Plastic Void Growth Research Articles

Flame retardancy and toughening properties of epoxy composites containing ammonium polyphosphate microcapsules and expanded graphite

Ammonium polyphosphate microcapsules (BM (polybenzoxazine modified) APP) were prepared through the in situ ring-opening polymerization of allyl group containing benzoxazine monomers on the surfaces of ammonium polyphosphate (APP), and they were significantly hydrophobic than the APP. A flame retardant system of epoxy (EP) resin was prepared with BMAPP and expanded graphite (EG). Flame retardancy, the thermal degradation behavior, a mechanical property of EP and EP/BMAPP/EG composites was investigated through limited oxygen index, vertical burning test, cone calorimetry (CONE), and the thermogravimetric analysis (TGA). The flame retardancy tests indicated that the EG could improve the thermal performance, promote the charring, and enhance the char quality of EP/BMAPP. Scanning electron microscopy (SEM) and Fourier-transform infrared spectroscopy (FTIR) were employed to analyze the morphology and composition of the char residue formed during CONE testing, and to understand the mechanism of char formation. The results of TG-FTIR confirmed the possible mechanism of flame retardancy of EP/BMAPP/EG in the gas phase during combustion. The EG content effects on Young’s modulus, the tensile strength, and the fracture toughness ( KIC) of the EP/BMAPP composites were also investigated. The KIC of the composites containing 1% of EG and 10% of BMAPP increased by approximately 76% and 153%, respectively, compared to the neat matrix and EP/BMAPP-10%. The SEM images of the fractured surface indicated that the enhanced toughness of EP/BMAPP/EG composites mainly attributed to the debonding of the BMAPP and the subsequent plastic void growth of the matrix, as well as the crack deflection effect of the BMAPP/EG.

Impact of ultrasonic assisted triangular lattice like arranged dispersion of nanoparticles on physical and mechanical properties of epoxy-TiO2 nanocomposites

Emerging ex-situ technique, ultrasonic dual mixing (UDM) offers unique and hitherto unapproachable opportunities to alter the physical and mechanical properties of polymer nanocomposites. In this study, triangular lattice-like arranged dispersion of TiO2 nanoparticles (average size ∼ 48 nm) in the epoxy polymer has been attained via concurrent use of a probe ultra-sonicator and 4 blades pitched impeller which collectively named as UDM technique. The UDM processing of neat epoxy reveals the generation of triangular lattice-like arranged nanocavities with nanoscale inter-cavity spacing. The UDM processing of epoxy-TiO2 nanocomposites reveals two unique features such as partial and complete entrapping of the nanoparticles by the nanocavities leading the arranged dispersion of particles in the epoxy matrix. Pristine TiO2 nanoparticles were dispersed in the epoxy polymer at loading fractions of up to 20% by weight. The results display that the arranged dispersion of nanoparticles is very effective at enhancing the glass transition temperature (Tg) and tensile properties of the epoxy at loading fractions of 10 wt%. We quantify a direct relationship among three important parameters such as nanoparticle content, cluster size, and inter-particle spacing. Our results offer a novel understanding of these parameters on the Tg and tensile properties of the epoxy nanocomposites. The tensile fracture surfaces revealed several toughening mechanisms such as particle pull-out, plastic void growth, crack deflection, crack bridging and plastic deformation. We show that a strong nanoparticle-matrix interface led to the enhanced mechanical properties due to leading toughening mechanisms such as crack deflection, plastic deformation and particle pull-out. We showed that the UDM has an inordinate prospective to alter the dispersion state of nanoparticles in viscous polymer matrices.

