Role of matrix modification on interlaminar shear strength of glass fibre/epoxy composites

Interlaminar shear properties of fibre reinforced polymer composites are important in many structural applications. Matrix modification is an effective way to improve the composite interlaminar shear properties. In this paper, diglycidyl ether of bisphenol-F/diethyl toluene diamine system is used as the starting epoxy matrix. Multi-walled carbon nanotubes (MWCNTs) and reactive aliphatic diluent named n-butyl glycidyl ether (BGE) are employed to modify the epoxy matrix. Unmodified and modified epoxy resins are used for fabricating glass fibre reinforced composites by a hot-press process. The interlaminar shear strength (ILSS) of the glass fibre reinforced composites is investigated and the results indicate that introduction of MWCNT and BGE obviously enhances the ILSS. In particular, the simultaneous addition of 0.5 wt.% MWCNTs and 10 phr BGE leads to the 25.4% increase in the ILSS for the glass fibre reinforced composite. The fracture surfaces of the fibre reinforced composites are examined by scanning electron microscopy and the micrographs are employed to explain the ILSS results.     

Residual strength distribution of impacted glass/epoxy laminates with embedded shape memory alloy at low temperature

This paper evaluated the strength reduction and probabilistic behaviors of the residual flexural strength for impacted glass/epoxy laminates with embedded shape memory alloy (SMA) wires at various temperatures. A series of impact tests were performed on base (glass/epoxy laminates without SMA wires) and SMA laminates (glass/epoxy laminates with embedded SMA wires) at temperatures of 293 K, 263 K and 233 K. Three point flexural tests were then carried out so as to investigate the post-impact strength at the aforementioned temperatures. Strength reduction behavior of impacted laminates could be described by Caprino’s residual strength prediction model. A probabilistic model was developed in order to estimate the variation in residual strength of the impacted laminates with temperature. As the temperature decreased, the variation in residual strength increased due to the embrittlement of the constituent materials of the laminates at lower temperatures. When compared to the base laminates, the SMA laminates exhibited a higher variation in residual strength, especially at lower temperatures.     

静电植绒法处理多壁碳纳米管改性玻纤织物/环氧树脂复合材料的制备及力学性能

为提高玻纤增强环氧树脂复合材料的力学性能,采用静电植绒法将多壁碳纳米管(MWCNTs)附着在玻纤织物表面,得到改性的玻纤织物。利用一种低黏度的环氧树脂和所制得的改性织物,采用真空辅助成型工艺(VARI)制备了MWCNTs改性格玻纤织物/环氧树脂复合材料层合板,表征了层合板的力学性能。对进行力学实验后的MWCNTs改性玻纤织物/环氧树脂复合材料试样断口进行了SEM和OPM观察。结果显示:与未添加MWCNTs的玻纤织物/环氧树脂复合材料层合板相比,添加了MWCNTs的层合板的拉伸强度降低了10.24%,弯曲强度降低了13.90%,压缩强度降低了17.33%,拉伸模量和弯曲模量分别提高了19.38%和16.04%,压缩模量提高了13%;MWCNTs与玻纤织物之间的结合较弱,在拉伸作用下,存在明显的脱粘和分层;将改性玻纤织物在200℃下热压处理2h后,制备的MWCNTs改性玻纤织物/环氧树脂复合材料层合板的力学性能均有所提高,热压处理后树脂与玻纤织物之间的界面结合得到改善。     

