Tensile strength of carbon-carbon composites (2)- interfacial strength and tensile strength

Interfacial strength:

The interfacial strength of the C-C was evaluated using a fiber-bundle push-out method illustrated in Fig3. In this methd, the interfacial fracture was induced by a load applied with a cylindrical indenter to a layer or fiber bundle, in which the fibers are placed parallel to the loading direction as shown in fig3 (b) and (c). Since the layer thickness of the 2D C-C was about 125um and bundle thickness, t, of 3D C-C was 200-300um, the diameter of the indenter was set to 50um, so as to pus the fibers within a layer or an original fiber bundle. The base plate possessed a 70 um-wide groove, which enabled a cylindrical C-C to be pushed out. Specimens of a thickness of approximately 200um were used for the FBP tests, and the load was applied using a screw-driven mechanical testing machine under a crosshead speed of 0.1 mm/min.

The advantage of the FBP method is that it is easier to perform than a single-giber push-out test. The single-fiber push-out tests were also conducted using a micro-Vickers indenter. However, thin specimens appropriate for the SFP test was extremely difficult to prepare, and in many cases resulted in the compressive failure of the fiber. Detailed comparison of results by the FBP and SFP tests were performed in the previous study, and it was concluded that both the tests resulted in similar results. Thus, the FBP test was adopted in the present study for the evaluations of interfacial debonding and sliding stresses.

Tensile strength of carbon fiber:

The hest treatment temperature (HTT) of the carbon fiber IM-600 is supposed to be around 1573-1773K. The C-C examined in this study were heat-treated at a temperature of 2273K or 2573K. Thus, the tensile strength of the fiber would be expected to differ from that given in the supplier’s data. In addition, the present study required the statistical distribution of the tensile strength of the fiber, i.e., the Weibull parameters. Hence, the tensile strengths of the IM600 monofilament were measured at least for 30 specimens after heat treatment at 2273K and 2573K using the method specified in JIS R 7606.

Optical micro-images and SEM images were obtained for polished and/ or fractured cross sections of the C-C before and after the tensile tests. During observation of these images, the diameters of IM600 fiber were found to vary depending on cycles of the HIP treatment. Thus, the diameters of IM600 monofilaments were evaluated on SEM images of polished cross sections perpendicular to fiber axis in the C-C.

The bulk density at each densification step of the C-C was determined by dividing the weight by its volume. The volume and weight were evaluated before the tabs were glued on the specimens. The volume was determined from the sample dimensions measured using a micrometer.

 

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