Unidirectional composites, thermal

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. 

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.

Carbon fiber/Si3N4 composites were fabricated using slip infiltration of fiber bundles, subsequently stacked in a plaster mold, dried, glass encapsulated and HfPed to form unidirectional composite test bars. Neither chemical reactions between fiber and matrix, nor thermal mismatch cracks in the matrix were observed. The bend fracture behavior of the composites was non-brittle with extensive fiber pullout. 

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. 

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... 

Lodha and Netravali [20, 105] also studied the effect of glycerin on the moisture content. Again, a similar positive relationship was found. As the glycerin content of the SA-SPl resin (with 20% SA) was lowered from 30% to 0%, the Young s modulus and the fracture stress increased from 181 MPa to 1096 MPa and from 6.1 MPa to over 20 MPa, respectively. At the same time the fracture strain decreased from 64.5% to 2.8%. The SA-SPl resin containing 20% stearic acid and 0% glycerin was used for fabricating flax yarn reinforced unidirectional composites. The TGA and DSC studies of the SA-SPl resin further confirmed that the SA-SPI resin was thermally more stable than the SPI resin. 

Fig. 11 Effect of consolidation temperature on the flexural stiffness of unidirectional composites produced by using coextruded PP tape technology. Over the range of consolidation temperature considered here, there is no significant effect on the mechanical properties of the final composite, indicating that this large temperature window would make composites based on coextruded tapes less susceptible to thermal relaxation during consolidation than systems with a smaller temperature processing window. Reproduced with kind permission from Sage Publieatioiis frran [159]...

The axial thermal conductivities of composites reinforced with glass, aramid, boron, and a number of the carbon fibers are relatively low. In fact. E-glass and aramid PMCs are often used as thermal insulators. As Table 5.5 shows, most PMCs have low thermal conductivities in the transverse direction, as a result of the low thermal conductivities of the matrix and the fibers in the radial direction. Through-thickness conductivities of laminates tend to be similar to the transverse thermal conductivities of unidirectional composites. 

As mentioned earlier, through-thickness thermal conductivities of laminates tend to be similar to the transverse thermal conductivities of unidirectional composites, which are relatively low. If laminate thickness is small, this may not be a severe limitation. However, low through-thickness thermal conductivity can be a significant issue for thick laminates and for very high thermal loads. This issue needs to be addressed on a case-by-case basis. 

Unidirectional composites have longitudinal (a, ) and transverse (a, ) coefficients of thermal coefficients given by... 

This is an important relationship. It states that the modulus of a unidirectional fibre composite is proportional to the volume fractions of the materials in the composite. This is known as the Rule of Mixtures. It may also be used to determine the density of a composite as well as other properties such as the Poisson s Ratio, strength, thermal conductivity and electrical conductivity in the fibre direction. 

A unidirectional glass fibte/epoxy composite has a fibre volume fraction of 60%. Given the data below, calculate the density, modulus and thermal conductivity of the composite in the fibre direction. 

Experimental results are presented that show that high doses of electron radiation combined with thermal cycling can significantly change the mechanical and physical properties of graphite fiber-reinforced polymer-matrix composites. Polymeric materials examined have included 121 °C and 177°C cure epoxies, polyimide, amorphous thermoplastic, and semicrystalline thermoplastics. Composite panels fabricated and tested included four-ply unidirectional, four-ply [0,90, 90,0] and eight-ply quasi-isotropic [0/ 45/90]s. Test specimens with fiber orientations of [10] and [45] were cut from the unidirectional panels to determine shear properties. Mechanical and physical property tests were conducted at cold (-157°C), room (24°C) and elevated (121°C) temperatures. 

Bowles, D.E. and Griffin, O.H. (1991a). Micromechanics analysis of space simulated thermal stresses in composites, part I Theory and unidirectional laminates. J. Reinforced Plast. Composites 10, 504-521. 

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