Core-shell rubber nanoparticle reinforcement and processing of high toughness fast-curing epoxy composites

To simultaneously address the lower toughness and the build-up of internal heat for fast-curing epoxy matrices, the influence of nominal 100 nm and 300 nm core-shell rubber (CSR) particles on the properties and rheo-kinetics were studied. The fracture energy was enhanced by a factor of 14.5, up to 2572 ± 84 J m−2 with 14.5 wt% of the nominal 300 nm diameter CSR particles, with evidence of cavitation and plastic void growth of the rubber core combined with shear band yielding of the epoxy matrix. These toughening mechanisms were modelled with an approximately linear increase up to 10 wt% for both particle types. At higher concentrations, deviation between the measured and modelled data was observed due to insufficient epoxy to dissipate additional energy. The CSR particles were not filtered out or damaged during the manufacturing of composites and reduced the total heat of reaction with a linear correlation, demonstrating a multi-functionality of simultaneous toughening and reduction of the exothermic peak.

Carbon nanotubes and core–shell rubber nanoparticles modified structural epoxy adhesives

In this study, multi-walled carbon nanotubes (MWCNTs) and core–shell rubber (CSR) nanoparticles were dispersed in an epoxy adhesive to produce conductive and tough structural adhesives. The thermo-mechanical properties, electrical conductivity, fracture toughness and lap shear strength of MWCNT–CSR–epoxy structural adhesives were studied. The addition of both MWCNTs and CSR nanoparticles into the epoxy adhesive increased the fracture energy significantly, from 397 J/m2 for the unmodified adhesive, to 2615 J/m2, for an adhesive modified with 10 wt% CSR nanoparticles and 0.3 wt% MWCNTs. The toughening mechanisms of the CSR nanoparticles were identified as cavitation of the rubber particles followed by plastic void growth and shear band yielding. The toughening enhancement of the MWCNTs was mainly attributed to the crack bridging, MWCNT break and crack path deflection. The addition of CSR nanoparticles into neat epoxy was found to affect the failure mode of the lap shear joints, from brittle, interfacial failure, to a ductile, cohesive failure. A moderate increase in the strength of the lap shear joint was also noted. The subsequent addition of MWCNTs further affected the failure mode of the lap shear joints. A mixture of interfacial and cohesive failure was observed for each joint configuration. The lap shear strength was observed to have increased significantly, by 39%, due to the addition of 0.5 wt% MWCNTs. The observed increase in lap shear strength with the addition of MWCNTs was attributed to the crack deflection caused by the MWCNTs.

Effect of annealing on the properties of polylactide/poly(butylene carbonate) blend

AbstractThe microstructure evolution of partial miscible polylactide/poly(butylene carbonate) (PLA/PBC) blend during annealing, including the variation in crystalline structure of PLA and PBC, the morphologies of PBC particles, and the relaxation of chain segments in the amorphous region of PLA, is investigated in detail. Based on the significant difference in melting temperature, phase separation arises as a result of the occurrence of crystallization of PLA and is further enhanced due to the crystallization of PBC. The glass transition temperature Tg of the PLA phase in the phase separation blend is significantly higher than that of the pure PLA due to increasing crystallinity. The increasing annealing time resulted in enhanced phase separation, as reflected in the harsher morphology, the decrease of molecular dynamics, and the reduction amount of the amorphous domain. It can be verified by the interaction parameter C values at various annealing conditions from the yield stress data. Therefore, blend is aptitude to elongation decrease and the toughening micromechanism changes, while the plastic deformation is the important energy dissipation process for quenched PLA/PBC blend. Debonding of the PBC particles followed by plastic void growth of the PLA matrix is the major toughening micro‐mechanisms for phase separation blend.

Toughened carbon fibre-reinforced polymer composites with nanoparticle-modified epoxy matrices

