Flexural properties loss of unidirectional epoxy/fique composites immersed in water and alkaline medium for construction application

Epoxy fique composites were evaluated for construction applications and compared with conventional wood used in construction. The composites studied were made with fique fibers treated using Na(OH) solution at 18 w/v%, untreated fique fibers were also used. The matrices were epoxy and epoxy with 5 wt.% of chemically modified C30B montmorillonite. Unidirectional composites of 90 mm × 20 mm × 4 mm were elaborated by pultrusion processing technique. The flexural properties loss occurred over 20 days of composites submitted to three types of environments: (i) water, (ii) saturated calcium hydroxide solution and (iii) mortar with w/c ratio of 0.45 and 540 kg/m³ of cement, cured in a saturated solution of lime stone at 50 °C. Results showed that fiber treatment and montmorillonite addition improved the flexural modulus and strength of composites in 40% and 34% respectively. Moreover the flexural properties of composites before and after ageing resulted comparable or even better than conventional wood used in construction.     

Tensile properties of real unidirectional Kevlar/epoxy composites

Mechanical behaviour, tensile strength and failure modes in real unidirectional Kevlar/epoxy composites, loaded parallel to the fibres, at volume fraction (V_f) range 0.26–0.73, were investigated. It was found that the measured tensile strengths deviated from the expected values calculated from the Rule of Mixture. The deviation, which was minimal at V_f of about 0.5, was mainly due to geometrical deficiencies typical of real composites. At V_f<0.5 it could be explained by non-homogeneous fibre spread and distribution of fibres. At V_f>0.5 the deviation was explained by the increasing lack of matrix between some adjacent fibres and by squeezing of fibres. The initial part of loading was typified by straightening out of non-axial fibres, accompanied by fibre/matrix debonding. The straightening process was completed at a stress level of about 0.6–0.7 of the composite strength. Matrix damage began at this stress level and continued to develop up to final failure. Failure of Kevlar fibres was noted to occur only at an extremely short loading interval coinciding with the catastrophic final failure. This was due to the small scatter of Kevlar fibre strength.     

PPESK改性环氧树脂基体及其单向复合材料的制备与性能研究

采用耐热等级较高的二氮杂萘联苯聚醚砜酮(PPESK)对"双酚A型环氧树脂"(DGEBA)进行共混改性,研究了该体系的玻璃化转变温度(Tg)、冲击韧性及断裂韧性(K<,IC>).结果表明:PPESK的加入提高了环氧树脂的耐热性及韧性,基体冲击强度最大提高了16%,K<,IC>值最大提高了108%.并采用连续S-玻璃纤维和...     

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.     

Piezoresistivity of unidirectional carbon/epoxy composites for multiaxial loading

The applied strain of carbon fibre reinforced plastics (CFRPs) is measurable by their electrical resistance changes. For damage monitoring of laminated CFRPs, piezoresistivity strongly affects the measured electrical resistance change through residual strain relief attributable to delamination cracks. Although several studies of CFRP laminates’ piezoresistivity have been published, this study uses single-ply CFRP for specific piezoresistivity measurements in four directions. A review of the theory of in-plane piezoresistivity reveals orthotropic properties of CFRP piezoresistivity. In the present study, piezoresistivity of multiaxial loading is derived, and the unsymmetrical piezoresistivity matrix is calculated using the measured piezoresistivity here. Effects of multiaxial loading in a misaligned unidirectional laminate are also discussed here. The misaligned laminate causes large shrink in the transverse direction during tensile tests; poor electrical contacts at electrodes increases the electric current in the transverse direction; these two effects cause decrease of electrical resistance for the poor electrical contact specimen with large fibre misalignment.     

