Self-sensing and dispersive evaluation of single carbon fiber/carbon nanotube (CNT)-epoxy composites using electro-micromechanical technique and nondestructive acoustic emission

Self-sensing and interfacial evaluation were investigated with different dispersion solvents for single carbon fiber/carbon nanotube (CNT)-epoxy composites by electro-micromechanical technique and acoustic emission (AE) under loading/subsequent unloading. The optimized dispersion procedure was set up to obtain improved mechanical and electrical properties. Apparent modulus and electrical contact resistivity for CNT-epoxy composites were correlated with different dispersion solvents for CNT. CNT-epoxy composites using good dispersion solvents exhibited a higher apparent modulus because of better stress transferring effects due to the relatively uniform dispersion of CNT in epoxy and enhanced interfacial adhesion between CNT and the epoxy matrix. However, good solvents exhibited a higher apparent modulus but lower thermodynamic work of adhesion, W_a for single carbon microfiber/CNT-epoxy composite. It is attributed to the fact that hydrophobic behavior with high advanced contact angle was observed for CNT-epoxy in the good solvent, which might not be compatible well with the carbon microfiber. Damage sensing was also detected simultaneously using AE combined with electrical resistance measurement. Electrical resistivity increased stepwise with progressing fiber fracture due to the decrease in electrical contact by the CNT.     

Inherent sensing and interfacial evaluation of carbon nanofiber and nanotube/epoxy composites using electrical resistance measurement and micromechanical technique

Inherent sensing of load, micro-damage and stress transferring effects were evaluated for carbon nanotube (CNT) and carbon nanofiber (CNF)/epoxy composites (with various added contents) by an electro-micromechanical technique, using the four-point probe method. Carbon black (CB)/epoxy composites, with conventional nanosize material added, were used for the comparison with CNT and CNF composites. Subsequent fracture of the carbon fiber in the dual matrix composites (DMC) was detected by acoustic emission (AE) and by the change in electrical resistance, ΔR due to electrical contacts of neighboring CNMs. Stress/strain sensing of the nanocomposites was detected by an electro-pullout test under uniform cyclic loading/subsequent unloading. CNT/epoxy composites showed the best sensitivity to fiber fracture, matrix deformation and stress/strain sensing, whereas CB/epoxy composite exhibited poorer sensitivity. From the apparent modulus (as a result of matrix modulus and interfacial adhesion), the stress transferring effects reinforced by CNT was highest among three CNMs. The thermodynamic work of adhesion, W_a as found by dynamic contact angle measurements of the CNT/epoxy composite as a function of added CNT content was correlated and found to be consistent with the apparent mechanical modulus. Uniform dispersion and interfacial adhesion appear to be key factors for improving both sensing and mechanical performance of nanocomposite. Thermally treated-CNF composites exhibited a slightly higher apparent modulus, whereas higher testing temperatures appeared to lower the apparent modulus.     

Pitch-matrix composites for electrical,electromagnetic and strain-sensing applications

Pitch-matrix composites for deicing, electromagnetic shielding and strain sensing have been developed by using carbon fiber (discontinuous) and carbon black as electrically conductive fillers. A composite with carbon fiber (5 vol%) as the sole filler is effective for strain sensing, which functions by the electrical resistivity increasing reversibly with tensile strain. A composite with carbon fiber (3.4 vol%) and carbon black (1.5 vol%) is less effective for strain sensing and is lower in tensile strength, modulus and ductility, but it is lower in the electrical resistivity. A composite with carbon black (7 vol%) as the sole filler is very high in resistivity, but exhibits high storage modulus. Either carbon fiber or carbon black as filler increases the storage modulus, decreases the resistivity, renders the ability to provide EMI shielding and increases the softening temperature.     

