碳纤维编织复合材料拉伸变形测量及声发射监测

采用声发射和数字图像相关互补技术,结合破坏断口微结构特征,研究碳纤维编织复合材料的损伤变形与失效机理。在复合材料试件拉伸加载的同时,实时获取变形特征和损伤声发射信号,分析复合材料力学响应与位移场、声发射特征的关系。结果表明,复合材料试件实时拉伸位移场、损伤破坏过程的声发射相对能量、撞击累积数及幅度等特征参数反映了复合材料表面变形与内部损伤演化过程。复合材料试件断裂时出现较多高持续时间、高幅度、高相对能量的声发射信号,宏观断口平齐,表现为脆性断裂。     

双轴向经编和三维正交机织增强复合材料拉伸性能实验研究

为对比双轴向经编和三维正交机织玻纤复合材料沿0°和90°方向拉伸性能,分别以0°/90°双轴向经编织物和三维正交机织物为增强体,E-2511-1A环氧树脂/2511-1BT固化剂为基体,通过VARTM成型,测试试样沿0°和90°拉伸性能。结果表明,该双轴向经编复合材料仅在0°方向拉伸强度和当量强度略高,弹性模量和当量模量均弱于三维正交机织复合材料,这与增强体纱线线密度和织造密度紧密相关。复合材料拉伸断裂截面显示,三维正交机织物内Z纱有效改善了双轴向经编复合材料易拉伸分层失效和断裂截面处纤维抽拔、脱粘等问题。     

三维机织复合材料力学性能分析与研究

通过对三维机织复合材料几何细观结构的研究,分析了三维机织复合材料的力学性能,采用椭圆形纤维束截面假设并结合实际的纱线形态建立了一种新的三维机织复合材料力学模型,对三维机织复合材料的拉伸、压缩和层间剪切强度进行了理论分析,并与实验结果进行了对比,验证了力学模型的正确性。     

铁磁非晶丝对复合材料受力状态的响应行为研究

采用VARTM工艺制备碳纤维增强树脂基复合材料和内嵌非晶丝的复合材料,通过铁磁非晶丝阻抗变化情况来分析复合材料的受力状态响应行为。试验结果表明：试件断裂损伤主要表现为基体裂纹的产生,层间出现分层断裂,材料损伤加剧直至整个试件发生断裂;在各拉伸应力下,铁磁非晶丝的阻抗均表现出随着拉伸应力的增加先增加后减小的趋势,验证了铁磁非晶丝监测复合材料的结构健康的可行性。     

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。     

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.     

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。     

长压缩应力-应变平台的聚氨酯弹性体泡沫的研究

制备了具有长压缩应力-应变平台的聚氨酯弹性体泡沫(PUEF),研究了硬段含量、交联度以及密度等对PUEF压缩应力-应变性能、压缩应力松弛及拉伸性能的影响。结果表明,硬段含量及密度对PUEF压缩应力-应变性能的影响较大,而交联度对压缩应力松弛及拉伸性能的影响较大。在相同的原料体系下,随着硬段含量及密度的增加,PUEF的应变降低,应力增大,压缩应力松弛增大,断裂伸长率降低,拉伸强度增大;随着交联点的增加,PUEF的压缩应力松弛降低,断裂伸长率下降,拉伸强度下降。     

二维与三维机织复合材料的力学性能实验研究

通过对超厚三维正交机织复合材料及二维机织层合板分别进行拉伸和压缩实验，研究比较两复合材料刚度和强度特性的差异；研究发现无论是三维机织材料的拉、压，还是二维层合板的拉、压的应力一应变曲线都可近似为直线关系，而且具有脆性破坏的特点；三维复合材料的拉、压强度要高于二维层合板，是由于不同的增强相结构及纤维含量造成；不同的破坏模式对材料强度影响很大。     

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.     

