The effect of gauge length on tensile strength and Weibull modulus of polyacrylonitrile (PAN)- and pitch-based carbon fibers

Carbon fibers are widely used as a reinforcement in composite materials because of their high specific strength and modulus. Current trends toward the development of carbon fibers have been driven in two directions; ultrahigh tensile strength fiber with a fairly high strain to failure (~2%), and ultrahigh modulus fiber with high thermal conductivity. Today, a number of ultrahigh strength polyacrylonitrile (PAN)-based (more than 6 GPa), and ultrahigh modulus pitch-based (more than 900 GPa) carbon fibers have been commercially available. In this study, the tensile strengths of PAN- and pitch-based carbon fibers have been investigated using a single filament tensile test at various gauge lengths ranging from 1 to 250 mm. Carbon fibers used in this study were ultrahigh strength PAN-based (T1000GB, IM600), a high strength PAN-based (T300), a high modulus PAN-based (M60JB), an ultrahigh modulus pitch-based (K13D), and a high ductility pitch-based (XN-05) carbon fibers. The statistical distributions of the tensile strength were characterized. It was found that the Weibull modulus and the average tensile strength increased with decreasing gauge length, a linear relation between the Weibull modulus, the average tensile strength and the gauge length was established on log–log scale. The results also clearly show that for PAN- and pitch-based carbon fibers, there is a linear relation between the Weibull modulus and the average tensile strength on log–log scale.     

The effect of high-temperature vapor deposition polymerization of polyimide coating on tensile properties of polyacrylonitrile- and pitch-based carbon fibers

Carbon fibers are widely used as a reinforcement in composite materials because of their high-specific strength and modulus. Current trends toward the development of carbon fibers have been driven in two directions; ultrahigh tensile strength fiber with a fairly high strain to failure (~2 %), and ultrahigh modulus fiber with high-thermal conductivity. Today, a number of ultrahigh strength polyacrylonitrile (PAN)-based (more than 6 GPa), and ultrahigh modulus pitch-based (more than 900 GPa) carbon fibers have been commercially available. In the present work, the tensile properties of polyimide-coated PAN-based (T1000GB, T300, and M60JB) and pitch-based (K13D and XN-05) carbon fibers have been investigated using a single-filament tensile test. The pyromellitic dianhydride/4-4′-oxydianiline polyimide coating was deposited on the carbon fiber surface using high-temperature vapor deposition polymerization (VDP_H). The Weibull statistical distributions of the tensile strength were characterized. The results clearly show that the VDP_H polyimide coating improves the tensile strength and the Weibull modulus of PAN- and pitch-based carbon fibers.     

Behavior of 3D orthogonal woven CFRP composites. Part I. Experimental investigation

This paper presents an experimental investigation of the mechanical behavior and failure mechanism of three-dimensional (3D) orthogonal woven CFRP composite panels. The 3D composite panels are preformed using Torayca T-300 (3K) carbon fiber, and then infused with the Epicote 828 epoxy resin. The nominal proportions of the stuffer yarn, the filler yarn and the warp weaver (or z yarn) are 1:1.2:0.2, respectively, and the overall fiber volume fraction is 43%. The 3D fiber architectures are measured and visualized in a micrograph form. Quasi-static tensile coupon tests are carried out to measure the in-plane Young's modulus, Poisson's ratio, tensile failure strengths and failure strains in both stuffer and filler yarn directions. Test results reveal that the average Young's modulus in the filler yarn direction is higher than that in the stuffer yarn direction, and the average failure strain in the filler yarn direction is lower than that in the stuffer yarn direction. The average failure strength in the filler yarn direction is slightly higher than that in the stuffer yarn direction. The fracture surfaces are studied using the scanning electron microscope (SEM) and the failure mechanism are then discussed. It is noted by studying the fracture surface that the fracture surface is always perpendicular to the loading direction. The crack causes the z yarn/matrix interface to debond. Also, the fracture of specimen cut along the x- (or stuffer yarn) direction causes filler yarn/matrix interface to debond and stuffer yarn to break, and the fracture of specimen cut along the y- (or filler yarn) direction causes stuffer yarn/matrix interface to debond and filler yarn to break. The testing results are then used to validate the developed models in Parts II and III of these series papers. In Part II, simplified analytical and finite element models are proposed to predict the mechanical property and failure strengths for the 3D orthogonal woven CFRP composites. In Part III, a curved beam model resting on an elastic foundation is presented to predict the tensile strength in the filler direction, and then to investigate the effect of some geometrical parameters on the tensile failure strength in the filler yarn direction.     

