Viscoelastic response and interlaminar delamination resistance of epoxy/glass fiber/functionalized graphene oxide multi-scale composites

Graphene oxide (GO) was functionalized using three different diamines, namely ethylenediamine (EDA), 4,4′-diaminodiphenyl sulfone (DDS) and p-phenylenediamine (PPD) to reinforce an epoxy/glass fiber (EP/GF) composite laminate, with the aim of improving the overall composite mechanical performance. Different mechanical characterization techniques were used to determine the mechanical performance, including: tensile stress strain, double cantilever beam (DCB) mode-I fracture toughness and dynamic mechanical thermal analysis (DMTA). Scanning electron microscopy (SEM) was used to support the results and conclusions. The results demonstrated remarkable enhancements in the mechanical performance of EP/GF composite laminates by incorporation of functionalized graphene oxide (FGO) nanofiller, whilst the mechanical performance of the GO reinforced composite only improved marginally. Finally, the mechanical performance of the EP/GF/FGO multi-scale composites was found to be dependent on the type of FGO functional groups; of which EDA exhibited the highest performance. These investigations confirmed that the EDA-FGO-reinforced EP/GF composites possess excellent potential to be used as multifunctional engineering materials in industrial applications.     

Mode-I interlaminar fracture behaviour of nanoparticle modified epoxy/basalt fibre-reinforced laminates

The effect of nano-reinforcements on fracture behaviour of bulk epoxy nanocomposites and mode-I interlaminar fracture toughness of filament-wound basalt fibre-reinforced laminates was studied. Fracture energy of the bulk epoxy nanocomposites significantly increased with acrylic tri-block-copolymer addition but remained unchanged with incorporation of nanoclay. Delamination fracture toughness was not influenced by the presence of nanoparticles in the matrix. Decreasing fibre volume fraction, on the other hand, significantly improved interlaminar fracture energy. Rigid fibres in these composites constrict the stress field ahead of the crack-tip. Hence, increasing resin content enhanced composite delamination energy by increasing the capacity for matrix deformation. Interlaminar crack propagation through the composite was observed to occur mainly by interfacial failure and matrix cracking.     

Improving the interlaminar fracture toughness of carbon fiber/epoxy composites using clustered microcapsules

The composite laminates are susceptible to delamination between reinforcing plies during their long-term service. In this paper, we propose a modified carbon fiber/epoxy composite laminate with embedded clustered dual-component microcapsules in order to increase the interlaminar fracture toughness of the lamina. The details of microcapsules were illustrated using scanning electron microscope (SEM). The modified CF/EP composite laminates were fabricated using hot-compaction technique. Mode I interlaminar fracture tests were conducted using double cantilever beam specimens, then the values of opening fracture toughness G_IC were calculated to evaluate the toughening effect of modified laminates. The toughening mechanism was revealed and discussed through micrographs of the fracture surfaces obtained by ultra-depth microscope and SEM. The results show that clustered microcapsules after polymerization are equal to special Z-pinning, significantly enhancing the ability of crack arrest, and largely and roundly improved the G_IC values of resultant composite laminates. Meanwhile, the clustered microcapsules and matrix resin formed a second-phase material layer, which also absorbed the fracture energy and suppressed the expansion of cracks.     

Fracture toughness and surface morphology of micro/nano sized fibrils-modified epoxy resin

Two kinds of micro/nano sized fibrils based on cellulose (MFC) and polyvinyl alcohol (PVA) were used as reinforcer for epoxy resin (EP) with different contents in the range from 0 to 0.3 wt %. PVA nanofibers with diameter about 40–80 nm were fabricated by electrospinning technique. The analysis of mechanical properties showed that by both adding MFC and PVA to EP the fracture toughness was increased. The SEM results showed that micro/nano sized fibers dispersed throughout epoxy resin, prevented and changed the path of crack growth.     

Surface modification of boron nitride by bio‐inspired polydopamine and different chain length polyethylenimine co‐depositing

We established a novel, easy, and versatile method of obtaining diverse and controllable interphases between epoxy resin and fillers. The method involved the co‐deposition of polydopamine (PDA) and polyethyleneimine (PEI) with different molecular lengths on boron nitride (BN) surface. The obtained PDA/PEI‐modified BN composites showed significantly improved mechanical properties, including tensile strength, toughness, and elongation at break. For example, the tensile strength, fracture toughness, and elongation at break of EP composite increased by 51%, 132%, and 170% compared with EP when the PEI molecular weight was 10 000, respectively. These results suggested that the interphases between BN and EP matrix can be adjusted by changing the molecular lengths of grafted modifiers, thereby offering a new method for the reasonable designing and exploitation of the BN‐based composite materials.     

