Material characterization of blended epoxy resins related to fracture toughness

The present study describes the effect of the macromolecular modifications on the fracture toughness of an epoxy system. We synthesized epoxy networks by the reaction of diglycidyl ether of bisphenol A (DGEBA) with methyl-tetrahydro-phthalic anhydride (MTHPA), initiated by a tertiary amine. Several materials were obtained by adding a high molecular weight monomer to one with low molecular weight (both based on DGEBA) at different concentrations. In every case, a stoichiometric amount of MTHPA was employed. The glass transition temperature and the Angell’s fragility index, derived from thermo-viscoelastic properties, were used to characterize the materials. Relationship between these two parameters and the fracture properties, including the fracture toughness and the microscopic roughness of the fracture surfaces observed by atomic force microscope (AFM), was then investigated. We found that there were direct correlations among the glass transition temperature, the fragility, the fracture toughness, and the roughness. This study revealed that both the glass transition temperature and the fragility are effective for characterizing material in relation to the fracture toughness of the blended epoxy resins.     

Synthesis and characterization of core–shell nanoparticles and their influence on the mechanical behavior of acrylic bone cements

Core–shell nanoparticles consisting of polybutyl acrylate (PBA) rubbery core and a polymethyl methacrylate (PMMA) shell, with different core–shell ratios, were synthesized in order to enhance the fracture toughness of the acrylic bone cements prepared with them. It was observed by TEM and SEM that the core–shell nanoparticles exhibited a spherical morphology with ca. 120 nm in diameter and that both modulus and tensile strength decreased by increasing the PBA content; the desired structuring pattern in the synthesized particles was confirmed by DMA. Also, experimental bone cements were prepared with variable amounts (0, 5, 10 and 20 wt.%) of nanoparticles with a core–shell ratio of 30/70 in order to study the influence of these nanostructured particles on the physicochemical, mechanical and fracture properties of bone cements. It was found that the addition of nanostructured particles to bone cements caused a significant reduction in the peak temperature and setting time while the glass transition temperature (T_g) of cements increased with increasing particles content. On the other hand, modulus and strength of bone cements decreased when particles were incorporated but fracture toughness was increased.     

The effect of dispersed paint particles on the mechanical properties of rubber toughened polypropylene composites

The influence of dispersed paint particles on the mechanical properties of rubber toughened PP was investigated. The matrix was basically a hybrid of PP, rubber and talc. Model systems with spherical glass bead filled matrix were also studied to examine the effect of filler shape and size. Properties like tensile strength, strain at break, impact strength, and fracture toughness were influenced by the dispersed inclusions. Tensile strength at yield decreased linearly according to Piggott and Leinder's equation. Strain at break decreased more drastically with paint particles than glass beads, revealing that irregularly shaped particles offered greater stress concentrations. The tensile strength and strain at break were less influenced by the size of paint particles whereas a slight decrease in the modulus values was observed with decreasing particle size. Impact strength and fracture toughness also decreased with increasing filler fraction. Lack of stress transfer between filler and matrix aided in reduction of impact strength. Decrease in fracture toughness was influenced by volume replacement and constraints posed by fillers. The size of paint particles had little effect on the impact strength and fracture properties at the filler concentration levels used in this investigation.     

Nanoparticle-induced enhancement in fracture toughness of highly loaded epoxy composites over a wide temperature range

The fracture toughness of an epoxy molding compound (EMC) has been enhanced over a wide temperature range by the addition of a very low volume fraction of silica nanoparticles to the EMC filled with micro-silica particles, which induces macroscopic crack deflection and plastic deformation in front of the crack tip. To evaluate the fracture toughness (G _IC) of these materials, the single edge notched bending (SENB) test was performed for a wide range of temperatures (from ambient temperature to 230°C). The fracture toughness of the nano-silica filled EMCs was found to be improved in this temperature range by as much as a factor of two. Investigation of the fracture surfaces revealed that the micro-silica particles are covered with deformed matrix materials, which implies that the silica nanoparticles induced the crack to move into the interface between the micro-silica particles. Fractography results suggest that the silica nanoparticles act as surface modifiers of the micro-silica particles, which results in crack deflection and plastic deformation.     

