Fracture Toughness Of Epoxy Research Articles

Toughness augmentation by fibrillation and yielding in nanostructured blends with recycled polyurethane as a modifier

In the present paper, we have carefully investigated the morphology and fracture mechanism of the recycled polyurethane (RPU)/epoxy blend system. The second phase (RPU) added to the epoxy resin has a positive effect on the overall mechanical properties. Interestingly, the recycled polymer has a remarkable effect on the fracture toughness of epoxy resin. The mechanism behind the fracture toughness improvement up on the addition of RPU was found to be very similar to that of the incorporation of hyperbranched polymers in epoxy resin. Brittle to ductile fracture was clear in the case of higher loadings such as 20 and 40 phr of RPU in the epoxy resin. The mechanism behind improvement of fracture toughness was found to fibrillation of the RPU phase which was evidenced by the fracture morphology. In fact the force applied to the epoxy matrix was effectively transferred to the added RPU phase due to its strong interaction with the epoxy phase. This effective transfer of force to the RPU phase protects the epoxy matrix without catastrophic failure and we observed 44% increase in G1C values at an addition of 40 phr RPU. This results in the extensive fibrillation of RPU which causes the generation of new surfaces. Thus the impact energy has been fully utilized by the RPU phase. The mechanism is termed as simultaneous reinforcing and toughening and normally reported as a result of cavitations and yielding. SEM, HRTEM and AFM analyses clearly demonstrated the fibrillated morphology of the fracture surface and the formation of nanostructures. This report is first of its kind in the case of both epoxy modification and the elastomer toughening.

Enhancement of Fracture Toughness of Epoxy Nanocomposites by Combining Nanotubes and Nanosheets as Fillers.

In this work the fracture toughness of epoxy resin has been improved through the addition of low loading of single part and hybrid nanofiller materials. Functionalised multi-walled carbon nanotubes (f-MWCNTs) was used as single filler, increased the critical strain energy release rate, GIC, by 57% compared to the neat epoxy, at only 0.1 wt% filler content. Importantly, no degradation in the tensile or thermal properties of the nanocomposite was observed compared to the neat epoxy. When two-dimensional boron nitride nanosheets (BNNS) were added along with the one-dimensional f-MWCNTs, the fracture toughness increased further to 71.6% higher than that of the neat epoxy. Interestingly, when functionalised graphene nanoplatelets (f-GNPs) and boron nitride nanotubes (BNNTs) were used as hybrid filler, the fracture toughness of neat epoxy is improved by 91.9%. In neither of these hybrid filler systems the tensile properties were degraded, but the thermal properties of the nanocomposites containing boron nitride materials deteriorated slightly.

Temperature effects on rigid nano‐silica and soft nano‐rubber toughening in epoxy under impact loading

ABSTRACTThe rigid nano‐silica and soft nano‐rubber toughening effects on neat epoxy under impact loading in a range of −50 to 80 °C were investigated. Nanosilica particles (20 nm) toughened neat epoxy at all temperatures with a maximum toughening efficiency at −50 °C and lower efficiency at elevated temperatures. In contrast, except at −50 °C, nano‐rubber particles (100 nm) showed the deterioration effect on the impact fracture toughness of epoxy resin. Scanning electron microscopy examinations revealed that the crack pinning and local epoxy deformation induced by rigid particles in term of nano‐silica/epoxy and nano‐rubber/epoxy interfacial debonding (at −50 °C) led to positive toughening efficiency on neat epoxy. However, at 20 and 80 °C, the rubber cavitations/void plastic growth was significantly suppressed under the impact loading, which led to the negative toughening efficiency on epoxy. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 45319.

Effects of the media and ageing condition on the tensile properties and fracture toughness of epoxy resin

This paper presents the water diffusion behaviour, tensile properties and fracture toughness of epoxy after it is aged in distilled and salt water in both steady and fluctuating conditions at a temperature of 50 °C. In steady and fluctuating conditions, the equilibrium water uptake of epoxy aged in salt water is lower than that in distilled water, but the diffusion rate is unaffected. The effects of the media and ageing condition on the tensile properties and fracture toughness are insignificant. The absorbed water (0.6–1.8%) improves the epoxy fracture toughness up to three times, which is indicated by the complex fracture surfaces of aged epoxy.

