HongBao Guo’s research while affiliated with Northwestern Polytechnical University and other places

The damage coupling effects of a two-dimensional carbon fibre-reinforced silicon carbide composite (2D-C/SiC) under proportional loading conditions were investigated using off-axial tensile and compressive tests. The applied load was translated into three proportional stress components in the fibre bundle directions of the composite. The tensile, compressive and shear stress-strain behaviours of the composite in the axial directions were obtained and analysed by mounting strain gauges on the tested specimens. Based on the stress-strain behaviours of the composite subjected to a single axial load, the coupling effects between damage caused by different stress components were characterised and analysed based on the strain deviations. Finally, SEM (scanning electron microscope) techniques were used to identify the damage mechanisms and failure modes of the tested specimens. The micro-damage model was built to explain the damage coupling effects of the composite. In addition, the influences of the damage coupling effects on the mechanical properties of the material were obtained.

Under monotonic, non-reverse and reverse cyclic load conditions, the evolution of the in-plane shear damage and the stress-strain behaviours of a two-dimensional Hi-Nicalon fibre-reinforced silicon carbide composite (2D-SiC/SiC) were investigated using Iosipescu tests. The stress-dependent shear damage evolution of the material was characterized by strain data, which were obtained from strain gauges mounted on the tested specimens. Under cyclic shear loading, the shapes of the hysteresis loops were explained by the fibre bending-bearing mechanism and the crack closure effect. The shear strengths under various loading procedures were also compared and analysed. Finally, SEM (scanning electron microscope) techniques were used to identify the shear failure modes of the tested material.

The macro-mechanical behaviors of 2D-woven SiC/SiC composites produced with CVI technique were investigated by uniaxial tensile loading and unloading tests and compressive tests, and the changes in elastic modulus and residual strain were studied. Damage evolution and failure mode were analyzed by AE(Acoustic Emission) parameters and optical macroscope photos. Tensile test shows that the tensile stress-strain curve is obvious bilinear. Significant tensile damage begins at a specific tensile stress level of about 175 MPa. After the occurrence of significant damage, the unloading modulus decrease linearly as tensile stress increases, while the residual strains increase. The compressive stress-strain curve is almost linear until fracture, and compressive modulus slightly increase due to initial defects (pores and cracks) closure. Fracture surface shows that cracks in matrix can grow through fibers easily, and fibers suffer high extent of stress concentration during tensile test. The failure mode of compressive specimen is similar to crush which is correspond to high compressive strength. The main compressive damage mechanisms include cracks between bundles, within 90° bundles, and finally shear fracture occurs w'ithin 0° bundles.

The applicability and limitation of several quadratic strength theories were investigated with respect to 2D-C/SiC and 2.5D-C/SiC composites. A kind of damage-based failure criterion, referred to as D-criterion, is proposed for nonlinear ceramic composites. Meanwhile, the newly developed criterion is preliminarily validated under tension-shear combined loadings. The prediction shows that the failure envelope given by D-criterion is lower than that from Tsai-Hill and Hoffman criteria. This reveals that the damage-based criterion is reasonable for evaluation of damage-dominated failure strength.