化学气相渗透法制备2维和2.5维C/SiC复合材料及其拉伸性能

采用化学气相渗透法制备了2维和2.5维碳纤维增强碳化硅(carbon-fiber-reinforced silicon carbide,C/SiC)复合材料,沿经纱(纵向)和纬纱(横向) 2个方向对2种复合材料进行了室温拉伸性能测试,并从预制体结构和原始缺陷分布的角度对比分析了两者力学性能之间的差异.结果表明:两种C/SiC复合材料均表现出明显的非线性力学行为,在经纱方向和纬纱方向上,2维C/SiC复合材料力学性能表现为各向同性,而2.5维C/SiC复合材料力学性能则表现出明显的各向异性:经纱方向上2.5维C/SiC复合材料的拉伸强度和拉伸模量(326 MPa,153 GPa)均高于2维C/SiC复合材料的(245 MPa,96 GPa),纬纱方向上的(145 MPa,62 GPa)均低于2维C/SiC复合材料的(239 MPa,90 GPa).两种复合材料的拉伸断裂行为均表现为典型的韧性断裂,并伴有大量的纤维拔出.两种复合材料中纱线断裂均呈现出多级台阶式断裂方式,但其断裂位置并不相同.2.5维C/SiC复合材料中由于经纱路径近似于正弦波,弯曲程度较大,在纱线交叉点处造成明显的应力集中,因此经纱多在纱线交叉点处断裂;而纬纱由于其路径近乎直线,应力集中现象不明显,因此纬纱断裂位置呈随机分布.2维C/SiC复合材料中经纱和纬纱由于其路径类似于2.5维C/SiC复合材料中的经纱,因此其断裂位置也多在纱线交叉点处.微观结构观察表明不同的编织结构是造成两种复合材料在不同方向上力学性能差异的主要原因.     

2维C/SiC复合材料的拉伸损伤演变过程和微观结构特征

通过单向拉伸和分段式加载-卸载实验,研究了二维编织C/SiC复合材料的宏观力学特性和损伤的变化过程.用扫描电镜对样品进行微观结构分析,并监测了载荷作用下复合材料的声发射行为.结果表明:在拉伸应力低于50MPa时,复合材料的应力-应变为线弹性;随着应力的增加,材料模量减小,非弹性应变变大,复合材料的应力-应变行为表现为非线性直至断裂.复合材料的平均断裂强度和断裂应变分别为23426MPa和0.6%.拉伸破坏损伤表现为:基体开裂,横向纤维束开裂,界面层脱粘,纤维断裂,层间剥离和纤维束断裂.损伤累积后最终导致复合材料交叉编织节点处纤维束逐层断裂和拔出,形成斜口断裂和平口断裂.     

三维针刺C/SiC复合材料的结构特征和力学性能

采用化学气相渗透法制备了在厚度方向上具有纤维增强的三维针刺碳纤维增强碳化硅(C/SiC)陶瓷基复合材料,复合材料的密度和气孔率分别为2.15 h/cm3和16%.三维针刺C/SiC复合材料中的针刺纤维将各层紧密结合在一起,其层间抗剪切强度显著提高,为95MPa,比二维碳布叠层C/SiC复合材料的剪切强度(35MPa)高171.4%.三维针刺C/SiC复合材料的拉伸强度和弯曲强度分别为159MPa和350MPa,断裂模式为非脆性断裂,包括:裂纹扩展、偏转,碳纤维的拉伸断裂和逐步拔出.     

三维针刺碳纤维增强SiC复合材料在加载-卸载下的拉伸行为

对T300碳纤维增强三维针刺碳纤维增强SiC(C/SiC)复合材料(纤维体积含量为30%)的单调和加载-卸载拉伸载荷下的拉伸行为进行了研究.结果表明:T300碳纤维增强三维针刺C/SiC复合材料的拉伸强度和断裂应变分别为129.6MPa和0.61%.单调和加载-卸载拉伸应力-应变曲线均为非线性变化,主要是复合材料中裂纹的扩展,界面相脱黏和滑移,以及纤维的逐步断裂和拔出所致,使得复合材料在拉伸载荷下呈非脆性破坏.卸载应力水平对卸载后的残余应变和再加载模量有较大影响.卸载应力小于80 MPa时,随着卸载应力的增加,残余应变线性增加,模量线性降低:卸载应力高于80MPa时,二者随着卸载应力的增加而呈二次函数快速变化.     

