Hybrid composite rods subjected to excessive bending loads

Glass fiber/carbon fiber/epoxy hybrid composite rods were investigated in this research for their resistance to excessive bending. The rods are presently being used as the load bearing component of the Aluminum Conductor Composite Core/Trapezoidal Wire (ACCC/TW™) design. The ACCC/TW™ design is one of the most serious candidates to replace the existing conductor designs based on steel and aluminum wires. The effects of mandrel size and thickness of the insulating glass fiber composite sheath on the axial compressive stress state during bending of the ACCC rod were numerically investigated by performing non-linear finite element analyses of the conductor wrapping process. In addition, two sets of compression experiments were performed on composite specimens in order to determine the ultimate compressive strength of the ACCC rod and of the carbon fiber composite alone. During the compression tests, acoustic emissions were monitored from the specimens to determine if a different failure process exists for the hybrid composite as opposed to a traditional uni-directional long fiber composite. Proof tests, and subsequent Scanning Electron Microscope (SEM) work of each type of composite were also performed to better understand the failure process. It was clearly demonstrated in this research that ACCC rods will be mechanically damaged by excessive bending over small diameter mandrels used for transportation and installation purposes. This work should be of great help to the manufacturers and potential users of the ACCC conductors around the world.     

Micromechanical simulation of tensile failure of discontinuous fiber-reinforced polymer matrix composites using Spring Element Model

The micromechanical damage and strength of discontinuous fiber-reinforced polymer matrix composites was simulated by the Spring Element Model (SEM), and SEM was compared with Periodic Unit-Cell (PUC) simulation to clarify the potential of SEM. Tensile failure simulations indicate that SEM can be effectively used to predict the strength of long discontinuous fiber reinforced composites. The transition between matrix cracking mode and fiber breaking mode is also discussed to clarify the fiber length at which SEM can be used to predict strength. In addition, the strengths predicted with SEM are compared with the results of experiments on long discontinuous fiber-reinforced thermoplastic composites.     

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.     

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.     

Basalt FRP rods for reinforcement and repair of timber

Limited research has been undertaken into the use of basalt fibre reinforced polymer (FRP) materials for the strengthening and repair of structural timber elements. This paper describes an experimental test programme in which the flexural performance of low-grade glued laminated timber was reinforced using bonded-in basalt FRP rods. Tension test results show that basalt FRP rods compare extremely well to the mechanical characteristics of glass FRP rods. Strengthened and repaired beams exhibited considerable ductility in contrast to brittle tension behaviour of the unreinforced sections. With the use of a modest reinforcement percentage of 1.4% strategically located in circular routed out grooves at the soffit of the beam, mean stiffness enhancements of 8.4% and 10.3% for the global and local measurements were achieved respectively and a mean improvement in the ultimate moment capacity of 23% was achieved in comparison to the unreinforced glulam beams. The distance of the reinforcement to the neutral axis was shown to be highly influential on the mechanical enhancements. The use of basalt FRP rods is seen to be highly effective as a repair technology for damaged timber elements. Strain profile readings from the beams which included the reinforcement demonstrated improved utilisation of the compression characteristics of the timber. In all testing, a good quality bond is reported between the basalt FRP and wood. There exists significant potential for the development of environmentally friendly engineered structural elements by combining timber based products with other natural materials such as basalt fibre reinforced polymers.     

Effect of adhesive fillet geometry on bond strength between microelectronic components and composite circuit boards

Printed circuit boards (PCBs) assembled with ball grid array (BGA) microelectronics packages were tested in a double cantilever beam (DCB) configuration. The results were compared for a filled and an unfilled underfill epoxy adhesive as well as a cyanoacrylate adhesive. The original fillet, formed in the underfilling process, was modified to create fillets of different sizes. Regardless of the underfill thermal and mechanical properties as well as its curing profile, the crack initiation load and the failure mode were solely a function of the size of the underfill fillet, and the failure always initiated within the PCB. Moreover, the strength of the underfilled solder joints was increased significantly (approximately 100%) by the presence of a relatively large fillet. This effect of the underfill fillet on the crack path and the fracture load was then examined in terms of differences in the stress states using a finite element model.     

Flexural strength of bidirectional hybrid epoxy composites reinforced by E glass and T700S carbon fibres

A study on the flexural properties of bidirectional hybrid epoxy composites reinforced by E glass and T700S carbon fibres in inter-ply configurations is presented in this paper. Test specimens are made by hand lay-up and their flexural properties are obtained by three point bend test in accordance with ASTM D790-07. For comparison, the flexural behaviour is also modelled numerically using finite element analysis (FEA), and analytically using the Classic Lamination Theory (CLT). It is shown from the results that in general, good agreement is found between the experimental data and the model predictions. The flexural strength decreases when partial laminas from a carbon/epoxy laminate are replaced by glass/epoxy laminas. No significant hybrid effects for the flexural strength are found from the experiments. However, simulation studies show that hybridisation can potentially improve the flexural strength.     

