Fracture Toughness Of Epoxy Composites Research Articles

Milled graphitic nanoparticle toughened epoxy composites via increased resistance to in-plane crack propagation

Fascinating features of 2D-based carbon materials like graphene have drawn increasing attention to fulfill the desire to enhance the fracture toughness of epoxy composites. However, challenges remain to improve their dispersibility in polymers and increase interaction ability. Thus, work is needed to synthesize alternative cost-effective 2D carbon nanofillers capable of delivering consistent performance as a reinforcer to monitor the fracture toughness of the epoxy composites. The present study demonstrates transforming graphite to ball-milled graphite (BMG) via an effective high-energy wet ball milling process. Herein a noteworthy size reduction is achieved for the BMG ranging from ∼217 to 227 nm. Microscopic investigation spotted an exfoliated 3D networked graphite nanosheet with nanopores. These BMGs were functionalized to attach amine-based silane moieties and were incorporated into the epoxy network. Consequently, even at higher loading levels, they showed their astonishing capability to enhance the crack far field resistance of nanocomposite resulting in significantly improved fracture toughness and energy to ∼2.31 MPa m1/2 and ∼11.30 kJ/m2, respectively. Improvement is ascribed to the better dispersing ability of BMG, which provides a consistent strong interfacial interaction with the epoxy network to modulate material resistance to fracture. Such environmentally friendly nanofillers must be industrially acceptable for synthesizing durable epoxy nanocomposite systems and can be utilized to prepare structural composites.

Effect of CNT filler and temperature on fracture toughness of epoxy composites reinforced with carbon fabric

AbstractIn present research the fracture behavior of carbon fabric‐epoxy composites (CE composites) was evaluated with the addition of 0.1, 0.2, 0.5, and 0.75 weight percentages of carbon nano tubes (CNTs) at different temperatures. The CE composites with different wt.% of CNT were fabricated using land layup method and cured by hot pressing. The fracture toughness of fabricated composites was evaluated in mode–I condition at different temperature. From the results it was shown that the highest fracture toughness was observed at 0.75 wt.% of reinforcement of CNT functional fillers. The fracture toughness of 0.75 wt.% CNT filled CE composites was increased by 19.73% compared to unfilled CE composites. The fracture toughness of CNT filled CE composites diminishes with increase in temperature and the composite filled with 0.75 wt.% CNT exhibits better fracture toughness than the unfilled CE composite at all the temperatures. The fracture toughness was improved by 57.76% and 71.65% at 40 and 60°C, respectively, at 0.75 wt.% reinforcement of CNT fillers. The fractured surfaces of composites were analyzed using Scanning Electron Microscope (SEM) to study the filler contribution to enhance the fracture toughness and failure mechanism of composites.

Effect of crack and vibration of waste tyre rubber hybrid composite for energy absorption applications

Composites reinforced with landfill waste materials have find the applications in engineering materials and they can lead to reduce the environmental pollutions. Waste tyre rubber, broken ceramic tiles and wood particles are creating the environmental hazard to the surroundings. In particular, the recycling of used tyre rubber is highly challenging, but it has very good property to absorb the energy. The reinforcement of rubber as single filler in composite has the limitations in processing and applications. Hence, the above waste materials are incorporated to prepare the composite in the present work. The fracture toughness and shear strength of composites were evaluated and compared with other combinations along with pure epoxy specimen. In order to find the application of composite in dynamic conditions the vibration analysis were done. The presents of rubber decreased the fracture toughness and at the same time the incorporation of ceramic largely improved the fracture toughness of epoxy composite. The shear strength of composites increased with the addition of ceramic and wood particles. But the rubber particle has the great influence on the damping behavior of ceramic base epoxy composites. The addition of ceramic with the epoxy increased the natural frequency and decreased the damping factor. This can be compensated by the inclusion of rubber with ceramic in epoxy resin matrix. 15 wt % addition of rubber with ceramic and epoxy increased the natural frequency of 18.52% and damping factor of 288% than 5 wt % of rubber with ceramic and epoxy. The natural frequency and damping factor of ceramic and rubber based epoxy composite have the highest amount of all combinations and can be used for vibration applications to absorb the energy at high frequencies.