The effect of surface modification with carbon nanotubes upon the tensile strength and Weibull modulus of carbon fibers

Carbon fibers are widely used as reinforcements in composite materials because of their high specific strength and modulus. Today, a number of ultrahigh strength polyacrylonitrile (PAN)-based (more than 6?GPa), and ultrahigh modulus pitch-based (more than 900?GPa) carbon fibers have been commercially available. In contrast, carbon nanotube (CNT) with the extremely high tensile strength have attracted attention as reinforcements. An interesting technique to modify the carbon fiber is CNT grafting on the carbon fiber surface. CNT-grafted carbon fibers offer the opportunity to add the potential benefits of nanoscale reinforcement to well-established fibrous composites to create micro-nano multiscale hybrid composites. In the present study, the tensile properties of CNT grown on T1000GB PAN- and K13D pitch-based carbon fibers have been investigated. Single filament tensile test at gauge lengths of 1, 5, and 25?mm were conducted. The effect of gauge length on tensile strength and Weibull modulus of CNT-grafted PAN- and pitch-based carbon fibers were evaluated. It was found that grafting of CNT improves the tensile strength and Weibull modulus of PAN- and pitch-based carbon fibers with longer gauge length (≥5?mm). The results also clearly show that for CNT-grafted and as-received PAN- and pitch-based carbon fibers, there is a linear relation between the Weibull modulus and the average tensile strength on log–log scale.     

Effects of geometry and specimen size on out-of-plane tensile strength of aligned CFRP determined by direct tensile method

The out-of-plane tensile strength of CFRP laminate determined by the direct tensile method varies with specimen geometry and size. This effect was first experimentally observed using aligned CFRP. To explain the geometry and size effects from a mechanical point of view, an analytical model combining Weibull statistics, including the concept of effective volume, and a fracture criterion under multi-axial loading was constructed on the basis of stress distributions calculated using the finite element method. The predicted out-of-plane tensile strength of aligned CFRP was found to be consistent with experimental results. Thus, the present model is useful for reducing experimentally determined out-of-plane tensile strength under complex stress distributions to that under a uniaxial and uniform stress distribution.     

Improvement of interlaminar mechanical properties of CFRP laminates using VGCF

In order to improve the interlaminar mechanical properties of CFRP laminates, hybrid CFRP/VGCF laminates have been fabricated by using a newly-developed method, i.e., powder method, where the powder of vapor grown carbon fiber (VGCF) is added at the mid-plane of [0°/0°]₁₄ CFRP laminates. Experimental results of double cantilever beam (DCB) tests indicate the improvement on the interlaminar mechanical properties of Mode-I fracture behavior with much higher critical load P_C and fracture toughness G_IC with VGCF interlayer. Crack propagation and fracture surface have also been observed to interpret this improvement mechanism. Moreover, based on experimental G_IC, numerical simulations using finite element method (FEM) with cohesive elements have been carried out to analyze the delamination propagation. The interlaminar tensile strength of hybrid CFRP/VGCF laminates, which is obtained by matching the numerical load–COD (crack opening displacement) curves to experimental ones, is higher than that of base CFRP laminates.     

A tensile strength model for unidirectional fiber-reinforced brittle matrix composite

A model for the ultimate tensile strength of unidirectional fiber-reinforced brittle matrix composite is presented. In the model, transverse matrix crack spacing and change in debonding length between the fiber and the matrix is continuously monitored with increasing applied load. A detailed approximate stress analysis, together with a Weibull failure statistics for fiber fracture, are used to determine the probability of fiber fracture and fiber fracture location in the composite. Results of the model are consistent with experimental data. It is suggested from the results that the strength and toughness of the composite are significantly influenced by the Weibull modulus of the fiber and the fiber/matrix interfacial shear stress. A higher fiber Weibull modulus results in a lower composite strength while a higher fiber/matrix interfacial shear stress results in a composite with higher strength but lower toughness. A moderate variation in matrix strength and fiber/matrix interfacial shear strength does not significantly affect the strength of the composite.     