Coupon Tests on z-Pinned and Unpinned Composite Samples for Damage Resistant Applications

The main objective of the work was to determine the mechanical properties of Z-pinned and unpinned composites samples. Tensile and interlaminar fracture tests under mode I have been carried out on thermoset polymer matrix composite samples composed of CYTEC 977-2 and carbon fibre yarn HTA-12K/35. The presence of the Z-pins has been found to significantly increase the interlaminar fracture toughness. The crack resistance improvement has been found to be accompanied by a small reduction of the in-plane stiffness properties for a Z-pins density of 2%. The results are presented in relation to the size and pinning quality and are rationalised on the basis of the location of Z-pin block within the sample.     

Improved mechanical properties of epoxy‐based composites with hyperbranched polymer grafting glass‐fiber

A glass‐fiber, grafted by hyperbranched polymer with hydroxyl group (GF‐HBPH), reinforced epoxy‐based composite was evaluated for mechanical properties and compared with the neat epoxy and silanized glass‐fiber, GF‐APS. The epoxy/GF‐HBPH composites were studied by attenuated total internal reflectance infrared spectroscopy, ¹H nuclear magnetic resonance spectroscopy, thermal gravimetric analysis, mechanical properties analysis, and field emission‐scanning electron microscopy. The results showed that the incorporation of GF‐HBPH could simultaneously enhance the mechanical properties of the epoxy composites. Field emission‐scanning electron microscopy images of the fracture surfaces of the test specimens were used to support the results and conclusions. Copyright © 2016 John Wiley & Sons, Ltd.     

Reinforcing epoxy resin through covalent integration of functionalized graphene nanosheets

A chemically converted graphene/epoxy (EP) resin nanocomposite has been developed through the use of the functionalized graphene nanosheets (FGNs). The FGNs were prepared via the reaction of amines with alkylcarboxyl groups attached to the graphite oxides in the course of a dicarboxylic acid acyl peroxide treatment. FGNs/EP composites were prepared by dissolving the FGNs in organic solvent followed by mixing with EP and curing agent. In this composite, the FGNs were able to create molecular entanglement with EP matrix by taking advantage of the reactions between amine groups of FGNs and EP groups of EP, thus the FGNs could be covalently integrated into the EP matrix and became part of the cross‐linked network structure rather than just a separated component. Great enhancement in the mechanical properties of the epoxy composite, such as the ultimate tensile strength and toughness, had been achieved with small loading (0.1 wt%) of FGNs by 17.0% and 262.2%, respectively. However, the FGNs reinforced EP composites showed a slight decrease in glass transition temperature (Tg). Copyright © 2014 John Wiley & Sons, Ltd.     

Comparison of static and fatigue interlaminar testing methods for continuous fiber reinforced polymer composites

Two different interlaminar fatigue testing methods have been compared by testing a carbon fiber reinforced epoxy (CF/EP) composite and a carbon fiber/multiwalled carbon nanotube reinforced epoxy (CF/MWCNT/EP) hybrid nanocomposite. The first, conventional fatigue testing method was the end-notched flexure (ENF) test, which was used as a reference. The second, novel technique was the fatigue interpretation of the double-notch shear (DNS) test. Both tests have been performed with static and cyclic loading to compare the evaluated properties of the different systems, the effect of transition from cyclic to fatigue loading and to demonstrate if the complex ENF test can be replaced by the simpler DNS test.The test results showed the slight beneficial effect of the nanoreinforcement in both static and cyclic load conditions, and the possibility to use the DNS test for fatigue testing of continuous fiber reinforced composites. The SEM micrographs taken of the fracture surfaces of the composites after the different interlaminar tests provide valuable data on the interlaminar failure phenomena of hybrid nanocomposites in both static and fatigue loading conditions.     