Flaky and fullerene-like WS2 nanoparticles as tribologic and toughening additives for epoxy

Just like MoS2, WS2 is known for its outstanding tribologic properties. When used as additives, both were found to considerably improve the tribologic behavior of epoxy, i.e., its coefficient of friction and wear resistance. The best improvements were obtained with WS2 or MoS2 nanoparticles, in particular if they had a fullerene-like morphology. Likewise, fullerene-like WS2 nanoparticles were shown to considerably enhance the fracture toughness of epoxy. It was thus hypothesized that the improved wear resistance could be due to the toughening effect rather than due to reduced friction. Our investigations showed that both flaky and fullerene-like WS2 nanoparticles can improve the fracture toughness of certain epoxy systems, while they can embrittle others. The beneficial effect on the epoxy’s wear resistance could not be confirmed either: The coefficient of friction and wear measured in pin-on-disc tests correlated insignificantly with the type or amount of nanoparticles used or the dispersion technique applied. The fact that the fracture toughness did not correlate with the measured wear suggests that the investigated epoxy system wears by adhesion rather than by abrasion. It is thus possible that tribologic additives like WS2 are unsuited for counteracting this wear mechanism. In a nutshell, both the toughening and the wear-reducing effect of flaky and fullerene-like WS2 nanoparticles seem to depend strongly on the particular epoxy system investigated.

Epoxy modification with poly(vinyl acetate) and poly(vinyl butyral). I. Structure, thermal, and mechanical characteristics

ABSTRACTEfficiency of the application of high strength heat resistant thermoplastics for improving fracture toughness and impact properties of epoxy resins motivated authors to try large‐scale production thermoplastics for the same purpose. Epoxy/anhydride systems were modified by up to 8 wt % poly(vinyl acetate) (PVAc) and up to 6 wt % poly(vinyl butyral) (PVB). In epoxy–PVAc blends it was possible to obtain morphologies with continuous thermoplastic phase. However, only sea‐island morphologies with a very small size of PVB‐rich phase were observed in epoxy–PVB matrices. The former type of morphology allowed a notable 2.4‐fold increase in the fracture toughness of epoxy resin and simultaneous up to 30% decrease in its' impact strength. The latter type of morphology caused a notably lower (45%) enhancement of the epoxy fracture toughness combined with a 50% increase in its' impact strength. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 44081.

Rheological properties and fracture toughness of epoxy resin/multi‐walled carbon nanotube composites

Multi‐walled carbon nanotubes (MWCNTs), surface‐treated via chemical functionalization, i.e., oxidation and amidation, were used to reinforce diglycidylether of bisphenol F (DGEBF) epoxy resin. The effects of the functionalization on the dispersion stability, rheological properties, and fracture toughness of DGEBF/MWCNT composites were investigated. The dispersion homogeneity of the MWCNTs in the epoxy matrix improved after functionalization. In addition, isothermal rheology measurements revealed that the DGEBF/dodecyl amine‐functionalized MWCNT (D‐MWCNT) composite had a longer gel time and higher activation energy of cross‐linking than the DGEBF/acid‐treated MWCNT (A‐MWCNT) composite. The fracture toughness of the former was also significantly higher than that of the latter; this resulted from the relatively high dispersion stability of the D‐MWCNTs in the epoxy matrix, owing to the presence of alkyl groups on the D‐MWCNT surface. POLYM. ENG. SCI., 55:2676–2682, 2015. © 2015 Society of Plastics Engineers

Fracture behaviour of epoxy nanocomposites modified with triblock copolymers and carbon nanotubes

The effect of the addition of a combination of rigid nanofillers and soft rubber nanoparticles on the fracture toughness of epoxy resins was studied. Carbon nanotubes (CNTs), as rigid nanofillers, and polystyrene-polybutadiene-poly(methyl metacrylate) (SBM) triblock copolymers, as rubber nanoparticles that self-assemble in the epoxy matrix, were used. Fracture tests according to ESIS TC4 protocol were performed on neat epoxy, epoxy reinforced with CNTs, epoxy nanostructured with SBM triblock copolymers and, finally, epoxy with CNTs and SBM triblock together. Finally, it was found that both fracture toughness and critical energy release rate increased significantly with the addition of rubber and rigid particles. The analysis of the fracture surfaces revealed the toughening micromechanisms.

Simultaneously enhanced tensile strength and fracture toughness of epoxy resins by a poly(ethylene oxide)-block-carboxyl terminated butadiene-acrylonitrile rubber dilock copolymer

PEG-b-CTBN diblock copolymer was synthesized by the esterification reaction and then used to modify epoxy via the formation of nanostructure.

