Testing of carbon fibers

Davies, P. Kausch, H.H., Williams, J.G. and 29 other researchers (1992). Round-robin interlaminar fracture testing of carbon fiber reinforced epoxy and PEEK composites. Composites Sci. Technol. 43, 129-136. 

Table 17.11 Dimensions and test conditions for compression testing of carbon fiber/epoxy...

Mechanical Properties and Stability at Elevated Temperature. One increasingly important characteristic of carbon fibers is their excellent performance at elevated temperatures. Strength tested in an inert environment remains constant or slightly increases to temperatures exceeding 2500°C. Amoco s high modulus pitch carbon fiber P-50 maintains approximately 80% of room temperature modulus at temperatures up to 1500°C, then decreases more rapidly to 30% at 2800°C (64). The rapid decrease in modulus is a result of increased atomic mobiHty, increa sing fiber plasticity. 

Schmitz et al. [184] tested various carbon fiber papers with different thicknesses as cathode DLs in PEM fuel cells. It was observed that the cell resistance dropped when the thickness of the DL increased thus, thicker materials are desired in order to improve the electrical conductivity. It was also mentioned that the optimal thickness for the DL is usually between the thinnest and the thickest materials because the two extremes give the lowest performance. In fact, in thin DLs, the water produced can fill pores within the material, resulting in flooding and the blockage of available flow paths for the oxygen. Similarly, Lin and Nguyen [108] concluded that thinner DLs (without MPLs) were more prone to liquid water accumulation than thicker ones. 

Soler, Hontanon, and Daza [268] tested two different FF designs with a number of carbon fiber paper and carbon cloth DLs in order to determine the best combination. They measured the pressure drop of the flow field in a nonactive fuel cell with each DL material with oxygen, air, and nitrogen. The researchers... 

Pitkethly, M.J. and Doble, J.B. (1990). Characterizing the fiber/matrix interface of carbon fiber-reinforced composites using a single fiber pullout test. Composites 21, 389-395. 

Fig. 5.18. Comparison of shear strengths of carbon fiber-epoxy matrix composites determined from three dificrcnt test methods. Fiber surface conditions as in Fig. 5.17. After Drzal and Madhukar (1993).

Recent tests have revealed surprisingly good fatigue and creep resistance for carbon/carbon composites. Figure 29 presents some results of torsion and flexure tests in which the fatigue properties of carbon-fiber-reinforced carbon (CFRC) 3D composites are compared with those of carbon-fiber-reinforced polymer (CFRP) 3D composites (53). 

Sinclair (1950) devised this loop test to estimate the strength and elastic modulus of fibers. There are many variants of this test, which is basically a kind of bend test. Greenwood and Rose (1974), among others, used such a test to evaluate Kevlar aramid fiber. Huttinger (1990) used such a loop or knot test to determine the deformation characteristics of carbon fibers. Figure 9.5 shows the knot test schematically. If d is the fiber diameter and is the loop diameter corresponding to a tensile strain of e in fiber, then we can write... 

Fig. 2. Experimental results of 4-point bending test for concrete beams with and without bonding of carbon fiber sheets.

Figure 4 shows typical failure surfaces obtained from tensile tests of the co-cured single and double lap Joint specimens. In the case of the co-cured single lap Joint, as the surface preparation on the steel adherend is better, a greater amount of carbon fibers and epoxy resin is attached to the steel adherend. Failure mechanism is a partial cohesive failure mode at the C ply of the composite adherend. In contrast with the co-cured single lap joint, failure mechanism of the co-cured double lap joint is the partial cohesive failure or interlaminar delamination failure at the 1 ply of the composite adherend because interfocial out-of-plane peel stress... 

One typical example of this behavior is the voltammogram of the ferro/ferricyanide couple (test reaction) that at carbon electrodes is less reversible than at noble metal electrodes. The kinetics of the test reaction in 1 M aqueous KCl was used as the reference to compare its electrochemical behavior on different carbon electrodes [20]. This electrochemical reaction occurs via an outer sphere mechanism and its rate depends on the electrolyte composition and can be increased by appropriate treatment of carbon electrodes, for instance, by application of a high current potential routine to electrodes of carbon fibers. Similar results have been obtained with glassy carbon surfaces that had been pretreated at 500°C under reduced pressure. An alternative activation method is based on careful electrode surface polishing [6]. 

Catalysts were prepared by impregnation of Pt inside the pore structure of carbon fibers. Care was taken to eliminate the active metal from the external surface of the support. A very high dispersion of Pt was measured. Four reactions were carried out in a fixed-bed reactor competitive hydrogenation of cyclohexene and 1-hexene, cyclization of 1-hexene, n-heptane conversion and dehydrogenation of cyclohexanol. Three types of carbon fibers with a different pore size and Pt-adsorption capacity along with a Pt on activated carbon commercial catalyst were tested. The data indicate a significant effect of the pore size dimension on the selectivity in each system. The ability to tailor the pore structure to achieve results drastically different from those obtained with established supports is demonstrated with heptane conversion. Pt on open pore carbon fibers show higher activity with the same selectivity as compared with Pt on activated carbon catalysts. 

Only a few studies published in the literature have examined the possibility to utilize carbon fibers as support with special shape-selectivity properties to prepare catalysts (5-7). llie purpose of this work is to test various carbon fibers as supports for Pt supported catalysts. Preliminary activity and selectivity data were obtained for hydrogenation, dehydrogenation and cyclization reactions. 

Kovacs, P. 1993. In vitro studies of the electrochemical behavior of carbon-fiber composites, in Composite Materials for Implant Applications in the Human Body Characterization and Testing, ASTM STP 1178. R.D. Jamison and L.N. Gilbertson, Eds. ASTM, Philadelphia, PA, pp. 41-52. 

Fibre-reinforced plastic composites - Determination of mode I interlaminar fracture toughness, Gic, for unidirectiOTially reinforced materials Testing methods for interlaminar fracture toughness of carbon fiber reinforced plastics... 

Nevertheless, some authors have provided some evidence of a flame inhibition effect of phosphorus. Schartel et al. have measured the EHC of epo y resins in a PCFC i.e. corresponding to complete combustion) and in a cone calorimeter. Epojy resins containing DOPO covalently linked to the network have been studied. All thermosets exhibited the same EHC in a PCFC (approximately 24 kJ g ), but in cone calorimeter tests the composites prepared from the phosphorus-containing resins (with 60 vol% of carbon fibers) exhibited a significantly lower EHC than the composite prepared from the phosphorus-free epoxy resin. In this study, a clear flame inhibition effect was evidenced. 

The commercial potential of updrawn quaternary calcium aiuminate giass fibers was tested in two stages. In the early 1960s, they were evaluated because they yielded higher moduli than those which couid then be achieved with siiicate glass fibers [36-37]. Timing for this development coincided with the onset of the commercial development of carbon fibers and no new aluminate or silicate glass fiber was commercialized until 1995. 

Temperature is one of the processing parameters which has a pronounced effect on the failure strength of carbon fibers. This effect is quite different for MP based and PAN based fibers. As the heat treatment temperature (HTT) is raised, the strength of a mesopitch based fiber, tested at room temperature, progressively increases [3] [38] [54-55]. Conversely, that of most PAN based fibers undergo a maximum at about 1300-1500 C (Figure 12). 

Tensile strength and modulus of carbon fibers inaease with increasing test temperature to about 1500 C [58]. Carbon fibers begin to creep above about 2000 C [47] [51] [59]. Creep is actuaily used during hot stretching to increase their stiffness or Young s modulus. 

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