Non-destructive damage sensing and cure monitoring of carbon fiber/epoxyacrylate composites with UV and thermal curing using electro-micromechanical techniques

Interfacial properties and cure monitoring of single-carbon fiber/thermosetting composites by thermal and different ultraviolet (UV) curing processes were investigated using electro-micromechanical test and electrical resistance measurement. During curing process the residual stress was monitored in terms of the electrical resistance and then they were compared to various curing conditions. In thermal cure the tensile strength and modulus of epoxyacrylate matrix were higher than those of UV cure, whereas the failure strain was lower. Interfacial shear strength (IFSS) increased gradually with elapsing UV exposing time and then saturated. For thermal cure the IFSS was significantly higher than that of UV cure, and cure shrinkage was observed due to matrix shrinkage and residual stress due to the difference in thermal expansion coefficient (TEC). The difference in electrical resistance, ΔR during thermal curing was larger than that of UV cure. In thermal cure apparent modulus indicating embedding matrix modulus and interfacial adhesion was highest and reaching time up to same stress was shortest. Thermal cure showed the strong durability against the IFSS deterioration after boiling test compared to UV cure.     

Interfacial evaluation and self-sensing on residual stress and microfailure of toughened carbon fiber/epoxy-amine terminated (AT)-polyetherimide (PEI) composites

Interfacial evaluation and self-sensing of tensile loading/subsequent unloading and microfailure detection of the carbon fiber/epoxy-amine terminated (AT)-polyetherimide (PEI) composites were investigated using micromechanical test and electrical resistance measurement with an aid of acoustic emission (AE). As AT-PEI content increased, both fracture toughness of epoxy-AT-PEI matrix and interfacial shear strength (IFSS) increased due to the optimized matrix modulus for energy absorption. With increasing curing temperature and time, the IFSS increased and then decreased. During curing process, the change in electrical resistance, ΔR increased gradually with adding AT-PEI contents because of different thermal and curing shrinkage of epoxy matrices. Moisture adsorption under durability test could cause to the change in matrix modulus and thus resulted in the change in electrical resistivity correspondently. Under changeable cyclic loading/subsequent unloading, apparent modulus and electrical resistivity during curing process were consistent well with the fracture toughness of epoxy modified with AT-PEI. In compressive test, the electrical resistivity decreased gradually initially and then increased rapidly during subsequent progress of microfailure including fiber fracture showing the buckling pattern.     

Shape memory and thermo-mechanical properties of shape memory polymer/carbon fiber composites

In this study, chopped carbon fiber reinforced trans-1, 4-polyisoprene (TPI) was developed via a proposed new manufacturing process with the aim of improving weak mechanical properties of bulk TPI bulk. Specimens of the developed shape memory polymer (SMP) composites were fabricated with carbon fiber weight fraction of 5%, 7%, 9%, 11% and 13%, respectively. Measured are the effects of chopped carbon fiber and temperature on: (a) shape recovery ratio and rate; (b) stress–strain relationship; (c) maximum tensile stress, strain and Young’s modulus; and (d) maximum stress and residual strain under a constant strain cyclic loading. In addition, SEM micrographs were also presented to illustrate the fracture surface. The present experimental results show that the SMP with 7% carbon fiber weight fraction appears to perform best in all the tests. This indicates that the 7% carbon fiber weight fraction could be the optimum value for the SMP developed using the proposed manufacturing process.     

Effects of thermal histories on interfacial properties of carbon fiber/polyamide 6 composites: Thickness,modulus, adhesion and shear strength

Interface thickness and modulus of carbon fiber (CF) reinforced polyamide 6 (PA 6) composites with different thermal histories are characterized as 331–394 nm and 0.24–0.30 times to fiber modulus, respectively. Transverse fiber bundle (TFB) test is firstly employed for evaluating semi-crystalline PA 6 interfacial adhesion. TFB Failure mechanisms are schematically given. Besides enhanced molecular entangling on fiber surface, increased matrix toughness is also found to have a great effect on improved TFB results. Droplet micro-debonding results show that decreasing cooling rate and increasing annealing temperature both decrease interfacial shear strength (IFSS) though residual PA 6 on carbon fiber surface increases. In the end, the above data are normalized together with some previous measured parameters. It shows that quenching of the CF/PA 6 composites and subsequent annealing are shown to give similar results as slow cooling. Relationships between each other are also discussed.     