In the current work, the microstructure and fracture performance of carbon fibre-reinforced polymer (CFRP) composites based upon matrices of an anhydride-cured epoxy resin (formulated with a reactive diluent), and containing silica nanoparticles and/or polysiloxane core–shell rubber (CSR) nanoparticles, were investigated. Double cantilever beam tests were performed in order to determine the interlaminar fracture energy of the CFRP composites, while the single-edge-notched bend specimen was employed to evaluate the fracture energy of the bulk polymers. The fracture energy of the bulk epoxy polymers increased from 173 J/m2 for the unmodified polymer to a maximum of 1237 J/m2 with the addition of 16 wt% of CSR nanoparticles. The toughening mechanisms were identified as (a) localised plastic shear yielding and (b) cavitation of the CSR particles followed by plastic void growth of the matrix. The steady-state propagation value of the interlaminar fracture energy of the CFRP composites increased with increasing nanoparticle concentration, from 1246 J/m2 for the unmodified epoxy matrix to a maximum of 1851 J/m2 with 4 wt% of silica nanoparticles and 8 wt% of CSR nanoparticles. Crack growth in the CFRP composites was dominated by fibre-bridging toughening mechanisms. The efficiency of the transfer of toughness from the bulk polymers to the carbon fibre composites was considered. The measured fracture energy of both bulk and composite materials decreased at a test temperature of −80 °C, compared with room temperature, i.e. 20 °C. Nevertheless, the toughening effects of both the silica and CSR nanoparticles on the bulk epoxy polymers and the CFRP composites, compared with the unmodified epoxy polymers, were still evident even at the lower temperature. Indeed, the toughening effect of the silica nanoparticles was greater at −80 °C than at room temperature.

Role of weak particle-matrix interfacial adhesion in deformation and fracture mechanisms of rigid particulate-filled poly(methyl methacrylate)

Toughening glassy thermoplastics such as poly(methyl methacrylate) (PMMA) without sacrificing modulus and thermomechanical stability is a valuable but challenging objective. Rigid particulate fillers have been found to improve toughness of some polymers with complex dependence on matrix ductility, particle size, and particle-matrix interfacial adhesion. We tested the effects of both strong and weak interfacial adhesion on deformation and fracture of a model system comprising PMMA filled with monodisperse 1µm diameter silica spheres. Fracture energy GIC of PMMA was found to increase by over 50% when filled with 1 v% of weakly bonded particles, while the force observed during melt compounding increased by less than 15% and Young's modulus increased systematically with filler loading. However, GIC decreased with filler loading above 1 v%. This behavior is consistent with a modified Kinloch-type model considering localized shear banding and plastic void growth around debonded particles at the crack tip. The ability of the matrix to deform via shear yielding and plastic void growth was confirmed by digital image correlation measurement of volumetric strain in uniaxial tension. We have extended Kinloch's model to account for shortening of the crack tip craze by the particles, which reduces the intrinsic toughness and toughenability of the PMMA matrix. Particles with strong interfacial adhesion generally reduced toughness. The experimental and modeling results suggest weakly bonded particles with size on the order of the crack-tip craze width may provide optimum toughening of glassy thermoplastics.

Mechanical and Thermal Properties of Al<sub>2</sub>O<sub>3</sub>-Filled Epoxy Composites

The paper presents studies of influences of platelet-shape alumina content on the morphology, mechanical and thermal properties of Al2O3/epoxy composites. The alumina particles were pre-treated with surface modifier, and its addition contents were ranged from 17 to 40vol.%. It was found that tensile strength and thermal conductivity increased with the increase of the alumina contents. The fracture surface investigation showed that several strengthening mechanisms, including particles pull-out, crack pinning, plastic void growth and deformation were the main factors for the increments of the tensile strength of the Al2O3/epoxy composites. Comparing with pure epoxy polymer, the creep strain values at 70 °C for 1800 s and recovery strain after 3600 s of 40vol.%Al2O3/epoxy composites was lower as 0.23% and 70%, respectively. It was due to effective prohibition of the slippage and disentanglement of epoxy polymer molecules by the rigid Al2O3.

