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.     

Interfacial and hydrophobic evaluation of glass fiber/CNT–epoxy nanocomposites using electro-micromechanical technique and wettability test

Interfacial evaluation of glass fiber reinforced carbon nanotube (CNT)–epoxy nanocomposites and the hydrophobicity of CNT–epoxy nanocomposites were investigated by micromechanical and wettability tests. The contact resistance of the CNT–epoxy nanocomposites was measured using a gradient specimen, containing electrical contacts with gradually-increasing spacing. The contact resistance of CNT–epoxy nanocomposites could be better valuated by mainly the two-point method. Due to the presence of hydrophobic domains on the heterogeneous surface, the static contact angle of CNT–epoxy nanocomposites was about 120°, which was somewhat lower than that for super-hydrophobicity (>150°). For surface treated glass fiber, tensile strength decreased dramatically, whereas tensile modulus exhibited little change despite the presence of flaws on the etched fiber surface. The interfacial shear strength (IFSS) between the etched glass fiber and the CNT–epoxy nanocomposites increased due to enhanced surface energy and roughness. As thermodynamic work of energy, W_a increased, both the mechanical IFSS and the apparent modulus increased.     

Nondestructive sensing evaluation of surface modified single-carbon fiber reinforced epoxy composites by electrical resistivity measurement

Nondestructive sensing of a single-carbon fiber reinforced epoxy composites was evaluated by the measurement of electrical resistivity under reversible cyclic loading. For the strain–stress sensing, the strain up to the maximum load of a bare carbon fiber itself is larger than that of carbon fiber composite. As curing temperature increased, apparent modulus up to the maximum load increased and the elapsed time became shorter. Higher residual stress might contribute to the improved interfacial adhesion. The strain up to the maximum load at low temperature was larger than that at higher temperature. The strain of electrodeposition (ED) treated carbon fiber was smaller than that of the untreated carbon fiber composite until the maximum load reached. This could be due to higher apparent modulus of composite based on the improved interfacial shear strength (IFSS). Since the electrical resistivity was responded well quantitatively with various parameters, such as matrix modulus, the fiber surface modification, the electrical resistivity measurement can be a feasible method of nondestructive sensing evaluation for conductive fiber reinforced composites inherently.     

Damage classification in carbon fibre composites using acoustic emission: A comparison of three techniques

Classifying the type of damage occurring within a structure using a structural health monitoring system can allow the end user to assess what kind of repairs, if any, that a component requires. This paper investigates the use of acoustic emission (AE) to locate and classify the type of damage occurring in a composite, carbon fibre panel during buckling. The damage was first located using a bespoke location algorithm developed at Cardiff University, called delta-T mapping. Signals identified as coming from the regions of damage were then analysed using three AE classification techniques; Artificial Neural Network (ANN) analysis, Unsupervised Waveform Clustering (UWC) and corrected Measured Amplitude Ratio (MAR). A comparison of results yielded by these techniques shows a strong agreement regarding the nature of the damage present in the panel, with the signals assigned to two different damage mechanisms, believed to be delamination and matrix cracking. Ultrasonic C-scan images and a digital image correlation (DIC) analysis of the buckled panel were used as validation. MAR’s ability to reveal the orientation of recorded signals greatly assisted the identification of the delamination region, however, ANN and UWC have the ability to group signals into several different classes, which would prove useful in instances where several damage mechanisms were generated. Combining each technique’s individual merits in a multi-technique analysis dramatically improved the reliability of the AE investigation and it is thought that this cross-correlation between techniques will also be the key to developing a reliable SHM system.     

Thermal conductive and electrical properties of polyurethane/hyperbranched poly(urea-urethane)-grafted multi-walled carbon nanotube composites

Hyperbranched poly(urea-urethane)-grafted multi-walled carbon nanotubes (HPU-MWCNTs) were incorporated in a polyurethane (PU) matrix based on poly(ethylene oxide-tetrahydrofuran) and aliphatic polyisocyanate resin as curing agent. The 9–12 nm thick HPU shell formed on the MWCNTs improved the dispersion of MWCNTs and enhanced the interfacial adhesion between the PU matrix and MWCNTs, leading to improvements in storage modulus and T_g of the composites and enhancement of the thermal stability of PU. Thus, composites with 0.5–1 wt% MWCNTs increased the thermal conductivity by about 60–70% when compared to, and retained the high electrical resistivity of, neat PU.     

The effective properties and local aggregation effect of CNT/SMP composites

A micromechanics model of the thermomechanical constitutive behavior and micro-structural inhomogeneity of carbon nanotubes (CNTs)/shape memory polymer (SMP) composites is presented. It is assumed that the CNTs are elastic and the SMP obeys a thermomechanical constitutive law. The effective properties of CNT/SMP composites are examined using a micro-mechanics method. The effect of CNT aggregation in the composite, frequently encountered in real engineering situations, is studied. The degree of aggregation is described by an aggregation coefficient, and the effective properties of SMP composites with aggregated CNTs are calculated using a stepping scheme. It is shown that the degree of CNT aggregation dramatically influences the effective properties of the CNT/SMP composites. A homogeneous microstructure leads to maximum levels of effective composite properties.     

