Improvements in mechanical properties of a carbon fiber epoxy composite using nanotube science and technology

Carbon fiber reinforced epoxy composite laminates, with strategically incorporated fluorine functionalized carbon nanotubes (f-CNTs) at 0.2, 0.3 and 0.5 weight percent (wt.%), are studied for improvements in tensile strength and stiffness and durability under both tension–tension (R = +0.1) and tension–compression (R = −0.1) cyclic loadings, and then compared to the neat (0.0 wt.% CNTs) composite laminate material. To develop the nanocomposite laminates, a spraying technology was used to deposit nanotubes on both sides of each four-harness satin weave carbon fiber fabric piece for the 12 ply laminate lay up. For these experimental studies the carbon fiber reinforced epoxy laminates were fabricated using a heated vacuum assisted resin transfer molding (H-VARTM®) method followed by a 2 soak curing cycle. The f-CNTs toughened the epoxy resin-fiber interfaces to mitigate the evolution of fiber/fabric-matrix interfacial cracking and delamination under both static and cyclic loadings. As a consequence, significant improvements in the mechanical properties of tensile strength, stiffness and resistance to failure due to cyclic loadings resulted for this carbon fiber reinforced epoxy composite laminate.     

[±θ/902]碳/环氧层板横向裂缝与分层的测试及分析

本工作对[±θ/90₂]_S和[0_n/90₂]_S两系列碳/环氧层板在拉仲载荷下的横向裂缝与分层损伤,进行了实验研究和有限元分析.采用声发射技术跟踪配合显微观测多向层板损伤过程,分析了θ角变化与力学性能、初始损伤、累积等的关系.表明实测横向开裂与分层结果和采用能量判据有限元计算预测比较,符合良好.同时在扫描电镜内进行各类层板压缩试验,动态观测破坏形貌,讨论了不同θ铺层角的微观破坏机理.     

CF/EP composite laminates with carbon black and copper chloride for improved electrical conductivity and interlaminar fracture toughness

An experimental study was conducted to improve the electrical conductivity of continuous carbon fibre/epoxy (CF/EP) composite laminate, with simultaneous improvement in mechanical performance, by incorporating nano-scale carbon black (CB) particles and copper chloride (CC) electrolyte into the epoxy matrix. CF/EP laminates of 65 vol.% of carbon fibres were manufactured using a vacuum-assisted resin infusion (VARI) technique. The effects of CB and the synergy of CB/CC on electrical resistivity, tensile strength and elastic modulus and fracture toughness (K_IC) of the epoxy matrix were experimentally characterised, as well as the transverse tensile modulus and strength, Mode I and Mode II interlaminar fracture toughness of the CF/EP laminates. The results showed that the addition of up to 3.0 wt.% CB in the epoxy matrix, with the assistance of CC, noticeably improved the electrical conductivity of the epoxy and the CF/EP laminates, with mechanical performance also enhanced to a certain extent.     

Cryogenic mechanical behaviors of carbon nanotube reinforced composites based on modified epoxy by poly(ethersulfone)

Cryogenic mechanical properties are important parameters for epoxy resins used in cryogenic engineering areas. In this study, multi-walled carbon nanotubes (MWCNTs) were employed to reinforce diglycidyl ether of bisphenol F (DGBEF)/diethyl toluene diamine (DETD) epoxy system modified by poly(ethersulfone) (PES) for enhancing the cryogenic mechanical properties. The epoxy system was properly modified by PES in our previous work and the optimized formulation of the epoxy system was reinforced by MWCNTs in the present work. The results show that the tensile strength and Young’s modulus at 77 K were enhanced by 57.9% and 10.1%, respectively. The reported decrease in the previous work of the Young’s modulus of the modified epoxy system due to the introduction of flexible PES is offset by the increase of the modulus due to the introduction of MWCNTs. Meanwhile, the fracture toughness (K_IC) at 77 K was improved by about 13.5% compared to that of the PES modified epoxy matrix when the 0.5 wt.% MWCNT content was introduced. These interesting results imply that the simultaneous usage of PES and MWCNTs in a brittle epoxy resin is a promising approach for efficiently modifying and reinforcing epoxy resins for cryogenic engineering applications.     