Synchronously enhanced thermal properties and fracture toughness of epoxy composites through melamine foam templated dispersion of carbon nanofibers

Fabricating the thermally conductive composites through incorporating fillers into polymer matrix is usually at the sacrifice of the fracture toughness. To resolve such contradiction, this work proposed a feasible but highly efficient method of constructing the thermally conductive path in the epoxy composite at extremely low filler content. The polyvinylpyrrolidone (PVP) modified carbon nanofibers (P-CNFs) were coated on the three-dimensional (3D) framework surface of the melamine foam (MF), then the epoxy resin was impregnated into the MF@P–CNF. The results showed that the assembled structure of MF@P-CNFs could be well reserved in the epoxy composite. Consequently, at extremely low P–CNF content (0.8 wt%), the MF@P–CNF/EP sample shows the largely enhanced thermal conductivity (0.4 W/m·K) and glass transition temperature (167 °C) compared with the pure epoxy sample (0.2 W/m·K and 162 °C). Specifically, the composite sample also shows the apparently enhanced fracture toughness. Therefore, it is believed that coating CNFs on the 3D interconnected framework of MF is a promising way to prepare the epoxy composites with good comprehensive properties.

Graphene oxide as multi-functional initiator and effective molecular reinforcement in PVP/epoxy composites

In this work, grafting of Polyvinylpyrrolidone (PVP) onto the surface of Graphene Oxide (GO) by following the ‘grafting from’ technique was carried out by the GO-initiated cationic polymerization of N-Vinylpyrrolidone monomers. The successful grafting of PVP onto GO sheets was confirmed with the help of various techniques like FTIR, XRD, Raman and XPS. The PVP polymer chain grafted GO (GO-g-PVP) was incorporated into the epoxy matrix as a reinforcement in order to investigate the effect in thermo-mechanical properties of epoxy. From static mechanical testing, it was observed that neat PVP adversely affects the mechanical (tensile) strength of epoxy, whereas the GO-g-PVP shows an improvement of about ~13% when compared to the virgin epoxy composites. Rheological data reveals the transformation of epoxy resin from the Newtonian behavior to shear thickening nature by the incorporation of GO-g-PVP. Dynamic mechanical analysis and the intensity of tan δ curves also ratify the rheological observation. The fracture toughness of epoxy composites showed a significant improvement of about 190% than the neat epoxy composites. Electron microscopes are employed to observe the Fractograms to systematically analyze the toughening mechanisms involved. Thermal stability and the temperature of glass transitions were optimized by TGA and DSC analysis.

Tensile and fracture properties of epoxy alumina composite: role of particle size and morphology

Epoxy composites reinforced with three different shapes of Al2O3 nanoparticles were synthesized by instant cross-linking process. Out of three types of Al2O3 particles, two were of rod shape particles termed as large nano rod (diameter 2–4 nm and length 100–200 nm), and small nano rod (diameter of 5–10 nm and length less than 50 nm) and third particle was spherical in shape (diameter of 27–43 nm). 0.5, 1.0, and 1.5 wt.%. of reinforcement was maintained constant. Reinforcement of low (1.0) wt.% of spherical Al2O3 nanoparticles is more promising in enhancing tensile strength and failure strain of epoxy composite. At 1.0 wt.% reinforcement of spherical Al2O3 nanoparticles, tensile strength and failure strain are improved by 10% and 29%, respectively. Quasi-static fracture toughness of epoxy composite was improved by reinforcement of all three types of Al2O3 nanoparticles. Fracture toughness of epoxy composite was enhanced by 52% at 1.5 wt.% inclusion of large Al2O3 nano rod. Crack deflection and crack bridging were the micro-mechanics accountable for improved fracture toughness of spherical Al2O3 composite. Crack tilting and twisting, particle pull out, along with crack deflection were the main contributors for increased fracture toughness of composites of Al2O3 nano rods.

