Carbon fiber composites thermal expansion coefficient

We propose to rationalize the observation by a phenomenon known as residual thermal stresses. Residual thermal stresses arise from the fact that carbon-fiber and epoxy have different thermal expansion coefficients and a quenching of the composite would conceivably produce residual stresses. Apparently, the quenching process may produce enough residual stresses to lower the toughness of the composite. In the absence of such residual stresses the free volume concept alone would predict a quenched glass to have larger amount of free volume and hence constitute a less brittle substance. 

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. 

Most satellites and spacecraft have large antennas made of carbon/epoxy composites to achieve dimensional stability. Carbon fibers have a negative coefficient of thermal expansion (CTE) and epoxies yield composites which have no thermal expansion or contraction within the temperature range corresponding to the space environment. As a result, both the shape of the antenna surface and Its precise alignment are properly maintained. 

Further improvement in mechanical properties of aramids appears possible since calculations based on perfect alignment and crystalline order predict a theoretical modulus of about 220-245 GPa. The properties of aramid fibers are compared in Table 5. Like carbon, aramid fibers have a negative thermal expansion coefficient in the longitudinal direction. Aramids have high impact resistance, and they have foimd their way into the markets of soft and rigid (composite) armors. 

The structure and properties of biofibers, mainly of cellulose, were described in this chapter. First, the hierarchy microstructure of natural plant fiber and then a variety of crystal modifications of cellulose were mentioned. The ultimate mechanical properties (modulus of 138 GPa and strength of 17.8 GPa) and thermal properties (thermal expansion coefficient of 10 order) were emphasized as quite excellent for cellulosic fiber, enough for use as reinforcement in the composites. With the manifestation of these intrinsic properties in macroscopic material, the oH-cellulose composite was shown to possess excellent mechanical properties, thermal resistance, and optical transparency, besides being composed of fully sustainable resources and hence, biodegradable. Nowadays, the interest in cellulosic nanocomposites has increased considerably [60, 61] and they are expected to be used in many fields such as electronic devices, vehicles, and windmills to replace glass and/or carbon fibers. 

In this study, a novel proach is plied to prepare polymer composites reinforced by both nanoparticles and long fibers. Carbon nanofibers were pre-boimd onto glass fiber mats, and then unsaturated polyester conposites were synthesized through vacuum assisted resin transfer molding. These composites were compared with those synthesized by pre-mixing carbon nanofibers into the polymer resin. Mechanical and thermal properties of composites were measured. Flexural strength and modulus of composites were improved with the incorporation of nanoparticles. It was also found that carbon nanofibers increased the glass transition temperature and reduced the thermal expansion coefficients of imsaturated polyester resin. 

Applied Sciences, Inc. has, in the past few years, used the fixed catalyst fiber to fabricate and analyze VGCF-reinforced composites which could be candidate materials for thermal management substrates in high density, high power electronic devices and space power system radiator fins and high performance applications such as plasma facing components in experimental nuclear fusion reactors. These composites include carbon/carbon (CC) composites, polymer matrix composites, and metal matrix composites (MMC). Measurements have been made of thermal conductivity, coefficient of thermal expansion (CTE), tensile strength, and tensile modulus. Representative results are described below. 

Thermal expansion-contraction of inorganic fillers is much lower compared with that of plastics. Therefore, the higher the filler content, the lower the coefficient of expansion-contraction of the composite material (see Chapter 10). Many inorganic nonmetallic fillers decrease thermal conductivity of the composite material. For example, compared with thermal conductivity of aluminum (204 W/deg Km) to that of talc is of 0.02, titanium dioxide of 0.065, glass fiber of 1, and calcium carbonate of 2-3. Therefore, nonmetallic mineral fillers are rather thermal insulators than thermal conductors. This property of the fillers effects flowability of filled plastics and plastic-based composite materials in the extruder. 

The resin has good adhesion to glass and carbon fibers and shows a lower coefficient of thermal expansion than epoxies and most plastics. As a matrix resin in a carbon composite, PEAR is said to surpass epoxy (Hercules/Hexcel 3501-6) by 48% in tensile, compression, hot/wet, compression-after-impact, and other fiber-dominated properties. Shear strength (matrix-dominated property) is reported at 50% higher than epoxy at room temperature and up to 80% higher under hot-wet conditions. 

---