The influence of nano/micro hybrid structure on the mechanical and self-sensing properties of carbon nanotube-microparticle reinforced epoxy matrix composite

Nano/micrometer hybrids are prepared by chemical vapor deposition growth of carbon nanotubes (CNTs) on SiC, Al₂O₃ and graphene nanoplatelet (GNP). The mechanical and self-sensing behaviors of the hybrids reinforced epoxy composites are found to be highly dependent on CNT aspect ratio (AR), organization and substrates. The CNT–GNP hybrids exhibit the most significant reinforcing effectiveness, among the three hybrids with AR1200. During tensile loading, the in situ electrical resistance of the CNT–GNP/epoxy and the CNT–SiC/epoxy composites gradually increases to a maximum value and then decreases, which is remarkably different from the monotonic increase in the CNT–Al₂O₃/epoxy composites. However, the CNT–Al₂O₃ with increased AR ⩾ 2000 endows the similar resistance change as the other two hybrids. Besides, when AR < 3200, the tensile modulus and strength of the CNT–Al₂O₃/epoxy composites gradually increase with AR. The interrelationship between the hybrid structure and the mechanical and self-sensing behaviors of the composites are analyzed.     

Advanced multifunctional properties of aligned carbon nanotube-epoxy thin film composites

Carbon nanotube (CNT)/epoxy composite films were successfully developed by a combination of layer-by-layer and vacuum-assisted resin transfer molding methods using directly chemical vapor deposition (CVD)-spun CNT plies. CNT fractions in the composite films were found to be dramatically enhanced as the number of CNT plies increased. The as-prepared CNT/epoxy composite films with 24.4 wt.% CNTs exhibited ~ 10 and ~ 5 times enhancements in their strength and Young's modulus, respectively, and high toughness of up to 6.39 × 10³ kJ/m³. Electrical conductivity reached 252.8 S/cm for the 20-ply CNT/epoxy films, which was 20 times higher over those of the CNT/epoxy composites obtained by conventional dispersion methods. This work proposed a route to fabricate high-CNT-fraction CNT/epoxy composites on a large scale. The high toughness of these CNT/epoxy composite films also makes them promising candidates as protective materials.     

Properties of graphene nanopowder and multi-walled carbon nanotube-filled epoxy thin-film nanocomposites for electronic applications: The effect of sonication time and filler loading

Graphene nanopowder (GNP) and multi-walled carbon nanotube (MWCNT)-filled epoxy thin-film composites were fabricated using ultrasonication and the spin coating technique. The effect of sonication time (10, 20 and 30 min) and GNP loading (0.05–1 vol%) on the tensile and electrical properties of GNP/epoxy thin-film composites was investigated. The addition of GNP decreased the material’s tensile strength and modulus. However, among the tested samples, the GNP/epoxy composites produced using 20 min of sonication time had a slightly higher tensile strength and modulus, with a lower electrical percolation threshold volume fraction. The effect of sonication time was supported by morphological analysis, which showed an improvement in GNP dispersion with increased sonication time. However, GNP deformation was observed after a long sonication time. The GNP/epoxy composites at different filler loadings showed higher electrical properties but slightly lower tensile properties compared with the MWCNT/epoxy composites fabricated using 20 min of sonication time.     

Some of the physical and mechanical properties of cement composites manufactured from carbon nanotubes and bagasse fiber

The objective of this investigation was to evaluate physical and mechanical properties of experimental cement type panels made from multi-wall carbon nanotubes (MWCNTs) and bagasse fiber. Three levels of MWCNTs, namely 0.5 wt.%, 1 wt.% and 1.5 wt.% were mixed with 10 wt.% and 20 wt.% of bagasse fiber in rotary type mixer. Thickness swelling, water absorption, bending characteristics and impact strength of the samples were evaluated. Based on the findings in this work the water absorption and thickness swelling of the nanocomposites decreased with increasing amount of the multi-wall carbon nanotubes content in the panels from 0.5% to 1.5%. On the other hand flexural modulus and impact strength of the panels were enhanced with increased percentage of carbon nanotubes. Panels having 0.5% MWCNTs exhibited the highest impact strength. Overall dimensional stability and strength properties of the samples were adversely influenced with increased amount bagasse fiber in the samples. It appears that using lower percentage of bagasse fiber or application of heat or chemical treatment to the raw material should be considered to improve negative influence of bagasse fiber on properties of the panels.     

