Multi-walled carbon nanotubes, with a typical length of 140 μm and a diameter of 120 nm, have been used to modify an anhydride-cured epoxy polymer. The modulus, fracture energy and the fatigue ...performance of the modified polymers have been investigated. Microscopy showed that these long nanotubes were agglomerated, and that increasing the nanotube content increased the severity of the agglomeration. The addition of nanotubes increased the modulus of the epoxy, but the glass transition temperature was unaffected. The measured fracture energy was also increased, from 133 to 223 J/m
2
with the addition of 0.5 wt% of nanotubes. The addition of the carbon nanotubes also resulted in an increase in the fatigue performance. The threshold strain-energy release-rate,
G
th
, increased from 24 J/m
2
for the unmodified material to 73 J/m
2
for the epoxy with 0.5 wt% of nanotubes. Electron microscopy of the fracture surfaces showed clear evidence of nanotube debonding and pull-out, plus void growth around the nanotubes, in both the fracture and fatigue tests. The modelling study showed that the modified Halpin–Tsai equation can fit very well with the measured values of the Young’s modulus, when the orientation and agglomeration of the nanotubes are considered. The fracture energy of the nanotube-modified epoxies was predicted, by considering the contributions of the toughening mechanisms of nanotube debonding, nanotube pull-out and plastic void growth of the epoxy. This indicated that debonding and pull-out contribute to the toughening effect, but the contribution of void growth is not significant. There was excellent agreement between the predictions and the experimental results.
An epoxy resin, cured with an anhydride, has been modified by the addition of silica nanoparticles. The particles were introduced via a sol–gel technique which gave a very well-dispersed phase of ...nanosilica particles which were about 20
nm in diameter. Atomic force and electron microscopies showed that the nanoparticles were well-dispersed throughout the epoxy matrix. The glass transition temperature was unchanged by the addition of the nanoparticles, but both the modulus and toughness were increased. The measured modulus was compared to theoretical models, and good agreement was found. The fracture energy increased from 100
J/m
2 for the unmodified epoxy polymer to 460
J/m
2 for the epoxy polymer with 13
vol% of nanosilica. The fracture surfaces were inspected using scanning electron and atomic force microscopies, and the results were compared to various toughening mechanisms proposed in the literature. The toughening mechanisms of crack pinning, crack deflection and immobilised polymer were discounted. The microscopy showed evidence of debonding of the nanoparticles and subsequent plastic void growth. A theoretical model of plastic void growth was used to confirm that this mechanism was indeed most likely to be responsible for the increased toughness that was observed due to the presence of the nanoparticles.
The present paper considers the mechanical and fracture properties of four different epoxy polymers containing 0, 10 and 20
wt.% of well-dispersed silica nanoparticles. Firstly, it was found that, ...for any given epoxy polymer, their Young’s modulus steadily increased as the volume fraction,
v
f, of the silica nanoparticles was increased. Modelling studies showed that the measured moduli of the different silica-nanoparticle filled epoxy polymers lay between upper-bound values set by the Halpin–Tsai and the Nielsen ‘no-slip’ models, and lower-bound values set by the Nielsen ‘slip’ model; with the last model being the more accurate at relatively high values of
v
f. Secondly, the presence of silica nanoparticles always led to an increase in the toughness of the epoxy polymer. However, to what extent a given epoxy polymer could be so toughened was related to structure/property relationships which were governed by (a) the values of glass transition temperature,
T
g, and molecular weight,
M
c, between cross-links of the epoxy polymer, and (b) the adhesion acting at the silica nanoparticle/epoxy-polymer interface. Thirdly, the two toughening mechanisms which were operative in all the epoxy polymers containing silica nanoparticles were identified to be (a) localised shear bands initiated by the stress concentrations around the periphery of the silica nanoparticles, and (b) debonding of the silica nanoparticles followed by subsequent plastic void growth of the epoxy polymer. Finally, the toughening mechanisms have been quantitatively modelled and there was good agreement between the experimentally-measured values and the predicted values of the fracture energy,
G
c, for all the epoxy polymers modified by the presence of silica nanoparticles. The modelling studies have emphasised the important roles of the stress versus strain behaviour of the epoxy polymer and the silica nanoparticle/epoxy-polymer interfacial adhesion in influencing the extent of the two toughening mechanisms, and hence the overall fracture energy,
G
c, of the nanoparticle-filled polymers.