PEK-C膜层间增韧碳纤维/环氧树脂复合材料的力学性能

为探究热塑性酚酞基聚醚酮（Polyaryletherketone with Cardo,PEK-C）树脂薄膜及膜厚对层间增韧碳纤维/环氧树脂复合材料力学性能的影响,利用浸渍提拉法制备了三种不同厚度（分别约为1 μm、10 μm、30 μm）的PEK-C膜,通过热压成型制备了层间增韧碳纤维/环氧树脂复合材料层合板,对其进行了Ⅰ型层间断裂韧性、冲击后压缩强度、层间剪切及弯曲性能测试,并利用SEM观察微观形貌及AFM扫描微观相图。结果表明:不同PEK-C膜厚增韧碳纤维/环氧树脂复合材料的Ⅰ型层间断裂韧性、冲击后压缩强度及层间剪切强度有不同程度提高,Ⅰ型层间断裂韧性及层间剪切强度以膜厚为10 μm最佳,分别增大了157.17%和17.57%,冲击后压缩强度以膜厚为30 μm最佳,达到了186.67 MPa,这是由于PEK-C与环氧树脂在热压固化过程中形成了双相结构,改善了材料韧性;但弯曲性能持续下降,强度及模量由未增韧的1 551 MPa、106 GPa分别降至30 μm时的965 MPa、79 GPa,这是由于PEK-C树脂扩散进入环氧树脂中,降低了纤维体积分数及材料刚度。      

Effect of post-curing on properties of carbon fibre–epoxy composites

Abstract

The effect of post-curing on the moisture absorption characteristics of Fibredux 914/T300 carbon fibre–epoxy composites, and hence on their thermomechanical behaviour, has been examined. Laminates 1 mm thick were post-cured at 190 or 210°C for 4 or 10 h. The various cross-link densities thus established had almost no effect on the moisture absorption behaviour. Interlaminar shear strength and torsion pendulum tests gave similar results, in that the cross-link density had almost no influence on the dynamic shear modulus or the mechanical dissipation factor. From these findings, environmental degradation of the composite is shown to depend on the content of absorbed water. The behaviour of the composite in hot, humid conditions therefore cannot be improved by post-curing treatment.

MST/400     

The mechanical properties of oxyfluorinated hybrids of glass fibre and polypropylene fibre

The mechanical properties of a low-cost system comprising orthophthalic polyester resin reinforced with hybrids of glass and polypropylene fibres were investigated. The fibres were oxyfluorinated to overcome the poor surface adhesion properties of polypropylene. Interlaminar shear tests, Izod-type impact tests and tensile tests were considered. It would be expected that increasing polypropylene fibre content corresponds with a decrease in mechanical properties due to the poor properties of polypropylene. Oxyfluorinated laminates containing approximately 25% and 50% polypropylene in the warp direction were, however, found to exhibit significant improvements in interlaminar shear strength, in peak shear stress under impact loading as well as in impact resistance over untreated glass fibre laminates. Scanning electron microscope images show that the reason for this improvement is that the interfacial bond between the polypropylene fibres and the resin is strengthened to such an extent that failure occurs within the polypropylene fibres rather than at the interface.     

Experimental study on mechanical behavior of laminates at low temperature

A series of tensile tests were conducted to research the mechanical behavior of the glass fiber and the carbon fiber reinforced epoxy laminates at low temperature (77 K). The specimens of laminates contained various stacking sequences (angle-ply) and notch geometry (central and edge notch). The curves of loading vs displacement of cross-head were recorded and the curves of stress-strain were got. The strengths of the various laminates were given too. Using a microscope-CCD imaging system, the growth of damage area was imaged. The concept of the energy dissipation density of laminates was presented and the data of the energy dissipation density for some laminates were got. The results showed that the strength and the energy dissipation density of laminates at 77 K are higher than those at 296 K.     

A single-lap shear specimen for determining the effect of strain rate on the interlaminar shear strength of carbon fibre-reinforced laminates

A new design of single-lap shear specimen for determining the effect of loading rate on the interlaminar shear strength of laminated composites is described. Finite element analyses are used to optimize the specimen geometry and minimize the variation in the shear stress and the magnitude of the normal stress along the interlaminar failure plane. Experimental results are obtained at a quasi-static and an impact rate of loading for the interlaminar shear strength parallel to the fibres in both unidirectional carbon/epoxy and unidirectional carbon/polyetheretherketone (peek) laminates and at interfaces across which the fibre orientation is 0°/90° and ±45°. Results for carbon/epoxy laminates are compared with those from an earlier investigation using a double-lap specimen geometry and show a similar small dependence on loading rate. No significant effect of loading rate was observed for the carbon/peek laminates.     