电热作用对碳纤维/树脂复合材料界面应力的影响电热作用对碳纤维/树脂复合材料界面应力的影响

碳纤维/树脂复合材料广泛应用于民用航空器结构中,在服役期间会受到复杂环境（湿热、腐蚀、复杂应力和电热作用等）的作用,低强度电流对碳纤维/树脂复合材料的影响受到的关注较少。以碳纤维/树脂复合材料为研究对象,根据碳纤维的温敏效应和通电时的电阻变化规律,计算出碳纤维单丝/环氧树脂复合试样的界面温度范围,之后采用拉曼光谱测试和单丝断裂实验研究了低强度电流对单丝复合体系界面应力和界面剪切强度的影响。结果表明:随着电流强度的提高,单丝复合体系的界面温度随之升高,电流为8 mA时,界面温度高达约200℃。随着电流强度的增大,单丝复合体系的界面压缩应力表现为先增大后减小的趋势,电流高于7 mA后,界面处树脂出现烧蚀降解破坏;单丝断裂实验结果表明随着电流强度增大,单丝复合体系的界面剪切强度呈现先升后降的趋势,在6 mA时界面剪切强度达到最大值62.39 MPa,而8 mA时界面剪切强度仅为34.95 MPa。      

Simultaneous global and local strain sensing in SWCNT-epoxy composites by Raman and impedance spectroscopy

Polymer composites can be benefited in many ways through the addition of carbon nanotubes (CNT). For instance, CNT can build up a percolated network within the polymer matrix, which results in a composite material with electrical conductivity and piezoresistive characteristics. This has very important implications for the realization of self-stress sensing structural composites. Moreover, the remarkable optical and transport properties of CNT permit to obtain information about the stress state of the composite at different scales. In the present work, the local and global stress response of SWCNT-epoxy composites is characterised by simultaneous Raman spectroscopic and electrical measurements on nanocomposite specimens submitted to different levels of surface strain. Both the Raman G′-band resonance frequency and the electrical resistance of the composite are found to change monotonically with strain until an inflection point is reached at ∼1.5% strain. Increased sensitivity of the piezoresistive network and lower load transfer efficiency occur beyond this strain level, and are considered to be the result of CNT slippage from the polymer. The reversibility of the stress sensitivity of the composites is verified by performing cyclic loading tests. Hysteresis loop are found to develop earlier on the Raman curves as in the resistance curves, which indicates that even at low strain levels, permanent damage is induced in the vicinity of carbon nanotubes. The use of Raman spectroscopy in combination with electrical methods provides a further insight on the stress sensing capabilities of CNT and the factors which affect the sensitivity and reproducibility of this behaviour.     

Electrodeposition of exfoliated graphite nanoplatelets onto carbon fibers and properties of their epoxy composites

Exfoliated graphite nanoplatelet (xGnP)/copper (Cu) coated carbon fibers were fabricated by electrophoretic deposition under different applied voltages. The electrical and mechanical properties of individual fibers and composites made from these fibers and epoxy resin were investigated. The electrical resistivity of xGnP/Cu coated single carbon fiber is lower than that of the uncoated control sample and decreases with increase in the applied voltage. The xGnP and metallic Cu were simultaneously deposited on the carbon fiber surface as a result of the electrochemical cell configuration. The interfacial shear strength decreases with applied voltage up to 30 V but increases with applied voltage of over 30 V. The interfacial shear strength for the coated samples except the 50 V treated sample is lower than that of control sample. The flexural strength and modulus of xGnP/Cu coated carbon/epoxy composites is higher than those of control sample due to the reinforcing effect of xGnP/Cu coated on the carbon fibers.     

Processing dependent morphology, interfacial interaction and shear behavior of short carbon fiber reinforced PEEK

Shear properties of compression-molded discontinuous AS4 carbon fiber reinforced poly(ether ether ketone) composites are evaluated with Iosipescu tests. It is found that both shear modulus and shear strength strongly depend on the molding conditions. A comparison between shear behavior of the composites and composites morphology reveals that fusion of the composite pellets and fiber/matrix interfacial interaction on molecular scale are the two dominant factors which determine the processing dependent shear properties of the materials studied.     