Influence of polyethyleneimine functionalized graphene on tribological behavior of epoxy composite

In order to improve the tribological properties of epoxy resin (EP), graphene was functionalized by an amine-riched polymer, polyethyleneimine (PEI) and then fabricated the P-GP/EP composites. The tribological behavior of P-GP/EP composites was investigated by wear testing machine and micro-scratch tester from macroscales and microscales, respectively. Meanwhile, the tensile strength and microhardness were also measured, and the fracture toughness of the composites was evaluated through micro scratch. The results showed that the friction and wear properties of P-GP/EP composites were improved both in macroscales and microscales, and the results of wear resistance were consistent under macro and micro tests. The wear rate in macroscale and volume loss in microscale were 76.76% and 51.34% lower than those of pure EP, respectively. The tensile strength, hardness and fracture toughness of the composites were also improved by the addition of P-GP. The fracture toughness of epoxy composites could be accurately evaluated based on linear elastic fracture mechanics (LEFM).

Effect of intralaminar hybridization on mode I fracture toughness of natural fiber-reinforced composites

The main objective of this work was to investigate the effect of intralaminar hybridization on the mode I fracture toughness of epoxy composites based on jute and sisal fabrics. Four types of composites were produced by the hand layup technique: sisal (S), sisal + curaua (S + C), jute (J) and jute + curaua (J + C). Double cantilever beam (DCB) tests were performed, and the modified beam theory (MBT) method and three different data criteria (i.e. the deviation from linearity (NL), the 5% offset/maximum load (5%/Max) and visual observation (VIS)) were used to evaluate the mode I fracture toughness (GIc). It was found that the GIc of S + C and J + C composites increased by hybridization of pure sisal and jute fabrics and that the intralaminar hybridization limited the crack propagation. An X-ray microcomputed tomography (µCT) equipment was used to visualize the delamination and the form of the front of cracks inside the hybrid composites.

A novel Fe2O3@APFS/epoxy composite with enhanced mechanical and thermal properties

Core-shell structured particle contains Fe2O3 as core and amino phenol formaldehyde resin as shell (Fe2O3@APFS) has been synthesized. The results of SEM and TEM test indicate that the obtained Fe2O3@APFS particle is monodisperse and possesses uniform core-shell structure. Its diameter is about 100 nm and the thickness of APFS layer is about 30 nm. The effects of Fe2O3@APFS on the physicochemical properties of cured epoxy composites have been systematically investigated. The cured Fe2O3@APFS/epoxy composites demonstrated enhanced mechanical and thermal properties. A maximum tensile strength of 94.3 MPa was obtained when 4 wt% loading Fe2O3@APFS particles was added. The fracture toughness of epoxy composites with 5 wt% Fe2O3@APFS loading reaches a value of up to 1.71 MPa·m1/2, which is 80% higher than that of pure epoxy resin. The glass transition temperature (Tg) of cured Fe2O3@APFS/epoxy composites was increased by 13.9 °C than that of pure epoxy resin. TEM observation illustrates that Fe2O3@APFS was monodisperse in epoxy matrix. Monodispersion of Fe2O3@APFS, covalent bond linked interface between Fe2O3@APFS and epoxy matrix and synergistic effect of Fe2O3 and APFS were responsible for the enhanced mechanical and thermal properties of epoxy composites. This work provides a new insight into the combination of soft and rigid fillers used to modify polymer.