Direct joining of carbon-fiber–reinforced plastic to an aluminum alloy using friction lap joining

A carbon-fiber–reinforced thermoplastic (polyamide 6 with 20 wt.% carbon fiber addition) and an aluminum alloy (A5052) were joined using friction lap joining. The joint characteristics were evaluated to investigate the effects of A5052 surface treatments and the joining speed on the joint properties. Carbon-fiber–reinforced thermoplastic and A5052 were joined via an interfacial magnesium oxide layer. Surface grinding of the A5052 generated the aluminum hydroxide on the alloy surface and increased the tensile shear strength of the joint. The tensile shear strength increased as the joining speed increased from 100 to 1600 mm min⁻¹, and decreased thereafter.     

Flexural fatigue behavior of synthesized graphene/carbon-nanofiber/epoxy hybrid nanocomposites

In the present research, effects of adding a combination of synthesized graphene nanosheets and carbon nanofibers (CNFs) on the flexural fatigue behavior of epoxy polymer have been investigated. Graphene nanosheets are synthesized based on a changing magnetic field. The flexural bending fatigue life of 0.5 wt.% of graphene/CNF/epoxy hybrid nanocomposites has been considered at room temperature. The samples were subjected to different displacement amplitudes fatigue loadings. Due to the addition of hybrid nanoparticles, a remarkable improvement in fatigue life of epoxy resin was observed in comparison with results obtained by adding 0.25 wt.% graphene or 0.25 wt.% CNF into the resin. Experimental observations show that at a strength ratio equal to 43% by using 0.5 wt.% of hybrid nanoparticles; 37.3-fold improvement in flexural bending fatigue life of the neat epoxy was observed. While, enhancement of adding only graphene or CNF was 27.4 and 24-fold, respectively.     

Effect of alkali treatment on interfacial bonding in abaca fiber-reinforced composites

Abaca fibers demonstrate enormous potential as reinforcing agents in composite materials. In this study, abaca fibers were immersed in 5, 10 or 15 wt.% NaOH solutions for 2 h, and the effects of the alkali treatments on the mechanical characteristics and interfacial adhesion of the fibers in a model abaca fiber/epoxy composite system systematically evaluated. After 5 wt.% NaOH treatment, abaca fibers showed increased crystallinity, tensile strength and Young’s modulus compared to untreated fibers, and also improved interfacial shear strength with an epoxy. Stronger alkali treatments negatively impacted fiber stiffness and suitability for composite applications. Results suggest that mild alkali treatments (e.g. 5 wt.% NaOH for 2 h) are highly beneficial for the manufacture of abaca fiber-reinforced polymer composites.     

Effect of dispersion conditions on the thermo-mechanical and toughness properties of multi walled carbon nanotubes-reinforced epoxy

In this work, multi wall carbon nanotubes (MWCNTs) dispersed in a polymer matrix have been used to enhance the thermo-mechanical and toughness properties of the resulting nanocomposites. Dynamic mechanical analysis (DMA), tensile tests and single edge notch 3-point bending tests were performed on unfilled, 0.5 and 1 wt.% carbon nanotube (CNT)-filled epoxy to identify the effect of loading on the aforementioned properties. The effect of the dispersion conditions has been thoroughly investigated with regard to the CNT content, the sonication time and the total sonication energy input. The CNT dispersion conditions were of key importance for both the thermo-mechanical and toughness properties of the modified systems. Sonication duration of 1 h was the most effective for the storage modulus and glass transition temperature (T_g) enhancement for both 0.5 and 1 wt.% CNT loadings. The significant increase of the storage modulus and T_g under specific sonication conditions was associated with the improved dispersion and interfacial bonding between the CNTs and the epoxy matrix. Sonication energy was the influencing parameter for the toughness properties. Best results were obtained for 2 h of sonication and 50% sonication amplitude. It was suggested that this level of sonication allowed appropriate dispersion of the CNTs to the epoxy matrices without destroying the CNT’s structure.     

