针刺C/C-SiC复合材料剪切非线性本构关系

为研究针刺C/C-SiC复合材料的剪切损伤行为，首先，进行了面内剪切加卸载实验，并利用SEM对复合材料的剪切破坏形貌进行了观测；然后，建立了一种塑性与损伤相结合的非线性本构模型描述复合材料的非线性力学行为，以幂函数描述等效塑性应变与等效应力的关系；最后，基于剪切强度的Weibull分布规律提出了一种指数型损伤变量表征剪切刚度的退化，并通过实验数据拟合得到模型中的参数。结果表明:复合材料在卸载后存在明显的残余应变，卸载模量随载荷的增加不断降低，表现出明显的剪切非线性特征；大量无纬布纤维束和纤维单丝拔出，且易在针刺部位发生破坏；由于针刺部位等缺陷的不规律分布，剪切强度存在一定的分散性，符合指数型Weibull统计分布规律；复合材料的剪切非线性主要由基体开裂和纤维/基体界面脱粘等内部损伤引起，从宏观上可以解释为塑性变形和刚度性能折减。所得结论表明本构模型能够很好地表征C/C-SiC复合材料的面内剪切非线性行为。 

Shear nonlinear constitutive relationship of needled C/C-SiC composite

In order to investigate the shear damage behavior of needled C/C-SiC composite,in-plane shear load-un-load experiments were conducted firstly,and the shear fracture morphologies of the composites were observed by SEM.Then,a plasticity-damage combined nonlinear constitutive model was established to describe the nonlinear mechanical behavior of the composite,and the power function was used to characterize the relationship between equivalent plastic strain and equivalent stress.Finally,an exponent-form damage variable was introduced to describe the shear stiffness degradation based on the Weibull distribution law of shear strength,and the parameters in model were determined by fitting of the experimental data.The results show that the composite has obvious residual de-formation after unloading,the unloading modulus degenerates with the increase of load continually,which exhibits obvious shear nonlinearity characteristic.A large number of fiber bundles and fiber single yarns in non-woven cloth were pulled out,and the failures tend to occur at needling positions.For the irregular distribution of imperfection at needling positions,shear strength possesses dispersibility at some extent,and conforms to exponent-form Weibull statistical distribution law.The shear nonlinearity of the composite was induced by inner damages such as matrix cracking and fiber/matrix interfacial debonding etc.,which can be described by plasticity deformation and stiffness properties degradation from a macro perspective.The conclusions obtained verify that the constitutive model can fully describe the in-plane shear nonlinear behavior of needled C/C-SiC composite.