Effect of Pickering emulsion on the mechanical performances and fracture toughness of epoxy composites

The improvement of mechanical properties and toughness of nanoparticles for epoxy composites was mostly dependent on the disperse state of nanoparticles in epoxy matrices. When the content of nanoparticles was higher than a threshold value, it was easy to aggregate and then affect the improvement effect. Pickering emulsion was prepared using SiO2 nanoparticles as emulsifier and functional monomer as oil phase. The influence of Pickering emulsion on the curing process was investigated. The effect of Pickering emulsion on the mechanical properties, toughness, and glass transition temperature (Tg) was studied. Impact and tensile fracture surface were observed by scanning electron microscopy (SEM). Results from differential scanning calorimeter (DSC), tensile, impact, and fracture toughness tests are provided. The results indicated that the introduction of Pickering emulsion can eliminate the residual stress and accelerate curing reaction. Epoxy composites were capable of increasing tensile strength by up to 29.9%, impact strength of three‐fold, fracture toughness of 35%, and Tg of 20.7°C in comparison with the reference sample. SEM images showed that SiO2 nanoparticles exhibit a good dispersion in epoxy matrix. The increases in mechanical properties, toughness, and Tg of epoxy composites were attributed to the “Second Phase Toughness” mechanism.

Multi-scale toughening of epoxy composites via electric field alignment of carbon nanofibres and short carbon fibres

The present paper demonstrates that multi-scale fillers such as carbon nanofibres (CNFs) and short carbon fibres (SCFs) can significantly improve the mode I fracture toughness of epoxy composites by various toughening mechanisms. A comparative assessment on the toughening performance promoted by CNFs and SCFs is presented along with the effects of aligning the filler normal to the crack growth using an applied alternating current (AC) electric field. For SCF concentrations of up to 1.5 wt%, with a concentration of CNFs of 1.0 wt%, the multi-scale, hybrid reinforcements additively toughen the epoxy polymer, with the measured fracture toughness being up to about fourteen times the value of the unmodified epoxy polymer. When subjected to an external AC electric field, these two reinforcements rapidly align along the direction of the electrical field in epoxy resin, with the CNFs concentrating between the ends of, and depositing on, the SCFs. For the same concentrations of SCFs and CNFs, the electric field induced alignment of the CNFs and the SCFs further increases the fracture toughness of the multi-scale toughened or hybrid epoxy polymer by up to twenty times that of the unmodified epoxy polymer. The intrinsic and extrinsic toughening mechanisms spanning the nano-to-millimetre length scale have been identified, based upon which an analytical model has been proposed.

STUDYING THE EFFECT OF CHEMICAL TREATMENT AND ORIENTATION ON FRACTURE TOUGHNESS OF KENAF FIBRE- EPOXY COMPOSITES

In this paper, the effect of surface modification and fibre arrangements of kenaf fibers on fracture toughness of epoxy composite was investigated. The chemical treatment of kenaf fibers (KFs) with 6 % NaOH was achieved, and composites with two different fibre arrangements (X and Y) directions were fabricated. Values of fracture toughness (KIc) measured of the compact tension (CT) specimens for both untreated kenaf fibre-reinforced epoxy (ut-KFRE) composites and treated kenaf fibre-reinforced epoxy (treated-KFRE) were much better than the neat epoxy. The KIc value of the treated-KFRE composite in Y-fibre direction was the highest of 2.74 MPa.m1/2 while it was 1.45 MPa.m1/2 for the neat epoxy. Different toughening mechanisms were noticed in the fracture surfaces of the composites in relation to the fibre reinforcement planar, they are shear yielding and fibre splaying with the X-direction and broken fibers, fibre pullout and fibre delamination with the Y-fibre orientation.

Clay/Polyaniline Hybrid through Diazonium Chemistry: Conductive Nanofiller with Unusual Effects on Interfacial Properties of Epoxy Nanocomposites.