Reinforcement effects of MWCNT and VGCF in bulk composites and interlayer of CFRP laminates

The reinforcement effects of two nanofillers, i.e., multi-walled carbon nanotube (MWCNT) and vapor grown carbon fiber (VGCF), which are used at the interface of conventional CFRP laminates, and in epoxy bulk composites, have been investigated. When using the two nanofillers at the interface between two conventional CFRP sublaminates, the Mode-I interlaminar tensile strength and fracture toughness of CFRP laminates are improved significantly. The performance of VGCF is better than that of MWCNT in this case. For epoxy bulk composites, the two nanofillers play a similar role of good reinforcement in Young’s modulus and tensile strength. However, the Mode-I fracture toughness of epoxy/MWCNT is much better than that of epoxy/VGCF.     

The influence of hygrothermal conditions on the damage processes in quasi-isotropic carbon/epoxy laminates

The effects of hygrothermal conditions on damage development in quasi-isotropic carbon-fiber/epoxy laminates are described. First, monotonic and loading/unloading tensile tests were conducted on dry and wet specimens at ambient and high temperatures to compare the stress/strain response and damage development. The changes in the Young's modulus and Poisson's ratio were obtained experimentally from the monotonic tensile tests. The critical stresses for transverse cracking and delamination for the above three conditions are compared. The delamination area is measured by using scanning acoustic microscopy (SAM) at various loads to discuss the effects of delamination on the nonlinear stress/strain behavior. Next, the stress distributions under tensile load including hygrothermal residual stresses are computed by a finite-element code and their effects on damage initiation are discussed. Finally, a simple model for the prediction of the Young's modulus of a delaminated specimen is proposed. It is found that moisture increases the critical stresses for transverse cracking and delamination by reducing the residual stresses while high temperature decreases the critical stresses in spite of relaxation of the residual stresses. The results of the finite-element analysis provide some explanations for the onset of transverse cracking and delamination. The Young's modulus predicted by the present model agrees with experimental results better than that predicted by conventional models.     

Enhancement of mechanical performance of epoxy/carbon fiber laminate composites using single-walled carbon nanotubes

Carbon nanotubes (CNT) in their various forms have great potential for use in the development of multifunctional multiscale laminated composites due to their unique geometry and properties. Recent advancements in the development of CNT hierarchical composites have mostly focused on multi-walled carbon nanotubes (MWCNT). In this work, single-walled carbon nanotubes (SWCNT) were used to develop nano-modified carbon fiber/epoxy laminates. A functionalization technique based on reduced SWCNT was employed to improve dispersion and epoxy resin-nanotube interaction. A commercial prepregging unit was then used to impregnate unidirectional carbon fiber tape with a modified epoxy system containing 0.1 wt% functionalized SWCNT. Impact and compression-after-impact (CAI) tests, Mode I interlaminar fracture toughness and Mode II interlaminar fracture toughness tests were performed on laminates with and without SWCNT. It was found that incorporation of 0.1 wt% of SWCNT resulted in a 5% reduction of the area of impact damage, a 3.5% increase in CAI strength, a 13% increase in Mode I fracture toughness, and 28% increase in Mode II interlaminar fracture toughness. A comparison between the results of this work and literature results on MWCNT-modified laminated composites suggests that SWCNT, at similar loadings, are more effective in enhancing the mechanical performance of traditional laminated composites.     

A generalized micromechanical approach for the analysis of transverse crack and induced delamination in composite laminates

The previously developed micromechanical approaches for the analysis of transverse cracking and induced delamination are limited for laminates with specific lay-ups such as cross-ply and specific loading conditions. In this paper a new micromechanical approach is developed to overcome such shortcomings. For this purpose, a unit cell in the ply level of composite laminate including transverse cracking and delamination is considered. Then, the governing equations for the stress and displacement fields of the unit cell are derived. The obtained approximate stress field is used to calculate the energy release rate for the propagation of transverse cracking and induced delamination. To show the capability of the new method, it is employed for the analyses of general laminates with [0/90]_s, [45/−45]_s, [30/−30]_s and [90/45/0/−45]_s lay-ups under combined loadings to calculate the energy release rate due to the transverse cracking and induced delamination. It is shown that the obtained energy release rates for transverse cracking and delamination initiation are in good agreement with the available results in the literature and finite element method. Furthermore, the occurrence priority of further transverse cracks and/or delamination at each damage state of the laminates will be discussed.     