Reinforcing potential of micro- and nano-sized fibers in the starch-based biocomposites

Starch-based biocomposites reinforced with jute (micro-sized fiber) and bacterial cellulose (BC) (nano-sized fiber) were prepared by film casting. Reinforcement in the composites is essentially influenced by fiber nature, and amount of loading. The optimum amount of fiber loading for jute and bacterial cellulose in each composite system are 60 wt% and 50 wt% (of starch weight), respectively. Mechanical properties are largely improved due to the strong hydrogen interaction between the starch matrix and cellulose fiber together with good fiber dispersion and impregnation in these composites revealed by SEM. The composites reinforced with 40 wt% or higher bacterial cellulose contents have markedly superior mechanical properties than those reinforced with jute. Young’s modulus and tensile strength of the optimum 50 wt% bacterial cellulose reinforced composite averaged 2.6 GPa and 58 MPa, respectively. These values are 106-fold and 20-fold more than the pure starch/glycerol film. DMTA revealed that the presence of bacterial cellulose (with optimum loading) significantly enhanced the storage modulus and glass transition temperature of the composite, with a 35 °C increment. Thermal degradation of the bacterial cellulose component occurred at higher temperatures implying improved thermal stability. The composites reinforced with bacterial cellulose also had much better water resistance than those associated with jute. In addition, even at high fiber loading, the composites reinforced by bacterial cellulose clearly retain an exceptional level of optical transparency owing to the effect of the nano-sized fibers and also good interfacial bonding between the matrix and bacterial cellulose.     

Study of the reinforcing mechanism and strain sensing in a carbon black filled elastomer

The mechanical and electrical properties of a styrene-butadiene rubber (SBR) matrix reinforced with carbon black, at various weight fractions were experimentally studied. Τhe electroconductive composites were used for strain sensing, under tension, by measuring together strain and electrical resistance. The storage and loss modulus decrement with strain were also investigated, on the basis of Payne effect, while a micromechanical model developed elsewhere was employed for the interpretation of the reinforcing mechanism of the composites examined.     

Development of sizing-free multi-functional carbon fibre nanocomposites

High quality multi-walled carbon nanotubes (CNTs) grown at high density using a low temperature growth method are used as an alternative material to polymer sizing and is utilised in a series of epoxy composites reinforced with carbon fibres to provide improved physical and electrical properties. We report improvements for sizing-sensitive mechanical and physical properties, such as the interfacial adhesion, shear properties and handling of the fibres, whilst retaining resin-infusion capability. Following fibre volume fraction normalisation, the carbon nanotube-modified carbon fibre composite offers improvements of 146% increase in Young’s modulus; 20% increase in ultimate shear stress; 74% increase in shear chord modulus and an 83% improvement in the initial fracture toughness. The addition of CNTs imparts electrical functionalisation to the composite, enhancements in the surface direction are 400%, demonstrating a suitable route to sizing-free composites with enhanced mechanical and electrical functionality.     

Dynamic mechanical and thermo-gravimetric analysis of Sansevieria cylindrica/polyester composite: Effect of fiber length,fiber loading and chemical treatment

The Sansevieria cylindrica (SC) fiber reinforced polyester matrix composites (SCFRPCs) were fabricated using compression molding machine. The influences of fiber length, fiber loading and chemical treatments of SCFRPCs over the mechanical and thermal stability were analyzed at different temperatures. The dynamic characteristics such as storage, loss modulus and damping were significantly influenced by the increase in fiber length and fiber loading but not in a geometric progression. Among various chemical treatments, the potassium permanganate treated SCFRPCs show the maximum increase in storage and loss modulus values. This result concluded that in addition to the reinforcing element (fiber length and wt% of fiber) the interfacial bonding between the fiber and the matrix plays a vital role in restricting the molecular mobility which was apparent from the storage modulus values. Efficient stress transfer at the interface is necessary to produce better dynamic properties rather than having more interfacial region. The change in morphology of cleaned and roughened SC fiber and the degree of interfacial adhesion between the fiber and matrix were studied using scanning electron microscope (SEM). The weight loss of SCFRPCs were also studied under varying temperatures with the help of thermo-gravimetric analysis (TGA).     

Smart Behavior of Carbon Fiber Reinforced Cement-based Composite

The electrical characteristics of cement-based material can be remarkably improved by the addition of short carbon fibers.Carbon fiber reinforced cement composite (CFRC) is an intrinsically smart material that can sense not only the stress and strain, but also the temperature. In this paper, variations of electrical resistivity with external applied load, and relation of thermoelectric force and temperature were investigated. Test results indicated that the electrical signal is related to the increase in the material volume resistivity during crack generation or propagation and the decrease in the resistivity during crack closure. Moreover, it was found that the fiber addition increased the linearity and reversibility of the Seebeck effect in the cement-based materials. The change of electrical characteristics reflects large amount of information of inner damage and temperature differential of composite, which can be used for stress-strain or thermal self-monitoring by embedding it in the concrete structures.     

