Effect of interfacial adhesion and statistical fiber strength on tensile strength of unidirectional glass fiber/epoxy composites. Part. II: comparison with prediction

An analytic model by Curtin and Takeda (Curtin WA, Takeda N. Tensile strength of fiber-reinforced composites: I. Model and effects of local fiber geometry. Journal of Composite Materials 1998;32(22):2042–59; Curtin WA, Takeda N. Tensile strength of fiber-reinforced composites: II. Application to polymer matrix composites. Journal of Composite Materials 1998;32(22):2060–81) is used to predict the ultimate tensile strength (UTS) of unidirectional (UD) glass fiber/epoxy composites with different interfacial adhesion and statistical fiber strength. Data for the fiber strength σ_c at the critical fiber length δ_c for five kinds of treated fibers are used for predicting UTS, which is obtained from both single fiber composite (SFC) and single fiber tension (SFT) tests. σ_c from SFC is attained using the Curtin theory on the fragmentation of SFC, while that from SFT is determined using a linear extrapolation of SFT data. Under good interface adhesion, the predicted UTS values based on the SFC data show the best agreement with measured ones at various fiber volume fractions, but a higher predicted value is obtained if the interface failure is matrix-controlled. For poor interfacial adhesion, the predicted UTS values are rather high compared to experimental ones due to the ineffective stress transfer. The predicted values based on the SFT data are much higher than the measured value for good interfacial adhesion.     

孔隙微观特征对碳纤维/环氧树脂复合材料横向拉伸强度的影响

通过有限元方法研究了相同孔隙率下孔隙的分布、尺寸和形状等微观特征对碳纤维增强环氧树脂复合材料单向板横向拉伸强度的影响。首先使用Matlab对复合材料微观图像进行处理,提取孔隙的半径分布。然后通过C++编写多种孔隙随机分布算法,包括可以生成不同分布孔隙、不同尺寸孔隙以及不同形状孔隙的随机分布算法。最后通过Python参数化生成代表性体积单元（RVE）,用有限元方法研究相同孔隙率下孔隙的分布、尺寸和形状对碳纤维/环氧树脂复合材料单向板横向拉伸强度的影响。研究结果显示,孔隙率相同时,碳纤维/环氧树脂复合材料的孔隙形状对横向弹性模量的影响较大,孔隙尺寸和形状对横向拉伸强度有较大的影响。     

拉应力对玻璃纤维增强聚合物基复合材料介电强度的影响

在拉应力条件下, 测试了聚合物基体和单向玻璃纤维增强聚合物基复合材料的介电强度, 探索了聚合物基体和玻璃纤维/聚合物复合材料的介电强度与拉应力的关系, 提出并证明了聚合物基体的介电强度与拉应力呈负指数关系, 复合材料中纤维与基体的界面是影响材料介电强度的主要因素。     

基于纤维束/环氧树脂复合材料试验的单向层合板横向拉伸强度预测方法

实验研究表明,纤维束/环氧树脂复合材料试件的横向拉伸强度与工程上常用的单向层合板横向拉伸强度在趋势上具有很好的相关性,但是数值上存在一定差距。本文使用两种碳纤维和两种环氧树脂制备了三种纤维束/环氧树脂复合材料和单向层合板,并分别测量了纤维束/环氧树脂复合材料和单向层合板的横向拉伸强度,以及环氧基体的拉伸强度。在实验基础上,应用Griffith断裂强度理论建立了纤维束/环氧树脂复合材料和单向层合板的横向拉伸强度的关系模型,通过两种复合材料实验的结果拟合了该模型中的参数。利用第三种复合材料实验进行校验,发现该模型预测的单向层合板横向拉伸强度与实测强度之间达到很好的一致性,相对偏差为9%。采用本文提出的方法,可以用较为简单的纤维束/环氧树脂复合材料和环氧基体拉伸试验预测单向层合板的横向拉伸强度。     