Damping characteristics of nanoclay filled hybrid laminates during medium velocity impact

The objective of this paper is to study the vibrational damping characteristics during medium velocity impact of nanoclay filled glass fiber reinforced epoxy hybrid laminates. A series of laminates with varying degree of nanoclay concentration (0–5 wt.%) and fiber weight fraction (25–75 wt.%) were prepared by vacuum assisted resin infusion molding (VARIM) method. The laminates were subjected to medium velocity projectile impact using in-house built gas gun set-up and the ballistic limit of laminates series was determined. The result indicated that during impact, the laminate undergoes vibrational damping. This damping property is a function of fiber weight fraction and orientation, nanoclay concentration and nanocomposite structure. A 42% increase of ballistic limit was observed for 5 wt.% nanoclay filled hybrid (50 wt.% fiber) when compared with unfilled composite. Structural and modal analysis of hybrids showed that the increased ballistic limit of nanoclay filled hybrids is due to the nanocomposite structure and improved damping and fracture properties.     

Phase morphology of nanofibre interlayers: Critical factor for toughening carbon/epoxy composites

Electrospun thermoplastic nanofibres were employed to toughen carbon/epoxy composites by direct deposition on carbon fibre fabrics, prior to resin impregnation and curing. The toughening mechanism was investigated with respect to the critical role of phase morphology on the toughening effect in carbon/epoxy composites. The influences of solubility in epoxy and melting characteristics of thermoplastics were studied towards their effects on phase structure and delamination resistance. For the three different thermoplastic nanofibre interlayers used in this work, i.e. poly(ε-caprolactone) (PCL), poly(vinylidene fluoride) (PVDF) and polyacrylonitrile (PAN) nanofibre interlayers, only PCL nanofibres produced toughening. Although cylinder-shaped fibrous macrophases existed in all three interlayer regions, only PCL nanofibres had polymerisation-induced phase separation with epoxy, forming ductile thermoplastic-rich particulate microphases on the delamination plane. These findings clearly show that the polymerisation-induced phase separation is critical to the interlayer toughening by thermoplastic nanofibres. An optimal concentration (15 wt.%) of PCL solution for electrospinning was found to produce composites with enhanced mode I interlaminar fracture toughness (G_IC), stable crack growth and maintained flexural strength and modulus.     

New development of self-damping MWCNT composites

Epoxy resin modified with nanofillers cannot be used alone for high performance structural applications due to their low-mechanical properties. Therefore, the objective of this work is to hybridize unidirectional and quasi-isotropic glass fiber composite laminates with 1.0 wt% multi-walled carbon nanotubes (MWCNTs). Results from flexural and damping characterizations showed that the flexural strength and modulus, storage modulus, and damping ratio of MWCNT/E nanocomposite are improved by about 7% ± 1.5% compared to neat epoxy. The enhancement in the flexural strength of quasi-isotropic laminate (20.7%) is about ten times higher than that for unidirectional laminate (2.1%). The flexural moduli of the nano-hybridized laminates are reduced by about 7.5–10.8%. Accordingly, the ultimate failure strain and damping properties are evidently improved. The improvement in damping ratio in some cases is about 100%. The high correlation coefficient (0.9995) between flexural and storage moduli suggests using the dynamic nondestructive tests for evaluation the elastic properties of composites.     

Mechanical and thermal behavior of non-crimp glass fiber reinforced layered clay/epoxy nanocomposites

Mechanical and thermal properties of non-crimp glass fiber reinforced clay/epoxy nanocomposites were investigated. Clay/epoxy nanocomposite systems were prepared to use as the matrix material for composite laminates. X-ray diffraction results obtained from natural and modified clays indicated that intergallery spacing of the layered clay increases with surface treatment. Tensile tests indicated that clay loading has minor effect on the tensile properties. Flexural properties of laminates were improved by clay addition due to the improved interface between glass fibers and epoxy. Differential scanning calorimetry (DSC) results showed that the modified clay particles affected the glass transition temperatures (T_g) of the nanocomposites. Incorporation of surface treated clay particles increased the dynamic mechanical properties of nanocomposite laminates. It was found that the flame resistance of composites was improved significantly by clay addition into the epoxy matrix.     