Flexural mechanical properties of thermally treated unidirectional and cross-ply Nicalon-reinforced calcium aluminosilicate composites

Long, square cross-section samples of a unidirectional and a cross-ply [0/90]3s silicon carbide (Nicalon) fibre calcium aluminosilicate glass-ceramic matrix composite have been subjected to a range of thermal treatments. They were held at temperatures up to 800 °C above room temperature for 1, 6 or 24 h then slowly cooled or quenched into water. The thermal cycle was repeated up to six times for a small number of samples. The effects of these thermal regimes on Young's modulus, onset of matrix cracking (as assessed by onset of non-linearity in the load–displacement curve) and flexure strength have been monitored using three-point flexure testing. In very broad terms, where clear trends emerged, the intermediate temperature differentials, i.e. 400–650 °C, have been found to have the most detrimental effects on properties, and this has been linked to expected changes in the carboneous interphase and its subsequent replacement by silica. © 1998 Kluwer Academic Publishers     

Transverse properties of unidirectional glass/epoxy composites: influence of fibre surface treatments

An investigation has been made on effects of fibre surface treatments on transverse mechanical behaviour of unidirectional glass/epoxy composites. Model composite plates were processed by filament winding using glass fibres coated with different sizings changing by their epoxy functionality and their reactivity towards the matrix.In the first part of the study, transverse tension and microindentation characterisations were performed in order to correlate the ultimate behaviour of the composite with interfacial properties. Experiments revealed that the most reactive sizings promote the highest interfacial strength and also increase ultimate properties of laminates in transverse tension. This feature was attributed to the high crosslink density of the polymer network in interfacial areas.In the second part, finite element calculations were used to evaluate local strain and stress concentration in a composite submitted to transverse tension conditions. The general trend for the evolution of composite failure strain as a function of interfacial strength has been established. The modelling showed that a transition of the composite failure mode occurs at a global strain of 1.15%, from an adhesive rupture at the fibre/matrix interface to a cohesive rupture in the matrix. In the domain of adhesive rupture, the value of the composite failure strain appeared to be directly governed by the interfacial strength.Therefore, improving interfacial strength by use of fibre sizings with high epoxy functionality could constitute an interesting way to reduce transverse brittleness of composite structures.     

海水浸泡对单向T700/环氧复合材料层间剪切性能的影响

对海水环境下严重影响单向T700/环氧复合材料层间剪切性能的因素进行了研究,考察了海水和蒸馏水5个浸泡周期(15、40、70、165和400d)后单向T700/环氧复合材料层间剪切性能和吸水率的变化,并利用SEM观测了单向T700/环氧复合材料的剪切断口微观形貌,揭示其性能衰减机制。结果表明:单向T700/环氧复合材料的吸水率随着浸泡时间的延长而增加,整体符合Fick吸水率变化规律;单向T700/环氧复合材料的层间剪切性能下降很明显,浸泡400d后层间剪切性能损失近10%,层间剪切性能的变化规律与吸水率的变化规律相似;单向T700/环氧复合材料的层间剪切性能对海水的敏感性大于对蒸馏水的敏感性;由试样断面微观形貌可以看出,海水的渗入破坏了树脂与纤维的界面,这是导致其层间剪切性能明显下降的主要原因,因此对于单向T700/环氧复合材料在海水中的应用,需要更加关注其层间剪切性能。     

基于介电性能研究碳纤维/环氧复合材料单向板电热作用机制

采用复合材料电热试验机对碳纤维/环氧复合材料(CF/EP)单向板进行0.05~0.2A·mm~(-2)直流电流通电处理,每次通电1.5h,同时对其电热作用下的表面温度场进行测量;分析电热作用后CF/EP试样的纵向体积电阻和介电性能变化;通过三点弯曲测试表征电热作用前后的弯曲性能。结果表明:试样的温度场随通电时间均呈现先迅速上升后稳态平衡的趋势。低于0.125A·mm~(-2)的电热作用使温度场分布均匀,纵向体积电阻降低以及介电性能提升;高于0.125A·mm~(-2)的电热作用使中间区域温度场高于两边区域,纵向体积电阻上升以及介电性能下降。介电常数和损耗角正切随测试频率增大而降低,交流电导率随频率增大先稳定后上升。受电热作用影响最大的损耗角正切可作为特征参量。弯曲性能测试表明,低于0.125A·mm~(-2)的电热作用使CF/EP试样的弯曲强度和弯曲模量最大提升11.8%和7.32%,而高于0.125A·mm~(-2)的电热作用使试样的弯曲强度和弯曲模量最大降低8.26%和6.52%。     