热残余应力对考虑微观孔隙碳纤维增强环氧树脂复合材料横向拉伸性能的影响

为研究由于材料固化产生的热残余应力对碳纤维增强环氧树脂复合材料横向拉伸性能预测结果的影响,发展了一种基于摄动算法的纤维和孔洞随机分布代表性体积单元（RVE）生成方法,建立更加接近真实材料微观结构的RVE模型。通过施加周期性边界条件,并赋予组分（纤维、基体和界面）材料本构关系,进而实现温度和机械荷载下模型的热残余应力和损伤失效分析。从结果中发现,材料固化过程会在纤维之间产生残余压应力,在模型孔隙周围产生沿加载方向的残余拉应力。所建立不含孔隙RVE模型的失效均是由于界面脱黏引起,材料固化在纤维之间产生的残余压应力会增加模型的预测强度。含有孔隙的RVE模型失效起始于孔隙周围的基体中,而材料固化在模型孔隙周围产生的热残余拉应力对含孔隙RVE模型预测的失效强度有降低作用。对于具有不同孔隙尺寸的RVE模型,模型的失效强度随着孔隙尺寸的增加而不断降低,但是热残余应力减弱了孔隙尺寸对模型预测结果的降低作用。对于具有不同长宽比椭圆形孔隙的RVE模型,热残余应力增加了孔隙长宽比对模型强度的降低作用。      

Torsion fatigue behavior of unidirectional carbon/epoxy and glass/epoxy composites

This paper presents results of the feasibility of carbon/epoxy composites (CFRP) as a future helicopter flexbeam material. Torsional behaviors of unidirectional CFRP and glass/epoxy composites (GFRP) with the same resin matrix were investigated. The initial torsional rigidity of CFRP was almost identical to that of GFRP. The torsional rigidities calculated using finite element analyses (FEA) agreed with the experimental results: the torsional rigidities are governed mainly by the material’s shear stiffness. Torsion fatigue tests were also conducted by controlling the angle of twist of the sinusoidal wave under a constant tensile axial load. No catastrophic failure occurred with either GFRP or CFRP, although decreased amplitudes of torque and torsional rigidities were observed according to the number of cycles. Results of X-ray CT inspections and numerical calculation by FEA revealed that degradation of a torsional rigidity is caused mainly by splitting crack propagation along the fiber direction. The torsion fatigue life of CFRP was superior to that of GFRP. Consequently, results confirmed that CFRP exhibits excellent properties as a torsional element of a helicopter flexbeam in terms of torsional rigidity and tension–torsion fatigue behaviors.     

Influences of interfacial bonding strength and scatter of fibre strength on tensile behaviour of unidirectional metal matrix composites

The influences of interfacial bonding strength and scatter of strength of fibres on tensile behaviour of unidirectional metal matrix composites, whose matrix has low yield stress in comparison to the strength of fibres, were studied using the Monte-Carlo simulation technique using two-dimensional model composites. The following results were found. The strength of composites increases with increasing bonding strength, especially when the bonding strength exceeds the shear yield stress of the matrix and then remains nearly constant. The strength of composites is very sensitive to bonding strength when the scatter of fibre strength is large, but not when it is small. The fracture mode varies from non-cumulative to cumulative with increasing scatter of fibre strength for both cases of weak and strong interfacial bondings. The fracture surface becomes irregular when bonding strength becomes low and scatter of fibre strength becomes large. The applicability of the Rosen and Zweben models and the rule of mixtures to predict the strength of composites was examined.     

Jute fibre/epoxy composites: Surface properties and interfacial adhesion

Jute fibres were surface treated in order to enhance the interfacial interaction between jute natural fibres and an epoxy matrix. The fibres are exposed to alkali treatment in combination with organosilane coupling agents and aqueous epoxy dispersions. The surface topography and surface energy influenced by the treatments were characterized. Single fibre pull-out tests combined with SEM and AFM characterization of the fracture surfaces were used to identify the interfacial strengths and to reveal the mechanisms of failure.     