γ辐照对玻璃布/环氧树脂层压板压缩性能的影响

玻璃纤维/环氧树脂复合材料具有高绝缘性和高力学性能,常作为支撑材料用于高能物理和核物理试验,但其力学性能在辐照环境中将发生变化。根据玻璃纤维/环氧树脂复合材料在新一代北京正负电子对撞机(BEPCII)中的应用要求,研究了玻璃纤维/环氧树脂复合材料中的玻璃布/环氧树脂层压板γ辐照前后的压缩性能。研究表明:经20kGy的γ辐照后,玻璃布/环氧树脂层压板的压缩强度由辐照前的320.21 MPa下降为315.05 MPa,下降了1.61%;经200kGy的γ辐照后,玻璃布/环氧树脂层压板的压缩强度下降为312.30 MPa,下降了2.47%。利用扫描电镜对辐照前后的试件断口进行微观形貌观察,发现辐照对玻璃布没有明显影响,但使玻璃布与环氧树脂的结合度有所降低,环氧树脂的碎片化趋于明显,红外谱图显示环氧树脂在辐照后的降解反应强于交联反应。     

Preparation and mechanical properties of carbon nanotube grafted glass fabric/epoxy multi-scale composites

In the present paper, carbon nanotubes (CNTs) were chemically grafted onto surfaces of the amino silane treated glass fabric by a novel chemical route for the first time to create 3D network on the glass fibers. The chemical bonding process was confirmed by Fourier transform infrared spectroscopy and scanning electron microscopy. The glass fabric/CNT/epoxy multi-scale composite laminates were fabricated with the CNT grafted fabrics using vacuum assisted resin infusion molding. Tensile tests were conducted on fabricated multi-scale composites, indicating the grafting CNTs on glass fabric resulted a decrease (11%) in ultimate tensile strength while toughness of the multi-scale composite laminates were increased up to 57%. Flexural tests revealed that the multi-scale composite laminates prepared with CNT grafted glass fabric represent recovering after first load fall. The interfacial reinforcing mechanisms were discussed based on fracture morphologies of the multi-scale composites.     

不同湿热条件下碳纤维/环氧复合材料湿热性能实验研究

对比研究了环氧5228A树脂及碳纤维/环氧5228A树脂复合材料层合板在3种湿热环境(水煮、70℃水浸,70℃85%相对湿度)下的湿热性能,考察了湿热条件对复合材料层间剪切性能的影响规律,并从吸湿特性、物理化学特性、树脂力学性能、湿应力等方面分析了不同湿热环境下复合材料性能衰减的机制。研究表明,碳纤维/高温固化环氧树脂复合材料层间剪切性能主要是由吸湿率决定,相同吸湿率不同湿热条件下性能的下降幅度基本相同;3种湿热条件下该树脂及其复合材料未发生化学反应、微裂纹等不可逆变化,复合材料层合板湿热老化机制主要是吸入水分后基体增塑和树脂、纤维湿应变不一致导致的湿应力对复合材料性能的负面作用。     

Adhesion strength of glass/epoxy composite embedded with heat-treated carbon black on the surface

The surface of a glass/epoxy composite material was embedded with oxidized carbon black by heat treatment to enhance the adhesion strength of the glass/epoxy composite structure. Quantitative chemical bonding analysis with X-ray photoelectron spectroscopy (XPS) was conducted to observe the chemical binding states of the surface of the carbon black particles with heat treatment. The morphological effects of the carbon black on the surface of the composite were observed using SEM and AFM. The surface free energies and lap shear strengths of the glass/epoxy adhesive joints whose adherends were embedded with oxidized carbon black were investigated in terms of the heat-treatment conditions and the amount of embedding. From the experimental results, embedding the heat-treated carbon black particles on the composite surface was found to greatly improve the adhesion strength of the composite due to the increased oxidation radicals on the carbon black surface.     

