Fracture Toughness Of Epoxy Research Articles

Improved fracture toughness of epoxy resin reinforced with micro‐sized lotus fibers pre‐ and post‐sodium hydroxide treatment

Improved fracture toughness of epoxy resin reinforced with micro‐sized lotus fibers pre‐ and post‐sodium hydroxide treatment

Effect of sodium hydroxide treatment on the fracture toughness of luffa natural fiber reinforced epoxy epikote 828

AbstractMicro‐sized luffa natural fibers (MLNFs) were derived from raw luffa natural fibers from Vietnam by undergoing treatment with sodium hydroxide. This study examined the impact of sodium hydroxide concentration, temperature, and treatment duration on the fracture toughness of epoxy resin 828 reinforced with MLNFs. The results showed that the fracture toughness of the composite, as measured by the critical‐stress‐intensity factor (KIC) and Izod impact strength, was improved after the treatment of the MLNFs with sodium hydroxide. Specifically, with the optimal sodium hydroxide treatment parameters of 6% NaOH at 70 °C for 6 h, the critical‐stress‐intensity factor increased by 93% (from 2.0 to 3.86 MPa m1/2) and the Izod impact strength rose by 44.6% (from 3.81 to 5.51 kJ/m2). The thermal properties and surface structure of MLNFs treated with NaOH were analyzed using thermogravimetric analysis (TGA) and scanning electron microscopy (SEM) images.

Mode-I analysis of glass fiber-reinforced polymer laminates toughened by interleaved polyamide 66 nanofibers and chopped carbon using acoustic emission

The fracture toughness of epoxy resins is weak, so enhancing its properties is essential. In this research it was found that interleaving chopped carbon fibers and polyamide 66 nano mats between fiberglass reinforced plastic laminates with 110 µm thickness, increased fracture toughness in Mode I by 35 and 112%, respectively. Acoustic emission technique showed that damage mechanisms begin at further displacements of load point in modified samples. A hierarchical clustering model was used to classify various failure types. It was observed that matrix cracking, fiber breakage, and matrix cracking were the major damage mechanisms for Reference, chopped carbon, and nanomodified samples, respectively. The cumulative energy released by nanomodified samples was 83% lower than that of Reference and chopped carbon samples. A scanning electron microscope was used to validate different types of damage mechanisms.

A comparative study of the cryogenic performance of CFRP composites with polyethersulfone/epoxy blends and electrospun polyethersulfone interleaves

Thermoslastic-modifers are used to enhance the fracture toughness of epoxy resins for cryogenic applications. This study presents a comparative analysis of the cryogenic performance of laminated fibre-reinforced composites, focusing on two different toughening approaches: interleaving electrospun polyethersulfone (PES) veils or epoxy/PES blends with varying PES loadings. In casted epoxy/PES blends, the particulate phase structure was formed with a PES content of 2.5–10 phr, transitioning to a co-continuous structure beyond 10 phr. Electrospun PES interleaves effectively enhanced interlaminar fracture toughness at both 293 and 77 K by 76 % and 48 %, respectively. These improvement levels achieved were comparable to those of laminates toughened with 7.5 phr PES/epoxy blend. The results presented suggest that interleaving composite laminates offers great potential in enhancing the durability of cryogenic tanks while being seamlessly integrated into manufacturing processes.

The synthesis of a secondary branched epoxy‐modified silicone resin and its application at low temperature

AbstractTo enhance the fracture toughness of epoxy resin at low temperature, a secondary branched epoxy‐terminated silicone resin (ESR‐6) was synthesized and incorporated into bisphenol A epoxy resin at different contents. The structure of ESR‐6 was characterized by Fourier transform infrared spectroscopy and nuclear magnetic resonance (1H NMR), and the fracture surface of the composites was observed by scanning electron microscope (SEM) and atomic force microscopy. At room temperature and − 70°C, the maximum values of elongation at break were 15.78% and 12.55% with 10 wt% ESR. Compared with those of neat epoxy resin, the values of elongation at break of the composite were increased by 50.86% and 36.12%. The results of dynamic mechanical analysis also showed that the toughness of the modified resin had been improved. The SEM images of the fracture surfaces suggested that the fracture mode of the modified resin changed from brittle one to plastic one because of the addition of ESR‐6, which further confirmed the toughening effect of ESR‐6. These research results may provide a new strategy for enhancing the low‐temperature toughness of epoxy resins.

Effects of Block Copolymer Terminal Groups on Toughening Epoxy-Based Composites: Microstructures and Toughening Mechanisms.

