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.     

先驱体转化-热压单向Cf/SiC复合材料的高温弯曲力学行为

研究了采用先驱体转化－热压烧结制备的单向C1／SiC复合材料室温,1573,1723,1923K温度下力学行为,并从显微结构的特征分析了单向Cf/SiC复合材料高温力学行为的变化原因,结果表明：C1/SiC复合的室温,1573,1723,1923K温度下弯曲强度分别为550,392,394,574MPa,弯曲模量分别为157,148,132,83GPa,Cf/SiC复合材料破坏时,其破坏方式将从室温和573K的分层断裂向1723K,1923K的脆性断裂转化,Cf/SiC显微结构的分析表明,在纤维周围和大晶粒间存在着大量的有一定结晶程度的玻璃相,它在高温时的软化对Cf/SiC复合材料的高温强度和弯曲模量变化规律起到重要的支配作用。     

三维编织复合材料有效模量的计算

三维编织复合材料的模型是计算机辅助设计复合材料的基础，但是目前的研究还很不成熟。本文应用三维编织复合材料的单胞模型，应用有限元分析软件ANSYS模拟了三维编织复合材料的编织结构，并模拟了得到了复合材料拉伸时的位移场和应力场，并由ANSYS的应力和应变结果计算了复合材料的等效弹性模量。其计算结果与实验结果符合较好。     

注塑机拉杆三角螺纹端卸载槽的多目标优化

针对注塑机拉杆三角螺纹在近卸载槽处发生早期断裂的问题，在用ANSYS软件对近卸载槽处三角螺纹进行有限元分析的基础上，就拉杆三角螺纹端卸载槽的几何特性建立参数优化模型，以控制三角螺纹在近卸载槽附近的最大应力、卸载槽的最大应力以及卸载槽加工量最小化为统一的目标函数。通过对拉杆在单向拉伸力作用下的多目标优化，可使拉杆三角螺纹牙根在卸载槽处的最大应力降低55％，在偏载作用下，其最大应力降低21．4％。在单向拉伸力作用下，拉杆卸载槽的最大应力比平均单向拉伸应力提高26．5％，使卸载槽能够较好地发挥其卸载的作用。     

聚碳硅烷浸渍裂解法制备的C／SiC材料研究

为提高C／SiC复合材料的机械性能、特别是断裂韧性，对增强体有特殊要求。本文研究了不同纤维增强形式对C／SiC复合材料机械性能的影响。结果表明，含C＋SiC过渡层的单向Cr增强SiC基复合材料的断裂韧性达到14．4MPa·m1/2，为未增韧的SiC的四倍，适当的界面结合是使不同的纤维增韧机制在较高的水平下协同作用的关键。四步法三维增强体是整体性的编织结构，经PCS浸清裂解循环8次,Vf＝48．0％，x：y：z＝4：1：1的四步法三维整体织物增强SiC基复合材料，ρ达到1·71g／cm3，σ\弯达到440MPa，σ剪达到32．3MPa，断裂韧性Kic达到12．68MPa·m1/2。     

二维机织碳纤维/碳化硅陶瓷基复合材料损伤分析

利用声发射技术全程监测二维机织C／SiC复合材料拉伸实验,通过声发射多参数分析法和断口显微观察,结合材料拉伸应力-变曲线,分析了二维机织C／SiC复合材料拉伸损伤演化过程和损伤机理。结果表明：材料拉伸损伤演化经历3个阶段：第一阶段为无损伤阶段,材料无损伤发生;第二阶段为损伤初始阶段．损伤主要为微裂纹开裂．并且微裂纹开裂基本上均匀发生在样品工作段;第三阶段为损伤加速阶段,损伤主要为宏观基体、界面开裂和纤维束断裂．井且集中发生在断口区域。损伤第二阶段与第三阶段的转换点在拉伸强度的76％左右,转换点的确定对二维机织C／SiC复合材料工程应用有重要意义。     

Cf/SiC复合材料生产工艺研究

Cf／SiC复合材料克服了单一SiC材料韧性低、烧结过程中晶粒长大造成强度下降等缺点，本文就Cf／SiC复合材料的生产工艺进行了综述。     

超高分子量聚乙烯纤维/有机玻璃复合材料摩擦磨损性能研究

用真空浸渍法成功制备出了超高分子量聚乙烯纤维／有机玻璃(UHMWPE／PMMA)复合材料，并对基体材料PMMA，单向超高分子量聚乙烯纤雏／有机玻璃复合材料以及三维编织超高分子量聚乙烯纤维／有机玻璃(即UHMWPE3D／PMMA)复合材料的摩擦磨损性能进行了研究。实验证明UHMWPE／PMMA复合材料具有优良的摩擦磨损性能。经过纤维增强的复合材料的摩擦磨损性能优于基体材料，三维编织纤维增强的复合材料其磨损远小于单向纤维增强的复合材料，但其摩擦系数没有显著变化。     