Response of hygrothermally aged GLARE 4A laminates under static and cyclic loadings

An experimental investigation focusing on the hygrothermal aging-structural degradation–mechanical property relationship of GLARE 4A laminates was conducted. Water immersion conditioning at 80 °C for up to 4 months was carried out on GLARE 4A laminates. It was found that although the outer aluminum layers effectively protected the glass/epoxy composite layers from hygrothermal attack, the composite layers absorbed moisture through the edges. Consequently, significant decrease in both, the tensile strength and fatigue life of the GLARE 4A laminates, was observed although no structural defects were apparently identifiable in the microstructures of the conditioned laminates. Detailed experimental investigation was conducted to study the mechanism of mechanical property decay due to hygrothermal aging. It is proposed that the strength of the S2-glass fibers was not fully realized due to the weakening of the fiber/matrix interface and the deterioration of the sizing, which consequently led to the reduction in the tensile strength and fatigue life of the GLARE 4A laminates. The stiffness degradation characteristics of GLARE 4A laminates under cyclic loading were also investigated.     

Evaluation of water degradation of vinylester and epoxy matrix composites by single fiber and composite tests

Degradation of the mechanical properties of vinylester and epoxy matrix composites exposed to water has been approached by monitoring the strengths of glass and carbon fibers and resins. In addition, the fiber/matrix (F/M) interface strengths and debond lengths of single-fiber composites were determined and test results were compared to test results of macroscopic composite specimens. The single-fiber tensile test results indicate a substantial loss of the tensile strength of glass fibers and the fragmentation tests reveal loss of F/M shear strength and substantial debonding for both glass and carbon fiber composites after water exposure. The transverse strengths of the composites are also degraded to large extents. The tests results identify water degradation of the F/M interface as a major strength limiting mechanism.     

Characterization of carbon/epoxy materials for structural repair of carbon/BMI structures

Characterization and compatibility of high-performance epoxy resins were investigated for on site repair of carbon/bismaleimide aircraft structures. First, processing of three bi-component resins was studied. Then, physical and mechanical characterizations were carried out on carbon fabric/repair resin composite laminates (materials A, B and C) before and after ageing and results are presented. One composite (material A) was chosen, according to industrial specifications, the main criterion being the glass transition temperature. The compatibility of this material with the carbon/BMI structures to be repaired was checked with lap shear tests. Compatibility between the selected repair material and carbon/epoxy composites, whose repair procedure is already known, was conducted and considered as reference point. The obtained results allowed to validate material A for on site step structural repair of the carbon/BMI structures of our industrial partner.     

Fatigue behaviour of glass fibre reinforced epoxy composites enhanced with nanoparticles

Nanoparticle reinforcement of the matrix in laminates has been recently explored to improve mechanical properties, particularly the interlaminar strength. This study analyses the fatigue behaviour of nanoclay and multiwalled carbon nanotubes enhanced glass/epoxy laminates. The matrix used was the epoxy resin Biresin® CR120, combined with the hardener CH120-3. Multiwalled carbon nanotubes (MWCNTs) 98% and organo-montmorillonite Nanomer I30 E nanoclay were used. Composites plates were manufactured by moulding in vacuum. Fatigue tests were performed under constant amplitude, both under tension–tension and three points bending loadings. The fatigue results show that composites with small amounts of nanoparticles addition into the matrix have bending fatigue strength similar to the obtained for the neat glass fibre reinforced epoxy matrix composite. On the contrary, for higher percentages of nanoclays or carbon nanotubes addition the fatigue strength tend to decrease caused by poor nanoparticles dispersion and formation of agglomerates. Tensile fatigue strength is only marginally affected by the addition of small amount of particles. The fatigue ratio in tension–tension loading increases with the addition of nanoclays and multi-walled carbon nanotubes, suggesting that both nanoparticles can act as barriers to fatigue crack propagation.     