Interlaminar fracture properties of weft-knitted/woven fabric interply hybrid composite materials

An experimental study has been undertaken to characterize the delamination behavior and tensile properties of interply hybrid laminated composites reinforced by interlock weft-knitted and woven glass fiber preform fabrics. The hybrid composites, comprising the alternate layers of interlock and uniweave fabrics, were compared to interlock knitted (only) and uniweave (only) composites with respect to delamination and tensile performances. Mode-I double cantilever beam and mode-II end-notched flexure tests were carried out to assess the interlaminar fracture toughness using aluminum-strip stiffened specimens. The mode-I and mode-II interlaminar fracture toughness values, G _IC and G _IIC, for the hybrid composite were about three and two times higher than that for the uniweave composite, respectively. The tensile strength and modulus of the hybrid composite were 315 MPa and 12.8 GPa in the wale direction, respectively, demonstrating that the strength and modulus were found to be slightly lower than those of the uniweave composite, and significantly improved in comparison with the interlock knitted composites.     

高温环境下碳纤维增强树脂复合材料的层间力学性能老化行为与失效预测

为了研究碳纤维增强树脂(CFRP)复合材料层间力学性能在高温环境中的老化失效行为,设计了CFRP复合材料层间拉伸和层间剪切实验,在高温(80℃)环境中进行0(未老化)、 120 h、 240 h、 360 h、 480 h、 600 h和720 h的老化测试,分析CFRP层间失效强度和失效形式随老化时间的变化规律,得到随高温老化的二次应力准则响应面。建立CFRP复合材料层间力学性能预测模型,得到不同老化衰减系数下的退化模型,并通过CFRP复合材料层间仿真模型进行了验证。结果表明:随着高温老化时间的增加,层间拉伸和层间剪切强度总体上都发生了一定程度的退化,层间拉伸时更容易发生碳纤维丝剥离,层间剪切发生局部的树脂剥离,纤维之间的分层更加明显,高温老化使树脂与纤维丝的界面结合力显著下降。通过CFRP复合材料层间力学性能随高温老化的二次应力准则,计算不同老化时间后的内聚力模型参数,预测CFRP复合材料在高温老化条件下的层间强度,发现仿真与实验误差小于10%,说明了CFRP复合材料层间失效预测模型的准确性。     

碳纤维增强树脂基复合材料层合板胶螺混合连接失效机制

为研究碳纤维增强树脂基复合材料(CFRP)层合板单搭接双螺栓胶螺混合连接失效机制,采用基于断裂能断裂准则的连续渐进退化方式,仿真CFRP层合板刚度退化,采用基于能量的B-K准则仿真胶层的损伤演化,建立胶螺混合连接结构渐进损伤三维有限元模型,有限元模型预测的最大失效载荷与实验结果吻合较好。搭接长度L_a为影响胶螺混合接头刚度和强度的重要几何参数,螺栓的位置不会明显影响接头的刚度,粘结面积越大,强度越大。胶螺混合接头在拉伸载荷作用下,由于二次弯曲效应的影响,螺栓向左倾斜,搭接区域的胶层损伤起始于搭接区域胶层外侧,并由外侧向内部扩展到钉孔附近,当胶层损伤扩展到钉孔附近时,螺栓承载增加,胶层和螺栓共同承载,此时CFRP层合板开始出现损伤;最终,左侧钉孔处的上层合板和右侧钉孔处的下层合板产生分层损伤并发生断裂。      

Numerical and experimental investigation on lap shear fracture of Al/CFRP laminates

This paper presents a new approach to numerically investigate the lap shear fracture of a hybrid laminate made of Carbon Fibre Reinforced Plastic (CFRP) and metal foil plies (e.g. aluminium), validated by corresponding experiments. The numerical Finite Element (FE) model of the hybrid laminate, subjected to lap shear fracture, is composed of five laminas with alternating metal/CFRP layers with cohesive elements lying within Al/CFRP interface. In the FE model, individual CFRP laminas are assumed as an orthotropic homogenized continuum under plane stress, and aluminium facesheets are modelled as an elastic–plastic continuum. The Al/CFRP interface is represented via quadratic cohesive elements, the constitutive law of which is an exponentially decaying law representing the degrading behaviour of the interface (implemented as user element in ABAQUS). The numerical model captures the experimentally obtained results with minimal error, and predicts the failure modes successfully. The influence of specimen geometry (e.g. overlap length, total length, and total width) on lap shear fracture response is analyzed in detail in this study, too, in order to confirm the specimen design for the test, as there is still no corresponding test standard for hybrid laminates.     