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.     

Interfacial microstructure and mechanical properties of carbon fiber composites by fiber surface modification with poly(amidoamine)/polyhedral oligomeric silsesquioxane

Polyhedral oligomeric silsesquioxane (POSS) was grafted onto carbon fiber surface using poly(amidoamine) (PAMAM) as a novel coupling agent at mild reaction conditions. Firstly, the reinforcement was designed with propagation of PAMAM on the fiber surface by in situ polymerization to improve the surface activities of carbon fiber. Secondly, the POSS further grafted on the fiber could significantly enhance fiber surface energy and wettability, which would greatly increase the interfacial strength of fiber-matrix. The microstructure and mechanical properties of carbon fiber and the resulting composites were investigated. The results indicated that PAMAM and POSS, which could significantly increase the surface roughness and wettability of carbon fiber, were successfully grafted on the fiber surface. Compared with the desized fiber composites, the interlaminar shear strength and the interfacial shear strength of the modified carbon fiber composites increased by 48% and 89%, respectively.     

Health monitoring of carbon/carbon,woven reinforced composites. Damage assessment by using advanced signal processing techniques. Part I: Acoustic emission monitoring and damage mechanisms evolution

The present work, first of two parts, deals with three types of woven carbon/carbon (C/C) composites having differentiations during the manufacturing procedure, which influences their fibre/matrix interface. All material types were tested under tensile loading in a load–unload–reload configuration, with online acoustic emission monitoring. Unsupervised pattern recognition algorithms were utilized to classify the acoustic emission (AE) data recorded during the tests. The resulted clusters, concluded by the analysis of AE hits, are associated with the damage mechanisms of the material, activated at the different load levels, and significant remarks were extracted regarding the damage evolution and its differentiation according to the different fibre/matrix interfaces. Emphasis is given on the impact of the different interface types upon the total mechanical behavior and damage accumulation at the test coupons. A qualitative evaluation of the interfaces using non-destructive testing data is also attempted. This first part intends to propose methodologies and procedures to analyze data from online acoustic emission monitoring in order to extract useful information regarding the damage evolution within C/C materials.     

Exploiting carbon nanotube networks for damage assessment of fiber reinforced composites

An approach for damage inspection of composite structures utilizing carbon nanotubes (CNT) networks is investigated. CNT are dispersed in an epoxy using a processing technique compatible with commonly employed composite manufacturing techniques and subsequently used as matrix for a structural glass fiber reinforced composite. The developed electrical conductivity of the composite system is verified experimentally. The electrically conductive CNT network within the GFRP is exploited through distributed electrical voltage measurements to sense and, ultimately, locate damage in the plane of the composite plate. Damage in the form of cracks or delamination interrupts the continuity of the CNT network separating and isolating regions of the conductive network. Employing electric potential fields these changes can become measurable and can provide information for inversely locating the damage. Electrical Resistance Tomography (ERT) is formulated and experimentally applied to measure changes in the potential fields and deliver electrical conductivity change maps which are used to identify and locate changes in the CNT networks. These changes are correlated to capture the damage in the composite. Different damage modes are studied to assess the capabilities of the technique. The technique shows sensitivity to very small damages; less than 0.1% of the inspected area. The solution of the inverse ERT problem delivers a conductivity change maps which offers an effective localization with nearly 10% error and an inspection area suppression of around 75%. The proposed methodology to create CNT networks enables the application of ERT for Non-Destructive Evaluation of composite materials, previously not possible due to lack of conductivity, thus offering damage sensing and location capabilities even in-situ.     

Detection of fibre fracture and ply fragmentation in thin-ply UD carbon/glass hybrid laminates using acoustic emission

This paper investigates the link between acoustic emission (AE) events and the corresponding damage modes in thin-ply UD carbon/glass hybrid laminates under tensile loading. A novel configuration was investigated which has not previously been studied by AE, where the laminates were fabricated by embedding thin carbon plies between standard thickness translucent glass plies to produce progressive fragmentation of the carbon layer and delamination of the carbon/glass interface. A criterion based on amplitude and energy of the AE event values was established to identify the fragmentation failure mode. Since the glass layer was translucent, it was possible to quantitatively correlate the observed fragmentation during the tests and the AE events with high amplitude and energy values. This new method can be used as a simple and advanced tool to identify fibre fracture as well as estimate the number and sequence of damage events that are not visible e.g. in hybrid laminates with thick or non-transparent layers as well as when the damage is too small to be visually detected.     