Controlled energy dissipation in fibrous composites. II: Microscopic failure mechanisms

Mechanical properties and microscopic fracture mechanisms of continuous fiber reinforced polymer composites were investigated. Perforated polyimide films (e.g. Kapton®) were added between composite prepreg layers to modify the interlaminar bonding strength. Addition of highly perforated films can increase the toughness of unidirectional glass/epoxy composites without an appreciable reduction in strength. The fibrous composites studied exhibit two fracture modes (compressive and tensile) when failed by three-point bending. In general, the compressive failure mode preceded the tensile failure mode. Real-time acoustic emission (AE) analysis was found to provide more fracture information which is otherwise not discernible from mechanical testing alone. The crack initiation stress level and the subsequent crack propagation mode were identified by real-time AE during deformation and by post-failure scanning electron microscopy fracture surface analysis.     

Compressive behaviour of rigid rod polymer fibres and their adhesion to composite matrixes

Abstract

The deformation behaviour of the new high performance polymer fibres, poly(p-phenylene benzobisoxazole) (PBO) and polypyridobisimidazole (PIPD) and their adhesion to an epoxy composite matrix have been investigated. Both fibres give well defined Raman spectra, and the deformation micromechanics of PBO and PIPD single fibres and composites were studied from stress induced Raman band shifts. Single fibre stress-strain curves were determined in both tension and compression, thus providing an estimate of the compressive strength of these fibres. It was found that the PIPD fibre has a higher compressive strength (~1 GPa) than PBO (~0·3 GPa) and other high performance polymer fibres, because hydrogen bond formation is possible between PIPD molecules. It has been shown that when PBO and PIPD fibres are incorporated into an epoxy resin matrix, the resulting composites show very different interfacial failure mechanisms. The fibre strain distribution in the PBO-epoxy composites follows that predicted by the full bonding, shear lag model at low matrix strains, but deviations occur at higher matrix strains due to debonding at the fibre/matrix interface. For PIPD-epoxy composites, however, no debonding was observed before fibre fragmentation, indicating better adhesion than for PBO as a result of reactive groups on the PIPD fibre surface.     

Effect of interfacial residual thermal stress on the fracture behavior of Cf/B4C composites prepared by spark plasma sintering

Carbon fiber reinforced boron carbide (Cf/B₄C) composites were fabricated with different Cfs contents by spark plasma sintering (SPS). The interfacial residual thermal stress (RTS) distribution in the Cf/B₄C composites and its effects on the fracture behavior of the composites was investigated based on Raman spectroscopy and finite-element (FE) calculation. The results show that the maximum RTS of Cf in as-produced composites were obtained at the edge of fibers. The B₄C matrix among fibers suffered a high compressive and tensile RTS simultaneously in radial and tangential directions, which caused the formation of micro-cracks and became the origin of fracture failure of the Cf/B₄C composites. The maximum relative density 98.7% and bending strength of 391.41 MPa were obtained when the Cf content was 2.5% while the maximum fracture toughness of 5.46 MPa·m^1/2 at 5% Cf content of Cf/B₄C sample. The sharp decreases of bending strength when increasing content of Cf cloth was mainly caused by high RTS in the composites.     

Acoustic emission and electrical resistance in SiC-based laminate ceramic composites tested under tensile loading

Acoustic emission and electrical resistance were monitored for SiC-based laminate composites while loaded in tension and correlated with damage sources. The ceramic matrix composites were composed of Hi-Nicalon Type S™ fibers, a boron-nitride interphase, and pre-impregnated (pre-preg) melt-infiltrated silicon/SiC matrix. Tensile load-unload-reload or tensile monotonic tests were performed to failure or to a predetermined strain condition. Some of the specimens were annealed which relieved some residual matrix compressive stress and enabled higher strains to failure. Differences in location, acoustic frequency and energy, and quantity of matrix cracking have been quantified for unidirectional and cross-ply type architectures. Consistent relationships were found for strain and matrix crack density with acoustic emission activity and the change in measured electrical resistance measured at either the peak stress or after unloading to a zero-stress state. Fiber breakage in the vicinity of composite failure was associated with high frequency, low energy acoustic events.     