Monotonic properties of unidirectional glass fiber (U)/random glass fiber (R)/epoxy hybrid composites

The main objective of the present paper is to study the tensile and bending behaviors of unidirectional glass fiber (U)/random glass fiber (R)/epoxy hybrid composites with total fiber volume fraction (V_fT) = 37%. Six kinds of laminated composites of average thickness 5.5 mm were manufactured using hand lay-up technique; i.e. [R]₅, [U/R/U/R/U], [U/0.5R/U]_S, [0.5R/U/U]_S, [U/U/0.5R]_S, and [U]₅. In bending test, notched and unnotched specimens were tested. For this purpose different circular notch sizes (D = 3, 6, 9 mm) were drilled at the specimen center. Tensile strength, tensile modulus, Poisson’s ratio, bending strength and bending modulus were determined experimentally. The effect of stacking sequences, random fiber relative volume fraction (V_fR/V_fT), and notch diameter on the mechanical properties of the mentioned composite types were studied. Failure modes of all specimens were investigated.     

Effect of interfacial silane network structure on interfacial strength in glass fibre composites

The effect of crosslinked structure of silane condensates on the interfacial strength between silane-treated glass fibre and epoxy resin was examined using a dialkoxysilane, γ-aminopropyldimethoxymethylsilane (APDS), and a trialkoxysilane, γ-aminopropyltrimethoxysilane (APTS). Glass fibres were treated with aqueous solutions of APDS, ATPS or mixtures of both to change the silane crosslink density, and then washed with methanol to leave only chemically bonded silane at the surface. As an index of interfacial strength, the interfacial stress transmissibility was determined using a modified single-filament test. The interfacial strength decreased in the order APDS, mixtures of APTS and APDS, and APTS, showing that siloxane crosslinking of the silane condensates reduces the interfacial strength. This result suggests that the crosslinked structure depresses the penetration of the resin into the interphase and hinders reaction between silane organofunctional groups and the resin. In addition, treatment of the glass fibre with the various APDS-APTS mixtures reduced the interfacial strength in all cases, compared with the strength after APDS treatment alone. This indicates that the silane interphases formed in the present system do not lead to the formation of interpenetrating networks with the resin molecules.     

Effect of interfacial mobility on flexural strength and fracture toughness of glass/epoxy laminates

Mechanical testing and surface fractography were used to characterize the fracture of E-glass fiber reinforced epoxy composites as a function of the silane coupling agent used. -Aminopropyltriethoxysilane (APS) and -aminobutyltriethoxysilane (ABS) were used because these have been shown to have different interfacial mobilities at multilayer coverage. The values of the properties studied generally increased from untreated c, as determined from a Mode I translaminar fracture toughness tests, for the untreated composites (10.5 ± 0.4 kJ/m²) was lower than that for the ABS-treated composites (14.3 ± 2.1 kJ/m²) which was lower than that for the APS-treated composites (17.1 ± 2.4 kJ/m²). Macroscopic observations showed that a larger fiber debonding area was formed in the crack tip region for the untreated glass composites, suggesting poorer bonding compared to those treated with coupling agents. Since these silanes have similar chemistry, the differences were attributed to differences in the interfacial mobility of the coupling agent layers.     

Transverse elastic moduli of unidirectional fiber composites with fiber/matrix interfacial debonding

An elastic contact model is developed to predict the transverse Young's modulus, Poisson's ratio and shear modulus of unidirectional fiber composites with interfacial debonding. The elastic deformation formulae of the fiber under contact pressure are derived by the use of elasticity theory. These results are then used in the formulation of an analytic boundaryelement method for solving the interfacial debonding problem. The two extreme cases of perfect bonding and the fiber-like void are also studied. On the basis of this theory, the upper and lower bounds of the transverse moduli for unidirectional fiber composites with imperfect fiber/matrix interfaces are provided. Numerical calculations of parametric studies are conducted for four composites, and some basic characteristics of the transverse elastic moduli of unidirectional fiber composites with interfacial debonding are presented.     