Impact damage characterisation of carbon fibre/epoxy composites with multi-layer reinforcement

Low-velocity impact tests were performed to investigate the impact behaviour of carbon fibre/epoxy composite laminates reinforced by short fibres and other interleaving materials. Characterisation techniques, such as cross-sectional fractography and scanning acoustic microscopy, were employed quantitatively to assess the internal damage of some composite laminates at the sub-surface under impact. Scanning electron microscopy was used to observe impact fractures and damage modes at the fracture surfaces of the laminate specimens. The results show that composite laminates experience various types of fracture; delamination, intra-ply cracking, matrix cracking, fibre breakage and damage depending on the interlayer materials. The trade-off between impact resistance and residual strength is minimised for composites reinforced by Zylon fibres, while that for composites interleaved by poly(ethylene-co-acrylic acid) (PEEA) film is substantial because of deteriorating residual strength, even though the damaged area is significantly reduced. Damages produced on the front and back surfaces of impact were also observed and compared for some laminates.     

Toughness,flexural, damping and interfacial properties of hybridized GFRE composites with MWCNTs

As the improved damping in fiber-reinforced composites can affect the other mechanical properties, therefore, the aim of this work is to investigate the effect of multiwall carbon nanotube (MWCNT) on the interfacial bond strength, flexural strength and stiffness, toughness and damping properties of hybridized glass-fiber reinforced epoxy (GFRE) composites. Nanophased epoxy resin was used to hybridize unidirectional and quasi-isotropic GFRE composites with [0/±45/90]_s and [90/±45/0]_s stacking sequences. Results from the interfacial characterizations of the hybridized composites showed improvement up to 30% compared to the control laminates. Hybridization of GFRE laminates with MWCNTs leads to decreasing the flexural and storage moduli, increasing flexural strength, toughness, natural frequencies and damping ratio. A high correlation coefficient of 0.9985 was obtained between static flexural and dynamic storage moduli. The highest flexural strength, flexural and storage moduli and natural frequency of quasi-isotropic laminate were observed for [0/±45/90]_s stacking sequence and vice versa for damping ratio.     

Improving interlaminar fracture toughness of carbon fibre/epoxy laminates by incorporation of nano-particles

Double-cantilever-beam tests were applied to investigate the mode I interlaminar fracture toughness of carbon fibre/epoxy laminates, in which the epoxy matrices were incorporated with rubber and silica nano-particles, either singly or jointly. It is shown that the toughness is improved owing to the presence of these nano-particles although nano-rubber is more effective than nano-silica. Further, by keeping the total particle weight percentage constant in epoxies (e.g., at 8 and 12 wt.%) filled with equal amount of nano-silica and nano-rubber, the interlaminar toughness values of the hybrid laminates are always higher than those with nano-silica filled epoxies but lower than those with nano-rubber filled matrices. Scanning electron microscopy examination of the delaminated surfaces of composite laminates filled with nano-particles revealed that cavitation of nano-rubber particles/void growth and debonding of nano-silica from epoxy matrix are responsible for the improved interlaminar toughness observed. It is also shown that the bulk toughness of nano-particle filled epoxies cannot be fully transferred to the interlaminar toughness of composite laminates, being limited by the constraint effect imposed by the carbon fibres. Finally, the role of fibre-bridging on the delaminated crack and hence delamination toughness is discussed.     

Acoustic structural health monitoring of composite materials : Damage identification and evaluation in cross ply laminates using acoustic emission and ultrasonics

The characterisation of the damage state of composite structures is often performed using the acoustic behaviour of the composite system. This behaviour is expected to change significantly as the damage is accumulating in the composite. It is indisputable that different damage mechanisms are activated within the composite laminate during loading scenario. These “damage entities” are acting in different space and time scales within the service life of the structure and may be interdependent. It has been argued that different damage mechanisms attribute distinct acoustic behaviour to the composite system. Loading of cross-ply laminates in particular leads to the accumulation of distinct damage mechanisms, such as matrix cracking, delamination between successive plies and fibre rupture at the final stage of loading. As highlighted in this work, the acoustic emission activity is directly linked to the structural health state of the laminate. At the same time, significant changes on the wave propagation characteristics are reported and correlated to damage accumulation in the composite laminate. In the case of cross ply laminates, experimental tests and numerical simulations indicate that, typical to the presence of transverse cracking and/or delamination, is the increase of the pulse velocity and the transmission efficiency of a propagated ultrasonic wave, an indication that the intact longitudinal plies act as wave guides, as the transverse ply deteriorates. Further to transverse cracking and delamination, the accumulation of longitudinal fibre breaks becomes dominant causing the catastrophic failure of the composite and is expected to be directly linked to the acoustic behaviour of the composite, as the stiffness loss results to the velocity decrease of the propagated wave. In view of the above, the scope of the current work is to assess the efficiency of acoustic emission and ultrasonic transmission as a combined methodology for the assessment of the introduced damage and furthermore as a structural health monitoring tool.     

