纤维束丝数对平纹编织C/SiC复合材料力学性能的影响

通过对2种丝束平纹编织碳纤维布增强SiC（C/SiC）复合材料的力学性能实验,研究了纤维束丝数（1 k和3 k）对复合材料性能的影响.实验结果表明：1 k C/SiC复合材料的拉伸模量、拉伸强度、压缩模量、压缩强度、面内剪切强度和弯曲强度分别为90.8 GPa,281.8 MPa,135.8 GPa,452.2 MPa,464.3 MPa和126.8 MPa,分别比3 k C/SiC高39%,15.8%,25%,132%,29.3%和30.2%.纤维束粗细不同是导致纤维束弯曲度和复合材料孔隙率差异的主要原因,对压缩强度的影响最大,对拉伸强度的影响最小.     

C/SiC拉伸强度与声发射演化的关联性

对平纹编织C/SiC复合材料样品拉伸破坏过程的声发射进行监测,采用基于改进遗传算法的无监督聚类方法对声发射信号进行模式识别,统计分析各类声发射模式的特征及其演化过程,结合断口分析,研究了C/SiC复合材料的拉伸强度、损伤机制与声发射信号演化之间的关系.结果表明:维断裂的声发射能量能够反映纤维/基体界面结合强度;低强度C...     

C/SiC拉伸强度与声发射演化的关联性EI北大核心CSCD

对平纹编织C/SiC复合材料样品拉伸破坏过程的声发射进行监测,采用基于改进遗传算法的无监督聚类方法对声发射信号进行模式识别,统计分析各类声发射模式的特征及其演化过程,结合断口分析,研究了C/SiC复合材料的拉伸强度、损伤机制与声发射信号演化之间的关系.结果表明:维断裂的声发射能量能够反映纤维/基体界面结合强度;低强度C/SiC材料中存在引起应力集中的基体富集区,在加载初期基体开裂事件占比超过50%;中强度C/SiC材料由于较强的界面,纤维损伤以单丝或部分纤维断裂事件为主;高强度C/SiC材料界面结合强度适中,纤维簇断裂是主要的失效模式.     

三维编织Cf/SiC复合材料的拉伸破坏行为

通过三维编织碳纤维(carbon fiber，Cf)／SiC复合材料样品单向拉伸以及单向拉伸加卸载实验．结合样品断口观察．从宏观上分析了三维编织Cf／SiC复合材料单向拉伸时的力学响应，为进一步描述三维编织Cf／SiC复合材料力学行为奠定了实验基础。实验结果表明：三维编织Cf／SiC复合材料单向拉伸时，卸载模量衰减与应力呈线性关系，残余应变的增加与应力呈二次函数关系。微裂纹主要在编织节点处萌生，沿纤维束界面扩展，最终在编织节点处汇合，导致样品发生破坏。     

SiC/SiC复合材料高温面内剪切强度测试方法研究

以三维四向编织方式的碳化硅纤维预制体为增强相,选用聚碳硅烷为先驱体浸渍剂,采用聚合物先驱体浸渍裂解工艺制备了SiC纤维增强SiC陶瓷基(SiC/SiC)复合材料,进而采用自主设计研制的陶瓷基复合材料高温面内剪切测试夹具对SiC/SiC复合材料进行高温面内剪切强度测试,分析研究了试样形状尺寸、加载速率、夹具材料等对SiC/SiC复合材料高温面内剪切强度测试结果的影响,并分析了夹具材料、测试环境等对测试夹具寿命的影响,最终优化确认出一套较优的针对SiC/SiC复合材料的高温面内剪切强度测试方法。     

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.     

纤维丝束大小对Mini-C/SiC拉伸性能与强度分布的影响

为了探究碳纤维丝束大小对纤维束复合材料碳/碳化硅(Mini-C/SiC)拉伸性能和强度分布的影响,采用化学气相浸渗(CVI)法制备了1k Mini-C/SiC和3k Mini-C/SiC复合材料.测试了C纤维束以及Mini-C/SiC复合材料的拉伸性能,并采用两参数Weibull分布模型分析了强度分布,同时还观察了拉伸断口形貌.结果表明:3k C纤维束表现出了明显的"聚拢效应",其拉伸性能和强度稳定性均优于1k C纤维束,而且其拉伸强度、Weibull模数、特征强度、延伸率和断裂功分别比1k C纤维束的高47％、13％、46％、54％和102％.同时,1k C纤维束发生韧性断裂,3k C纤维束发生脆性断裂.3k Mini-C/SiC复合材料的拉伸性能和强度稳定性均优于1k Mini-C/SiC复合材料,其拉伸强度、Weibull模数、特征强度、延伸率和断裂功分别比1k Mini-C/SiC复合材料提高了67％、69％、63％、92％和216％,而且两者的拉伸断裂方式均为典型的脆性断裂.纤维体积分数高是大纤维丝束复合材料3k Mini-C/SiC拉伸性能和强度稳定性优于小纤维丝束复合材料1k Mini-C/SiC的主要原因.     