PLA/wood biocomposites: Improving composite strength by chemical treatment of the fibers

A resol type phenolic resin was prepared for the impregnation of wood particles used for the reinforcement of PLA. A preliminary study showed that the resin penetrates wood with rates depending on the concentration of the solution and on temperature. Treatment with a solution of 1 wt% resin resulted in a considerable increase of composite strength and decrease of water absorption. Composite strength improved as a result of increased inherent strength of the wood, but interfacial adhesion might be modified as well. When wood was treated with resin solutions of larger concentrations, the strength of the composites decreased, first slightly, then drastically to a very small value. A larger amount of resin results in a thick coating on wood with inferior mechanical properties. At large resin contents the mechanism of deformation changes; the thick coating breaks very easily leading to the catastrophic failure of the composites at very small loads.     

Numerical simulation of strain-rate dependent transition of transverse tensile failure mode in fiber-reinforced composites

This study numerically simulates strain-rate dependent transverse tensile failure of unidirectional composites. The authors’ previous study reported that the failure mode depends on the strain rate, with an interface-failure-dominant mode at a relatively high strain rate and a matrix-failure-dominant mode at relatively low strain rate. The present study aims to demonstrate this failure-mode transition by a periodic unit-cell simulation containing 20 fibers located randomly in the matrix. An elasto-viscoplastic constitutive equation that involves continuum damage mechanics regarding yielding and cavitation-induced brittle failure is used for the matrix. A cohesive zone model is employed for the fiber–matrix interface, considering mixed-mode interfacial failure. For the results, the relationship between failure modes and the strain rate is consistent with the authors’ previous studies.     

Simultaneously enhanced cryogenic tensile strength and fracture toughness of epoxy resins by carboxylic nitrile-butadiene nano-rubber

Epoxy resins are important matrices for composites. Carboxylic nitrile-butadiene nano-rubber (NR) particles are employed to improve the tensile strength and fracture toughness at 77 K of diglycidyl ether of bisphenol-F epoxy using diethyl toluene diamine as curing agent. It is shown that the cryogenic tensile strength and fracture toughness are simultaneously enhanced by the addition of NR. Also, the fracture toughness at room temperature (RT) is enhanced by the addition of NR. On the other hand, the tensile strength at RT first increases and then decreases with further increasing the NR content up to 5 phr. 5 phr NR is the proper content, which corresponds to the simultaneous enhancements in the tensile strength and fracture toughness at RT. Moreover, the comparison of mechanical properties between 77 K and room temperature indicates that the tensile strength, Young’s modulus and fracture toughness at 77 K are higher than those at RT but the failure strain shows the opposite results. The results are properly explained by the SEM observation.     

Modeling the residual strength of carbon fiber reinforced composites subjected to cyclic loading

Fatigue and residual strength data available in literature were modeled with a modified two-parameter wear-out model based on strength degradation. The model explicitly accounts for the maximum applied stress and the stress ratio and requires a limited number of experimental data to predict with accuracy the fatigue life of a series of polymer-based composites. In this paper a substantial modification of the model is proposed in order to enhance its capability in predicting the residual strength kinetics with emphasis to the “sudden drop” of strength before catastrophic failure. It is argued that the strength degradation kinetics under given loading conditions can be obtained from the statistical distribution of cycles to failure under the same loading conditions. From the new approach no new parameters are introduced, limiting to a minimum the experimental data needed to predict the residual strength. The strength degradation law reliability is verified on three different materials data sets appeared in literature. The results indicate that both the fatigue life and the residual strength are related to the statistical distribution of the static strength.     

High performance composite nozzle for the improvement of cooling in grinding machine tools

Although grinding is one of the most versatile machining operations that can be used to produce surface finish up to the micrometer level; it often induces thermal damage to a ground surface and higher power consumption if careful selection of grinding parameters are not made. The main aim of this study is to investigate the effect of a newly developed composite structure with enhanced coolant delivery system for optimizing grinding processes in comparison to the nozzles commercially available. The current investigation further aims to correlate the effect of tool geometry developed on grinding process conditions through experiment and modeling by means of Computational Fluid Dynamics (CFD) and Finite Element Analysis (FEA). The results show that with the use of the newly developed composite nozzle, a 30% decrease in coolant waste was achieved. Besides, it was found that the new nozzle yielded approximately 60–80% percentage pump pressure and power reduction compared to commercially available nozzles.     

