2D-C/SiC复合材料的单轴拉伸力学行为及其强度

UNIAXIAL TENSILE STRESS-STRAIN BEHAVIOR AND STRENGTH OF PLAIN WOVEN C/SIC COMPOSITE

通过单调拉伸和循环加卸载试验,研究了平纹编织C/SiC复合材料的损伤演化过程及其应力-应变行为.结果表明,残余应变、卸载模量和外加应力的关系曲线与拉伸应力-应变曲线具有类似的形状.基于剪滞理论和混合率建立了材料的损伤本构关系和强度模型,分析计算表明,残余应变主要由裂纹张开位移和裂纹间距决定,而卸载模量主要由界面脱粘率决定;材料的单轴拉伸行为主要由纵向纤维束决定,横向纤维对材料的整体模量和强度贡献较小.理论模拟结果与试验值吻合较好.

Fiber reinforced ceramic matrix composites（CMCs） are widely investigated in recent years in order to obtain their damage mechanisms,failure modes and mechanical properties.A well-accepted fact is that the interface between the fibers and the ceramic matrix has great effects on the mechanical behavior of CMCs.The tough and nonlinear stress-strain behavior of CMCs can mainly be attributed to matrix cracking and interface debonding.Therefore,how these two factors affect the stress-strain relationship and the final strength of CMCs needs to be deeply studied. In this paper,the damage evolution and the tensile stress-strain behavior of 2D-C/SiC composite were investigated under monotonic and stepwise incremental loadings and unloadings.The strain is measured by 25 mm extensometer.The experimental results show that the stress-strain behavior of the material is obviously nonlinear;the unloading modulus decreases while the inelastic strain increases with increasing of the unloading stress;the relational curves of residual strain and unloading modulus versus the applied stress have analogous form with the tensile stress-strain curve;the fracture of the composite is quasi-brittle,i.e.catastrophic. Based on the shear-lag theory and the rule of mixture,a concise constitutive equation and a tensile strength calculation model of the damaged material were established.The simulated stress-strain curve is in good agreement with the experimental data when the applied stress is lower than 100 MPa.The difference between the theoretical curve and the experimental data when the applied stress is higher than 100 MPa mainly comes from the local strain concentrations within the seriously damaged zone,because large crack openings usually appear in this region from the experimental view.The analysis results reveal that the residual strain mainly depends on the crack opening displacement and the crack spacing,while the unloading modulus is significantly dependent on the interface debonding rate.Moreover,the uniaxial tensile behavior of 2D-C/SiC composite is mostly dominated by the longitudinal fiber bundles;the contributions of transverse fibers to the material＇s modulus and strength are obviously small. In the strength model,the failure criteria proposed by Tsai-Hill was applied to obtain the off-axes strength, and then the curved fiber bundle in the real material structure was equivalent to the straight.Furthermore, the rule of mixture was modified and the expression for strength calculation was presented.And,the predicted tension strength ranges from 210.5 MPa to 265.6 MPa for the 2D-C/SiC composite with 0.15μm thick carbon interphase,which is also in good agreement with the experimental data.