Influence of process parameters on mechanical performance and bonding area of AA2024/carbon-fiber-reinforced poly(phenylene sulfide) friction spot single lap joints

The effects of friction spot joining process parameters on the bonding area and mechanical performance of single lap joints were investigated using full-factorial design of experiments and analysis of variance. On one hand, the main process parameters with significant influence on the bonding area were joining pressure, tool rotational speed and joining time. On the other hand, tool rotational speed and joining pressure displayed the highest influence on the lap shear strength of the joints followed by tool plunge depth, whereas the joining time was not statistically significant. The interaction between the rotational speed and joining time was the only interaction with a significant effect on the mechanical performance. Joints with ultimate lap shear forces varying between 1698 ± 92 N and 2310 ± 155 N were obtained. It was observed that generally a larger bonding area as a result of higher heat input leads to an increased mechanical performance of the joints. The generated regression model by the analysis of variance was used to identify an optimized set of parameters for increasing the lap shear strength of the joints to 2280 ± 88 N. Furthermore, the process temperature was monitored, which varied in the range of 370–474 °C.     

Reinforcing brittle and ductile epoxy matrices using carbon nanotubes masterbatch

In this study, the mechanical and thermal properties of epoxy composites using two different forms of carbon nanotubes (powder and masterbatch) were investigated. Composites were prepared by loading the surface-modified CNT powder and/or CNT masterbatch into either ductile or brittle epoxy matrices. The results show that 3 wt.% CNT masterbatch enhances Young’s modulus by 20%, tensile strength by 30%, flexural strength by 15%, and 21.1 °C increment in the glass transition temperature (by 34%) of ductile epoxy matrix. From scanning electron microscopy images, it was observed that the CNT masterbatch was uniformly distributed indicating the pre-dispersed CNTs in the masterbatch allow an easier path for preparation of CNT-epoxy composites with reduced agglomeration of CNTs. These results demonstrate a good CNT dispersion and ductility of epoxy matrix play a key role to achieve high performance CNT-epoxy composites.     

Grafting of nano-TiO2 onto flax fibers and the enhancement of the mechanical properties of the flax fiber and flax fiber/epoxy composite

In this article, a flax fiber yarn was grafted with nanometer sized TiO₂, and the effects on the tensile and bonding properties of the single fibers and unidirectional fiber reinforced epoxy plates were studied. The flax fiber yarn was grafted with nanometer sized TiO₂ through immersion in nano-TiO₂/KH560 suspensions under sonification. The measured grafting content of the nano-TiO₂ ranged from 0.89 wt.% to 7.14 wt.%, dependent on the suspension concentration. With the optimized nano-TiO₂ grafting content (∼2.34 wt.%), the tensile strength of the flax fibers and the interfacial shear strength to an epoxy resin were enhanced by 23.1% and 40.5%, respectively. The formation of Si–O–Ti and C–O–Si bonds and the presence of the nano-TiO₂ particles on the fiber surfaces contributed to the property enhancements. Unidirectional flax fiber reinforced epoxy composite (V_f = 35.4%) plates prepared manually showed significantly enhanced flexural properties with the grafting of nano-TiO₂.     

Comparison of carbon nanofiller-based polymer composite adhesives and pastes for thermal interface applications

Graphite nanoplatelets (GNP), carbon black (CB) and carbon nanotubes are extensively researched to produce thermal interface materials (TIMs). This work reports comparison of interfacial thermal conductance (ITC) of carbon nanofiller-based polymer composite adhesives and pastes. The results show that total thermal contact resistance (TTCR) of GNP/rubbery epoxy composite was the same as that of an equivalent glassy epoxy composite. Although CB-based rubbery epoxy and silicone composites can be applied as thin bondlines, their TTCRs were significantly higher than GNP/rubbery epoxy. GNP incorporation into CB/rubbery epoxy composite improves the ITC of the CB/rubbery epoxy composites but the performance of CB/GNP/rubbery epoxy was inferior to GNP/rubbery epoxy. The thermal paste of GNP/polyetheramine had TTCR of 4.8 × 10^− 6 m²·K/W which is comparable to commercial TIM-paste. The paste produced with silicone had relatively poor ITC versus that prepared with polyetheramine. The paste having smaller particle sized GNPs offers lower TTCR than that prepared with large sized GNPs. The GNP/rubbery epoxy adhesives produced from precursor pastes gave the lowest TTCRs in comparison with the other adhesives. This study suggests that GNPs offer potential for enhancing ITC of TIMs and that ITC of adhesives depends on fillers' thermal conductivity and their interfacial contact with substrates.     