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An epoxy resin, cured using an anhydride hardener, has been modified by the addition of pre-formed polysiloxane core-shell rubber (S-CSR) particles with a mean diameter of 0.18 μm. The glass ...transition temperature, Tg, of the cured unmodified epoxy polymer was 148 °C, and this was unchanged after the addition of the S-CSR particles. The polysiloxane rubber particles had a Tg of about −100 °C. Atomic force microscopy showed that the S-CSR particles were well-dispersed in the epoxy polymer. The addition of the S-CSR particles reduced the Young's modulus and tensile strength of the epoxy polymer, but at 20 °C the fracture energy, GIc, increased from 117 J/m2 for the unmodified epoxy to 947 J/m2 when 20 wt% of the S-CSR particles were incorporated. Fracture tests were also performed at −55 °C, −80 °C, and −109 °C. The results showed that the measured fracture energy of the S-CSR-modified epoxy polymers decreased significantly below room temperature. For example, at −109 °C, a fracture energy of 481 J/m2 was measured using 20 wt% of S-CSR particles. Nevertheless, this value of toughness still represented a major increase compared with the unmodified epoxy polymer, which possessed a value of GIc of 174 J/m2 at this very low test temperature. Thus, a clear fact that emerged was that the addition to the epoxy polymer of the S-CSR particles may indeed lead to significant toughening of the epoxy, even at temperatures as low as about −100 °C. The toughening mechanisms induced by the S-CSR particles were identified as (a) localised plastic shear-band yielding around the particles and (b) cavitation of the particles followed by plastic void growth of the epoxy polymer. These mechanisms were modelled using the Hsieh et al. approach 33,49 and the values of GIc of the S-CSR-modified epoxy polymers at the different test temperatures were calculated. Excellent agreement was found between the predictions and the experimentally measured fracture energies. Further, the experimental and modelling results of the present study indicated that the extent of plastic void growth was suppressed at low temperatures for the S-CSR-modified epoxy polymers, but that the localised shear-band yielding mechanism was relatively insensitive to the test temperature.
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•An epoxy polymer has been modified with polysiloxane core-shell rubber particles.•The epoxy had Tg of about 148 °C and the particles had a Tg of about −100 °C.•AFM showed that the particles were well-dispersed in the epoxy polymer.•The addition of 20 wt% particles increased GIc from about 120 J/m2 to 950 J/m2.•The particles also significantly toughened the epoxy polymer even at about −100 °C.
The microstructure and fracture performance of an epoxy resin cured with an anhydride hardener containing silica nanoparticles and/or polysiloxane core-shell rubber (CSR) nanoparticles were ...investigated in the current work. The effect of adding a reactive diluent, i.e. hexanediol diglycidylether, to the epoxy resin was also investigated. The fracture energy of the neat (i.e. unmodified) epoxy polymer increased slightly from 125 J/m2 to 172 J/m2 due to the addition of 25 wt% of the reactive diluent to the epoxy. The fracture energy of the unmodified epoxy polymer increased to 889 J/m2 when 20 wt% of the CSR nanoparticles were added to the epoxy without any reactive diluent being present. However, the results show that the increase in fracture energy due to the addition of the CSR nanoparticles particles was much more marked in the case when 25 wt% of the reactive diluent was present, e.g. an increase to 1237 J/m2 with the addition of 16 wt% of CSR nanoparticles. Furthermore, while the subsequent addition of silica nanoparticles, to give hybrid epoxy polymer nanocomposites, i.e. which contained both silica and CSR nanoparticles, produced only modest increases in the fracture energy in the case of the epoxy with the reactive diluent additive present, some synergistic effects on the toughening were noted. No significant improvements in toughness were found for the hybrid epoxy polymer nanocomposites without reactive diluent. The measured toughness of the hybrid materials can be related to the degree of dispersion of both nanoparticle phases in the epoxy polymer matrix. The toughening mechanisms were identified and the experimentally measured values of toughness were in good agreement with modelling studies.
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•Epoxy nano composites are fabricated using silica nanoparticles and core shell rubber.•Fracture toughness of epoxy hybrid nanocomposites depends on nano morphology.•Shear banding and void growth are identified toughening mechanisms.•Fracture toughness can be predicted analytically.