基于模量/应变波耦合特性的FBG碳纤维增强塑料复合材料拉伸断裂监测

从光纤Bragg光栅（FBG）反射中心波长随碳纤维增强塑料复合材料（CFRP）拉伸试件表面应变变化敏感特性的角度,详细研究了拉伸过程中FBG中心波长的拉伸变化速率（即CFRP的宏观弹性模量）与复合材料内部断裂饱和状态的相关性和断裂瞬间试件表面的应变波响应特性,即：在拉伸过程中,CFRP拉伸试件的宏观弹性模量随着内部断裂的发生而不断减小,且在试件出现明显应力松弛状态前趋于平稳;应力松弛状态出现时,断裂区域表面接收的应变波响应略大于其他区域。通过设计相应排布形式,将FBG与CFRP断裂监测相结合,提出了一种基于FBG传感的CFRP断裂分阶段监测方法。该方法中传感探头直接与CFRP试件耦合,测量及传导光路全光纤化,可实现对CFRP断裂状态的绝对监测。     

循环荷载作用对CFRP筋力学性能的影响

应用于桥梁工程的碳纤维增强复合材料（CFRP）筋通常受循环荷载作用,导致其力学性能的退化,这将影响桥梁结构的受力。为了考察循环荷载作用对CFRP筋力学性能的影响,首先利用静载试验对CFRP筋的初始力学性能进行了检测,然后结合静载试验结果并利用疲劳试验考察了各阶段循环荷载作用对CFRP筋弹性模量、松弛及抗拉强度的影响。研究结果表明,极限拉力作用下CFRP筋的弹性模量较初始状态高约5%,对于CFRP筋构件的变形对结构内力有较大影响的结构,在设计阶段建议适当地考虑CFRP材料的非线性问题;桥梁工程中,经200万次正常使用设计循环荷载作用的CFRP筋,其弹性模量和松弛性能较初始状态未发生明显退化,经应力幅为4.3%的极限抗拉强度的循环荷载作用后,CFRP筋的抗拉强度提高了1.2%,但应力幅提高至7%的极限抗拉强度时,其抗拉强度与初始状态相比无明显变化。     

电化学氧化对高强高模碳纤维表面结构及力学性能的影响

利用循环伏安多重扫描法分析了不同电解质的氧化能力及其氧化特点,讨论了在表面氧化处理中不同电解液体系对高模高强碳纤维力学性能的影响,提出了适合高强高模碳纤维表面处理的工艺条件,并通过Raman光谱、XPS与SEM的表征,研究了电化学氧化对高强高模碳纤维表面结构及力学性能的影响。研究结果表明,与NH₄H₂PO₄溶液相比,用NH₄H₂PO₄与CH₃COONH₄复合的电解质溶液对碳纤维进行表面处理,能大幅度提高纤维表面含氧官能团,而且纤维表面sp²杂化碳原子相对含量也较多,在提高了碳纤维/环氧树脂复合材料层间剪切强度(ILSS)的同时,还较好地保持了高强高模碳纤维本体力学性能。当CH₃COONH₄与NH₄H₂PO₄的物质的量之比为2:1时,碳纤维/环氧树脂复合材料的ILSS与未处理纤维相比提高了168%,而碳纤维拉伸强度却下降很小,此复合电解质溶液是一种较为理想的对高强高模碳纤维进行表面改性的电解质体系。     

Tensile fracture of soft and hard PZT

Power applications generate high stresses which can damage piezoceramic components. In this study tensile fracture of several types of PZT (hard/soft) is investigated. After validation of the specimen geometry by means of numerical simulation, samples are led to failure using a specific device. Weibull law parameters enable the characterisation of the tensile strength distribution and highlight clear differences between soft and hard ceramics. A fractographic approach emphasises the specificities of the fracture mode and the fracture origin for each type of samples.     