Fracture toughness of zirconia nanoparticle-filled dental composites

The fracture toughness of dental composites containing zirconia nanoparticles dispersed in a bisphenol A glycol dimethacrylate-based monomer blend (GTE) was studied for several yttria contents. Three-point bend test bars with and without a notch were tested at ambient temperature to determine elastic modulus, flexure strength, and fracture toughness. The ZrO₂ nanoparticles increased the fracture toughness of the nanocomposites compared to previous results for the matrix and Schott glass-filled nanocomposites. X-ray diffraction analyses revealed mostly tetragonal ZrO₂ in the nanocomposites before and after testing, in agreement with a theoretical analysis. The enhancement in fracture toughness in ZrO₂-filled nanocomposites was caused mainly by the higher values of particle toughness and interface toughness in GTE/ZrO₂ compared to those of GTE/Schott glass nanocomposites.     

Preparation, microstructure and mechanical properties of metal-particulate/glass-matrix composites

Composites with a borosilicate glass matrix containing different concentrations of vanadium particles were fabricated by powder metallurgy and hot-pressing. The mechanical properties and fracture behaviour of the composites were assessed by a range of techniques. Young's modulus, fracture strength in bending, and fracture toughness increased with vanadium content. By virtue of the good interfacial bonding and low residual internal stresses, an effective crack-particle interaction during fracture was achieved. The fracture toughness of composites containing 30 vol. % of vanadium inclusions was approximately 65 % higher than that of the unreinforced glass. Experimental values for the fracture toughness increment were in good qualitative agreement with the predictions of theoretical models in the literature. Extensive plastic deformation of the vanadium inclusions was not found, however. This was attributed mainly to the constraint imposed by the rigid matrix surrounding the particles and to possible embrittlement of the particles during composite fabrication at high temperatures. The brittleness index (B) of the composites was calculated and its relevance for characterisation of the ductile versus brittle behaviour of brittle-matrix composites is discussed.     

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 328 J/m² for the control to 842 J/m² with 15 wt% 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.     

The toughness of epoxy polymers and fibre composites modified with rubber microparticles and silica nanoparticles

The present paper investigates the effect of adding silica nanoparticles to an anhydride-cured epoxy polymer in bulk and when used as the matrix of carbon- and glass-fibre reinforced composites. The formation of ‘hybrid’ epoxy polymers, containing both silica nanoparticles and carboxyl-terminated butadiene-acrylonitrile (CTBN) rubber microparticles, is also discussed. The structure/property relationships are considered, with an emphasis on the toughness and the toughening mechanisms. The fracture energy of the bulk epoxy polymer was increased from 77 to 212 J/m² by the presence of 20 wt% of silica nanoparticles. The observed toughening mechanisms that were operative were (a) plastic shear-yield bands, and (b) debonding of the matrix from the silica nanoparticles, followed by plastic void-growth of the epoxy. The largest increases in toughness observed were for the ‘hybrid’ materials. Here a maximum fracture energy of 965 J/m² was measured for a ‘hybrid’ epoxy polymer containing 9 wt% and 15 wt% of the rubber microparticles and silica nanoparticles, respectively. Most noteworthy was the observation that these increases in the toughness of the bulk polymers were found to be transferred to the fibre composites. Indeed, the interlaminar fracture energies for the fibre-composite materials were increased even further by a fibre-bridging toughening mechanism. The present work also extends an existing model to predict the toughening effect of the nanoparticles in a thermoset polymer. There was excellent agreement between the predictions and the experimental data for the epoxy containing the silica nanoparticles, and for epoxy polymers containing micrometre-sized glass particles. The latter, relatively large, glass particles were investigated to establish whether a ‘nano-effect’, with respect to increasing the toughness of the epoxy bulk polymers, did indeed exist.     