Effect of micro/nano white bamboo fibrils on physical characteristics of epoxy resin reinforced composites

As a green filler comprising both nano- and micro-sized fibrils, micro/nano white bamboo fibrils (MWBFs) were treated with a silane coupling agent (S-MWBFs) prior to their introduction in an epoxy resin (EP). The sequential processes of steam explosion, alkaline treatment, and micro grinding were used to prepare to MWBFs. The effects of the S-MWBFs on various characteristics of the cured EP such as compatibility, fracture toughness, morphology, mechanical property, and flame retardation were studied. The fracture toughness and mechanical characteristics of both the storage modulus and tanδ maximum increased following addition of the S-MWBFs to the EP. Notably, the fracture toughness of the EP with 0.3 wt% of S-MWBFs was 22.2% higher than that of unmodified resin. Scanning electron microscopy presented somewhat rougher surfaces with shear deformation and tortuously twisting cracks, resulting in a higher fracture toughness in S-MWBF-modified epoxy samples. However, fire testing showed that the presence of S-MWBFs increased the burning rate of the EP.     

Preparation of (Fe)MIL-101 on short carbon fibers to improve the flame retardancy,smoke suppression,and mechanical of epoxy resin

Nano (Fe)MIL-101 particles were grafted on the short carbon fibers (SCFs) by in situ growth method to prepare (Fe)MIL-101@SCFs. The flame-retarded composites of epoxy resin (EP) were fabricated with combination of (Fe)MIL-101@SCFs and ammonium polyphosphate (APP). The composites showed good flame retardancy, smoke suppression, and mechanical properties simultaneously. The main heat release rate peak of the flame-retarded composites was reduced and delayed evidently in comparison with pristine EP. The high amount of residual char with coherent and dense structure was formed owing to the catalytic char formation of (Fe)MIL-101 as well as the strengthening action of SCF. The improvement in mechanical properties of the flame-retarded composite was due to the reinforcement effects of (Fe)MIL-101@SCFs and its action of interfacial adjustment. This research solved the contradiction between the flame retardancy and mechanical properties of EP, and proposed a new method to prepare the mechanically reinforced and flame retardant EP.     

Polymer blends based on epoxy resin and polyphenylene ether as a matrix material for high-performance composites

The application of poly(2,6-dimethyl-1,4-phenylene ether), PPE, as a matrix material for continuous carbon fibre reinforced composites has been studied. Due to the intractable nature of PPE melt impregnation is not feasible and a novel impregnation route, using epoxy resin as a reactive solvent, was developed. The introduction of epoxy resin results in enhanced flow and a reduced processing temperature, enabling the processing of PPE and the preparation of high quality composites. Upon curing, phase separation is initiated and epoxy resin is converted into a second phase. In composites, epoxy resin preferentially accumulates at the polar fibre surface, resulting in an epoxy layer around the fibres, providing a high level of interfacial adhesion. For a high fibre volume fraction (> 50%) this results in the ultimate morphology of epoxy coated fibres in a neat PPE matrix. Due to this unique morphology the composite materials reveal outstanding mechanical properties in terms of interlaminar toughness and impact performance.     

中空多壳层TiO2填充对环氧树脂复合材料力学性能的影响

为改善环氧树脂(EP)材料的力学性能, 采用次序模板法合成了TiO₂中空多壳层结构(HoMS)材料, 利用偶联剂对获得的TiO₂ HoMSs进行接枝改性后填充到EP中, 制备了TiO₂ HoMSs/EP复合材料; 并与单壳层TiO₂中空结构进行比较, 研究了壳层数和偶联剂改性对复合材料力学性能的影响规律. 结果表明, 随着壳层数的增加, 复合材料的力学性能增强, 并且偶联剂改性的TiO₂ HoMSs可进一步提高复合材料的力学性能. 在该体系中, 经硅烷偶联剂KH-560改性后的三壳层TiO₂ HoMSs(3S-TiO₂ HoMSs)/EP复合材料的拉伸强度、 断裂伸长率和冲击强度可分别达到71.66 MPa, 7.4%和35.81 kJ/m². 扫描电子显微镜(SEM)断面形貌表征结果显示, 相较于纯EP材料, TiO₂ HoMSs/EP复合材料的断面更加粗糙, 说明TiO₂ HoMSs材料起到了吸收外界应力和阻碍裂纹扩展的作用, 提高了复合材料的韧性, 提升了复合材料的冲击性能.     