Graphene nanoplatelet-modified epoxy: effect of aspect ratio and surface functionality on mechanical properties and toughening mechanisms

Graphene has the potential to act as a high-performance reinforcement for adhesives or fibre composites when combined with epoxy polymer. However, it is currently mostly available not as single high aspect ratio sheets but as graphene nanoplatelets (GNPs), comprising stacks of graphene sheets. GNPs of a range of lateral size, thickness, aspect ratio and surface functionality were used to modify an anhydride-cured epoxy polymer. The morphology, mechanical properties and toughening mechanisms of these modified epoxies were investigated. The GNPs were sonicated in tetrahydrofuran (THF) or n-methyl-pyrrolidone (NMP) to facilitate dispersion in the epoxy. The use of THF resulted in large agglomerates, whereas more finely dispersed stacks of GNPs were observed for NMP. The maximum values of modulus (3.6 GPa at 1 wt%) and fracture energy (343 J/m\(^2\) at 2 wt%) were measured for the epoxy modified with an intermediate platelet size of approximately 4 \(\upmu \)m, compared to 2.9 GPa and 96 J/m\(^2,\) respectively, for the unmodified epoxy. The Young’s modulus was highly dependent on the dispersion quality, whereas the fracture energy was independent of the degree of GNP dispersion. The larger agglomerates of the GNPs which were dispersed in THF toughened the epoxy by crack deflection, whereas the GNPs dispersed in NMP showed platelet debonding, pull-out and plastic void growth of the epoxy. This work indicates that reinforcement and toughening can be achieved at much lower contents than for conventional modifiers. Further, achieving a good dispersion is crucial to the engineering application of these materials, and intermediate-sized graphene achieves the best balance of properties.

The Effect of High Concentration and Small Size of Nanodiamonds on the Strength of Interface and Fracture Properties in Epoxy Nanocomposite.

The concentration and small size of nanodiamonds (NDs) plays a crucial role in the mechanical performance of epoxy-based nanocomposites by modifying the interface strength. Herein, we systemically analyzed the relation between the high concentration and small size of ND and the fracture properties of its epoxy-based nanocomposites. It was observed that there is a two-fold increase in fracture toughness and a three-fold increase in fracture energy. Rationally, functionalized-NDs (F-NDs) showed a much better performance for the nanocomposite than pristine NDs (P-NDs) because of additional functional groups on its surface. The F-ND/epoxy nanocomposites exhibited rougher surface in contrast with the P-ND/epoxy, indicating the presence of a strong interface. We found that the interfaces in F-ND/epoxy nanocomposites at high concentrations of NDs overlap by making a web, which can efficiently hinder further crack propagation. In addition, the de-bonding in P-ND/epoxy nanocomposites occurred at the interface with the appearance of plastic voids or semi-naked particles, whereas the de-bonding for F-ND/epoxy nanocomposites happened within the epoxy molecular network instead of the interface. Because of the strong interface in F-ND/epoxy nanocomposites, at high concentrations the de-bonding within the epoxy molecular network may lead to subsequent cracks, parallel to the parent crack, via crack splitting which results in a fiber-like structure on the fracture surface. The plastic void growth, crack deflection and subsequent crack growth were correlated to higher values of fracture toughness and fracture energy in F-ND/epoxy nanocomposites.

Investigation of the Role of Rubber Particle Cavitation Resistance on the Toughening Mechanisms of Epoxy Resins

The occurrence of rubber particle cavitation in rubber modified epoxy resins has generated some controversy in the literature and role of cavitation resistance in toughening mechanisms has remained an enigma. In this study, hydroxyl terminated polybutadiene (HTPB) and synthesized epoxy terminated polybutadiene (ETPB) have been used to toughen an epoxy resin. FTIR and titration techniques were used to monitor the ETPB synthesis. Mean fracture toughness, KIC, of 0.65 MPa.m0.5 was measured for unmodified epoxy polymer. The measured toughness was increased by the addition of HTPB to 1.23 MPa.m0.5 and ETPB to 1.80 MPa.m0.5. The cavitation zone of the HTPB modified sample was bigger than that of ETPB modified sample which showed the higher cavitation resistance of ETPB particles. SEM studies showed volumetric expansion of rubbery phase in slow crack growth region of both samples which proved the plastic void growth. The results showed that the elevated cavitation resistance of ETPB did not suppress plastic void growth. The optical microscopy of crack tips showed shear banding which was more significant in ETPB modified sample. SEM micrographs of fracture surface of HTPB modified samples showed no tail formation while pinning tails were present in ETPB modified samples. The results showed that higher cavitation resistance activated crack pinning process and induced larger shear banding zone in ETPB modified samples. Deflection and forking of the main crack into secondary cracks was also observed in ETPB modified samples while these phenomena were not present in HTPB modified samples.