碳纤维-玻璃纤维层内混杂单向增强环氧树脂复合材料拉伸性能

采用不同混杂比的碳纤维-玻璃纤维层内经向混编单轴向织物制备了混杂纤维增强环氧树脂复合材料, 研究了不同混杂结构和不同混杂比的碳纤维-玻璃纤维/环氧树脂复合材料拉伸性能的变化及破坏形式。0°拉伸结果表明:同种混杂织物的不同混杂结构中, 碳纤维相对集中的完全对齐结构强度最高, 不同混杂比织物的完全对齐结构强度相当;碳纤维-玻璃纤维/环氧树脂复合材料的模量遵循混合定律。90°拉伸结果表明:纤维与树脂间的界面结合强度为碳纤维/树脂>玻璃纤维/树脂, 碳纤维-玻璃纤维/环氧树脂复合材料的强度、模量与材料厚度方向上界面的不同形式(单一或交替界面、碳纤维或玻璃纤维的分布位置等)有关, 与碳纤维的含量基本无关。      

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。      

Demonstration of pseudo-ductility in unidirectional hybrid composites made of discontinuous carbon/epoxy and continuous glass/epoxy plies

A new, partially discontinuous architecture is proposed to improve the mechanical performance of pseudo-ductile, unidirectional (UD) interlayer carbon/glass hybrid composites. The concept was successfully demonstrated in different laminates with high strength and high modulus carbon and S-glass epoxy UD prepregs. The novel hybrid architecture provided pseudo-ductile tensile stress–strain responses with a linear initial part followed by a wide plateau and a second linear part, all connected by smooth transitions. The best hybrid configuration showed 60% improvement in modulus compared to pure glass, 860 MPa plateau stress and 2% pseudo-ductile strain. The initial modulus, the plateau stress and the overall tensile stress–strain response of each specimen configuration were predicted accurately.     

单向连续碳纤维-玻璃纤维层间混杂增强环氧树脂基复合材料的力学性能

为了研究连续单向纤维的层间混杂方式对复合材料力学性能及破坏方式的影响,采用碳纤维-玻璃纤维体积比为1∶1,以拉-挤成型法制备了具有不同层间混杂结构的连续单向纤维增强环氧树脂基复合材料,并研究了不同层间混杂结构的连续单向碳纤维-玻璃纤维增强环氧树脂基复合材料的力学性能及破坏形式。结果表明：具有层间混杂结构的复合材料抗拉强度处于纯碳纤维/环氧树脂复合材料和纯玻璃纤维/环氧树脂复合材料之间,复合材料的拉伸断裂方式为劈裂;具有层间混杂结构的复合材料的层间剪切强度均优于纯碳纤维/环氧树脂复合材料和纯玻璃纤维/环氧树脂复合材料,复合材料的剪切断裂方式为层间断裂。     

Flexural and interlaminar shear strength properties of carbon fibre/epoxy composites cured thermally and with microwave radiation

The ease of heating an epoxy resin with microwaves depends, among other factors, on the dielectric properties of its components at the frequency of the radiation used. The majority of the papers published on the microwave curing of reinforced epoxy resin composites have used widely available DGEBA type resins and amine hardeners such as 4,4′-diaminodiphenylsulphone (DDS). This paper investigates the use of two epoxy systems where the choice of resin and hardener was based on their measured dielectric loss factors. System 1 contained a resin and hardener with higher loss factors than those used in System 2. The two systems are formulated with polyetherimide (PEI) as a toughening agent. Unidirectional carbon fibre prepregs were prepared from both systems. Composites were laid up from these prepregs, which were then cured in three different ways: autoclave curing, partial autoclave curing followed by microwave post-curing, and microwave curing. System 1 composites had greater flexural properties and interlaminar shear strengths than System 2 composites when autoclave cured. Flexural properties and interlaminar shear strengths were greater for System 2 in the microwave post-cured composites. When fully microwave-cured the properties were similar. In the microwave-cured composites the flexural and interlaminar shear properties were influenced by the structure of the phase separated PEI and the void content.     

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.     