Fracture toughness and electrical conductivity of epoxy composites filled with carbon nanotubes and spherical particles

The attainment of both high toughness and superior electrical conductivity of epoxy composites is a crucial requirement in some engineering applications. Herein, we developed a strategy to improve these performances of epoxy by combining the multi-wall carbon nanotubes (MWCNTs) and spherical particles. Two different types of spherical particles i.e. soft submicron-rubber and rigid nano-silica particles were chosen to modify the epoxy/MWCNT composites. Compared with the binary composites with single-phase particles, the ternary composites with MWCNTs and spherical particles offer a good balance in glass transition temperature, electrical conductivity, stiffness and strength, as well as fracture toughness, exhibiting capacities in tailoring the electrical and mechanical properties of epoxy composites. Based on the fracture surface analysis, the complicated interactions between multiscale particles and the relative toughening mechanisms were evaluated to explain the enhancement in fracture toughness of the ternary composites.     

Fracture mechanisms of epoxy-based ternary composites filled with rigid-soft particles

Epoxy composites filled with different amounts of aggregate-free silica nanoparticles and phase-separated submicron rubber particles were fabricated to study the synergistic effect of multi-phase particles on mechanical properties of the composites. Compared with binary composites with single-phase particles, the ternary composites with both rigid and soft particles offer a good balance in stiffness, strength and fracture toughness, showing capacities in tailoring the mechanical properties of modified epoxy resins. It was observed that debonding of silica nanoparticles from matrix in the ternary composites was less pronounced than that in the binary composites. Moreover, the rubber particles became smaller and their shape tends to be irregular, affected by the presence of rigid silica nanoparticles. The toughening mechanisms in the epoxy composites were evaluated, and the enlarged plastic deformation around the crack tip, induced by the combination of rigid and soft particles, seems to be a dominant factor in enhancing fracture toughness of the ternary composites.     

Simultaneous improvement of the strength and fracture toughness of MgO-Al2O3-SiO2 glass/Mo composites by the microdispersion of flaky Mo particles

The effect of shape and volume percent of Mo particles on theflexural strength and fracture toughness of MgO-Al₂O₃-SiO₂(MAS) glass/Mo composites was investigated. The flexural strengthand fracture toughness of composites depends heavily on Mo particleshapes, and there is greater improvement in composites reinforcedwith flaky rather than massive Mo particles. In the compositesreinforced with flaky Mo particles, fracture toughness increases withvolume percent of Mo and, at 50 vol% Mo, is 11.6 MPam,which is approximately 6.7 times higher than that of the matrix. Increases in fracture toughness of composites reinforced with flakyMo particles is greater than with SiC whiskers, SiC platelets, SiC particles or ZrO₂ particles. Fabricating composites reinforcedwith flaky Mo particles is an effective toughening technique capableof simultaneously improving the strength and toughness of brittlematerials, such as monolithic Al₂O₃ and MAS glass, by utilizing plastic deformation of ductile phase.     

聚酰亚胺颗粒层间增韧碳纤维/邻苯二甲腈树脂复合材料

邻苯二甲腈树脂是一种新型的高性能热固性树脂,具有优良的力学性能和耐高温性能,而邻苯二甲腈树脂本身的脆性限制了其用作结构材料方面的应用。本文采用热塑性聚酰亚胺(PI)颗粒对邻苯二甲腈树脂复合材料进行层间增韧改性,研究改性前后复合材料的耐热性能和力学性能。研究发现,使用PI对邻苯二甲腈复合材料进行改性时,随着掺入量的增加,复合材料的玻璃化转变温度降低。PI颗粒的引入会显著提高复合材料韧性,10wt% PI改性复合材料层间剪切强度提高了41.2%,15wt% PI改性复合材料的Ⅰ型层间断裂韧性提高了156.3%。复合材料的层间能够清晰地观察到颗粒的存在;PI的质量分数进一步提高时,出现粒子团聚缺陷,导致复合材料的层间剪切强度下降。此外PI增韧邻苯二甲腈树脂复合材料在380℃下的层间剪切强度与未改性复合材料数值相当,该温度下PI颗粒的含量已不是影响复合材料韧性的主要因素。      