Simultaneously tough and conductive rubber–graphene–epoxy nanocomposites

This work investigates the effect of adding graphene nanoplatelets (GNP) and either a phase-separating carboxyl-terminated butadiene acrylonitrile rubber (CTBN) or a polysiloxane core–shell rubber (CSR) to an anhydride-cured epoxy polymer. The effect of adding a reactive diluent to the resin was also investigated. The relationship between the microstructure and the resultant electrical and mechanical properties was investigated. The fracture energy of the unmodified epoxy polymer increased from 125 to 668 J/m2 with the addition of 9 wt% CTBN and 12.5 % reactive diluent. The subsequent addition of GNP to the rubber systems decreased the fracture energy. The epoxy nanocomposites modified with only GNP exhibited only a modest increase in measured fracture energy. The major toughening mechanisms in the rubber-modified formulations were observed to be shear band yielding and cavitation of the rubber particles followed by plastic void growth of the epoxy matrix. The electrical conductivity of the hybrid systems was also investigated. It was observed that the conductivity of the nanocomposites improved when 0.5 wt% of GNP was added although this improvement was lost in a CTBN–GNP system while the conductivity was further improved in the CSR–GNP system over the GNP only system with low-CSR particle loadings. It is demonstrated that this behaviour can be directly attributed to the microstructure of the nanocomposite. The results demonstrate that separation of nanomodified phases at the microscale can be used to develop simultaneously tough and conductive composites.

Temperature effect on nano-rubber toughening in epoxy and epoxy/carbon fiber laminated composites

Abstract The nano-rubber toughening effects on neat epoxy and epoxy/carbon fiber composites under a range of temperatures from −80 °C to 50 °C were investigated. Nano-rubber particles (100 nm) toughened significantly neat epoxy at all temperatures with a maximum toughness value at 20 °C and lower values on either side. The influence of temperature on the delamination fracture toughness of epoxy/carbon fiber composites with nano-rubber filled epoxy matrices compared to those with neat epoxy matrices mirrored the same trend of nano-rubber particles on epoxy. However, except at −50 °C, the toughness of nano-rubber/epoxy cannot be fully transferred to the composites against delamination. Optical (OM) and scanning electron microscopy (SEM) examinations revealed that different extent of plastic void growth and matrix shear yielding led to different toughening efficiency on (a) bulk epoxies and (b) epoxy/carbon fiber laminates with varying intensity of crack-wake bridging and delamination crack growth.

A Thermomechanically Consistent Constitutive Theory for Modeling Micro-Void and/or Micro-Crack Driven Failure in Metals at Finite Strains

Within a continuum approximation, we present a thermomechanical finite strain plasticity model which incorporates the blended effects of micro-heterogeneities in the form of micro-cracks and micro-voids. The former accounts for cleavage-type of damage without any volume change whereas the latter is a consequence of plastic void growth. Limiting ourselves to isotropy, for cleavage damage a scalar damage variable [Formula: see text] is incorporated. Its conjugate variable, the elastic energy release rate, and evolution law follow the formal steps of thermodynamics of internal variables requiring postulation of an appropriate damage dissipation potential. The growth of void volume fraction [Formula: see text] is incorporated using a Gurson-type porous plastic potential postulated at the effective stress space following continuum damage mechanics principles. Since the growth of micro-voids is driven by dislocation motion around voids the dissipative effects corresponding to the void growth are encapsulated in the plastic flow. Thus, the void volume fraction is used as a dependent variable using the conservation of mass. The predictive capability of the model is tested through uniaxial tensile tests at various temperatures [Formula: see text]C, [Formula: see text]C]. It is shown, via fracture energy plots, that temperature driven ductile-brittle transition in fracture mode is well captured. With an observed ductile-brittle transition temperature around [Formula: see text]C, at lower temperatures fracture is brittle dominated by [Formula: see text] whereas at higher temperatures it is ductile dominated by [Formula: see text].