Mechanical behavior and optimization of multidirectional laminate specimens under delamination by bending

Delamination process in the multidirectional laminates under bending loading conditions is often accompanied by excessive deformations ahead of the crack tip and non-linearities due to large displacements and transverse microcracks, which appear in the most strained plies especially for thin materials. Since linear elastic fracture mechanics cannot be directly used in order to characterize delamination, our objective is to highlight, experimentally and analytically, the delamination peculiarities on DCB specimens, then to optimize specimen thickness to the value, which makes it possible to avoid the appearance of transverse cracking before starting of delamination. The material used is E-glass/epoxy multidirectional laminates. The results of optimization obtained are in good agreement with the experimental data. __________ Translated from Problemy Prochnosti, No. 6, pp. 66–78, November–December, 2006.     

Simulation of fatigue performance of cross-ply composite laminates

The fatigue life of cross-ply composite laminates was evaluated using a statistical model. A modified shear-lag analysis was applied to describe the cycle-number-dependent stiffness reduction and consequent stress redistribution processes in the laminates resulted from both progressive transverse matrix cracking in transverse plies and local delamination at tips of transverse cracks. From the strength degradation behaviour and the static strength distribution of 0° plies as well as the fatigue behaviour of 90° plies, the fatigue life of cross-ply laminates with various types of lay-up can be simulated from the model. Predictions of fatigue performance are compared with experimental data for [0/90₂] _s , [02/90₂] _s and [0₂/90₄] _s graphite/epoxy cross-ply laminates: good agreements are obtained.     

Reinforcing epoxy nanocomposites with functionalized carbon nanotubes via biotin–streptavidin interactions

We report on the preparation of nanocomposites consisting of biofunctionalized single-walled carbon nanotubes (BF-SWCNTs) reinforcing an ultraviolet curable epoxy polymer by means of biotin–streptavidin interactions. The as-produced laser ablation SWCNTs are biofunctionalized via acid oxidization based purification process and non-covalent functionalization using surfactant, followed by grafting the resulting nanotubes with biomolecules. The biotin-grafted nanotubes are capable of interacting with epoxy groups in presence of streptavidin molecules by which chemical bridges between BF-SWCNTs and epoxy matrix are formed. The biomolecules grafted to the nanotubes surface not only facilitate the load transfer, but also improve the nanotube dispersion into the epoxy matrix, as observed by optical imaging and scanning electron microscopy. Mechanical characterization on the nanocomposite microfibers demonstrates considerable enhancement in both strength (by 76%) and modulus (by 93%) with the addition of only 1 wt.% of BF-SWCNTs. The electrical measurements reveal a clear change in electrical conductivity of nanocomposite microfibers reinforced with 1 wt.% of BF-SWCNTs in comparison to the microfibers containing solely purified carbon nanotubes. These multifunctional nanocomposite materials could be used to fabricate macro and microstructures for a wide variety of applications such as high strength polymer nanocomposite and potential easily-manipulated biosensors.     

Quantification of flexural fatigue life and 3D damage in carbon fibre reinforced polymer laminates

Carbon fibre reinforced polymer (CFRP) laminated composites have become attractive in the application of wind turbine blade structures. The cyclic load in the blades necessitates the investigation on the flexural fatigue behaviour of CFRP laminates. In this study, the flexural fatigue life of the [+45/−45/0]_2s CFRP laminates was determined and then analysed statistically. X-ray microtomography was conducted to quantitatively characterise the 3D fatigue damage. It was found that the fatigue life data can be well represented by the two-parameter Weibull distribution; the life can be reliably predicted as a function of applied deflections by the combined Weibull and Sigmodal models. The delamination at the interfaces in the 1st ply group is the major failure mode for the flexural fatigue damage in the CFRP laminate. The calculated delamination area is larger at the interfaces adjacent to the 0 ply. The delamination propagation mechanism is primarily matrix/fibre debonding and secondarily matrix cracking.