The concept of conductive network structure in thermoset matrix without sacrificing the inherent mechanical properties of thermoset polymer (e.g., epoxy) is investigated here using "hairy" bentonite fillers. The latter were prepared through the in situ polymerization of aniline in the presence of 4-diphenylamine diazonium (DPA)-modified bentonite (B-DPA) resulting in a highly exfoliated bentonite-DPA/polyaniline (B-DPA/PANI). The nanocomposite filler was mixed with diglycidyl ether of bisphenol A (DGEBA), and the curing agent (4,4'-diaminodiphenylsulfone) (DDS) at high temperature in order to obtain nanocomposites through the conventional melt mixing technique. The role of B-DPA in the modification of the interface between epoxy and B-DPA/polyaniline (B-DPA/PANI) is investigated and compared with the filler B/PANI prepared without any diazonium modification of the bentonite. Synergistic improvement in dielectric properties and mechanical properties points to the fact that the DPA aryl groups from the diazonium precursor significantly modify the interface by acting as an efficient stress transfer medium. In DPA-containing nanocomposites, unique fibril formation was observed on the fracture surface. Moreover, dramatic improvement (210-220%) in fracture toughness of epoxy composite was obtained with B-DPA/PANI filler as compared to the weak improvement of 20-30% noted in the case of the B/PANI filler. This work shows that the DPA diazonium salt has an important effect on the improvement of the interfacial properties and adhesion of DGEBA and clay/PANI nanofillers.

Nano-Halloysite Concentration Effects on Fracture Toughness of Diverse Epoxy Nanocomposites

Abstract We experimentally report material design of halloysite nanotube (HNT)/epoxy composites, focusing on the effects of HNT concentration on thermal and mechanical properties, especially fracture toughness, of diverse epoxy composites with as-received and phenylphosphonic-acid (PPA)-treated HNTs prepared by mechanical mixing or ball-milling homogenization. It is demonstrated that, with HNT added in the region of 0.0–10.0 wt. %, significantly reinforced fracture toughness of the epoxy composites can be achieved. The epoxy composites prepared by ball-milling homogenization have much more uniform HNT size and dispersion than those prepared by simple mechanical mixing, enhancing their fracture toughness. The morphology of treated HNTs changes from nanotubes to nanoplatelets; as a result, with a substantial increase in the total contact area between HNT and epoxy and enhancing the fracture toughness of epoxy composites. This higher HNT concentration and the higher fracture toughness are achieved for various epoxy composites. However, the optimal concentration of HNT is 5.0 wt. % in this study. The addition of further HNT achieves only marginal fracture toughness enhancement and more negative effects appear, such as HNT concentration gradient in cured epoxy composites, high potential decrease in glass transition temperature (Tg), and potential immature tensile failure.

Mechanical behavior and fracture toughness of epoxy composites reinforced with combination of fibrous and spherical nanofillers

Epoxy nanocomposites modified with multiwalled carbon nanotubes (MWNTs), rubber nanoparticles (RNPs), and the combinations of MWNTs and RNPs were prepared. The effects of multiphase reinforcements on mechanical and fracture properties of epoxy resin were investigated. With combined use of RNPs and MWNTs, the ternary nanocomposites exhibit simultaneous enhancement in stiffness, strength and fracture toughness. Maximum increase of 101% in KIC and 294% in GIC of the ternary composites were achieved in this study. A modified model was developed to predict the modulus of the ternary composites based on the Halpin‐Tsai equation, which was proved to match the experimental results exactly. DSC, TEM, SEM, and AFM studies were carried out to evaluate the composition and microstructure of the binary and ternary composites. POLYM. COMPOS., 36:2147–2156, 2015. © 2014 Society of Plastics Engineers

Functionalised silica/epoxy nanocomposites with enhanced fracture toughness for large-scale applications

A small amount of commercial functional nanosilica was mechanically mixed with epoxy to enhance the composite fracture toughness. Nanosilicas with amino and epoxide functional groups show strong interfaces with epoxy, which suppress large aggregations and enhance resin-wettability, hence enhancing the fracture toughness of epoxy composites. Compared with other reports, less nanosilica content was needed to achieve the same fracture toughness values or similar enhancement ratio. Due to their commercial availability, the low-cost of the raw material and simple fabrication method, those nanocomposites have the potential for large-scale applications.