The effect of adding carbon nanotubes to glass/epoxy composites in the fibre sizing and/or the matrix

Carbon nanotubes (CNTs) were integrated in glass fibres epoxy composites by either including CNTs in the fibre sizing formulation, in the matrix, or both. The effects of such controlled placement of CNTs on the thermophysical properties (glass transition temperature and coefficient of thermal expansion) and the Mode I interlaminar fracture toughness of the composites were studied. The present method of CNT-sizing of the glass fibres produces an increase of almost +10% in the glass transition temperature and a significant reduction of ?31% in the coefficient of thermal expansion of the composites. Additionally, the presence of CNTs in the sizing resulted in an increased resistance of crack initiation fracture toughness by +10%, but a lowered crack propagation toughness of ?53%. Similar trends were observed for both instances when CNTs were introduced only in the matrix and in combination of both matrix and sizing.     

Fracture toughness of metal reinforced glass composites

The effect of hardness and strength of particulate reinforcements on the toughening of a glass matrix composite have been investigated. Spherical particles of two gold-based alloys were blended with a low-fusing glass powder; the mixture was hot-pressed, and disc-shaped specimens prepared for fracture toughness testing using the strength/flaw method. Scanning electron microscopy was used to examine fracture surfaces. It was found that the softer, more ductile alloy was a more effective toughening additive than the harder alloy.     

Influence of poly(acrylic acid) content on the fracture behaviour of glass polyalkenoate cements

A linear elastic fracture mechanics approach (LEFM) was used to study glass polyalkenoate cements as a function of the poly(acrylic acid) content. Cement specimens were tested at three time intervals after mixing; one, seven and twenty eight days. Two series of cements were investigated one with a glass volume fraction of 0.4 and the other with a glass volume fraction of 0.5. The fracture toughness, toughness, Young's modulus and un-notched fracture strength increased significantly with the percentage polyacid content. The Young's modulus increased with time for all the cement samples studied. In many cases the moduli values at twenty eight days were twice the values at one day. This is consistent with increased ionic crosslinking of the polyacrylate matrix. The toughness increased with the polyacid content as predicted by the chain pull-out model for fracture and did not change significantly on increasing the glass volume fraction from 0.4 to 0.5. Fracture toughness and Young's modulus increased significantly with glass volume fraction consistent with the residual glass particles acting as a reinforcing filler.     

The mechanical properties of PMMA and its copolymers with ethyl methacrylate and butyl methacrylate

The mechanical properties of polymethyl methacrylate and copolymers formed with both ethyl methacrylate and butyl methacrylate were investigated. Six polymers were produced by bulk polymerization, measured for molecular weight and glass transition temperature, T _g and assessed for modulus of elasticity and fracture toughness. Increasing the concentration of ethyl methacrylate or butyl methacrylate resulted in a linear decrease in the glass transition temperature, modulus of elasticity, and fracture toughness. A comparison of testing environments revealed that the modulus of elasticity was reduced when conditioned and tested in water at 37 °C compared to ambient laboratory conditions for all polymers. Similar comparisons of the fracture toughness showed an increase for testing in water at 37 °C; however, this was not significant for the lower T _g compositions. Both modulus of elasticity and fracture toughness were strongly correlated with the glass transition temperature and composition.     