A comparative study of the load transfer mechanisms of the carbon nanotube reinforced polymer composites with interfacial crystallization

With the help of shear-lag theory, load transfer analysis is performed on the carbon nanotube reinforced polymer composites with interfacial crystallization of different morphologies, including transcrystallinity layer (TCL) and nanohybrid shish-kebab (NHSK) structures. By comparison, we find that the TCL structures can ease the burden of the CNT while the NHSK structures can lead to a fluctuating distribution of the axial stress in the CNT. Both structures can improve the effective elastic modulus of the composites, though the effect of the TCL structures is more pronounced. Besides, the enhancement of the load transfer efficiency of the composites is also observed, the study of the interfacial stress on different kinds of interfaces shows that the reinforcing effect of the TCL structures is sensitive to both the CNT/crystalline polymer interface and crystalline polymer/amorphous polymer interface, while the major decisive factor for the NHSK structures is confined to be the CNT/crystalline polymer interface because of the interlocking effect. Based on these features, some suggestions are given for tailoring the high-performance carbon nanotube reinforced polymer composites.     

Structural health monitoring of glass fiber reinforced composites using embedded carbon nanotube (CNT) fibers

In the present work, carbon nanotube (CNT) fibers had been embedded to glass fiber reinforced polymers (GFRP) for the structural health monitoring of the composite material. The addition of the conductive CNT fiber to the non-conductive GFRP material aims to enhance its multi-function ability; the test specimen’s response to mechanical load and the insitu CNT fiber’s electrical resistance measurements were correlated for sensing and damage monitoring purposes. It is the first time this fiber is used in composite materials for sensing purposes; CNT fiber is easy to be embedded and does not downgrade the material’s mechanical properties. Various incremental loading–unloading steps had been applied to the manufactured specimens in tension as well as in three-point bending tests. The CNT fiber worked as a sensor in both, tensile and compression loadings. A direct correlation between the mechanical loading and the electrical resistance change had been established for the investigated specimens. For high stress (or strain) level loadings, residual resistance measurements of the CNT fiber were observed after unloading. Accumulating damage to the composite material had been calculated and was correlated to the electrical resistance readings. The established correlation between these parameters changed according to the material’s loading history.     

Damage detection of glass fiber reinforced composites using embedded PVA–carbon nanotube (CNT) fibers

Polyvinyl alcohol–carbon nanotube (PVA–CNT) fibers were embedded in glass fiber reinforced plastic composites and used as strain sensors for damage monitoring of the composite. Sensing of the structural integrity of the composite was made by the in situ measurement of the electrical resistance of the embedded PVA–CNT fiber during the mechanical tests. The multi-functional materials were tested in tensile progressive damage accumulation (PDA) tests. These tests aimed to seek the electrical response of untreated and pre-stretched PVA–CNT fibers with known level of progressively induced damage to the composite. The advantages and disadvantages of each PVA–CNT fiber used as a sensor are analyzed; the electrical resistance readings of the PVA–CNT fibers were correlated with known parameters that express the induced damage of the composite.     

Analysis of effective elastic modulus for multiphased hybrid composites

Short fiber reinforced composites inherently have fiber length distribution (FLD) and fiber orientation distribution (FOD), which are important factors in determining mechanical properties of the composites. Since the internal structure has a direct effect on the mechanical properties of the composites, a Micro-CT was used to observe the three dimensional structure of fibers in the composites and to acquire FLD and FOD. It was successful to investigate FLD, FOD, and fiber orientation states and to predict the elastic modulus of the hybrid system. Since hybrid composites used in this study consist of three phases of particles, glass fibers, and matrix, theoretical hybrid modeling is required to consider reinforcing effects of both particles and glass fibers. Interaction between the particles and matrix was considered by using a perturbed stress–strain theory, the Tandon–Weng model. In addition, the laminating analogy approach (LAA) was used to predict the overall elastic modulus of the composite. Theoretical prediction of hybrid moduli indicated that there was a possibility of poor adhesion between glass fibers and matrix. The poor interfacial adhesion was confirmed by morphological experiments. This theoretical and experimental platform is expected to provide more insightful understanding on any kinds of multiphased hybrid composites.