玻璃纤维/环氧树脂复合材料与镀层界面的Ni颗粒强化工艺

针对玻璃纤维/环氧树脂复合材料与镀层结合界面强度低的问题,基于复合材料/镀层间的机械互锁原理及传统塑料基体化学镀工艺,提出通过增强颗粒的桥接作用,增加含有增强颗粒的过渡层来强化镀层界面的复合材料金属化方法。对金属化后的玻璃纤维/环氧树脂复合材料试件采用拉伸试验法测量镀层的结合强度,并通过截面和断面的显微观测,分析了增强颗粒对于镀层界面的强化机制;同时获得了玻璃纤维/环氧树脂复合材料表面粗糙度和增强颗粒质量分数对含有过渡层的镀层结合强度的影响规律。结果表明:采用上述金属化方法可以显著提高镀层的界面强度,与传统的金属化工艺制备试件相比,玻璃纤维/环氧树脂复合材料在不同表面粗糙度下,镀层结合强度平均提高161%;同时,镀层的结合强度随着增强颗粒质量分数的增加,呈现先增大后减小的趋势,当增强颗粒的质量分数为50%时,镀层的结合强度达到最大。      

Oxygen plasma processing and improved interfacial adhesion in PBO fiber reinforced epoxy composites

The effects of oxygen plasma processing on the improved interfacial adhesion properties of poly(1,4-phenylene-cis-benzobisoxazole) (PBO) fiber reinforced epoxy composites have been investigated in this paper. Both As-spun (AS) and high-modulus (HM) PBO fiber systems were studied. The characterization techniques included microscopy, surface analysis, and composite interfacial adhesion tests. The results showed that the high-modulus fiber surface free energy could be increased significantly by 42.2% from 46.2 to 65.7 mJ/m², while the tensile strength was only slightly decreased by 3.4% from 5.87 to 5.67 GPa. In addition, the interfacial adhesion strength of PBO fiber reinforced epoxy composite was improved by 37.5% from 32.5 to 44.7 MPa for the HM fiber system. The improvement has been attributed to the enhanced cohesive failure that dissipated more fracture energy.     

Effect of interfacial bonding strength on tensile ductility of multilayered steel composites

The effect of the bonding strength of the laminate interface on ductility in the tensile deformation of multilayered steel composites was investigated. Multilayered steel composites consisting of alternating layers of as-quenched martensitic and austenitic steels were prepared with various bonding strengths, ranging from weak bonding obtained by alpha-cyanoacrylate adhesive to strong bonding obtained by cold rolling with a subsequent heat treatment. Tensile tests and peel tests were conducted to investigate the relationship between tensile behavior and bonding strength at the interface. It was demonstrated that tensile ductility could be markedly enhanced as the bonding strength increased, and that three types of tensile fracture behavior were identified depending on the bonding strength of the interface.     

中等应变率下单向玻璃纤维增强环氧树脂基复合材料的剪切本构关系

采用真空辅助成型工艺制备单向玻璃纤维增强环氧树脂基的[±45°]_8s复合材料试样,通过专用试验设备开展恒定应变率下的面内剪切性能研究,应变率范围为3×10^-4~128.4 s^-1。以Khan-Huang本构关系模型表达形式为基础,考虑应变率效应,建立了一种单向玻璃纤维增强环氧树脂基复合材料在中等应变率下的剪切本构模型,通过最小二乘法和遗传算法获得了最优本构参数。结果表明,单向玻璃纤维增强环氧树脂基复合材料的剪切性能具有应变率敏感性,剪切强度随着应变率的提高逐渐增大,在128.4s^-1时极限强度提高了35.5%。建立的本构关系模型能够准确反映剪切性能与应变率的关系,可用于中等应变率条件下的剪切性能预测。     

The effect of polybutadiene interlayer on interfacial adhesion and impact properties in oxygen-plasma-treated UHMPE fiber/epoxy composites

In order to improve the interfacial adhesion and impact properties of ultra-high modulus polyethylene (UHMPE) fiber/epoxy composites at the same time, the fiber coating technique was combined with the oxygen plasma treatment. The UHMPE fiber was treated with oxygen plasma and thin polybutadiene (PB) coating was introduced. PB coating decreased the interfacial adhesion and increased the impact property of the oxygen-plasma-treated UHMPE fiber/epoxy composites. However, oxygen-plasma-treated and PB-coated UHMPE fiber/epoxy composites show improved interfacial adhesion, flexural properties and impact property in comparison with the untreated control UHMPE fiber/epoxy composites. Oxygen plasma treatment introduces micro-pittings on the UHMPE fiber surface. These micro-pittings improved interfacial adhesion and flexural properties and decreased impact properties through mechanical interlocking. Thin PB coating cannot exclude this mechanical interlocking effect completely and there are imperfect wetted UHMPE fiber surface regions in which effect mechanical interlocking can occur. Stress transfer through the viscous PB interlayer also contributes to the interfacial adhesion and flexural properties of PB-coated UHMPE fiber/epoxy composites. The impact property of PB-coated UHMPE fiber/epoxy composites is due to low modulus PB interlayer.     

