To predict the nonlinear stress-strain behavior and the rupture strength of orthotropic ceramic matrix composites (CMCs) under macroscopic plane stress, a concise damage-based mechanical theory including a new constitutive model and two kinds of failure criteria was developed in the framework of continuum damage mechanics (CDM). The damage constitutive model was established using strain partitioning and damage decoupling methods. Meanwhile, the failure criteria were formulated in terms of damage energy release rate (DERR) in order to correlate the failure property of CMCs with damage driving forces, and the maximum DERR criterion and the interactive DERR criterion were suggested simultaneously. For the sake of model evaluation, the theory was applied to a typical CMC with damageable and nonlinear behavior, that is, 2D-C/SiC. The damage evolution law, strain response and rupture strength under incremental cyclic tension along both on-axis and off-axis directions were completely investigated. Comparison between theoretical predictions and experimental data illustrates that the newly developed mechanical theory is potential to give reasonable and accurate results of both stress-strain response and failure property for orthotropic CMCs.

陶瓷基复合材料典型结构件的承载性能研究不足.针对2D-C/SiC槽型梁开展弯扭组合加载下的损伤-破坏特性试验研究,通过变形监测和裂纹观测,分析了槽型梁试件的承载性能、损伤过程及其失效机理.结果 表明,复杂加载下,分层和屈曲是2D-C/SiC槽型梁的主要损伤形式,其破坏过程非常复杂,应变响应通常发生波动和突变,力学性能严重劣化,极限承载能力较低.与完好试件相比,翼缘开孔试件的损伤-断裂机理发生改变,分层起始载荷与局部屈曲载荷明显降低.

基于陶瓷基复合材料在多轴应力作用下的各向异性损伤演化机制,提出损伤解耦分析的模型和方法,实现定量描述损伤分量之间的耦合影响效应.考虑损伤引起的内应力强化,通过引入有效应力的概念,对微细观损伤造成的材料承载性能衰减进行了表征,并提出复杂应力条件下强度失效判别的最大有效应力判据和二次有效应力判据.采用平纹编织C/SiC复合材料,开展了轴向拉伸加卸载、偏轴拉伸加卸载和面内剪切加卸载试验,进行了损伤演化和强度失效分析.模型和试验结果对比分析表明,本文提出的强度理论具有合理性,预测结果准确.

This paper presents a finite element (FE) analysis of the mechanical responses for stitched CFRP laminates under different mechanical loads. Firstly, the through-thickness stitch was simplified to z-pin like reinforcement with a uniform displacement constraint on the upper and lower surfaces of the laminate. Then, a mesoscale 3D representative volume element (RVE) of the stitched composite was proposed and modeled in the FE code ABAQUS, where the reinforcing stitch, composite layers and interfaces were built. A 3D Hashin damage model and built-in cohesive elements were respectively used to predict the mechanical failure of the stitch and the damageable behavior of cohesive interfaces. Simulation results reveal the progressive damage and rupture processes of the RVE under tensile and shear mechanical loads, and macroscopic nonlinear load-displacement responses of the mesoscale model are also captured.

The tensile damage behaviors of 2.5D self-healing C/SiC composite were studied by experi?ments. And the results show that the modulus of weft and warp directions almost have no change until the tensile stress reaches 50 MPa. After that, the residual strain increases with the tensile stress gradually. Based on the stress distribution of the major crack plane, two strength models (brittle fracture model and ductile fracture model) of unidirectional reinforced self-healing C/SiC composite were built, and the strength of 2.5D self-healing C/SiC composite in weft and warp directions were predicted. The results show that the failure mode is brittle fracture and the predicted result of brittle fracture agrees well with the experi?ment result.

将二维编织结构简化为(0°/90°)s正交铺层结构.采用含损伤变量的剪滞分析理论,解得双向等轴拉伸载荷下,0°层和90°层开裂后各层的应力分布;基于随机基体裂纹演化理论,随机纤维损伤和最终失效理论,确定了0°层和90°层沿纤维方向的应力-应变关系,以及切线拉伸模量与施加载荷之间的关系;然后,将切线拉伸模量代入正交铺层结构的剪滞分析中,进而预测出二维编织陶瓷基复合材料在双向等轴拉伸载荷下的应力-应变关系.预测结果表明:在双向等轴拉伸载荷下,二维编织陶瓷基复合材料的横向和纵向应力-应变曲线基本相同,与单向加载时的应力-应变曲线相近.

对含有不同切口损伤的复合材料层合板试件进行了拉伸试验,采用电阻应变计同步测量切口损伤前缘区域随载荷的变化,测定含切口损伤层合板的剩余强度,并讨论了损伤长度和损伤角度对剩余强度的影响规律.建立含切口损伤复合材料层合板有限元模型,分析了含切口损伤复合材料层合板的拉伸失效行为,计算了含切口损伤复合材料层合板的剩余强度,确定了剩余强度与切口损伤状态的关系.计算结果与试验结果具有较好的一致性.