Suitable structure of thermosetting CFRP sheet for cold/warm forming

The forming processes at room temperature and under a warm condition have been investigated for carbon-fiber-reinforced plastic (CFRP) sheets consisting of thermosetting resin and continuous fibers used for mass production. While CFRPs consisting of thermosetting resin have the advantage of high strength, to subject them to press forming, in contrast to carbon-fiber-reinforced thermoplastics (CFRTPs), which consist of thermoplastic resin. When CFRP sheets are formed into the desired shape through plastic deformation, a higher-strength structural material easily applicable to mass production is obtained. To improve the formability of thermosetting CFRP sheets while retaining their strength, a suitable structure allowing plastic deformation under warm condition is proposed. The tensile stress obtained by a tensile test and the bending properties obtained by a stretch-bending test indicate the strength and formability, respectively. A stretch-bending test is a bending test in which a tensile load is placed on a sheet, and it has the characteristics of a bending test and a deep drawing test. A suitable structure containing prepreg layers to allow plastic deformation based on the relationship between the bending load and the results of specimen observations is revealed.     

Prediction of prestraining effect on fracture toughness of high strength structural steel by local approach

In the present study, the tension and fracture toughness tests on high strength structural steel of Q420 were carried out in different conditions of non-prestraining and prestraining. The results indicated that the prestrain increased the yield stress and tensile strength, but decreased the fracture toughness. Meanwhile, the local parameters m and σ_u controlling the brittle fracture were obtained using finite element method (FEM) analysis. Based on the Weibull stress fracture criterion, the prestraining effect on the fracture toughness was predicted from fracture toughness results of the virgin material by the local approach. The prediction was in good agreement with the experimental results. It certified that the critical Weibull stress obeys the two-parameter Weibull distribution in the local approach, and the fracture behaviour of the material with prestrain can be characterised well by the local approach.     

The effect of a compliant polyimide nanocoating on the tensile properties of a high strength PAN-based carbon fiber

The effect of a compliant polyimide nanocoating on the tensile strength of a polyacrylonitrile-based high tensile strength (T1000GB) carbon fiber was investigated. The pyromellitic dianhydride/4-4′-oxydianiline polyimide nanocoating was deposited by high-temperature vapor deposition polymerization. The thickness of the polyimide coating was about 100 nm. The tensile strength and Weibull modulus of nanocoated and uncoated fiber bundles were evaluated using a polyimide-impregnated bundle-composite. The results clearly demonstrated that the compliant polyimide nanocoating is effective in improving the tensile strength and Weibull modulus of T1000GB carbon fiber.     

混杂比对碳纤维-玄武岩纤维混杂增强环氧树脂基复合材料弯曲性能的影响

将玄武岩纤维置于混杂铺层的压缩侧,研究了碳纤维-玄武岩纤维混杂增强环氧树脂基复合材料的弯曲性能及混杂比对其弯曲性能的影响。通过对试样进行三点弯曲试验得到了材料的弯曲性能,并通过扫描电子显微镜观察材料的失效模式。与纯碳纤维增强环氧树脂基复合材料相比,当混杂比为16.7%和33.3%时,混杂复合材料的弯曲强度明显提升,弯曲强度分别提高12.4%和15.2%,但是其弯曲模量随着混杂比的提升而降低。混杂后的材料及玄武岩纤维增强环氧树脂基复合材料的失效位移都高于碳纤维增强环氧树脂基复合材料,断裂韧性明显提升。从侧面观察可以发现不同铺层在压缩侧、拉伸侧和中间层有不同的失效形式。      

ZrB_2基超高温陶瓷复合材料的高温拉伸损伤行为

开展了SiC(20vol%)-石墨(15vol%)/ZrB2复合材料室温及高温拉伸性能实验,发现高温时复合材料的拉伸强度和弹性模量有所降低,并且具有明显的非线性特征。引入热损伤来表征弹性模量随温度的衰减规律,利用强度统计分析方法确定单向应力状态下材料的机械损伤演化方程,建立了材料在热力耦合条件下的高温拉伸损伤非线性本构模型。分析表明:随着温度的升高,SiC-石墨/ZrB2复合材料的热损伤和机械损伤不断增加,延性增强,且脆性-延性破坏转变温度范围为1 250~1 350℃。