Cyclic Olefin Copolymer Interleaves for Thermally Mendable Carbon/Epoxy Laminates

Thin cyclic olefin copolymer (COC) foils were used as intrinsic thermoplastic healing agents in carbon fiber (CF)-reinforced epoxy laminates. COC films were produced by hot pressing and were interleaved in the interlaminar regions between each EP/CF lamina, during the hand layup fabrication of the laminates. Three samples were produced, i.e., the neat EP/CF laminate without COC, and two laminates containing COC layers with a thickness of 44 μm and 77 μm, respectively. It was observed that the fiber volume fraction decreased, and the porosity increased with the introduction of COC layers, and this effect was more evident when thick films were used. These two effects, combined with the sub-optimal adhesion between COC and EP, caused a decrease in the mechanical properties (i.e., the elastic modulus, flexural strength, interlaminar shear strength and interlaminar fracture toughness) of the laminates. Specimens subjected to mode I interlaminar fracture toughness test were then thermally mended under pressure by resistive heating, through the Joule effect of conductive CFs. A temperature of approximately 190 °C was reached during the healing treatment. The healing efficiency was evaluated as the ratio of critical strain energy release rate (G_IC) of the healed and virgin specimens. Healed specimens containing COC layers of 44 μm and 77 μm exhibited a healing efficiency of 164% and 100%, respectively. As expected, the healing treatment was not beneficial for the neat EP/CF laminate without COC, which experienced a healing efficiency of only 2%. This result proved the efficacy of COC layers as a healing agent for EP/CF laminates, and the effectiveness of resistive heating as a way to activate the intrinsic healing mechanism.     

Synergic improvement of DGEBA/CSP/HBP composite on mechanical behavior

Hyperbranched polymer with amino end groups (HBPA) and core-shell particle (CSP, which is fabricated through grafting HBPA onto the surface of silica nanoparticle) were incorporated into an epoxy matrix to fabricate a high performance composite. The effects of CSPs contents on the mechanical properties of composites were studied, discussing the results from tensile, flexural, and impact tests. The composites revealed noticeable improvements in tensile strength, elongation, flexural strength and impact strength in comparison to the neat epoxy or epoxy/HBPA system. The glass transition temperature (T_g) was also improved by the addition of CSP. Field emission scanning electron micrograph (FESEM) indicated that HBPA could favorable improve the compatibility between CSP and epoxy matrix. And the toughening mechanisms were the synergic effect of shearing deformation, phase separation, crack propagation, crack deflection, and crack pinning.     

Effect of Pickering emulsion on the mechanical performances and fracture toughness of epoxy composites

The improvement of mechanical properties and toughness of nanoparticles for epoxy composites was mostly dependent on the disperse state of nanoparticles in epoxy matrices. When the content of nanoparticles was higher than a threshold value, it was easy to aggregate and then affect the improvement effect. Pickering emulsion was prepared using SiO₂ nanoparticles as emulsifier and functional monomer as oil phase. The influence of Pickering emulsion on the curing process was investigated. The effect of Pickering emulsion on the mechanical properties, toughness, and glass transition temperature (Tg) was studied. Impact and tensile fracture surface were observed by scanning electron microscopy (SEM). Results from differential scanning calorimeter (DSC), tensile, impact, and fracture toughness tests are provided. The results indicated that the introduction of Pickering emulsion can eliminate the residual stress and accelerate curing reaction. Epoxy composites were capable of increasing tensile strength by up to 29.9%, impact strength of three‐fold, fracture toughness of 35%, and Tg of 20.7°C in comparison with the reference sample. SEM images showed that SiO₂ nanoparticles exhibit a good dispersion in epoxy matrix. The increases in mechanical properties, toughness, and Tg of epoxy composites were attributed to the “Second Phase Toughness” mechanism.     

Graphene oxide/epoxy composites with enhanced fracture toughness for liquid hydrogen storage

Graphene oxide (GO)/epoxy composites cured by aliphatic dibasic acids have been prepared. The influences of structure of aliphatic dibasic acid and loading of GO on curing process and mechanical properties of epoxy composites were studied. The results show that the reaction activities, gel time of corresponding epoxy-acid system and tensile strength of the formed epoxy resins decrease with the increase of the chain length of aliphatic dibasic acids. Both fracture toughness (>1.96 MPa⋅m^1/2) and elongations at break (>6%) increase with the increase of the chain length of aliphatic dibasic acids. The introduction of GO is helpful to increase the mechanical properties and the gas transmission coefficient of GO/epoxy composites. A maximum of tensile strength and elongations at break were obtained when the loading of GO is 0.6 wt%. The gas transmission coefficient of GO/epoxy composite increases with the increase of GO loading. The excellent mechanical properties and gas leakage resistance coefficient of the formed epoxy composites provides potential application in many fields where conventional brittle epoxy resins are inapplicable.     

