Influence of z-pin length on the delamination fracture toughness and fatigue resistance of pinned composites

The effect of z-pin length on the mode I and mode II delamination toughness and fatigue resistance of z-pinned carbon-epoxy composites is investigated. Experimental testing and mechanical modelling reveals that both the mode I fracture toughness and fatigue resistance increase with the z-pin length due to increased bridging traction loads generated by elastic stretching and pull-out of the pins. The opposite trend occurs for mode II toughness, which decreases with increasing z-pin length due to lower traction loads arising from restrictions on the shear-induced rotation and pull-out of the pins. The mode II fatigue resistance is increased by z-pinning, although it is not dependent on the z-pin length. Increasing the z-pin length beyond a critical size also changes the mode I and mode II delamination fracture and fatigue processes from single to multiple cracking. The effect of z-pin length on the delamination toughening and fatigue strengthening mechanisms is determined.     

Effect of cellulose nano fiber (CNF) on fatigue performance of carbon fiber fabric composites

The effect of cellulose nano fibers (CNF): micro-fibrillated cellulose and bacteria cellulose fibers were investigated on the fatigue life of carbon fiber (CF) fabric/epoxy (EP) composites. Epoxy used as the matrix was physically modified with CNF in advance before fabricating the laminates. The high cycle fatigue strength was significantly improved at 0.3 wt% CNF. There exists an appropriate CNF content which makes the fatigue life longest. An increase of adhesive strength between CF and matrix results due to physical modification with CNF. The adhesive strength much increases with increasing the CNF content. Almost no interfacial debonding occurs at 0.8 wt% CNF content when CF breakage takes place. On the other hand, some debonding occurs along CFs from the breaking point at 0.3 wt% CNF. Debonding is more significant in the case of no CNF addition to the matrix. An appropriate interfacial strength brought at 0.3 wt% CNF is the key of fatigue life extension.     

Bridging effect on mode I fatigue delamination behavior in composite laminates

This paper discusses the bridging effect of fibres on mode I fatigue delamination growth in unidirectional and multidirectional polymer composite laminates based on a series of double cantilever beam (DCB) tests. From the results, there is sufficient evidence that fibre bridging can decrease the crack growth rate da/dN significantly, and using only one fatigue resistance curve to determine the delamination behavior in composite materials with large-scale fibre bridging may be inadequate. The bridging created in fatigue delamination is different from that of quasi-static delamination at the same crack length. So it is incorrect to use the resistance curve (R-curve) from quasi-static delamination tests to normalize fatigue delamination results.     

Effects of stretching on mechanical properties of aligned multi-walled carbon nanotube/epoxy composites

Composites based on epoxy resin and differently aligned multi-walled carbon nanotube (MWCNT) sheets have been developed using hot-melt prepreg processing. Aligned MWCNT sheets were produced from MWCNT arrays using the drawing and winding technique. Wavy MWCNTs in the sheets have limited reinforcement efficiency in the composites. Therefore, mechanical stretching of the MWCNT sheets and their prepregs was conducted for this study. Mechanical stretching of the MWCNT sheets and hot stretching of the MWCNT/epoxy prepregs markedly improved the mechanical properties of the composites. The improved mechanical properties of stretched composites derived from the increased MWCNT volume fraction and the reduced MWCNT waviness caused by stretching. With a 3% stretch ratio, the MWCNT/epoxy composites achieved their best mechanical properties in this study. Although hot stretching of the prepregs increased the tensile strength and modulus of the composites considerably, its efficiency was lower than that of stretching the MWCNT sheets.     

Comparison of drying methods for the preparation of carbon fiber felt/carbon nanotubes modified epoxy composites

Carbon fiber felt with carbon nanotubes (CNTs) were prepared by immersing three-dimensional (3D) felt into CNT aqueous solution (with dispersant) followed by removing water with different drying methods. Epoxy resin was then introduced into the felt to obtain 3D fiber felt/CNTs modified epoxy composites. This paper highlights the effect of drying method on macro-morphologies of the felt, morphological dispersion of CNTs and some relevant properties of the composites, including electrical conductivity and flexural performance. The results demonstrate that compared to the commonly used heat drying method, freeze drying technique possesses obvious advantages for the fabrication of fiber felt/CNT modified epoxy composites.     