Two kinds of 2D C/SiC composite beams, I-section and U-section are fabricated by CVI technique. The mechanical responses of the beams under different loading cases are investigated experimentally and numerically. Experimental results show that strain response can successfully record the sudden local-failure accident in the structural components. Meanwhile, the gradual interior damage evolution can also be clearly reflected. On the other hand, method of infrared thermal wave imaging was used to examine the total damage extent. And, large delamination was detected in each tested beam. With those measured data and photographs, the complex load-strain relations and the fracture modes of each beam were analyzed and discussed. The failure properties of both I-beam and U-beam were also presented, respectively. Finally, finite element method was applied to simulate the structural response of the I-section beams under plane-bending and skew-bending conditions. The ultimate strengths were also predicted and further compared with experimental data.

The experiments have been developed for static tensile loading up to the ultimate load on notched wide laminates. The effect of notch length and angle on the laminates was considered. The numerical analyses have been developed using commercial code to simulate the failure procedure and predict the residual strength of notched laminate. Equivalent stiffness method was developed to modify the traditional progressive damage method. All failure predictions have a reasonably good correlation with the test results.

经典唯象强度理论适用于正交各向异性线弹性体.对于非线性纤维增强复合材料,通过加卸载试验和损伤力学的分析方法,可以得到一种虚拟的线性化应力-应变关系;依据损伤等效假设,针对线性损伤和非线性损伤,对基于应力的经典二次失效准则进行变换,建立了一种基于损伤的强度理论,即"D失效判据",这一强度理论可以作为经典判据的补充和扩展.针对平纹编织C/SiC复合材料的拉/剪组合试验,进行了实例计算,结果表明:利用D失效判据预测的失效包络线比蔡-希尔准则的预测曲线低,而且,失效曲线的形式与材料的损伤演化规律相关.

In this work, a macroscopic non-linear constitutive model accounting for damage, inelastic strain and unilateral behavior is proposed for the 2D plain-woven C/Si C composite. A set of scalar damage variables and a new thermodynamic potential expression are introduced in the framework of continuum damage mechanics. In the deduced constitutive equations, the material’s progressive damage deactivation behavior during the compression loading is described by a continuous function, and different deactivation rates under uniaxial and biaxial compression loadings are also considered. In damage evolution laws, the coupling effect among the damage modes and impediment effect of compression stress on the development of shear damage in different plane stress states are taken into account. Besides, the general plasticity theory is applied to describing the evolution of inelastic strain in tension and/or shear stress state. The Tsai–Wu failure criterion is adopted for strength analysis. Additionally, the material model is implemented as a user-defined material subroutine(UMAT) and linked to the ABAQUS finite element software, and its performance is demonstrated through several numerical examples.

对含30％质量分数短玻璃纤维注塑增强PA66材料,按照与注塑方向成0°、30°、45°、60°和90°裁取试验件,进行偏轴拉伸试验,得到了5组试验件的载荷-位移曲线、拉伸模量和拉伸强度.应用45°偏轴拉伸测定了材料的剪切弹性模量.通过对偏轴拉伸模量和强度对比,以及应用复合材料计算剪切模量的结果对比应用各向同性计算的结果,证明了短纤维复合材料并不能当做传统各向同性材料来处理分析,是具有各向异性的材料.分析表明,连续纤维偏轴拉伸模量计算方法对于短纤维复合材料仍然适用,蔡-希尔失效判据也能较好的适用于这种材料.从试验结果和偏轴模量以及强度计算发现,60°是最差承载角.

基于剪滞理论,建立了单向纤维增强陶瓷基复合材料的加卸载理论模型,分析了基体长碎块和短碎块对材料迟滞力学行为的不同影响.通过拉伸循环加卸载试验,获得了2D-Si C/Si C复合材料的迟滞应力-应变行为.依据材料基体损伤特点,将试验结果代入长碎块对应理论推导结果,计算得到了4个表征材料组份性能的参数:基体开裂应力为90 MPa,热残余应力为19 MPa,界面脱粘能为3.1 Jm2,界面滑移力为74 MPa.最后结合少量短碎块的存在对试验结果的影响,定性分析了计算结果的偏差.结果表明,获得的材料组分性能参数具有较小的分散性,并能够准确表征材料整体的力学行为.

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.

