Mechanical Properties And Fracture Mechanisms Of Carbon / Carbon Composites

Mechanical properties of C/C composite used as aircraft brake disks were systematically studied firstly in this dissertation. The experiment results of mechanical properties of C/C composites made by different process( pure CVD densified, pure pitch impregnation densified, "CVD+pitch impregnation densified", "CVD+Furan resin impregnation densified" and "CVD+B4C Furan resin impregnation densified") were obtained with Electron multi-function test machine, and through X-ray diffraction, optical microscope, scanning electron microscope(SEM) observation to fracture surface, the crack propagation of C/C composite was analyzed in detail, then the factors that influence mechanical properties were further discussed and the failure mechanism of C/C composite was elementarily put forward.Results showed that, the graphitization was among 40-80% for samples with resultant density over 1.80 g/cm3 no matter what densification process they had undergone, the heat conductivity was among 40-82 w.m-1.k-1 and 17-41 w.m1'.k-1 for parallel and vertical direction. And samples with the highest graphitization degree (g=78.4%) had the best heat conductivity (X x/=81.67 w.nv1.k-1, A ?=40.85 w.m-1.k-1). The bending, shear, parallel compressive and vertical compressive strength were among 85~150MPa, 9~16MPa, 55~110MPa and 90~250MPa, respectively. For the samples with same resultant density, their mechanical properties were equivalent to each other, except that pure pitch impregnation densified samples had lower compressive strength.By comparing with the effect of density, graphitization degree, carbon matrix on different materials' mechanical properties, it was appropriately thought that, for samples densified by the same process (such as pure CVD densified or pure pitch impregnation densified), the higher the resultant density, the higher the mechanical strength, for those made by combined process with comparable density, mechanicalstrength would get improved by increasing the ratio of CVD carbon. While to those densified by pure Furan resin impregnation, there exist no linear relationship between strength and initial CVD density. Test results of B4C Furan resin impregnation densified samples under different heat treatment temperature (HTT) indicated that strengths would decrease with increased HTT. However, on different materials, the effect of HTT would be different due to different carbon matrix, namely, though with the same graphitization degree, there maybe exist great difference in mechanical properties.Failure modes of materials were concluded by analyzing their fracture morphology with SEM observation and stress-strain curves. During 3-point bending short beam test, except that samples densified by pure pitch impregnation showed brittle failure for their strong interface between fibers and pitch derived carbon, others showed "pseudo-plastic" failure. Generally speaking, the fracture for bending and shear samples were three-dimensional. Since there being series of small-angled cracks along the central axis, and materials would fracture by demonstrating multi-delaminating. During vertical compression tests, in addition to delaminating phenomenon(releasing stress concentration by crack propagation along inter-laminar interface) , the samples fractured along the plane 45 ?to central axis because of the maximal shear stress on this plane. If the load was parallel, the failure would occur with delaminating, kinks, shear failure or the mixed mode, mainly depending on inter-laminar bonding strength ,transfigure strength and crack concentration in addition.