Hybrid nanoreinforced carbon/epoxy composites for enhanced damage tolerance and fatigue life

Hybrid nano/microcomposites with a nanoparticle reinforced matrix were developed, manufactured, and tested showing significant enhancements in damage tolerance properties. A woven carbon fiber reinforced polymer composite, with the polymer (epoxy) matrix reinforced with well dispersed carbon nanotubes, was produced using dispersant-and-sonication based methods and a wet lay-up process. Various interlaminar damage tolerance properties of this composite, including static strength, fracture toughness, fatigue life, and crack growth rates were examined experimentally and compared with similarly-processed reference material produced without nanoreinforcement. Significant improvements were obtained in interlaminar shear strength (20%), fracture toughness (180%), shear fatigue life (order of magnitude), and fatigue crack growth rate (factor of 2). Observations by scanning electron microscopy of failed specimens showed significant differences in fracture surface morphology between the two materials, related to the differences in properties and providing context for understanding of the enhancement mechanisms.     

Dynamic fracture of carbon nanotube/epoxy composites under high strain-rate loading

We investigate dynamic fracture of three types of multiwalled carbon nanotube (MWCNT)/epoxy composites and neat epoxy under high strain-rate loading (${10}^5''–''{10}^6$ s⁻¹). The composites include randomly dispersed, 1 wt%, functionalized and pristine CNT/epoxy composites, as well as laminated, ∼50 wt% CNT buckypaper/epoxy composites. The pristine and functionalized CNT composites demonstrate spall strength and fracture toughness slightly higher and lower than that of neat epoxy, respectively, and the spall strength of laminated CNT buckypaper/epoxy composites is considerably lower; both types of CNTs reduce the extent of damage. Pullout, sliding and immediate fracture modes are observed; the fracture mechanisms depend on the CNT–epoxy interface strength and fiber strength, and other microstructures such as the interface between CNT laminates. Compared to the functionalized CNT composites, weaker CNT–epoxy interface strength and higher fiber strength lead to a higher probability of sliding fracture and higher tensile strength in the pristine CNT composites at high strain rates. On the contrary, sliding fracture is more pronounced in the functionalized CNT composites under quasistatic loading, a manifestation of a loading-rate effect on fracture modes. Despite their helpful sliding fracture mode and large CNT content, the weak laminate–laminate interfaces play a detrimental role in fracture of the laminated CNT buckypaper/epoxy composites. Regardless of materials, increasing strain rates leads to pronounced rise in tensile strength and fracture toughness.     

Augmented fatigue performance and constant life diagrams of hierarchical carbon fiber/nanofiber epoxy composites

We report the results of an extensive multi-stress ratio experimental study on the axial fatigue behavior of an all-carbon hierarchical composite laminate, in which carbon nanofibers (CNFs) are utilized alongside traditional micron-sized carbon fibers. Primary carbon fibers were arranged in matrix-dominated biax ±45° lay-ups in order to establish matrix and matrix/fiber interaction based performance. CNFs were matrix dispersed by three-roll calender milling. Results indicate that the CNF-reinforced composites collectively possess improved fatigue and static properties over their unmodified counterparts. Large mean lifetime improvements of 150–670% were observed in fully compressive, tensile and tensile dominated loadings. Enhancements are attributed to the high interface density and damage shielding effect of the CNFs within the matrix. Further improvements are believed to occur when the nanofibers arrest and redistribute small scale, slowly propagating matrix cracks at low applied stresses. These results highlight the ability of a nanometer-sized reinforcing phase to actively participate and enhance matrix properties while moving toward a cost effective alternative to current material solutions.     

Application of infrared thermography for the characterization of damage in braided carbon fiber reinforced polymer matrix composites

The focus of this study is to assess, using infrared thermography, the fatigue behavior and the corresponding damage states of a textile polymeric composite plate, as a prerequisite step in the development of damage based life prediction models for such advanced composite materials. Monotonic (quasi-static) loading test results confirmed that the dominant damage mechanism is cracking in the braider yarns, which was monitored using thermographic images and confirmed by edge replication microscopic observations. Fatigue results confirmed that the saturation of braider yarn cracks during cyclic loading corresponded to changes in the stiffness degradation rate as well as the surface temperature profile. This was confirmed by edge replication and scanning electron microscopic analysis. The reported results and observations provide an important step in the validation of thermography as a powerful non-destructive evaluation tool for monitoring the development of fatigue damage as well as predicting the damage states of laminated composite materials in general, and braided polymeric composite materials in particular.     