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。     

连续纤维复合材料环形试样蠕变行为研究

对连续玻璃纤维复合材料进行了拉伸蠕变试验研究,为了模拟复合材料在压力容器中的受力状态并减少夹具加持力对试样的影响,采用环形复合材料试样拉伸蠕变试验方法。对复合材料环形试样的拉伸强度及不同应力等级下的拉伸蠕变性能进行了研究,并基于时间?应力等效原理,通过双对数法拟合出压力容器50年使用寿命时复合材料的最大蠕变应力,为复合材料压力容器的设计提供支持。并基于时间?应力等效原理,通过双对数法拟合出压力容器50年使用寿命时复合材料的最大蠕变应力应低于其拉伸强度的44.4 %。     

热处理工艺对Mini C/SiC复合材料拉伸性能和强度分布的影响

采用5种工艺制备了C纤维束增韧SiC陶瓷基复合材料(Mini C/SiC),研究了热处理工艺对不同制备工艺条件下Mini C/SiC复合材料拉伸性能和强度分布的影响。实验结果表明:在不进行热处理的Mini C/SiC复合材料中引入热解炭(PyC)界面相可提高拉伸性能和强度稳定性。与不进行热处理的MiniC/SiC复合材料相比,对引入PyC界面相复合材料的C纤维束和/或PyC界面相进行热处理均可提高拉伸性能。热处理温度小于等于1700℃时,先对C纤维进行热处理然后再沉积PyC界面相的MiniC/SiC复合材料,其拉伸性能最好。热处理温度为2 000℃时,先对C纤维沉积PyC界面相然后再进行热处理的Mini C/SiC复合材料,其拉伸性能最好。热处理温度对Mini C/SiC复合材料变形行为有着显著的影响,热处理温度不同时,复合材料表现出了不同的变形行为。     

Interlaminar shear behaviors of 2D needled C/SiC composites under compressive and tensile loading

The interlaminar shear strength of 2D needled C/SiC composites was measured using the double-notch shear test method. Interlaminar shear tests were performed under compressive and tensile loading. Shear stress–strain response and shear strain field evolution were studied using the digital image correlation (DIC) technique. The results show that the interlaminar shear strength of the specimen using the compressive loading method is 15% higher than that of the tensile loading method. Severe shear strain concentration was observed near the upper notch of the tensile loading specimen. Acoustic emission (AE) was utilized to monitor the damage during the tests. Typical damage mechanisms were categorized according to AE signal characteristics. The statistical results show that more matrix cracks were produced in the tensile loading specimen and no separate fiber/matrix debonding signal was detected in both specimens.     

Analysis of the nonlinear behavior of adhesives in bonded assemblies—Comparison of TAST and Arcan tests

This paper describes a study in which the shear behavior of a structural epoxy adhesive has been measured using the standard thick adherend shear test (TAST) specimen and a modified Arcan test. A numerical study of the TAST test taking into account the nonlinear behavior of the adhesive and the finite deformations of the adhesive joint, shows that there is a localization of plastic zones close to the adhesive–substrate interface near the free edge of the adhesive. Experimental tests carried out with steel and aluminum substrates and with various adhesives also show that failure initiates in this region. These edge effects in the TAST fixture can lead to an incorrect analysis of the behavior of the adhesive (for instance, underestimation of the shear stress in the joint at failure), particularly when an adhesive failure mode is dominating. The modified Arcan fixture provides a more homogeneous stress state. A similar improvement of the TAST fixture is proposed.     

Compression behavior of woven glass/epoxy specimens using a new end‐loaded hybrid specimen

A new compression specimen was applied to woven glass/epoxy laminates. The specimen consists of epoxy layers cast on the sides of the laminate to prevent buckling. Thin‐sheet aluminum ends enable alignment and avoid crushing under end loading, which does not require any special fixture. The compression stress–strain behavior of the laminate was obtained from the specimens by discounting the previously measured stress–strain curve of the epoxy backings. Despite the higher scatter in compression tests, the average modulus was practically identical to the tensile modulus. Moreover, failure occurred away from the ends in nearly all of the specimens tested. The average compressive strength was 84% of the tensile strength and consistent with the flexural strength measured in four‐point bending tests. The present compression specimen could, therefore, become an interesting alternative to the more elaborate standard test methods available. Nevertheless, this new compression testing approach needs further evaluation involving application to other materials. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers     