低压化学气相渗透法制备2.5维SiCf/SiC复合材料

采用低压化学气相渗透法制备了具有和不具有热解炭界面层的2.5维连续SiC纤维增强的SiC复合材料(SiCf/SiC).SiC纤维的体积分数为30%和41%.所制备复合材料的气孔率为20%左右.当纤维为30%时,沉积有0.1 μm热解炭界面层的复合材料的弯曲强度由未加热解炭界面层的232MPa增加到328MPa,而且材料由灾难性断裂转变为非灾难性断裂.在同一制备条件下,纤维体积分数为41%的SiCf/SiC比30%的SiCf/SiC具有更高的气孔率.纤维为41%时,热解炭界面层厚度为0.1 μm的SiCf/SiC的弯曲强度只有244MPa,但是它具有更高的韧性和更长的纤维拔出长度.     

芳纶/碳纤维混杂机织复合材料的力学性能研究

分别以芳纶和碳纤维(CF)为经、纬向纱,采用小样织机制备了不同经纬密度的芳纶/CF混杂平纹织物;以环氧树脂为基体,采用真空辅助树脂转移成形方法制备芳纶/CF混杂织物增强复合材料(简称复合材料);分析了复合材料的力学性能。结果表明:在经向纱(简称经纱)、纬向纱(简称纬纱)的纤维种类分别相同时,复合材料的弯曲强度随经向密度增大而增大;以芳纶为经纱时,复合材料的弯曲模量和拉伸模量随着经纱的密度增大而减小,而复合材料的拉伸强度和断裂伸长率随着经纱密度的增大而增大;以CF为经纱时,复合材料的弯曲模量随着经纱密度的增大而增大,且弯曲强度和弯曲模量均高于以芳纶为经纱时的复合材料。     

制备工艺对亚麻增强热塑性复合材料拉伸力学性能的影响

将增强体亚麻纱线和基体丙纶复丝制成pp/亚麻包覆纱后,进行织造,织物用层合热压法制成复合材料.制备工艺中,包覆纱法对复合材料的拉伸强度最好;麻含量50%的复合材料的拉伸强度达到最佳;当纬纱密度相同时,随着经纱密度的增大经向的拉伸强力和拉伸弹性模量也随之增大,而纬向的却随之减小,当经纱密度相同时,随着纬纱密度的增大,经向的拉伸强力和拉伸弹性模量随之减小,纬向的随之增大.     

芳纶纤维/AFR树脂复合材料界面粘结性能的研究

为了改善芳纶纤维复合材料的界面粘结性能,合成了一种新型树脂(AFR)作为基体,以未经任何表面处理的芳纶纤维作增强材料,制备了芳纶纤维/AFR复合材料。采用测定表面能、接触角、层间剪切强度、横向拉伸性能和扫描电镜观察形貌等方法,从宏观和微观等方面研究了芳纶纤维/AFR复合材料的界面粘结性能。结果表明,AFR树脂与芳纶纤维有相近的表面能,AFR树脂溶液与芳纶纤维的接触角为42.8°,而环氧树脂(EP)与芳纶纤维的接触角为68°,说明AFR树脂对芳纶纤维的润湿性优于EP树脂;芳纶/AFR复合材料的层间剪切强度、横向拉伸强度和纵向拉伸强度分别为74.64MPa、25.34MPa和2256MPa,比芳纶/EP复合材料的相应强度分别提高了28.7%、32.5%和13.4%,其复合材料破坏面的形貌也说明芳纶纤维与AFR树脂之间的界面粘结性能较好。     

柔韧棉秆皮纤维/PLA复合材料的制备与性能

采用抄造与热压相结合的方法,制备了薄且柔韧的棉秆皮纤维/聚乳酸(PLA)复合材料。该复合材料是由聚乳酸包覆、填充的棉秆皮纤维网,具有较好的柔韧性。研究了棉秆皮纤维与聚乳酸质量比和热压时间对复合材料结构、拉伸性能和弯曲刚度的影响。结果表明,棉秆皮纤维与PLA质量比为1/1,热压15 min(温度190℃)为最佳条件。该条件制得的复合材料厚(0. 126±0. 006) mm,干、湿态下的拉伸强度分别为12. 15、4. 88 MPa,断裂伸长率分别为1. 55%、1. 5%,杨氏模量分别为1 126、508 MPa,弯曲刚度为0. 35 N·cm2/cm。该研究为纤维增强热塑性复合材料的制备方法提供了新的思路,也有利于提高废弃棉秆皮的利用率。     