Thermo-mechanical properties of hyperbranched polymer modified epoxies

The thermo-mechanical properties of hyperbranched polymer-epoxy blends and their dependence on hyperbranched polymer shell chemistry were investigated. Hyperbranched polymers were shown to be able to increase resin toughness by inducing both a heterogeneous and homogeneous morphology. While the former was better performing in terms of toughness, the latter showed satisfactory toughness together with complete transparency. In order to understand fracture toughness enhancement, toughening mechanisms as well as the properties of both matrix and particles were studied. Particle composition was derived by combining dynamic mechanical analysis and the Fox equation. This resulted in an evaluation not only of particle composition but also of glass transition temperature and stiffness, whose value was cross-checked by a micro-mechanical model. The complete picture concerning particle and matrix properties, as well as toughening mechanisms and their dependence on hyperbranched polymer shell chemistry, finally enabled defining the optimum molecular design of the hyperbranched polymers in order to achieve the desired fracture toughness.     

Nanostructured systems based on SBS epoxidized triblock copolymers and well-dispersed alumina/epoxy matrix composites

In the present research, the effect of addition of (1 wt.% and 3 wt.%) alumina nanoparticles (Al₂O₃) to epoxy modified by poly(styrene-b-butadiene-b-styrene) (SBS) epoxidized triblock copolymer was studied. The microstructure of final hybrid composites was studied with atomic force microscopy (AFM). Composites showed homogeneously dispersed Al₂O₃ nanoparticles in the epoxy matrix containing polystyrene (PS) microphase separated nanodomains. Dynamic mechanical analyses (DMA), flexural and fracture toughness investigations were carried out. The glass transition temperature of epoxy matrix has been retained unchanged by the addition of Al₂O₃ nanoparticles. The nanostructured epoxy systems based on SBS epoxidized triblock copolymer and well-dispersed Al₂O₃ nanoparticles allowed an increase in fracture toughness maintaining the transparency and stiffness of neat epoxy.     

Mechanical properties and fracture behaviors of epoxy composites with multi-scale rubber particles

In this work, we developed a strategy to balance the toughness and thermal resistance of epoxy composites by incorporating the multi-scale rubber particles. Two types of rubber i.e. the phase-separation-formed submicron liquid rubber (LR) and preformed nano-scale powered rubber (PR) particles were chosen as tougheners. It was found that the combination of these multi-scale rubber particles not only provides superior efficiency in enhancing the impact resistance of epoxy composites, but also results in balanced glass transition temperature. In particular, the highest gain in impact strength was obtained for the ternary composites containing 9.2 wt% submicron liquid rubber and 9.2 wt% nano-sized powered rubber which were ∼112% higher than the maximum enhancements of ∼49% and ∼66% for the corresponding binary composite systems with the single-phase rubber, respectively. The damage zone observation and fracture surface analysis suggested that the combined use of multi-scale particles was effective to promote matrix plastic deformation including void growth and shear banding induced by the improved rubber cavitation/debonding, which is likely responsible for the highly improved impact resistance of the ternary composites.     

Fracture behaviour of hybrid glass matrix composites: thermal ageing effects

The thermal aging of a glass matrix composite reinforced by short carbon fibres as well as by ZrO₂ particles (hybrid composite) was investigated at temperatures in the range 500–700 °C for exposure durations of 24 h in air. The mechanical properties of as-received and aged samples were evaluated at room temperature by using the three-point flexure chevron notch technique. The fracture toughness values of as-received specimens were in the range 2.6–6.4 MPa m^1/2. Fracture toughness was affected by the thermal aging conditions. For thermal aging at temperatures <700 °C, degradation of fibre–matrix interfaces occurred and therefore the apparent fracture toughness and flaw tolerant resistance decreased. For the most severe ageing conditions tested (700 °C/24 h), fracture toughness values dropped to 0.4 MPa m^1/2. Significant degradation of the material was detected for this aging condition, mainly characterised by porosity formation in the matrix as a result of softening of the glass and oxidation of the carbon fibres.