Exploring biomass based carbon black as filler in epoxy composites: Flexural and thermal properties

Carbon blacks (CB), derived from bamboo stem (BS-CB), coconut shells (CNS-CB) and oil palm empty fiber bunch (EFB-CB), were obtained by pyrolysis of fibers at 700 °C, characterized and used as filler in epoxy composites. The results obtained showed that the prepared carbon black possessed well-developed porosities and are predominantly made up of micropores. The BS-CB, CNS-CB and EFB-CB filled composites were prepared and characterized using scanning electron microscope (SEM) and thermogravimetric analyzer (TGA). The SEM showed that the fractured surface of the composite indicates its high resistance to fracture. The CBs–epoxy composites exhibited better flexural properties than the neat epoxy, which was attributed to better adhesion between the CBs and the epoxy resin. TGA showed that there was improvement in thermal stability of the carbon black filled composites compared to the neat epoxy resin.     

石墨烯/环氧树脂复合材料的介电性能研究

通过Staudenmaier法制备了完全氧化的氧化石墨(GO),并通过高温热膨胀制备了单层石墨烯(graphene)。用FT-IR和TG对GO的氧化程度、含氧官能团进行了表征,用SEM和TEM对天然石墨(NG)、GO和graphene的微观结构进行了分析。利用超声共混法制备了graphene/环氧树脂介电纳米复合材料,介电性能的测试表明,graphene的加入使环氧树脂介电常数大幅提高,当graphene添加量为0.25%(质量分数)时,材料介电常数达到25,是纯环氧树脂的4倍,介电损耗0.11。这为石墨烯在介电储能方面的应用和低成本介电复合材料的制备提供了新思路。     

Effect of pectin and hemicellulose removal from hemp fibres on the mechanical properties of unidirectional hemp/epoxy composites

The objective of this study was to investigate the effect of pectin and hemicellulose removal from hemp fibres on the mechanical properties of hemp fibre/epoxy composites. Pectin removal by EDTA and endo-polygalacturonase (EPG) removed epidermal and parenchyma cells from hemp fibres and improved fibre separation. Hemicellulose removal by NaOH further improved fibre surface cleanliness. Removal of epidermal and parenchyma cells combined with improved fibre separation decreased composite porosity factor. As a result, pectin removal increased composite stiffness and ultimate tensile strength (UTS). Hemicellulose removal increased composite stiffness, but decreased composite UTS due to removal of xyloglucans. In comparison of all fibre treatments, composites with 0.5% EDTA + 0.2% EPG treated fibres had the highest tensile strength of 327 MPa at fibre volume content of 50%. Composites with 0.5% EDTA + 0.2% EPG → 10% NaOH treated fibres had the highest stiffness of 43 GPa and the lowest porosity factor of 0.04.     

Splitting initiation and propagation in notched unidirectional graphite/epoxy composites under tension-tension cyclic loading

The initiation and propagation of splitting damage in notched unidirectional graphite/epoxy composites under tension-tension cyclic loading were investigated experimentally. One-hundred-and-twenty [0°]₈ specimens, notched at the center with a 3·175, 6·350, or 9·525 mm hole or slit, were tested. Thirty specimens were loaded monotonically to failure to determine the static splitting stress. Ninety specimens were cycled at maximum loads corresponding to 80%, 70%, and 60% of their predicted static splitting stress. The stress ratio of the load cycle was maintained at a nominal value of $\text{R}{}_{\mathrm{\sigma }}\text{= 0·1}$, while the frequency was changed with the stress amplitude to maintain the same loading rate. The results show that the static splitting stress is sensitive to both notch type and size and that the split length varies linearly with the natural logarithm of the number of accumulated load cycles. The intercept of this linear relation depends on the notch type, notch size, and stress level while its slope depends only on the notch size and stress level. Empirical functional forms are derived using multiple linear regression analyses of the test data.     

Water jet and abrasive water jet cutting of unidirectional graphite/epoxy composite

Unidirectional graphite/epoxy composite material has been machined by water jet and abrasive water jet cutting processes. Topography and morphology of the machined surfaces were evaluated with surface profilometry and scanning electron microscopy. The surface characteristics in terms of roughness and the micromechanisms of material removal for both processes were analysed and compared. Abrasive water jet surface characteristics of graphite/epoxy were found to be significantly different from those of the water jet cutting process and micromechanical behaviour of material removal was strongly dependent on the fibre orientation.