as anisotropic thermal expansion of carbon coating on SiC fibers. In the fiber direction, linear thermal expansion is controlled by the SiC fiber, and in the direction perpendicular to the fiber, it is controlled by the RBSN matrix. 

A large number of studies have been made on a of polymer matrix composites of epoxy, polyimide and other polymers reinforced with carbon and Kevlar fibers [117-123]. The carbon and graphite fibers are characterized by slightly negative (Xi in their axial directions and very large positive aj in their radial directions [122-124]. The unidirectional and bidirectional composites based on them are found to demonstrate highly anisotropic thermal expansion behavior. In unidirectional composites,... 

Figure 4.10 Design of unidirectional composite laminae Halpin-Tsai rule of mixtures. Relationship of coefiticient of thermal expansion to volume fraction.

The thermal expansion behavior of unidirectional carbon fiber reinforced Cu matrix composites has been determined by Korb et al [104]. 

Korb G, Korab J, Groboth G, Thermal expansion behaviour of unidirectional carbon-fiber reinforced copper-matrix composites, Composites Part A-Appl Sci Manuf, 29(12), 1563-1567, 1998. 

Most unidirectional composites are manufactured from prepreged layers of yarns. Each layer is called a lamina and a group of lamina is called a laminate. Each lamina in the laminate can have a specific direction based on the design requirements starting from an arbitrary direction. The only important matter to remember is that these laminates must have a mirror symmetry around their neutral axis otherwise they will warp immediately after manufacture and cannot be straightened. This happens due to the difference in the thermal expansion coefficients between the fibers and the matrix and between different lamina causing one side to permanently expand more than... 

The in-plane mechanical, viscoelastic and thermal properties of a satin weave carbon fabric impregnated with an amine cured epoxy resin were studied by Abot and co-workers [74]. The in-plane quasi-static behaviour including the failure modes under tension, compression and shear and all the mechanical properties including elastic moduli and strengths were determined. The viscoelastic properties including the glass transition temperature were also measured as well as the coefficients of thermal expansion. These measured properties for the fabric composites were also compared with their corresponding ones for a unidirectional composite with the same fibre and matrix. 

Parallel to the fibre axis, the thermal expansion behaviour is controlled only by the thermal expansion of the C fibre, whereas the thermal expansion perpendicular to the fibre axis is dominated mainly by the carbonaceous matrix. In Fig. 14, the thermal expansion and the coefficient of thermal expansion of unidirectionally reinforced C/C composites parallel and perpendicular to the fibre axis is shown versus temperature. In Fig. 14a, curves are plotted of composites with polyimide as the matrix precursor with different final heat treatment temperatures of 1500°C, 2000 C, and 2700°C. Figure 14b illustrates, in comparison with the polyimide-based composites, the expansion behaviour of pitch-based composites reinforced with different types of graphitized C fibres. 

Figure 15 shows measurements of linear thermal expansion and coefficients of thermal expansion at temperatures between 4.2 K and room temperature, which were preformed with unidirectionally reinforced C/C composites parallel to the fibre axis. The composites are fabricated with two different carbon matrix precursors, CT pitch and CT pitch modified by elemental sulfur, as well as two different fibre types, Sigrafil HF (type II) and Sigrafil HM (type I). The C/C composites consist of 50 V/o fibre volume fraction and are densified four times with a final heat treatment of 1000°C. Details of the measuring apparatus are described by Hartwig et al, ... 

Effective Thermal Expansion Coefficients of Unidirectional Composites... 

The orientation averaging method can also be applied to thermal expansion coefficients. To account for the effect of fiber orientation distribution, one writes the thermoelastic constitutive equation for the unidirectional composite as... 

Carbon fiber reinforcements enhance the modulus and strength of a plastic and reduce the density and thermal coefficient of expansion. They also lower the coefficient of friction and provide excellent resistance to most environmental exposure conditions and chemicals. The properties of the final composite are very much dependent on the degree of orientation of the fibers within the resin matrix. When the fibers are in complete aligmnent (e.g., unidirectional or all fibers are at an orientation of 0°), the properties of the composite are more affected by the fiber than if the fibers are randomly aligned. 

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