Interlaminar Fracture Toughness Characterization of Laminated Composites: A Review

Abstract

Interlaminar fracture toughness had been the subject of great interest for several years and is still interesting to the research community. In this article, a comprehensive analysis of fracture toughness in FRP laminates is presented. Primarily, toughness studies are undertaken on glass and carbon fiber reinforced composites under mode-I and mode-II loading conditions. The fracture behavior and its failure pattern depend on a number of parameters: fiber sizing/coating, matrix modification, insert film, fiber volume fraction, stacking sequence, specimen geometry, loading rate and temperature change. In fact, a state-of-the-art process enables increasing fracture resistance with “matrix toughening by carbon nanotubes (CNT) inclusion”. It enables production of materials having ultra-high strength and low weight. The present study has highlighted the available techniques of CNT incorporation: mechanical mixing, grafting and interleaving. Other aspects, such as the dispersion level, matrix viscosity, fiber surface roughness, loading weight %, bonding strength with epoxy, height and density of grown CNT, energy absorption mechanism during delamination, etc., have been examined as well. Although a clear correlation of all these parameters with fracture toughness is hard to establish, there is growing understanding of the surface-grown CNTs and interleaving processes as they ensure significant increase in fracture toughness.     

Interlaminar and ductile characteristics of carbon fibers-reinforced plastics produced by nanoscaled electroless nickel plating on carbon fiber surfaces

In this work, a new method based on nanoscaled Ni-P alloy coating on carbon fiber surfaces is proposed for the improvement of interfacial properties between fibers and epoxy matrix in a composite system. Fiber surfaces and the mechanical interfacial properties of composites were characterized by atomic absorption spectrophotometer (AAS), scanning electron microscopy (SEM), X-ray photoelectron spectrometry (XPS), interlaminar shear strength (ILSS), and impact strength. Experimental results showed that the O(1s)/C(1s) ratio or Ni and P amounts had been increased as the electroless nickel plating proceeded; the ILSS had also been slightly improved. The impact properties were significantly improved in the presence of Ni-P alloy on carbon fiber surfaces, increasing the ductility of the composites. This was probably due to the effect of substituted Ni-P alloy, leading to an increase of the resistance to the deformation and the crack initiation of the epoxy system.     

Effect of phase change microcapsules on the thermo-mechanical,fracture and heat storage properties of unidirectional carbon/epoxy laminates

This work aims at producing and characterizing unidirectional carbon/epoxy composites containing different fractions of paraffin microcapsules (MC) for thermal management applications. The viscosity of the epoxy/MC mixtures increases with the MC content, thereby increasing the final matrix weight and volume fraction and reducing that of the fibers. This is at the basis of the decrease in mechanical properties of the laminates with high MC concentration (the elastic modulus decreases up to 53% and the flexural strength up to 67%), but the application of theoretical models shows that this decrease is only due to the lower fiber volume fraction, and not to a change in the properties of the constituents or the fiber/matrix interaction. The MC phase is preferentially distributed in the interlaminar zone, which leads to a thickening of this region and a decrease in matrix-related properties, such as the interlaminar shear strength, which decreases of up to 70%. However, a modest MC fraction causes an increase in the mode I interlaminar fracture toughness of 48%, due to the introduction of new toughening mechanisms. On the other hand, an excessive MC content lets the crack propagating through the matrix and not at the fiber/matrix interface, thereby reducing the toughening mechanism provided by fiber bridging. For the thermal properties, the phase change enthalpy increases with the MC fraction up to 48.7 J/g, and this is reflected in better thermal management performance, as proven by thermal imaging tests. These results are promising for the development of multifunctional polymer composites with thermal energy storage and thermal management properties, and future works will be focused on a deeper study of the micromechanical properties of PCM microcapsules and on the improvement of the capsule/matrix adhesion.