Relatively tough epoxy-blend polymers are now commercially available for use as adhesives and as the matrices for fibre composites. Nevertheless, another failure property which may be of equal, or ...even of greater, importance in some applications is the resistance of the epoxy polymer to cyclic-fatigue loading. However, the cyclic-fatigue behaviour of epoxy polymers has not been studied in great detail, especially for epoxy polymers where the material has been modified by forming a polymer blend in order to increase its toughness under quasi-static test rates or impact test rates. Therefore, a major aim of the present work has been to undertake a novel investigation of a range of rubber and thermoplastic materials to modify an epoxy polymer to study whether both a relatively high toughness and a significantly improved cyclic-fatigue behaviour can be simultaneously achieved in a given formulation. The unmodified epoxy-polymer possessed a value of the fracture energy, GIc, of 495 J/m2 and a value for the threshold value of the maximum strain-energy release rate in a fatigue cycle, Gth, (below which no significant crack growth occurs) of 155 J/m2. Several epoxy-polymer blends have been identified which do show major increases in these values and probably the best combination of such properties were for the epoxy-polymers modified with a poly(polypropylene-glycol)-based polyurethane (PU) modifier: either when used by itself or as a ‘hybrid’ polymer-blend in combination with core–shell rubber (CSii) particles, based upon a styrene-butadiene rubber core. For these PU-based epoxy polymers the values of GIc and Gth were found to increase to values of about 2475 J/m2 and 445 J/m2, respectively. The mechanisms of toughening that were induced by the addition of the polymer-blend modifier revealed that the presence of a multiphase in the epoxy-blend polymer was a critical requirement in achieving relatively high values of GIc and Gth. This was due to the second-phase particles initiating plastic deformation of the epoxy-matrix phase, which was the major source of energy dissipation and toughening. In turn, the extent of energy dissipated by the plastic deformation of the epoxy-matrix phase is clearly greatly influenced by the degree of ductility exhibited by this phase of the epoxy-blend polymer. Thus, another important feature of the degree of toughening observed is the effect that the modifier has upon the yield stress and plastic failure strain of the epoxy-matrix phase.
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•A range of rubber and thermoplastic materials are used to modify an epoxy polymer.•Results in dramatic increases in both the toughness and the cyclic-fatigue behavior.•Best simultaneous increases achieved in ‘hybrid modified epoxy-polymer blends’.•The toughening mechanisms are also identified.
In the present paper, for the first time, we report the substantial increase in toughness that may be achieved when such nano-SiO2 particles are well dispersed in a hot-cured single-part epoxy ...polymer, and again the synergistic effect of having a multiphase structure based upon both nano-SiO2 particles and rubbery particles is clearly demonstrated. It should be noted that achieving a relatively high toughness in a hot-cured single-part epoxy polymer represents a far greater challenge than in a two-part epoxy formulation. This is because the former invariably has a significantly higher glass transition temperature, Tg, and a lower molecular weight between crosslinks; and both of these features inhibit the plastic deformation of the epoxy matrix which is a major toughening mechanism.
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.
We present a non-oxidative production route to few layer graphene via the electrochemical intercalation of tetraalkylammonium cations into pristine graphite. Two forms of graphite have been studied ...as the source material with each yielding a slightly different result. Highly orientated pyrolytic graphite (HOPG) offers greater advantages in terms of the exfoliate size but the source electrode set up introduces difficulties to the procedure and requires the use of sonication. Using a graphite rod electrode, few layer graphene flakes (2nm thickness) are formed directly although the flake diameters from this source are typically small (ca. 100–200nm). Significantly, for a solvent based route, the graphite rod does not require ultrasonication or any secondary physical processing of the resulting dispersion. Flakes have been characterized using Raman spectroscopy, atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS).
The growth of delaminations in polymer-matrix fibre composites under cyclic-fatigue loading in operational aircraft structures has always been a very important factor which has the potential to ...significantly affect the service-life of such structures. The recent introduction by the Federal Aviation Administration (FAA) of a ‘slow growth’ approach to the certification of composites has further focused attention on the experimental data and the analytical tools needed to assess the growth of delaminations under fatigue loads. Specific attention is given to the test and data-reduction procedures required to determine a ‘valid’ rate of fatigue crack growth (FCG), da/dN, versus the range of the energy release-rate, ΔG, (or the maximum energy release-rate, Gmax, in a cycle) relationship (a) to characterise and compare different types of composites, and (b) for designing and lifing in-service composite structures. Now, fibre-bridging may occur behind the tip of the advancing delamination and may cause very significant retardation of the FCG rate. Such retardation effects cannot usually be avoided when using the Mode I double-cantilever beam test to ascertain experimentally the fatigue behaviour of composites, so that a means of estimating a valid (i.e. ideally a ‘retardation-free’ or, at least, a very low-retardation) relationship is needed. The present paper presents a novel methodology, that is based on a variant of the Hartman-Schijve equation, to ascertain a valid, ‘retardation-free’, upper-bound FCG rate curves.
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