Tribological performance of carbon nanotube–graphene oxide hybrid/epoxy composites

An investigation is conducted on the effect of the hybrid of multi-wall carbon nanotubes (MWCNTs) and graphene oxide (GO) nanosheets on the tribological performance of epoxy composites at low GO weight fractions of 0.05–0.5 phr. The MWCNT amount is kept constant at 0.5 phr, which is typical for CNT/epoxy composites with enhanced mechanical properties. Friction and wear tests against smooth steel show that the introduction of 0.5 phr MWCNTs into the epoxy matrix increases the friction coefficient and decreases the specific wear rate. When testing the tribological performance of MWCNT/GO hybrids, it is shown that at a high GO amount of 0.5 phr, the friction coefficient is decreased below that of the neat matrix whereas the wear rate is increased above that of the neat matrix. At an optimal hybrid formulation, i.e., 0.5 phr MWCNTs and 0.1 phr GO, a further increase in the friction coefficient and a further reduction in the specific wear rate are observed. The specific wear rate is reduced by about 40% down to a factor of 11 relative to the neat epoxy when the GO content is 0.1 phr.     

Electrically conductive carbon nanotube/polypropylene nanocomposite with improved mechanical properties

Conductive polymer nanocomposites based on carbon nanotubes (CNTs) have wide range of applications in the electronics and energy sectors. For many of these applications, such as the electromagnetic interference (EMI) shielding, high nanofiller loading is typically needed to achieve the desired properties. The high nanofiller concentration deteriorates the composite's tensile strength due to the increase in nanofiller aggregation. In this work, highly conductive CNT/polypropylene (PP) nanocomposite with improved tensile strength was prepared by melt mixing. The effects of CNT content on the processing behavior, microstructure, mechanical and electrical properties of the nanocomposite were investigated. Scanning electron microscopy was used to investigate the composite microstructure. Good level of CNT dispersion with remarkable adhesion at the CNT/PP interface was observed. Based on a theoretical model, the interfacial strength was estimated to be in the range of 36–58 MPa. As a result of this microstructure, significant enhancement in ultimate tensile strength was reported with the increase of CNT content. The tensile strength of the 20 wt.% CNT/PP nanocomposite was 80% higher than that of the unfilled PP. Moreover, and due to the good dispersion of CNT particles, an electrical percolation threshold concentration of 0.93 wt.% (0.5 vol.%) was obtained.     

Defect features and mechanical properties of friction stir lap welded dissimilar AA2024–AA7075 aluminum alloy sheets

5 mm-Thick dissimilar AA2024-T3 and AA7075-T6 aluminum alloy sheets were friction stir lap welded in two joint combinations, i.e., (top) 2024/7075 (bottom) and 7075/2024. The influences of process conditions (welding speed and joint combination) on defects (hook and voids) features and mechanical properties of joints were investigated in detail. It was found that the hook deflects largely upwards into the stir zone (SZ) at lower welding speeds (50, 150 mm/min) in both combinations. The process conditions significantly affect the hook geometry which in return affects the lap shear strength. In all 2024/7075 joints, voids appear and the joints fracture from the tip of hook on AS along the SZ/TMAZ (thermomechanically affected zone) interface in lap shear test (tensile fracture mode). In 7075/2024 joints, the hook on RS horizontally extends a large distance into the bottom stir zone at higher welding speeds (225, 300 mm/min). The joints fracture in three modes: shear fracture along the lap interfaces, tensile fracture and the mix fracture of both. In both joint combinations, the lap shear strength generally increases with the increase of welding speed. 7075/2024 Joints show higher failure load than 2024/7075 joints at lower welding speeds while the opposite result appears at higher welding speeds.     