Mechanical properties of epoxy composites filled with silane-functionalized graphene oxide

In this work, the effects of as-produced GO and silane functionalized GO (silane-f-GO) loading and silane functionalization on the mechanical properties of epoxy composites are investigated and compared. Such silane functionalization containing epoxy ended-groups is found to effectively improve the compatibility between the silane-f-GO and the epoxy matrix. Increased storage modulus, glass transition temperature, thermal stability, tensile and flexural properties and fracture toughness of epoxy composites filled with the silane-f-GO sheets are observed compared with those of the neat epoxy and GO/epoxy composites. These findings confirm the improved dispersion and interfacial interaction in the composites arising from covalent bonds between the silane-f-GO and the epoxy matrix. Moreover, several possible fracture mechanisms, i.e. crack pinning/deflection, crack bridging, and matrix plastic deformation initiated by the debonding/delamination of GO sheets, were identified and evaluated.

Effect of hybridization of liquid rubber and nanosilica particles on the morphology, mechanical properties, and fracture toughness of epoxy composites

A liquid carboxyl-terminated butadiene–acrylonitrile copolymer (CTBN) and SiO2 particles in nanosize were used to modify epoxy, and binary CTBN/epoxy composites and ternary CTBN/SiO2/epoxy composites were prepared using piperidine as curing agent. The morphologies of the composites were observed by scanning electron microscope (SEM) and transmission electron microscope (TEM), and it is indicated that the size of CTBN particles increases with CTBN content in the binary composites, however, the CTBN particle size decreases with the content of nanosilica in the ternary composites. The effects of CTBN and nanosilica particles on the mechanical and fracture toughness of the composites were also investigated, it is shown that the tensile mechanical properties of the binary CTBN-modified epoxy composites can be further improved by addition of nanosilica particles, moreover, obvious improvement in fracture toughness of epoxy can be achieved by hybridization of liquid CTBN rubber and nanosilica particles. The morphologies of the fractured surface of the composites in compact tension tests were explored attentively by field emission SEM (FE-SEM), it is found that different zones (pre-crack, stable crack propagation, and fast crack zones) on the fractured surface can be obviously discriminated, and the toughening mechanism is mainly related to the stable crack propagation zone. The cavitation of the rubber particles and subsequent void growth by matrix shear deformation are the main toughening mechanisms in both binary and ternary composites.

Fracture toughness of nano- and micro-spherical silica-particle-filled epoxy composites

The effects of particle size and volume fraction on the mode I fracture toughness of epoxy composites filled with spherical silica particles were investigated through experiments and an analytical model. Spherical silica particles with various particle diameters ranging from 1.56μm to 240nm and volume fractions from 0 to 0.35 were added to epoxy resin. We found that the fracture toughness of the composites was approximately linear with respect to the reciprocal of the product of (i) the square root of the mean distance between the particle surfaces and (ii) the normalized mean stress in the matrix given by the equivalent inclusion method.

Temperature dependence of fracture toughness of silica/epoxy composites: Related to microstructure of nano- and micro-particles packing

Temperature dependence of the fracture toughness of epoxy composites reinforced with nano- and micro-silica particles was evaluated. Epoxy composites containing varied composition ratios Φ SP of spherical nano- and micro-silica particles, 240 nm and 1.56 μm, were prepared at a fixed volume fraction ( V P = 0.30). The thermo-viscoelasticity and fracture toughness of the composites and neat epoxy were measured at 143 K, 185 K, 228 K, 296 K, 363 K, and 399 K. Experimental results revealed that fracture toughness strongly depended on the microstructure of nano- and micro-particles bidispersion as well as its interactions with the matrix at all temperature, but depended on toughened matrix due to increase in mobility of matrix at the relaxation temperatures.