2D-SiC/SiC陶瓷基复合材料的拉伸本构模型研究

通过单向拉伸试验,研究了2D-SiC/SiC复合材料的应力-应变行为.结果表明,材料单向拉伸应力-应变曲线表现出明显的双线性特征,且线弹性段较长.通过试件断口照片,分析了2D-SiC/SiC复合材料单向拉伸破坏机理和损伤模式.基于对损伤过程的假设,建立了二维连续纤维增强陶瓷基复合材料的双线性本构模型,并将其应用于2D-SiC/SiC复合材料的应力-应变曲线模拟,模拟结果与试验值吻合很好.同时,分析计算表明,2D-SiC/SiC复合材料的单轴拉伸行为主要由纵向纤维柬决定,横向纤维对材料的整体模量和强度贡献很小.     

Preparation and performance of braid‐reinforced poly(vinyl chloride) hollow fiber membranes

A novel braid‐reinforced (BR) poly(vinyl chloride) (PVC) hollow fiber membrane was fabricated via dry‐wet spinning process. The mixtures of PVC polymer solutions were uniformly coated on the tubular braid which contained polyester (PET) and polyacrylonitrile (PAN) fibers. The influences of braid composition on structure and performance of BR PVC hollow fiber membranes were investigated. The results showed that the prepared BR PVC hollow fiber membranes were composed of two layers which contained separation layer and tubular braid supported layer when the PET and PET/PAN hybrid tubular braids were used as the reinforcement. But the sandwich structure appeared when the PAN tubular braid was used as the reinforcement, which revealed outer separation layer, tubular braid supported layer and the inner polymer layer. The BR PVC hollow fiber membranes that prepared by PET/PAN hybrid tubular braid had favorable interfacial bonding state compared with the membrane prepared by pure PET or PAN tubular braid. The pure water flux of the BR PVC hollow fiber membranes that prepared by the PET/PAN hybrid tubular braid were lower than that prepared by pure PET or PAN tubular braid, but the rejection of Bovine serum albumin was opposite. The tensile strength of prepared BR PVC hollow fiber membrane was higher than 50 MPa. Both of the tensile strength and elongation at break decreased with the increase of the PAN filaments in the PET/PAN hybrid tubular braid. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45068.     

Micromechanical model of time-dependent damage and deformation behavior for an orthogonal 3D-woven SiC/SiC composite at elevated temperature in vacuum

This paper presents a micromechanical model to predict the time-dependent damage and deformation behavior of an orthogonal 3-D woven SiC fiber/BN interface/SiC matrix composite under constant tensile loading at elevated temperature in vacuum. In-situ observation under monotonic tensile loading at room temperature, load–unload tensile testing at 1200 °C in argon, and constant load tensile testing at 1200 °C in vacuum were conducted to investigate the effects of microscopic damage on deformation behavior. The experimentally obtained results led to production of a time-dependent nonlinear stress–strain response model for the orthogonal 3-D woven SiC/SiC. It was established using the linear viscoelastic model, micro-damage propagation model, and a shear-lag model. The predicted creep deformation was found to agree well with the experimentally obtained results.     

Improving bonding strength between a hydrophilic coating layer and poly(ethylene terephthalate) braid for preparing mechanically stable braid‐reinforced hollow fiber membranes

A braid‐reinforced hollow fiber membrane with mechanically stable coating layer was prepared by coating a blended polymer dope solution on an alkaline‐treated poly(ethylene terephthalate) (PET) braid. The alkaline treatment was carried out to endow the PET braid surface with more polar groups and better hydrophilicity. The results showed that the bonding strength between the hydrophilic coating layer and modified PET braid was about two times as great as that between the coating layer and original PET braid, while the pure water permeability (PWP) of the membrane remained unchanged when the PET braid was simply treated in 3 wt % potassium hydroxide (KOH) solution at 90 °C for 1 h or 1 wt % KOH solution for 6 h. The proposed modification approach proved to be a facile, low‐cost, and effective method to improve bonding strength between the coating layer and the braid, while other properties, such as PWP and morphology of the coating layer, of the braid‐reinforced hollow fiber membranes were not altered, indicating promising potential in membrane engineering. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46104     

Bulge forming of braided thermoplastic composite tubes under axial compression and internal pressure

In this study, the bulge forming of fiber reinforced thermoplastic (FRTP) composite braided tubes was studied as a new forming technique utilizing fiber orientation rearrangement (the trellis effect) at temperatures near the matrix melting point. It is shown that free bulge forming with the trellis effect can easily result in high expansion without buckling only by axial compression. This is when the stress of the matrix is small enough to rearrange the fiber orientation of the braid. To improve the surface quality and dimensional accuracy, bulge forming with closed dies and internal pressure was also examined experimentally. The appropriate conditions for axial penetration and internal pressure are established. The kinematics of braid is also considered, and calculations from the model are evaluated from experiments. In addition, the kinematic state of braid after deformation is simulated by calculation from the geometric braid model. It is found that the “normalized profile” of the bulged tube does not change during the process and the thickness of the matrix depends on changing fiber orientation.     

