A Modified Anti-Symmetric Four-Point Bending Method for Testing C/C Composites under Biaxial Shear and Compression

Carbon fiber-reinforced carbon matrix (C/C) composites are attractive alternative materials for the fabrication of various high-temperature structural components subjected to complex loading conditions. A modified anti-symmetric four-point bending (MAFPB) method for testing a V-notched beam specimen was developed in this study for the investigation of the mechanical behavior of C/C composites under biaxial shear and compression. A finite element model of the specimen was generated to examine the effects of the notch radius and notch spacing on the uniformity and distribution of the stresses in the gauge area. Uniform shear and normal stress were observed in the gauge section of a specimen with a blunted notch and appropriate notch spacing. The three-dimensional (3D) mechanical behaviors of a C/C composite determined by MAFPB tests under pure shear and various biaxial shear–compression stress states were compared with those determined by Iosipescu and off-axis compression experiments. The MAFPB test was found to produce reliable estimates of the mechanical behaviors and failure mechanisms of C/C composites under a wide range of biaxial shear–compression stress states. The shear fracture stress of a 3D C/C composite was particularly observed to significantly decrease with increasing compressive stress component.