The compressive and tensile behavior of a 0/90 C fiber woven composite at high strain rates

An investigation into the compressive and tensile behavior of a carbon fiber reinforced resin matrix composite at high strain rates is carried out using a split Hopkinson bar. All the dynamic tests are performed under the condition of stress equilibrium and constant strain rate. The results of the compressive tests show that the failure strength and strain of the composite increase with the increase of strain rate. A plateau is observed in a typical stress–strain curve which prompts further study into the failure mechanism by monitoring the failure process with a high-speed camera. The three-phase failure mechanism of on-impact compression, crack-induced unloading, and crack deviation-caused further condensation, is found to have greatly increased the strength and toughness of the composite. In the tensile tests, an increase of strain rate produces a reduced fracture angle and extended crack path. In this process, more failure energy is absorbed, thus the failure strength and strain of the composite are improved. The Cowper–Symonds model of strain rate dependency indicates that the material has a higher tensile strength than compressive strength, and the strain rate sensitivity is more noticeable at high stain rates than quasi-static conditions.     

Application of a gripping system to test a uniaxial graphite fiber-reinforced composite (PMR 15/celion 6000) in tension at 316°C

A gripping system has been developed to test uniaxial, 0° orientation PMR 15/Celion 6000 composites at elevated temperatures. The method involves compression of grit-blasted laminate between grit-blasted metal to give a non-slipping interface for load transfer. Tensile testing at both 316°C and room temperature indicated that deformation was elastic to fracture and that the variation in tensile properties for one laminate is the same as that for several panels. In addition, the tensile properties for uniaxial PMR 15/Celion 6000 are identical at 316°C and room temperature. For nominally 51 volume percent fiber, the elastic modulus is 119.6 GPa, the fracture stress is 1370 MPa, and the strain to fracture is about 1.15 percent. In addition, data are presented which indicate that the gripping system can be used for long term, elevated temperature testing of composite materials.     

The contribution of thermal stresses to the failure of Kevlar fabric composites

The mismatch in thermoelastic properties between fiber and matrix in Kevlar 49 fabric-epoxy composites is shown to result in significant thermal stresses with cool down from processing temperatures. Cooling generates local transverse tensile stresses that can potentially initiate microcracking at ambient conditions. A temperature reduction also places the curved fiber in the fabric composite in axial compression. This compression adds to the bending strain in the fiber, resulting in significant local reduction of its inherently low compressive load-bearing capability. The combination of thermal stresses and external compressive loads that are below ultimate values can cause local compressive failure of the fiber. The kink bands formed as a result of compressive failure of Kevlar fiber are expected to cause debonding between fiber and matrix and, therefore, are also potential sites for crack initiation. Thus, thermal stresses can contribute to the initiation of at least two damage mechanisms that may severely limit the compressive and flexural fatigue strength of Kevlar fabric composites at and below ambient temperature.     

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.     

Creep damage resistance of ceramic–matrix composites

The tensile creep and creep fracture properties in air at 1300 °C are documented for two ceramic fibre-reinforced ceramic–matrix composites (CFCMCs). These recently developed materials were produced with woven bundles of Hi-Nicalon™ fibres reinforcing either A1₂O₃ or enhanced SiBC matrices, allowing data comparisons to be made with similar CFCMCs having different fibre–matrix combinations. The results confirm that the longitudinal fibres govern the rates of strain accumulation and crack growth, but the fracture characteristics are determined by fibre failure caused by oxygen penetration as matrix cracks develop. The analysis then suggests that carbon fibre-reinforced doloma–matrix composites could offer a combination of creep-resistant fibres and creep damage-resistant matrices suitable for long-term load-bearing service in high-temperature oxidizing environments.