Interlaminar shear properties of polymer matrix composites: Strain rate effect

Studies are carried out on interlaminar shear behavior of typical polymer matrix composites under high strain rate shear loading. Torsional split Hopkinson bar (TSHB) apparatus is used for the studies in the shear strain rate range of 496–1000/s. Experimental details, specimen configuration and development, data acquisition and processing are presented. Interlaminar shear strength and shear modulus are presented as a function of shear strain rate. The results are presented for typical plain weave carbon/epoxy and plain weave E-glass/epoxy composites. For comparison, studies are presented at quasi-static loading. It is observed that the interlaminar shear strength at high strain rate is enhanced compared with that at quasi-static loading. Further, it is observed that the interlaminar shear strength increases with increasing shear strain rate within the range of shear strain rate considered.     

γ射线辐照对碳纤维表面接枝改性的影响

为提高碳纤维/环氧树脂复合材料的界面粘结性能, 采用γ射线共辐照接枝方法对碳纤维表面改性, 利用X光电子能谱仪(XPS)、 扫描电子显微镜(SEM)、 电子万能材料试验机, 研究了在缩乙二醇丙酮溶液和环氧氯丙烷丙酮溶液中经200 kGy剂量的γ射线辐照接枝后, 碳纤维的表面化学元素及官能团组成、 表面形貌、 复合材料剪切断面形貌及其层间剪切强度(ILSS)的变化。研究表明, 缩乙二醇类接枝液的接枝效果较理想, 碳纤维接枝率达7%; 辐照处理碳纤维表面O/C比值和含氧官能团含量增加, 以此制备的碳纤维/环氧复合材料的ILSS提高, 最大提高率达31.2%; 同时还发现辐照接枝后的碳纤维表面粗糙度增大。     

Mechanical,thermal and fire retardation behaviours of nanoclay/vinylester nanocomposites

The dispersion of montmorillonite (MMT) in vinylester for preparing nanoclay/vinylester gel coat was reported. Two sets of MMT/vinylester specimens, namely Type 1 and Type 2, were prepared for comparative studies. Type 1 specimens were prepared using ultrasonication only, and Type 2 specimens were prepared using both ultrasonication and twin-screw extrusion. According to XRD and TEM results, Type 2 specimens showed lower levels of nanoclay agglomeration and higher levels of exfoliation. DSC results showed that the glass transition temperatures of Type 2 specimens are higher than those of Type 1 specimens. TGA results showed that the residual weight of 4 wt.% MMT/vinylester of Type 1 was 7.38%, while the corresponding value of Type 2 was 13.5%, indicating lower thermal degradation in the latter. MMT/vinylester/glass and MMT/vinylester/carbon specimens were fabricated and tested for mechanical and fire retardation behaviours. Type 2 based nanocomposite laminates showed greater values of ultimate tensile strength, flexural strength, interlaminar shear strength, impact strength, horizontal burning rate, and vertical burning rate than Type 1 based laminates. SEM images of tensile fractured surfaces revealed that Type 2 based laminates have no or less agglomeration of nanoclay than Type 1 based laminates.     

Effect of new epoxy matrix for T800 carbon fiber/epoxy filament wound composites

A new epoxy resin matrix with good adherence to T800 carbon fibers (T800 CFs) in filament winding was developed by addition of hardener and resin diluter. Interfacial behavior of the T800 CF/epoxy composites was analyzed according to the Naval Ordnance laboratory (NOL) ring test, short-beam-shear test and fracture surface observation. Meanwhile, scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) were used in analysis of the interfacial behavior. The interfacial properties of the T800 CF/epoxy filament wound composites were improved by optimizing the matrices through increasing the toughness and reducing the viscosity, which is an important factor in influencing the wettability of T800 CFs. The Interlaminar shear strength (ILSS) of the unidirectional T800 CF/epoxy composites and the tensile strength of NOL-ring in this work reached to 123 and 2570 MPa, respectively. Also, the interfacial adhesion was much improved by the chemical reactions between the new matrix and the sizing on the T800 CFs.