極低温熱サイクル疲労を受ける平織炭素繊維布強化複合材料へのフィラー充填が窒素ガスのリーク防止に及ぼす影響

Carbon fiber reinforced plastics (CFRP) are widely used as structural components and are regarded as the major candidates for reducing the structural weight. Especially, application of CFRP to the cryogenic liquid fuel tanks is one of the most yearnings for achieving the drastic weight reduction. To apply CFRP for the fuel tank instead of the aluminium alloy, behaviors of CFRP such as fuel leakage or gas permeation under the cryogenic temperature should be cleared. The purpose of this study is to improve the nitrogen gas permeation properties of Plain-Woven CFRP. To improve the permeation properties of CFRP (V-CFRP), the nanoclay filler, which is montmorillonite is compounded (M-CFRP). The leakage pathway of nitrogen gas through the laminates is also revealed in order to identify the major factor of leakage. The leak rate of CFRP after cryothermal fatigue tests was increased as cyclic number increases. This is because that the interfacial debonding and micro cracks were initiated in CFRP due to cryothermal cycling. The increase of leak rate due to crack initiation was significantly higher than that due to the nitrogen diffusion through the matrix. The micro crack density and crack length of M-CFRP were improved compared to V-CFRP. This is because that the fracture toughness of epoxy resin improved by filling the montmorillonite. Therefore, the leak rate of M-CFRP was decreased, and the permeation properties can be improved by filling montmorillonite. The velocity of diffusion of M-CFRP until reaching steady leakage was lower than that of V-CFRP. It should be said that the montmorillonite, which is the inorganic material plays the role of the obstacle for nitrogen molecule.

Synergistic Toughening in Ternary Silica/Hollow Glass Spheres/Epoxy Nanocomposites

Herein, the fracture toughness of ternary epoxy systems containing nanosilica and hollow glass microspheres (HGMS) is investigated. The experimental measurements reveal synergistic fracture toughness in some hybrid compositions: The incorporation of 10 phr of HGMS and nanosilica alone modify the fracture toughness of epoxy by 39% and 91%, respectively. However, use of 10 phr hybrid modifier can enhance the fracture toughness of the resin up to 120%. Observations reveal different toughening mechanisms for the blends i.e., plastic deformation for silica nanoparticles and crack bifurcation for HGMS. Both of these toughening mechanisms additively contribute to the synergism in ternary epoxies.

Mechanical and thermal properties of epoxy/silicon carbide nanofiber composites

The silicon carbide (SiC) nanofibers (0.1, 0.25, and 0.5 phr), produced by self‐propagating high‐temperature synthesis (SHS), are used to reinforce the epoxy matrix cured with an anhydride hardener. Morphological studies reveal a better dispersion of SiC nanofibers and a good level of adhesion between nanofiber and the matrix in composites with lower (0.1 and 0.25 phr) nanofiber loading. The flexural studies show that a maximum increase in flexural properties is obtained for composites with 0.25 phr SiC nanofiber. The fracture toughness of epoxy is found to increase with the incorporation of SiC nanofibers, and 0.25 phr SiC nanofiber loading shows maximum fracture toughness value. The possible fracture mechanisms that exist in epoxy/SiC nanofiber composites have been investigated in detail. Thermogravimetric analysis reveals that SiC nanofibers are effective fillers to improve the thermal stability of epoxy matrix. Copyright © 2014 John Wiley & Sons, Ltd.

Fracture Characteristics of Shear Adhesive Dissimilar Joint

In this study, the effect of bond thickness upon shear strength and fracture toughness of epoxy adhesively bonded joint with dissimilar adherents was addressed. The bond thickness, t between the adherents was controlled to be ranged between 0.1 mm and 1.2 mm. Finite element analyses were also executed by commercial ANSYS 11 code to investigate the stress distributions within the adhesive layer of adhesive joint. As a result, shear strength of adhesive joint reduces with increasing bond thickness. The strength of shear adhesive joint was also depended on elastic modulus of adherent. Moreover, the failure of dissimilar adherents bonded shear joint originated at a location with critical stress-y which was the interface corner of ALYH75/epoxy. In the case of shear adhesive joint with an interface crack, the fracture also occurred at the ALYH75/epoxy interface even in the steel-adhesive-aluminum (SEA) specimens. Fracture toughness, Jc of aluminum-adhesive-steel (AES) joints was similar to those of SES and demonstrates strong dependency upon bond thickness. Furthermore, the interface crack in SEA specimen has relatively large fracture resistance if compared to those in AES specimen. Finally, Kc fracture criterion was found to be appropriate for shear adhesive joints associated with adhesive fracture.

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

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

Modification of dicyandiamide-cured epoxy resin with different molecular weights of polyethylene glycol and its effect on epoxy/glass prepreg characteristics

Epoxy resins are widely used in the manufacture of structural composites; however, the inherent brittleness of epoxy matrices has restricted their applications. In this work, a diglycidyl ether of bisphenol-A epoxy resin cured by dicyandiamide (DICY) was modified by adding polyethylene glycol (PEG) with molecular weights of 2000 and 4000 g/mol in amounts of 10, 20, and 30 phr. The effect of these PEGs on the mechanical properties and fracture toughness of epoxy resin was studied. Toughness (the area under the stress–strain curve) increased approximately four- and eightfold upon the addition of 20 and 30 phr of the PEGs, respectively. Results of this study also showed a 73% increase in fracture toughness (KIC) and increases in 300% and 425% in fracture energy (GIC) following the addition of 30 phr of PEGs 2000 and 4000, respectively. Subsequently, prepregs having 50 wt% resin content were prepared. Then relevant prepreg tests such as rheology, resin flow, and lap shear strength were performed. Increasing the PEGs continuously improved resin flow. Scanning electron micrographs (SEMs) of unmodified samples and some PEG-modified samples were examined. SEM showed homogeneous blends with no phase separation occurring.