Through Thickness Mechanical Properties of Chemical Vapor Infiltration and Nano‐Infiltration and Transient Eutectic‐Phase Processed SiC/SiC Composites

The through thickness (interlaminar) shear strength and trans‐thickness tensile strength of three different nuclear‐grade SiC/SiC composites were evaluated at room temperature by the double‐notched shear and diametral compression tests, respectively. With increasing densification of the interlaminar matrix region, a transition in failure locations from interlayer to intrafiber bundle was observed, along with significant increases in the value of the interlaminar shear strength. Under trans‐thickness tensile loading, cracks were found to propagate easily in the unidirectional composite. The 2D woven composite had a higher trans‐thickness tensile strength (38 MPa) because the failure mode involved debonding, fiber pull‐out and fiber failure.     

A Test Method to Measure Shear Strength of Ceramic Joints at High Temperatures

The asymmetrical four-point bend (AFPB) method, which provides a pure shear stress state in the middle of the test specimen, was utilized to measure the shear strength of the joints between Nicalon-fiber-reinforced SiC composites. The small test specimens were prepared as butt-joints with uniform depth (no notch). It was shown experimentally that the position of loading points is critical to induce shear fracture at the joint without the failure of base material. The appropriate positions for the present test specimen were determined and, as a result, an articulated ceramic fixture was designed. This test method is economical and involves small test specimens with simple geometry, and can be used at high temperatures.     

Cracking Resistance of Silicon Carbide Composites by Single- and Double-Notched Specimen Techniques

This paper aims to evaluate fracture resistance of nanoinfiltration transient eutectic phase-sintered (NITE)–silicon carbide fiber-reinforced silicon carbide matrix (SiC/SiC) composites, using the single-edge notched bend (SENB) test and the double-notch tensile (DNT) test. Both SENB and DNT test results identified notch insensitivity of NITE–SiC/SiC composites. With the fact of notch insensitivity, unique proportional limit stress, and fracture strength were identified regardless of the presence of artificial notches. By applying the nonlinear fracture mechanics, micro- and macrocracking energies were separately estimated. Specifically, lower microcrack formation energy was identified compared with conventional low-stiffness composite system. Finally, superior crack resistance of NITE–SiC/SiC composites was clearly demonstrated.     

Strength of a Ceramic at High Temperatures Under Biaxial and Uniaxial Tension

A design is presented for a fixture to test concentric-ring loaded disks and four-point loaded plates at high temperatures. In this fixture the loading plunger is guided by flexure plates to determine the load path with accuracy. The uniaxial and equibiaxial tensile strengths of a brittle material were measured at 982°C with this fixture. For lapped surfaces, the equibiaxial tensile strength of dense, isostatically pressed, sintered alumina was 16.0% lower than the uniaxial tensile strength. The uniaxial strength at 982°C was 42.3% lower than the previously reported value for comparable tests at room temperature. The effect of frictional forces and lateral constraint, of specimen and loading geometry, and of local stresses on the nominal stress are discussed.     

Experimental and numerical investigations of crack face adhesive bonding effect on the mixed-mode fracture strength of PMMA

Experimental and numerical investigations have been conducted to evaluate the effect of adhesive bonding of crack surfaces on the mixed-mode (I and II) fracture strength and effective stress intensity geometry/loading factor of a plate with an edge crack. The experimental tests were carried out on five batches of simple edge crack and specimens in which adhesive bonding was used on crack faces at different distances from the crack tip. The cracked specimens made from poly methyl-methacrylate rectangular plates. The specimens’ fracture strength was obtained by employing a tensile testing machine at different loading angles using a modified Arcan fixture. In the numerical part, finite element simulations were used to model the test specimens and thereby establishing their stress intensity geometry/loading factors. The results show that the adhesive bonding of the crack surfaces has a significant effect on reducing the equivalent mixed-mode stress intensity factor for all loading angles. The bonded specimens show considerable fracture force enhancement compared to the simple edge crack specimens.     

Characterization of Interlaminar Shear Strength of Ceramic Matrix Composites

The interlaminar shear strengths of three ceramic matrix composites have been characterized using a double-notch shear (DNS) test. The material systems investigated are plain woven C/SiC, plain woven SiC/SiC, and cross-plied SiC/calcium aluminosilicate-II. The use of the double-notch shear test for measuring the interlaminar shear strength of ceramic matrix composites is evaluated first. Numerical stress analyses are performed to investigate the effect of DNS specimen length, notch distance, and specimen supporting jig on the stress distribution in the expected fracture plane and the interlaminar shear strength. The numerical findings are then compared with an analytical model proposed elsewhere and correlated with the experimental results. The validity of this test technique has been established.