The optimization of friction spot welding process parameters in AA6181-T4 and Ti6Al4V dissimilar joints

Friction spot welding is a relatively new solid-state joining process able to produce overlap joints between similar and dissimilar materials. In this study, the effect of the process parameters on the lap shear strength of AA6181-T4/Ti6Al4V single joints was investigated using full-factorial design of experiment and analyses of variance. Sound joints with lap shear strength from 4769 N to 6449 N were achieved and the influence of the main process parameters on joint performance was evaluated. Tool rotational speed was the parameter with the largest influence on the joint shear resistance, followed by its interaction with dwell time. Based on the experimental results following response surface methodology, a mathematical model to predict lap shear strength was developed using a second order polynomial function. The initial prediction results indicated that the established model could adequately estimate joint strength within the range of welding parameters being used. The model was then used to optimize welding parameters in order satisfy engineering demands.     

Influence of carbon nanotube (CNT) on the mechanical properties of LLDPE/CNT nanocomposite fibers

The present study shows the effect of adding CNT to linear low-density polyethylene (LLDPE) to produce LLDPE/CNT nanocomposite fibers. The LLDPE/CNT fibers were produced by melt extrusion process using a twin-screw extruder, in a controlled temperature from 160 °C to 275 °C. Further, melt extrusion process was followed by drawing of fibers at the room temperature. Three different weight percentages, 0.08, 0.3 and 1 wt.% of CNT were studied for producing nanocomposite fibers. The addition of 1 wt.% CNT in the LLDPE fiber has increased the tensile strength by 38% (350 MPa). The addition of 0.08 and 0.3 wt.% CNT in the fiber matrix has improved the ductility by 87% and 122%, respectively. Similarly, improvement in the toughness was observed by 63% and 105% for LLDPE fibers with 0.08 wt.% and 0.3 wt.% CNT respectively. The increase in the mechanical properties of the composite fibers was attributed to the alignment and distribution of CNT in the LLDPE matrix. The dispersion of CNT in the polymeric matrix has been revealed by SEM. The study shows that the small addition of CNT when properly mixed and aligned will increase the mechanical properties of pristine polymer fibers.     

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.     

Single-walled carbon nanotube–epoxy composites for structural and conductive aerospace adhesives

Single-walled carbon nanotubes (SWCNTs) were incorporated at low loading (up to ∼1 wt%) into an unfilled aerospace-grade epoxy system, to impart electrical conductivity while maintaining structural bonding capability, as a route for development of a structural and conductive adhesive. At these low SWCNT loadings the tensile properties were maintained or improved, while strength decreased in a higher loading case. The structural bonding performance of composite-to-composite joints, evaluated in lap-shear and peel tests, was reasonably maintained for adhesives containing 0.5 wt% or 1 wt% SWCNTs. In the case of the 0.5 wt% SWCNT–adhesive, peel and lap-shear strength were unchanged while the addition of 1 wt% resulted in 30% increase of peel strength but the lap-shear strength was reduced by 10–15%. For 1 wt% SWCNT–adhesives, conductivities as high as 10⁻¹ S m⁻¹ and typically ∼10⁻³ S m⁻¹ were achieved. Joint electrical resistance measured between aluminum adherends was larger than predicted by the bulk conductivity, but was reduced by a post-treatment step resulting in apparent joint conductivities within one order of magnitude of the bulk samples.     

Strain and damage monitoring in carbon-nanotube-based composite under cyclic strain

The resistive behavior of multi-walled carbon nanotube (MWCNT)/epoxy resins, tested under mechanical cycles and different levels of applied strain, was investigated for specimens loaded in axial tension. The surface normalized resistivity is linear with the strain for volume fraction of MWCNTs between 2.96 × 10⁻⁴ and 2.97 × 10⁻³ (0.05 and 0.5% wt/wt). For values lower than 0.05% wt/wt, close to the electrical percolation threshold (EPT) a non-linear behavior was observed. The strain sensitivity, in the range between 0.67 and 4.45, may be specifically modified by controlling the nanotube loading, in fact the sensor sensitivity decreases with increasing the carbon nanotubes amount. Microscale damages resulted directly related to the resistance changes and hence easily detectable in a non-destructive way by means of electrical measurements. In the fatigue tests, the damage is expressed through the presence of a residual resistivity, which increases with the amount of plastic strain accumulated in the matrix.