Improved bending fatigue life of Kevlar 29 braid by the use of an impregnating medium

Kevlar 29 fiber has been widely considered for the manufacture of very-long high-performance cables. Due to low transverse strength fiber-on-fiber rubbing leads to rapid deterioration. The usefulness of impregnating Kevlar 29 braids with resin to overcome this drawback has been evaluated by performing reverse bending fatigue tests. Braids of identical construction, one of them being impregnated with a polyurethane resin by a patented process, were fatigued to failure on a purpose-built rig under varying applied loads. Fatigue tests were also carried out to 30 percent and 50 percent of total braid life and residual strength values measured. It was found that high applied loads (～50 percent of ultimate) lead to premature braid failure dominated by a creep mechanism. Both braids showed similar behavior, although the impregnated braid was superior. At low applied loads, however, where the failure mechanisms was dominated by wear or internal abrasion, it was seen that resin impregnation could increase braid life by a factor of four. Resin impregnation coupled with bending fatigue significantly stiffens braids, as was demonstrated by tensile testing up to braid failure.     

Effects of Fiber Architecture on Matrix Cracking for Melt-Infiltrated SiC/SiC Composites

The matrix cracking behavior of slurry cast melt-infiltrated SiC matrix composites consisting of Sylramic-iBN fibers with a wide variety of fiber architectures were compared. The fiber architectures included 2D woven, braided, 3D orthogonal, and angle interlock architectures. Acoustic emission was used to monitor in-plane matrix cracking during unload–reload tensile tests. Two key parameters were found to control matrix-cracking behavior: the fiber volume fraction in the loading direction and the area of the weakest portion of the structure, that is, the largest tow in the architecture perpendicular to the loading direction. Empirical models that support these results are presented and discussed.     

用有限元法研究短纤维／橡胶复合材料内应力传递

采用有限元法对短纤维／橡胶复全材料内应力传递和分配规律进行了初步探讨。结果表明：纤维末端的界面剪切应力和纤维中部的拉伸应力均最大，纤维末端仍传递一定的拉伸应力；纤维体积分数越大，纤维末端端应力集中越严重；纤维长径比越大，纤维上的最大拉伸 力也越大，但增幅越于平缓；纤维模量增加，纤维上的拉伸应力集中也增加，但增减缓，基南模量增加，界面剪切应力集中和纤维拉伸应力集中均下降，但剪切应力和拉伸应力的红色     

Multiple Cracking and Tensile Behavior for an Orthogonal 3-D Woven Si-Ti-C-O Fiber/Si-Ti-C-O Matrix Composite

This paper presents experimental results for the multiple microcracking and tensile behavior of an orthogonal 3-D woven Si-Ti-C-O fiber (Tyranno™ Lox-M)/Si-Ti-C-O matrix composite with a nanoscale carbon fiber/matrix interphase and processed using a polymer impregnation and pyrolysis route. Based on microscopic observations and unidirectional tensile tests, it is revealed that the inelastic tensile stress/strain behavior is governed by matrix cracking in transverse (90°) fiber bundles between 65 and 180 MPa, matrix cracking in longitudinal (0°) fiber bundles between 180 and 300 MPa, and fiber fragmentation above 300 MPa. A methodology for estimation of unidirectional tensile behavior in orthogonal 3-D composites has been established by the use and modification of existing theory. A good correlation was obtained between the predicted and measured composite strain using this procedure.     

三维管状编织物的制备及力学性能实验研究

本文介绍了两种三维管状编织物的制备方法,一种采用传统四步法编织,另一种采用针织和编织相结合的新型编织方法(多轴向编织)加工而成。对比分析了两种结构管状编织物的拉伸和弯曲力学性能。结果表明,在纤维体积含量相同的情况下,多轴向编织结构拉伸性能不如四步法编织结构,但弯曲性能接近四步法编织结构;轴纱的加入使多轴向管状编织物的拉伸性能变弱,弯曲性能增强。本文研究结果对于上述两种管状编织物的结构设计与性能研究具有重要的指导作用。     

A new model for the cracking process and tensile ductility of Strain Hardening Cementitious Composites (SHCC)

Strain Hardening Cementitious Composites (SHCC) are materials exhibiting tensile hardening behavior up to several percent strain accompanied by the formation of fine multiple cracks. Their tensile ductility is governed by the spacing and opening of cracks, which depend on the stress transfer between the fibers and the matrix. In this article, a new analytic model which takes into consideration the effects of non-uniform matrix strength, post-cracking increase in fiber bridging stress and fiber rupture on stress transfer and multiple cracking behavior of SHCC is developed. Using material parameters within the range reported in the literature, simulation results can reach reasonable agreement with test data on SHCC for two different fiber contents. The effect of fiber length on tensile behavior of SHCC is then simulated to illustrate the applicability of the model to material design. The new model should be helpful to the micromechanics-based design of SHCC for various ductility requirements.