Challenges in fracture toughness modelling of nanocomposites

ness of hybrid polymer composites is whether or not the particles’ agglomerations are considered. Note that even in microparticle reinforced polymers the distribution of the particles is locally inhomogeneous because they get into contact as a consequence of production. In nanoparticle filled elastomers particles agglomerate and the agglomerates build even larger structures. Consequently, if we consider the close neighbourhood of a single particle there exists a much higher particle density than in larger distances. Such behaviour can be described for example by pair distribution functions (DOI: 10. 1016/j.compositesb.2014.01.002). Close particles cause high stress or strain concentrations in the matrix in between. This fact must be considered in strength and fracture toughness modelling. Filled polymer composites have been studied extensively because of their technological and scientific importance (DOI: 10.1007/BF01160743; DOI: 10.016/j. compositesb.2008.01.002; DOI: 10.1016/0266-3538 (88)90009-7). Generally, crack tip blunting, crack pinning, localized shear bands, debonding of particles as plastic voiding and localized plastic shear yielding were found to be responsible for the increase of toughness in thermoset composites. Models that consider such effects are able to describe some of the experimental results properly (DOI: 10.1007/s10853009-4064-9; DOI: 10.1016/j.compscitech.2009.12. 024). Also the effect of particle size on plastic void growth has been taken in account (DOI: 10.1016/ j.mechrescom.2015.2.002). Homogeneous particle distribution cannot be expected in ‘real’ materials. Increased number of nearest neighbours in microparticle filled composites and agglomerate formation in nanocomposites must be considered. Stiffness is not influenced by these events because at small deformations no debonding occurs. However, if strength or fracture toughness are computed, such local heterogeneities should be considered. As basic mathematical and experimental data are available, the modelling should involve this structural level. In the first step any mathematical pair distribution function can be used to calculate the number of particles close to the chosen one in the ensemble. High stress concentration in between such a pair may initiate debonding. The ligament in between the debonded particles becomes unloaded and their outer region overloaded accompanied with yielding. Debonding does not contribute much to energy dissipation but triggers matrix yielding that enhances the toughness. Debonding of particles leads to a reduction of tensile strength, too. Let us go in this direction to get a deeper understanding in the mechanical and fracture mechanical behaviours!

The fatigue behaviour of ZnO nano-particle modified thermoplastics

The present paper describes an investigation into the effects on the fatigue behaviour of adding up to 1.00 vol.% ZnO nano-rods with a diameter of 13 nm and an aspect ratio of three to two amorphous styrene acrylonitrile copolymers. Two acrylonitrile contents, i.e. 24% and 34% respectively, were studied. Fracture mechanics based fatigue tests were conducted at 5 Hz and a Paris Law analysis approach was followed. The fatigue threshold increased notably with the addition of the ZnO nano-rods. The increases in toughness were analysed using an analytical model of plastic void growth around the particles and this enabled the surface energy per unit area for the particle-matrix debonding process to be deduced. Good particle distributions were achieved only for very small volume fractions (<0.3%) above which the occurrence of agglomerated particles and consequential toughness declines were observed. In the fatigue threshold region, surface micrographs showed clear evidence of debonding and plastic void growth and the average measured void diameter agreed closely with that predicted by the model.

Toughening performance of glass fibre composites with core–shell rubber and silica nanoparticle modified matrices

The fracture energies of glass fibre composites with an anhydride-cured epoxy matrix modified using core–shell rubber (CSR) particles and silica nanoparticles were investigated. The quasi-isotropic laminates with a central 0°/0° ply interface were produced using resin infusion. Mode I fracture tests were performed, and scanning electron microscopy of the fracture surfaces was used to identify the toughening mechanisms.The composite toughness at initiation increased approximately linearly with increasing particle concentration, from 328J/m2 for the control to 842J/m2 with 15wt% of CSR particles. All of the CSR particles cavitated, giving increased toughness by plastic void growth and shear yielding. However, the toughness of the silica-modified epoxies is lower as the literature shows that only 14% of the silica nanoparticles undergo debonding and void growth. The size of CSR particles had no influence on the composite toughness. The propagation toughness was dominated by the fibre toughening mechanisms, but the composites achieved full toughness transfer from the bulk.