Synergistic effect of siloxane modified aluminum nanopowders and carbon fiber on electrothermal efficiency of polymeric shape memory nanocomposite

The present study reports an effective approach of significantly enhancing electrothermal efficiency and shape recovery performance of shape memory polymer (SMP) nanocomposite, of which shape recovery was induced by electrically resistive heating. Metallic aluminum (Al) nanopowders synthesized from Al³⁺ solution were chemically grafted onto carbon fiber. Siloxane groups were grafted onto surfaces of the Al nanopowders to enhance the interfacial bonding between the carbon fiber and SMP matrix via van der Waals force and covalent bond, respectively. The siloxane modified Al surfaces could improve both the electrically induced shape recovery performance and electrothermal efficiency through facilitating the electrically resistive heating from carbon fiber into the SMP matrix. Effectiveness of the synergistic effect between siloxane modified Al surface and carbon fiber was demonstrated to achieve the electrical actuation for SMP nanocomposites at a low electrical voltage below 4.0 V.     

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.     

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.     

Use of piezoelectric as acoustic emission sensor for in situ monitoring of composite structures

In this paper, the influence of the integration of several sensors in composite structures is investigated. The plates and the structures in simple shapes, composed of laminated and sandwich materials, are considered. The mechanical behaviour, the acoustics activity and the location of damage sources in various structures with and without piezoelectric implant are compared. The analysis of results allowed a better identification of the influence of the impact of piezoelectric implant on the mechanical behaviour of different structures under different loads. Then, the analysis and the observation of Acoustic Emission (AE) signals led to the identification of the main acoustic signatures of different damage modes dominant in each type of composite materials (laminates and sandwich). Viewpoint comparison between integrated and non-integrated structures, acoustic activity is more significant in the case of integrated material. The location of the sources of damage has shown that acoustic events occurred far from the positions of integrated sensors.     

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.     

Comparative evaluation of fiber treatments on the creep behavior of jute/green epoxy composites

This work presents the short term creep behavior of novel treated jute fabric reinforced green epoxy composites. Jute fabric was treated with CO₂ pulsed infrared laser, ozone, enzyme and plasma. The treated jute fibers were characterized by scanning electron microscopy (SEM). Composites were prepared by hand layup method and compression molding technique. The creep and dynamic mechanical tests were performed in three-point bending mode by dynamic mechanical analyzer (DMA). The creep strain was experiential to increase with temperature. The treated composites exhibited less creep strain than untreated one at all temperatures. The best result in terms of creep deformation is presented by laser treated composite which dominantly exhibited elastic behavior rather than viscous behavior, especially at higher temperatures. The Burgers four parameters model was used to fit the experimental creep data using R statistical computing software. A good agreement between experimental data and theoretical curves were obtained. Dynamic mechanical analysis results revealed the reduction in the tangent delta peak height of treated composites, might be due to improvement in fiber/matrix interfacial adhesion. The degree of interfacial adhesion between the jute fiber and green epoxy was also anticipated using adhesion factor obtained through DMA data and laser treated composite revealed the better interlocking of fibers and matrix at the interface.     

Molecular dynamics simulations of orientation induced interfacial enhancement between single walled carbon nanotube and aromatic polymers chains

In this work, molecular dynamics simulations were utilized to probe the interfacial enhancement between aromatic polymers and single walled carbon nanotube (SWCNT) induced by molecular orientation. Two aromatic polymers, polyphenylene sulfide (PPS) and polystyrene (PS) were chosen for comparison study. It was found that orientation of polymer chain could bring about an obvious promotion in interfacial interaction for both systems. In PPS/SWCNT systems, the increased interfacial interaction energy was due to the easy formation of offset π–π stacking, while in PS/SWCNT systems the formation of edge-to-face π–π stacking contributed to the enhancement. Polymer/SWCNT composites were also constructed and a similar interfacial enhancement was observed as well. The mechanism of the orientation induced enhancement was a combination of forming more π–π stacking and better coating effect. This will help to deepen the understanding of interfacial interaction in aromatic polymers/carbon nanotubes composites and guide the fabrication of high performance materials.     

Application of continuously-monitored single fiber fragmentation tests to carbon nanotube/carbon microfiber hybrid composites

To assess the effect of carbon nanotube (CNT) grafting on interfacial stress transfer in fiber composites, CNTs were grown upon individual carbon T-300 fibers by chemical vapor deposition. Continuously-monitored single fiber composite (SFC) fragmentation tests were performed on both pristine and CNT-decorated fibers embedded in epoxy. The critical fragment length, fiber tensile strength at critical length, and interfacial shear strength were evaluated. Despite the fiber strength degradation resulting from the harsh CNT growth conditions, the CNT-modified fibers lead to a twofold increase in interfacial shear strength which correlates with the nearly threefold increase in apparent fiber diameter resulting from CNT grafting. These observations corroborate recently published studies with other CNT-grafted fibers. An analysis of the relative contributions to the interfacial strength of the fiber diameter and strength due to surface treatment is presented. It is concluded that the common view whereby an experimentally observed shorter average fragment length leads to a stronger interfacial adhesion is not necessarily correct, if the treatment has changed the fiber tensile strength or its diameter.