通过理论和试验方法,分析了平纹编织C/SiC复合材料的偏轴拉伸性能.在室温下进行(0°/90°)和(±45°)、(30°/60°)试件的轴向拉伸试验和(0°/90°)试件的面内剪切试验.结果表明,材料的面内力学性能与材料纤维的方向角有明显的关系.经典层合板理论在初始低应力阶段仍然适用于平纹编织C/SiC复合材料,但随着载荷的增大,由于拉剪损伤耦合效应的影响,计算的应力-应变曲线逐渐偏离试验曲线.根据试验数据,给出了拉剪耦合的影响公式,对经典层合板理论模型进行修正,由修正后模型得到的计算应力-应变曲线与实测曲线吻合很好.

为了研究国产2DSiC/SiC复合材料的拉伸损伤行为以及低周循环载荷作用下的力学性能,通过试验和建立加卸载细观力学模型,对其拉伸加卸载行为进行了探讨.建立了单向连续纤维增强陶瓷基复合材料加卸载细观力学模型,得到了初始加载、卸载和重新加载时的应力应变关系;利用断裂统计方法得到了基体裂纹数随应力变化的关系和复合材料失效判断条件.经过应力转化,将该模型应用于国产二维编织SiC/SiC复合材料.对单向加载试件,采用正交试验方法和最小二乘法得到基体Weibull模量和界面剪切阻力,通过控制材料失效强度与试验结果一致,得到纤维Weibull模量.由上述参数确定的2D-SiC/SiC复合材料拉伸循环加卸载应力应变曲线与实测曲线吻合很好.通过Matlab编程得到2D-SiC/SiC复合材料单向加载时基体开裂过程图.结果表明,2DSiC/SiC复合材料失效时,基体裂纹分布相对比较均匀;基体裂纹数随应力单调增加,未出现持平段,表明材料失效时,基体裂纹还没有达到饱和.

为了研究国产2D-SiC/SiC复合材料的拉伸损伤行为以及低周循环载荷作用下的力学性能，通过试验和建立加卸载细观力学模型，对其拉伸加卸载行为进行了探讨。建立了单向连续纤维增强陶瓷基复合材料加卸载细观力学模型，得到了初始加载、卸载和重新加载时的应力-应变关系；利用断裂统计方法得到了基体裂纹数随应力变化的关系和复合材料失效判断条件。经过应力转化，将该模型应用于国产二维编织SiC/SiC复合材料。对单向加载试件，采用正交试验方法和最小二乘法得到基体Weibull模量和界面剪切阻力，通过控制材料失效强度与试验结果一致，得到纤维Weibull模量。由上述参数确定的2D-SiC/SiC复合材料拉伸循环加卸载应力-应变曲线与实测曲线吻合很好。通过Matlab编程得到2D-SiC/SiC复合材料单向加载时基体开裂过程图。结果表明，2D-SiC/SiC复合材料失效时，基体裂纹分布相对比较均匀；基体裂纹数随应力单调增加，未出现持平段，表明材料失效时，基体裂纹还没有达到饱和。

This paper reports an experimental investigation on the macroscopic mechanical behaviors and damage mechanisms of the plain-woven (2D) C/SiC composite under in-plane on- and off-axis loading conditions. Specimens with 15°, 30°, and 45° off-axis angles were prepared and tested under monotonic and incremental cyclic tension and compression loads. The obtained results were compared with those of uniaxial tension, compression, and shear specimens. The relationships between the damage modes and the stress state were analyzed based on scanning electronic microscopy (SEM) observations and acoustic emission (AE) data. The test results reveal the remarkable axial anisotropy and unilateral behavior of the material. The off-axis tension test results show that the material is fiber-dominant and the evolution rate of damage and inelastic strain is accelerated under the corresponding combined biaxial tension and shear loads. Due to the damage impediment effect of compression stress, compression specimens show higher mechanical properties and lower damage evolution rates than tension specimens with the same off-axis angle. Under cyclic tension–compression loadings, both on-axis and off-axis specimens exhibit progressive damage deactivation behaviors in the compression range, but with different deactivation rates.

试验研究了2.5维自愈合C/SiC复合材料的压缩力学行为,根据材料的细观结构特点,建立了压缩载荷下的损伤力学模型,得到了经纬向压缩的非线性应力应变关系,预测结果与试验值吻合较好.结果表明,经向和纬向的力学行为不同,纬向的压应力逐渐增大时,切线模量逐渐增大,压缩强度为270.05 MPa,而经向压应力逐渐增大时,层间损伤逐渐发生,经纱承受的弯矩越来越大,切线模量逐渐降低,压缩强度为128.66 MPa.

以C/SiC和SiC/SiC复合材料为典型代表的连续纤维增韧陶瓷基复合材料，因为其在高温环境下稳定且优异的力学性能，以及类似于金属材料的韧性断裂破坏行为，使得其在某些特定的材料使用领域，如航空航天领域，具有无可替代的重要地位。