Investigation into the toughening mechanism of epoxy reinforced with multi-wall carbon nanotubes

Abstract This study reports the reinforcement and fracture toughening mechanism of pristine multi-walled carbon nanotubes (MWCNTs) on epoxy matrix. The tensile strength and fracture energy (G IC) of the epoxy polymer increased simultaneously upon the addition of a small amount of MWCNTs. The fracture surfaces of single-edge-notch three-point bending test specimens were analysed by scanning electron microscopy, and the double-notch four-point bending technique was used to investigate the fracture process by transmission electron microscopy, respectively. MWCNT pull-out and subsequent plastic void growth were found; meanwhile, fracture of MWCNTs was observed along the crack propagation path. The theoretical model of shearing band initiated by the stress concentrations around the MWCNTs is the dominant toughening mechanism. While the crack bridging of MWCNTs and the plastic void growth of epoxy also have a toughening effect.

Epoxy nanocomposites containing magnetite-carbon nanofibers aligned using a weak magnetic field

Novel magnetite-carbon nanofiber hybrids (denoted by “Fe3O4@CNFs”) have been developed by coating carbon nanofibers (CNFs) with magnetite nanoparticles in order to align CNFs in epoxy using a relatively weak magnetic field. Experimental results have shown that a weak magnetic field (∼50 mT) can align these newly-developed nanofiber hybrids to form a chain-like structure in the epoxy resin. Upon curing, the epoxy nanocomposites containing the aligned Fe3O4@CNFs show (i) greatly improved electrical conductivity in the alignment direction and (ii) significantly higher fracture toughness when the Fe3O4@CNFs are aligned normal to the crack surface, compared to the nanocomposites containing randomly-oriented Fe3O4@CNFs. The mechanisms underpinning the significant improvements in the fracture toughness have been identified, including interfacial debonding, pull-out, crack bridging and rupture of the Fe3O4@CNFs, and plastic void growth in the polymer matrix.

Effect of core–shell rubber (CSR) nano-particles on mechanical properties and fracture toughness of an epoxy polymer

A DGEBA epoxy resin cured using dicyandiamide hardener, was modified by using two types of CSR nano-particles over a range of volume fractions from 0 vol.% to 38 vol.%. The size and microstructure of CSR particles were studied and the measured diameters are 203 nm (with a 16.9 nm shell thickness) and 74.1 nm respectively. Addition of CSR nano particles increased the glass transition temperature due to the interaction between matrix and particles. Addition of CSR particles increased the ductility and Poisson's ratio of epoxy, but reduced the Young's modulus and tensile strength. Young's modulus, Poisson's ratio and tensile strength were well predicted by theoretical models using an effective volume fraction of rubber. An optimum CSR content was found to exist, when the fracture energy increased from 343 J/m2 for matrix to 2671 J/m2 for epoxy modified by 30 vol.% of CSR particles, and then started to decline. The main toughening mechanisms were proved to be debonding of CSR particles from matrix, followed by plastic voids growth, which was accompanied by shear bands yielding. Large-scale plastic void growth was discovered at the subcritical tip of double notch four points bending test (DN-4PB), and the values of fracture energy were found to be proportional to the size of plastic damage zone measured from subcritical crack tip of DN-4PB. This proved the plastic void growth is the major toughening mechanism, which correlated well with the results of fracture energy modelling. A theoretical model was used to predict the fracture toughness increment due to the shear band yielding and plastic void growth. The analytical values showed good agreement with the experimental data, and further demonstrated the proposed toughening mechanism.