聚氨酯/废旧聚乙烯纤维复合材料的制备

以废旧超高相对分子质量聚乙烯(UHMWPE)纤维为增强材料,制备了聚氨酯(PU)/UHMWPE复合材料,并采用电子万能拉力试验机、抗冲击强度测定仪、热线法快速热导仪等研究了硅烷偶联剂KH550及其用量、UHMWPE纤维用量对PU/UHMWPE复合材料性能的影响。结果表明:UHMWPE纤维质量分数为3%,KH550质量分数为5.0%时,制备的PU/UHMWPE复合材料的综合性能较为优异,拉伸强度、断裂伸长率和悬臂梁缺口冲击强度分别为17.0 MPa,35.8%,11.8 MPa,导热系数为0.023 W/(m·K)。     

Compressive Properties and Fracture Behavior of Two-and-a-Half-Dimensional C/SiC Composites

Uniaxial compression tests for two-and-a-half-dimensional carbon fiber-reinforced silicon carbide composites along the warp and weft directions were conducted at room temperature. The results show that the stress–strain behaviors for both loading directions exhibited great anisotropy. The compressive strength was greater in the weft direction than in the warp direction, which is attributed to good confinement of the weft yarns and resisting roles of the warp yarns in the former. Microscopic examinations reveal that the fracture surfaces had good correlation with the weave architecture, and the failure mechanism was characterized by transverse shear resulting from fiber microbuckling in the axial yarns.     

In-situ characterization on crack propagation behavior of SiCf/SiC composites during monotonic tensile loading

The microstructure and crack propagation path of 2.5D SiC_f/SiC composites were observed by synchrotron radiation x-ray computed micro tomography (SR-μCT) equipped with in-situ tensile device. The results showed that the pore morphologies of the SiC_f/SiC composites are mainly divided into three types in three-dimension space: interconnected pores, isolated pores and micro pores in fiber bundles. The crack initiation occurred at the root of the defects under in-situ tensile load and the crack was perpendicular, parallel to the stress axis or mixed mode to propagate. At the interface scale between fiber and matrix, the crack deflection will be controlled by physical parameters such as fracture energy release rate and the modulus of elasticity. At the fiber bundle scale, the crack is easy to shear propagate along the interface between weft and warp fiber bundles due to the existence of the mechanical bonding and residual tensile stress.     

Dynamic fracture of C/SiC composites under high strain-rate loading: microstructures and mechanisms

We investigate dynamic fracture of C/SiC composites under high strain-rate compression or tension with split Hopkinson pressure bar (SHPB) and gas gun loading. Components of the as-fabricated composites are mapped and quantified with X-ray computed tomography, including C fibers and fiber bundles, SiC matrix, and inter- and intrabundle voids. Compression loading is applied along the out-of- and in-plane directions by SHPB at strain rates of 10²–10³ s⁻¹ along with in situ X-ray phase contrast imaging. Out-of-plane direction compression and tension are examined with gas gun impact at strain rates 10⁴–10⁵ s⁻¹. For the out-of-plane loading, compression induces fracture via void collapse and shear damage banding, while delamination dominates fracture for the in-plane direction compression. With increasing strain rates, the compression failure modes transit from interbundle to intrabundle fracture of SiC, and then to fiber and bundle breaking. Tensile failure involves delamination, fiber pullout and fiber breaking. In contrary to normal solids, dynamic tensile or spall strength decreases with increasing impact velocities, owing to compression-induced predamage before subsequent tensile loading.     

平纹机织玄武岩纤维/环氧树脂复合材料冲击拉伸行为

以常规机织工艺生产织物增强体,以真空辅助树脂转移模塑法(VARTM)制备成型复合材料,研究单层平纹玄武岩长丝增强环氧树脂复合材料在准静态和高应变率加载下的拉伸性能。准静态和高应变率拉伸试验分别在MTS-810.23试验仪和分离式霍普金森拉杆(SHTB)测试系统上完成。试验结果表明该材料的力学性能具有应变率依赖性:随着应变速率的增加,拉伸模量和拉伸强度单调增加,失效应变单调减小,弹性能先增加后减小。材料的失效破坏特征也呈现明显的应变率效应:准静态拉伸时,材料断口整齐,树脂的破碎少,几乎没有纤维的抽拔和经纬向纤维束间的滑移;高应变率拉伸时,材料断口参差错乱,树脂完全破碎,纤维束抽拔严重、相互崩裂和滑移,织物增强体结构的整体性破坏严重。     