Thermomechanical behavior of PA6.6 composites subjected to low cycle fatigue

In this study, manifold experiments were conducted to investigate the thermomechanical behavior of short E-glass fiber-reinforced polyamide 6.6 composites subjected to low cycle fatigue loadings. Different hygrometric states, fiber configurations and loading rates were considered. Mechanical, thermal and energy responses of composite specimens were recorded using photomechanic techniques. The influence of water content, fiber orientation and loading rate on these thermomechanical responses was systematically analysed.The mechanical findings indicated that the ratcheting phenomenon was more pronounced for humid composites reinforced with fibers oriented transversely and subjected to a low loading rate. Moreover, the order of magnitude in self-heating was greater for transversal fiber composites conditioned at high relative humidity and subjected to a 10 Hz loading rate. From a thermodynamic standpoint, we also noticed that high proportions of the mean stored energy rate were obtained at a high loading rate, with values exceeded 64%. These values were noticeably altered by the water content and fiber angles, i.e. lower as the relative humidity increased and higher as the fiber angles increased.     

The tensile fatigue behaviour of a silica nanoparticle-modified glass fibre reinforced epoxy composite

An anhydride-cured thermosetting epoxy polymer was modified by incorporating 10 wt.% of well-dispersed silica nanoparticles. The stress-controlled tensile fatigue behaviour at a stress ratio of R = 0.1 was investigated for bulk specimens of the neat and the nanoparticle-modified epoxy. The addition of the silica nanoparticles increased the fatigue life by about three to four times. The neat and the nanoparticle-modified epoxy resins were used to fabricate glass fibre reinforced plastic (GFRP) composite laminates by resin infusion under flexible tooling (RIFT) technique. Tensile fatigue tests were performed on these composites, during which the matrix cracking and stiffness degradation was monitored. The fatigue life of the GFRP composite was increased by about three to four times due to the silica nanoparticles. Suppressed matrix cracking and reduced crack propagation rate in the nanoparticle-modified matrix were observed to contribute towards the enhanced fatigue life of the GFRP composite employing silica nanoparticle-modified epoxy matrix.     

A comparative study of fatigue behaviour of flax/epoxy and glass/epoxy composites

Experimental investigations on flax and glass fabrics reinforced epoxy specimens, i.e. FFRE and GFRE, submitted to fatigue tests are presented in this paper. Samples having [0/90]_3S and [±45]_3S stacking sequences, with similar fibre volume fractions have been tested under tension–tension fatigue loading. The specific stress-number of cycles to failure (S–N) curves, show that for the [0/90]_3S specimens, FFRE have lower fatigue endurance than GFRE, but the [±45]_3S FFRE specimens offer better specific fatigue endurance than similar GFRE, in the studied life range (<2 × 10⁶). Overall, the three-stage stiffness degradation is observed in all cases except for [0/90]_3S FFRE specimens, which present a stiffening phenomenon of around 2–3% which could be related to the straightening of the microfibrils.     

Acoustic emission analysis for characterisation of damage mechanisms in fibre reinforced thermosetting polyurethane and epoxy

Acoustic emission analysis is used to investigate microscopic damage mechanisms and damage progress in unidirectional glass and carbon fibre reinforced composites. Under static loading the influence of fibre orientation on damage initiation and propagation is determined. A novel polyurethane matrix system significantly enhances material performance in terms of crack initiation load levels, crack growth, damage tolerance and off-axis tensile strength. Hysteresis measurements during stepwise increasing dynamic load tests highlight the effect of fibre–matrix-adhesion and resin fracture toughness in unidirectional 0° fibre reinforced composites. Acoustic detection of beginning fibre breakage correlates with a significant increase of loss work per cycle.     

Aligning carbon nanotubes using bulk acoustic waves to reinforce polymer composites

Bulk acoustic waves (BAWs) are used to align multi-walled carbon nanotubes (MWCNTs) in polymer composite materials. MWCNTs are first dispersed in the liquid state of a thermoset resin and aligned using standing BAWs. Cross-linking of the resin fixates the aligned MWCNTs in the polymer matrix material. We have quantified the alignment obtained with this method on the macro, micro, and nanoscale, and it is found to be similar to other alignment techniques such as stretching, slicing, and wet spinning. The elastic modulus and ultimate tensile strength of composite material specimens with aligned MWCNTs, fabricated using this technique, are evaluated and compared with specimens consisting of randomly oriented MWCNTs and resin material without MWCNTs. Different MWCNT loading rates are considered. The elastic modulus of composite material specimens with only 0.15 weight percent aligned MWCNTs is observed to be 44% higher than specimens with randomly oriented MWCNTs, and 51% higher than specimens without MWCNTs. However, further increasing the MWCNT loading rate does not significantly increase the elastic modulus and ultimate tensile strength, likely because of insufficient dispersion of MWCNTs in the thermoset matrix material.     

