Uniaxial tensile stress-strain behavior and strength of plain woven C/SiC composite

Fiber reinforced ceramic matrix composites (CMCs) are widely investigated inrecent years in order to obtain their damage mechanisms, failure modes andmechanical properties. A well-accepted fact is that the interface betweenthe fibers and the ceramic matrix has great effects on the mechanicalbehavior of CMCs. The tough and nonlinear stress-strain behavior of CMCs canmainly be attributed to matrix cracking and interface debonding. Therefore,how these two factors affect the stress-strain relationship and the finalstrength of CMCs needs to be deeply studied.In this paper, the damage evolution and the tensile stress-strain behaviorof 2D-C/SiC composite were investigated under monotonic and stepwiseincremental loadings and unloadings. The strain is measured by 25 mmextensometer. The experimental results show that the stress-strain behaviorof the material is obviously nonlinear; the unloading modulus decreaseswhile the inelastic strain increases with increasing of the unloadingstress; the relational curves of residual strain and unloading modulusversus the applied stress have analogous form with the tensile stress-straincurve; the fracture of the composite is quasi-brittle, i.e. catastrophic.Based on the shear-lag theory and the rule of mixture, a conciseconstitutive equation and a tensile strength calculation model of thedamaged material were established. The simulated stress-strain curve is ingood agreement with the experimental data when the applied stress is lowerthan 100 MPa. The difference between the theoretical curve and theexperimental data when the applied stress is higher than 100 MPa mainly comesfrom the local strain concentrations within the seriously damaged zone,because large crack openings usually appear in this region from theexperimental view. The analysis results reveal that the residual strainmainly depends on the crack opening displacement and the crack spacing,while the unloading modulus is significantly dependent on the interfacedebonding rate. Moreover, the uniaxial tensile behavior of 2D-C/SiCcomposite is mostly dominated by the longitudinal fiber bundles; thecontributions of transverse fibers to the material's modulus and strengthare obviously small.In the strength model, the failure criteria proposed by Tsai-Hill wasapplied to obtain the off-axes strength, and then the curved fiber bundle inthe real material structure was equivalent to the straight. Furthermore, therule of mixture was modified and the expression for strength calculation waspresented. And, the predicted tension strength ranges from 210.5 MPa to265.6 MPa for the 2D-C/SiC composite with 0.15 \mu m thick carboninterphase, which is also in good agreement with the experimental data.