橡胶改性对硼纤维/环氧单向复合材料力学性能的影响

对环氧树脂进行液体丁腈橡胶改性, 并采用缠绕无纬布层压成型工艺制备了硼纤维/环氧单向复合材料。测试了环氧树脂液体丁腈橡胶改性前后硼纤维/环氧单向复合材料的力学性能, 研究了硼纤维/环氧单向复合材料的纵向拉伸破坏模式。结果表明, 基体中的10%液体丁腈橡胶使硼纤维/环氧单向复合材料的拉伸强度、 弯曲强度、 层间剪切强度和断裂延伸率分别提高了18.42%、 13.39%、 28.45%和43.40%, 但其拉伸和弯曲模量稍有下降。基体中含10%液体丁腈橡胶的硼纤维/环氧单向复合材料的纵向拉伸破坏模式为界面层的内聚破坏和脱黏破坏共存的混合破坏。      

低温暴露对碳纤维/环氧树脂复合材料拉伸力学性能的影响

针对低温暴露对碳纤维/环氧树脂(CF/EP)复合材料力学性能影响进行研究,对低温0℃、?20℃、?40℃、?60℃暴露100 h、200 h、300 h、400 h、500 h后,对CF/EP的复合材料拉伸力学性能影响展开研究,利用SEM电镜扫描分析损伤机制,根据试验结果提出了一种预测CF/EP复合材料低温暴露后剩余强度的预测公式。试验结果表明,在长时间低温暴露后,CF/EP复合材料拉伸强度随低温暴露时间的增长呈现出先增后降的趋势;低温暴露时间低于300 h时,CF/EP复合材料拉伸强度随温度下降先增后降,暴露时间高于300 h后,拉伸强度随温度下降逐渐降低;CF/EP复合材料拉伸弹性模量随低温暴露时间的增长呈现逐渐上升趋势,温度越低,上升趋势越明显。SEM结果表明,低温暴露后,纤维与环氧树脂黏结程度增强,有利于荷载传递,CF/EP复合材料拉伸强度增大,破坏形貌上表现为纤维上包裹更多树脂;长时低温暴露后,由于纤维与基体收缩系数不同导致微裂纹产生,在受到荷载时裂纹进一步扩散,不利于荷载传递,使拉伸强度下降,破坏形貌上表现为纤维成束凝集,纤维束间距增大。基于初始试验,本文提出了一种基于初始试验的CF/EP复合材料低温暴露后剩余强度预测模型,试验与预测结果吻合较好,由于考虑了同种材料在不同低温和暴露时间耦合作用下的等效作用,可减少相同材料在不同低温与暴露时间下的试验次数,因此具备一定参考价值。      

Tension behavior of unidirectional glass/epoxy composites under different strain rates

By considering wide applications of composite materials, having a proper knowledge of them under dynamic loading is necessary. In order to study the effects of strain rates on the behavior of the materials, special testing machines are needed. Most of the research in this field is focused on applying real loading and gripping boundary conditions on the testing specimens. In this study, behavior of unidirectional glass fiber reinforced polymeric composites under uni-axial loading is determined at quasi-static and intermediate strain rates of 0.001–100 s⁻¹. The tests were performed using a servo-hydraulic testing apparatus equipped with a strain rate increase mechanism. For performing the tests, a jig and a fixture are designed and manufactured. The performance of the test jig was evaluated and found to be adequate for testing of composites. Dynamic tests results are compared with the results of static tensile tests carried out on specimens with identical geometry. Experimental results show a significant increase of the tensile strength by increasing the strain rate. The tensile modulus and strain to failure are also observed to increase slightly by increasing the strain rate.