Hysteresis loops of carbon fiber-reinforced ceramic-matrix composites with different fiber preforms

The hysteresis loops of C/SiC ceramic-matrix composites (CMCs) with different fiber preforms, i.e., unidirectional, cross-ply, 2D and 2.5D woven, 3D braided, and 3D needled at room temperature have been investigated. Based on fiber slipping mechanisms, the hysteresis loops models considering different interface slip cases have been developed. The effects of fiber volume fraction, matrix cracking density, interface shear stress, interface debonded energy, and fibers failure on hysteresis loops, hysteresis dissipated energy, hysteresis width, and hysteresis modulus have been analyzed. An effective coefficient of fiber volume fraction along the loading direction (ECFL) was introduced to describe fiber preforms. The hysteresis loops, hysteresis dissipated energy and hysteresis modulus of unidirectional, cross-ply, 2D and 2.5D woven, 3D braided and 3D needled C/SiC composites have been predicted.     

Comparison of the mechanical hysteresis of carbon/ceramic-matrix composites with different fiber preforms

The mechanical hysteresis of four ceramic matrix composites with different carbon fiber preforms, i.e. needled C/SiC, 2D C/SiC, 2.5D C/SiC, and 3D C/SiC, was investigated and compared during cyclic reloading-unloading tests. An effective coefficient of the fiber volume fraction in the direction of loading (ECFL) was defined to characterize fiber architectures of the preforms. It is shown that an increase in permanent strain and a decrease in stiffness with the applied stress were strongly affected by the ECFL. The thermal residual stress (TRS) and ultimate tensile strength of the composites are predicted theoretically related to the ECFL, and then validated by experimental results and microstructural observations. The predicted results not only demonstrate good agreement with experimental measurements, but also explain why differences in the composite ECFL result in substantial variations in TRS.     

In-Plane Cracking Behavior and Ultimate Strength for 2D Woven and Braided Melt-Infiltrated SiC/SiC Composites Tensile Loaded in Off-Axis Fiber Directions

The tensile mechanical properties of ceramic matrix composites (CMC) in directions off the primary axes of the reinforcing fibers are important for the architectural design of CMC components that are subjected to multiaxial stress states. In this study, two-dimensional (2D)-woven melt-infiltrated (MI) SiC/SiC composite panels with balanced fiber content in the 0° and 90° directions were tensile loaded in-plane in the 0° direction and at 45° to this direction. In addition, a 2D triaxially braided MI SiC/SiC composite panel with a higher fiber content in the ±67° bias directions compared with the axial direction was tensile loaded perpendicular to the axial direction tows (i.e., 23° from the bias fibers). Stress–strain behavior, acoustic emission, and optical microscopy were used to quantify stress-dependent matrix cracking and ultimate strength in the panels. It was observed that both off-axis-loaded panels displayed higher composite onset stresses for through-thickness matrix cracking than the 2D-woven 0/90 panels loaded in the primary 0° direction. These improvements for off-axis cracking strength can in part be attributed to higher effective fiber fractions in the loading direction, which in turn reduces internal stresses on weak regions in the architecture, e.g., minicomposite tows oriented normal to the loading direction and/or critical flaws in the matrix for a given composite stress. Both off-axis-oriented panels also showed relatively good ultimate tensile strength when compared with other off-axis-oriented composites in the literature, both on an absolute strength basis as well as when normalized by the average fiber strength within the composites. Initial implications are discussed for constituent and architecture design to improve the directional cracking of SiC/SiC CMC components with MI matrices.     

制备工艺对亚麻增强聚丙烯复合材料拉伸性能的影响

以亚麻纤维为增强体,与聚丙烯（PP）长丝进行丝束级共混,形成PP包覆亚麻的纱线结构,利用机织工艺织成二维机织布,作为复合材料的预制件。采用层合热压方法制备PP／亚麻复合材料板材。通过对板材拉伸性能测试及扫描电镜（SEM）拉伸断口形貌分析,研究了不同纤维体积分数、织造密度及织造组织等因素对复合材料拉伸性能的影响。结果表明,在选取最优热压温度与压力的条件下,纤维体积分数为50％的板材性能最优;经向密度相同时,拉伸性能随着纬向密度的增加而提高;经、纬向密度均相同时,斜纹3／1组织的板材性能最优,纬向最大拉伸强度可达92．42 MPa。