Rapidly cured epoxy/anhydride composites: Effect of residual stress on laminate shear strength

The drive towards rapid cure thermosetting composites requires a better understanding of the residual stresses that develop during curing. This study investigates the impact of residual stresses on the interlaminar shear strength of resin-infused epoxy/anhydride carbon-fibre laminates. The magnitude of the residual stress was varied by changing the initial injection cure temperature between 75 °C and 145 °C. The corresponding cycle times and the final glass transition temperature of the resin were also measured. The experimentally measured chemical shrinkage and thermal expansion properties of the resin after vitrification were used as inputs to a finite element analysis to calculate the peak residual stresses in the composite. An increase in the initial cure temperature from 85 to 135 °C resulted in an increase of 25% in the residual stress, which led to an experimentally measured reduction in the composite’s short beam shear strength of approximately 16% (8 MPa), in good agreement with model prediction.     

A survey of test methods for multiaxial and out-of-plane strength of composite laminates

This review paper gives an overview of test methods for multiaxial and out-of-plane strength of composite laminates, with special consideration of non-crimp fabrics (NCF) and other textile systems. Tubular and cruciform specimens can provide arbitrary in-plane loading, while off-axis and angle-ply specimens provide specific biaxial loadings. Tensile and compressive out-of-plane strength may be determined by axial loading of specimens with a waisted gauge section, while bending of curved specimens allow determination of the out-of-plane tensile strength. Tests suited for out-of-plane shear strength include the short beam shear test, the inclined double notch test and the inclined waisted specimen. Testing of arbitrary tri-axial stress states using tubular or cruciform specimens with superimposed through-the-thickness loading is highly complex and significant problems have been reported in achieving the intended stress states and failure modes. Specific tri-axial stress states can be obtained by uniaxial loading of specimens with constrained expansion, as in the die channel test.     

Impact and post-impact damage characterisation of hybrid composite laminates based on basalt fibres in combination with flax,hemp and glass fibres manufactured by vacuum infusion

The impact and flexural post-impact behaviour of ternary hybrid composites based on epoxy resin reinforced with different types of fibres, basalt (B), flax (F), hemp (H) and glass (G) in textile form, namely FHB, GHB and GFB, has been investigated. The reinforcement volume employed was in the order of 21–23% throughout. Laminates based exclusively on basalt, hemp and flax fibres were also fabricated for comparison. Hybrid laminates showed an intermediate performance between basalt fibre reinforced laminates on the high side, and flax and hemp fibre reinforced laminates on the low side. As for impact performance, GHB appears to be the worst performing hybrid laminate and FHB slightly overperforms GFB. In general, an increased rigidity can be attributed to all hybrids with respect to flax and hemp fibre composites. The morphological study of fracture by SEM indicated the variability of mode of fracture of flax and hemp fibre laminates and of the hybrid configuration (FHB) containing both of them. Acoustic emission monitoring during post-impact flexural tests confirmed the proneness to delamination of FHB hybrids, whilst they were able to better withstand impact damage than the other hybrids.     

Micromechanical modeling to determine the compressive strength and failure mode interaction of multidirectional laminates

Computational models to predict the compressive strength of carbon fiber reinforced polymer matrix composites are proposed here, motivated by the failure mechanisms observed in compression tests. Delamination, fiber kink-banding and their interaction are seen to dominate the failure response. An upscaled semi-homogenized laminate model is developed to predict the observed compressive response of multidirectional laminates. A generalized 2-D formulation is presented to determine the interfaces most susceptible to delamination. Subsequently, cohesive elements are added along these interfaces to introduce delamination capability in the model. Predictions of the model are compared against experimental data, and are found to be in agreement with respect to compressive strength and failure modes. Further, the effect of stacking sequence on the compressive strength and failure mode is investigated.     

Finite element modeling of lamb wave propagation in anisotropic hybrid materials

A finite element approach for modeling of acoustic emission sources and signal propagation in hybrid multi-layered plates is presented. Modeling results are validated by Laser vibrometer measurements and comparison to calculated dispersion curves. We investigate hybrid plates as typically found in composite pressure vessels, composed of fiber reinforced polymers with arbitrary stacking sequences and attached metal or polymer materials. Hybrid plate thickness, the ratio between anisotropic and isotropic materials and material properties are varied. Lamb-wave propagation in a geometry representative of a pressure vessel is modeled. It is demonstrated, that acoustic emission sources in multi-layered structures can cause Lamb-waves superimposed by guided waves within the individual layers.     

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.     

Impact strength of composites with nano-enhanced resin after fire exposure

Composite materials have been widely used in several engineering applications. However, there are very few studies about the effects of nanoclays on the impact strength of laminates after exposure to the fire. Therefore, this paper intends to study this subject and the impact performance was analysed by low velocity impact tests carried out at different incident impact energy levels. For better dispersion and interface adhesion matrix/clay, nanoclays were previously subjected to a silane treatment appropriate to the epoxy resin. The exposure to the fire decreases the maximum load and increases the displacement in comparison with the respective values obtained at room temperature. Mathematical relationships are proposed to estimate the maximum impact force and displacement, based on the total impact energy and flexural stiffness. Finally, a decrease of the elastic recuperation can be found, independently of the benefits introduced by the nanoclays.     

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.