Enhancement of mechanical properties of carbon fabric/epoxy composites using micro/nano-sized bamboo fibrils

Micro/nano-sized bamboo fibrils (MBFs) were fabricated from raw bamboo using the alkaline treatment technique and the mechanical extraction method (micro-grinding). The fracture toughness of epoxy resin significantly increased by 84.6% from 0.639MPam1/2 to 1.18MPam1/2 due to 0.8wt.% MBF inclusion into the resin in comparison with the epoxy resin without MBF. Both mode-I interlaminar fracture toughness of carbon fabric/epoxy (CF/EP) composites at crack initiation and propagation also improved by 35.3% and 47.3%, respectively when 0.8wt.% MBF was dispersed in the epoxy matrix. The fatigue life of modified composites at all of MBF content up to 0.8wt.% dramatically increased 12–25 times longer than those of unmodified composites at high cycle fatigue while the tensile strength slightly increased. Scanning electron microscopy (SEM) observations revealed that MBF delayed the onset of matrix crack, and reduced the crack growth in the CF/EP composites.

Thermal Stability and Fracture Toughness of Epoxy Resins Modified with Epoxidized Castor Oil and Al2O3Nanoparticles

nanoparticles, Epoxidized castor oil, Thermal stabilities, Fracture toughness IntroductionEpoxy resins are used widely in non-structural (coatings,flooring, and painting) and structural (casting, tooling, andmatrices for advanced composites) applications owing totheir outstanding mechanical performance, good chemicaland electrical resistance, and superior dimensional stability.

Effect of poly(amic acid) and polyimide on the adhesive strength and fracture toughness of epoxy resin

We investigate the effect of polyimide and poly(amic acid) contents on the properties of epoxy resin. Charpy impact strength increases by 80% and 70% with the addition of 10 wt.% poly(amic acid) and 15 wt.% polyimide, respectively. However, the flexural strength is improved by 95% and 73% (as compared with the neat epoxy resin) by the addition of 5 wt.% polyimide and 5 wt.% poly(amic acid), respectively. Moreover, the addition of 10 wt.% polyimide resulted in a twofold increase in the impact adhesive strength, while the tensile adhesive strength of the epoxy increases by 70% with only 5 wt.% polyimide. The infrared spectra show that the obtained compositions containing polyimide or poly(amic acid) exhibit semiinterpenetrating polymer network structures characterized by a relatively high flexibility and very good mechanical properties.

Toughening Self-healing Epoxy Resin by Addition of Microcapsules

Microcapsules, filled with healing agent, not only has the ability to heal automatically the epoxy materials but also it can increase the virgin fracture toughness of epoxy. We investigated the effect of microcapsules on the fracture toughness of epoxy resins. The fracture toughness are measured using a tapered double-cantilever beam (TDCB) specimen. The fracture toughness depend strongly on the concentration of microcapsules added to the epoxy. Fracture of the neat epoxy is brittle, exhibiting a mirror fracture surface. Addition of 10 wt.% poly (urea-formaldehyde) (PUF) yields up to 58.2% increase in fracture toughness and induced a change in fracture plane morphology to hackle markings. Toughening mechanisms was study by observing the morphology of fracture plane by SEM.

Thermomechanical and fracture properties of exfoliated nanoclay nanocomposites

This article investigates the dispersion of nanoclay and its effect on the properties of nanoclay/epoxy nanocomposites developed using a high-shear mixing technique. Epon 828, Nanomer 1.30 E, and a compatible amine-curing agent (Epikure W) were the materials used. The dispersion of nanoclay was studied using XRD and TEM at different levels of magnification. Even though nanoclays were found to have exfoliated based on XRD’s d-spacing data (>8 nm), they remained as tactoids based on TEM data. The effect of curing on the dispersion of nanoclay was also studied using XRD analysis, which showed polymerization of resin in-between the nanoclay layers during curing. This polymerization was only helpful for the exfoliation of nanoclay, it could not completely delaminate and disperse the individual nanoclay layers. The nanocomposites were characterized using dynamic mechanical analysis and ASTM standard fracture tests. Measured elastic modulus was compared with the predicted modulus using modified Halpin—Tsai model. The experimental value was less than the predicted value and the difference was attributed to the poor dispersion of nanoclays. However, addition of 5.3 wt% nanoclay increased the storage modulus and the fracture toughness of epoxy resin by 21 % and 16%, respectively. The fracture morphologies indicated that the major toughening mechanism was due to crack deflection around nanoclay tactoids.