Waterproof characteristics of nanoclay/epoxy nanocomposite in adhesively bonded joints

When adhesively bonded joints are exposed to a moist environment, the tensile load capability of the joint is significantly decreased because moisture absorption weakens the mechanical properties of epoxy adhesive. In this paper, a nanoclay with excellent penetration resistance properties was used as a filler in epoxy adhesive in order to enhance adhesive strength in moist environments. The water absorption of the epoxy adhesive and the adhesive strength of the adhesively bonded joints were measured in water absorption experiments with respect to the weight fraction of the nanoclay and the moisture exposure time. These results showed that the tensile load capability of the nanoclay-filled adhesively bonded joint was greatly enhanced, even in a moist environment, because the nanoclay reduced water absorption into the epoxy adhesive as well as into the interface between the epoxy adhesive and the steel adherend and increased the strength of the epoxy adhesive itself.     

Thermal conductivity of epoxy composites with a binary-particle system of aluminum oxide and aluminum nitride fillers

Aluminum oxide and aluminum nitride with different sizes were used alone or in combination to prepare thermally conductive polymer composites. The composites were categorized into two systems, one including composites filled with large-sized aluminum nitride and small-sized aluminum oxide particles, and the other including composites filled with large-sized aluminum oxide and small-sized aluminum nitride. The use of these hybrid fillers was found to be effective for increasing the thermal conductivity of the composite, which was probably due to the enhanced connectivity offered by the structuring filler. At a total filler content of 58.4 vol.%, the maximum values of both thermal conductivities in the two systems were 3.402 W/mK and 2.842 W/mK, respectively, when the volume ratio of large particles to small particles was 7:3. This result was represented when the composite was filled with the maximum packing density and the minimum surface area at the same volume content. As such, the proposed thermal model predicted thermal conductivity in good agreement with experimental values.     

Strain sensing in polymer/carbon nanotube composites by electrical resistance measurement

In this work multiwall carbon nanotubes (MWCNTs) dispersed in a polymer matrix have been used for strain sensing of the resulting nanocomposite under tensile loading. This was achieved by measuring the relative electrical resistance change (ΔR/R₀) in conductive polyvinylidenefluoride (PVDF)/MWCNTs nanocomposites prepared by melt-mixing with varying filler content from 0.5 wt.% to 8 wt.%. Two main parameters were systematically studied. The PVDF/MWCNTs mixing procedure that results in a successful MWCNTs dispersion, and the effect of MWCNTs content on material’s sensing behaviour. The samples were subjected to tensile loading and the longitudinal strain was monitored together with the longitudinal electrical resistance. The results showed that MWCNTs dispersed in insulating PVDF matrix have the potential to be used as a sensitive network to monitor the strain levels in polymer/carbon nanotube nanocomposites as the deformation level of each sample was being reflected by the resistance changes.     

Preparation of continuous carbon nanotube networks in carbon fiber/epoxy composite

In order to optimize carbon nanotube (CNT) dispersion state in fiber/epoxy composite, a novel kind of CNT organization form of continuous networks was designed. The present work mainly discussed the feasibility of preparing continuous CNT networks in composite: Fiber fabric was immersed into CNT aqueous solution (containing dispersant) followed by freeze drying and pyrolysis process, prior to epoxy infusion. The morphologies of fabric with CNTs were observed by Scanning Electron Microscope. The relationship between CNT networks and flowing epoxy resin was studied. Properties of composite, including out-of-plane electrical conductivity and interlaminar shear strength (ILSS), were measured. The results demonstrated that continuous and porous CNT networks formed by entangled CNTs could be assembled in fiber fabric. Most part of them were preserved in composite due to the robustness of network structures. The preserved CNT networks significantly improved out-of-plane electrical conductivity, and also have an effect on ILSS value.     

Multi-step microwave reduction of graphite oxide and its use in the formation of electrically conductive graphene/epoxy composites

The multi-step MW reduction technique was developed in this study to obtain reduced graphene oxides; EG, RGO-1, and RGO-2 with MW irradiation time of 1, 2, and 3 min, respectively. Results of TGA, IR, and elemental analysis demonstrated that the degree of reduction of GO increased with increasing the MW irradiation time. Overall, 3 min of MW irradiation of GO in 3 steps was sufficient to obtain highly reduced GO (C/O ratio 10.38 by elemental analysis). The electrical percolation threshold of composites was observed as 1 wt% and 0.3 wt% for RGO-1 and RGO-2, respectively. Even at 0.5 wt% loading of RGO-2 in epoxy, the T_g value of the composite increased by 10 °C, indicating a strong interfacial interaction between graphene and epoxy resin.