Despite the considerable research attention paid to block copolymer (BCP)-toughened epoxy resins, the effects of their terminal groups on their phase structure are not thoroughly understood. This study fills this gap by closely examining the effects of amino and carboxyl groups on the fracture toughness of epoxy resins at different temperatures. Through the combination of scanning electron microscopy and digital image correlation (DIC), it was found that the amino-terminated BCP was capable of forming a stress-distributing network in pure epoxy resin, resulting in better toughening effects at room temperature. In a 60 wt.% silica-filled epoxy composite system, the addition of a carboxyl-terminated BCP showed little toughening effect due to the weaker filler/matrix interface caused by the random dispersion of the microphase of BCPs and distributed silica. The fracture toughness of the epoxy system at high temperatures was not affected by the terminal groups, regardless of the addition of silica. Their dynamic mechanical properties and thermal expansion coefficients are also reported in this article.

Predicting the Effect of Hardener Composition on the Mechanical and Fracture Properties of Epoxy Resins Using Molecular Modeling

Improving the toughness of brittle epoxy while keeping its high strength-to-weight ratio is challenging, as these two properties work against each other. Fracture processes are difficult to ascertain with experiments, as they occur at nanoscopic lengths and time scales and require higher efficiency than what can be attained with atomistic simulations. To overcome this challenge, we utilize a recently developed chemistry specific coarse-grained model to examine two different hardeners, diamine 4,4′-methylenebis(cyclohexylamine) (PACM) and propylene oxide diamine (Jeffamine), to cure bisphenol A diglycidyl ether (DGEBA) at varying stoichiometries and understand how hardener composition influences the elasticity, yield strength, and fracture toughness of epoxy resins. The results indicate that PACM mainly contributes to the modulus and that long Jeffamine chains increase fracture toughness by making the epoxy ductile, whereas short Jeffamine chains significantly improve the yield strength. Longer Jeffamines also lead to larger voids and the formation of fibrils that carry a significant amount of stress and contribute to toughness. Interestingly, the Ashby plots reveal that epoxies with intermediate-length Jeffamine chains (D800 and D2000) outperform other systems, as the toughness enhancement from flexible Jeffamine chains and the stiffness due to PACM help to overcome the strength–toughness trade-off. Our modeling framework and findings establish a path toward resin design from predictive multiscale models with no empirical input and reveal new insights into the molecular failure mechanisms of epoxy resins.

The Effect of Flame Retardant-Aluminum Trihydroxide on Mixed Mode I/II Fracture Toughness of Epoxy Resin.

Fire resistance is a major issue concerning composite materials for safe operation in many industrial sectors. The design process needs to meet safety requirements for buildings and vehicles, where the use of composites has increased. There are several solutions to increasing the flame resistance of polymeric materials, based on either chemical modification or physical additions to the material’s composition. Generally, the used flame retardants affect mechanical properties either in a positive or negative way. The presented research shows the influence of the mixed-mode behavior of epoxy resin. Fracture toughness tests on epoxy resin samples were carried out, to investigate the changes resulting from different inorganic filler contents of aluminum trihydroxide (ATH). Three-point bending and asymmetric four-point bending tests, with different loading modes, were performed, to check the fracture behavior in a complex state of loading. The results showed that the fracture toughness of mode I and mode II was reduced by over 50%, compared to neat resin. The experimental outcomes were compared with theoretical predictions, demonstrating that the crack initiation angle for higher values of KI/KII factor had a reasonable correlation with the MTS prediction. On the other hand, for small values of the factor KI/KII, the results of the crack initiation angle had significant divergences. Additionally, based on scanning electron microscopy images, the fracturing of the samples was presented.

Synergistically enhanced performance of epoxy resin by block copolymer and multi‐walled carbon nanotubes

AbstractThis work aims to investigate the performance of the epoxy resin modified by poly(ethylene glycol)‐b‐poly(propylene glycol)‐b‐poly(ethylene glycol) (PEO‐PPO‐PEO) block copolymers and multi‐walled carbon nanotubes (MWCNTs). It is detected that when PEO‐PPO‐PEO block copolymers and MWCNTs are simultaneously added, synergistic toughening effects for epoxy resin achieve without loss of other properties. Specifically, with the introduction of 5 wt% PEO‐PPO‐PEO block copolymers, the critical stress intensity factor (KIc) of the epoxy resin is increased by 65.7% in contrast with the pure epoxy resin by the reason of the epoxy matrix shear yielding triggered by cavitation of PEO‐PPO‐PEO nanophases and by 28.7% with the introduction of 0.25 wt% MWCNTs owing to the pull‐out and crack bridging of MWCNTs, but other properties are decreased. However, with the simultaneous introduction of 5 wt% PEO‐PPO‐PEO and 0.25 wt% MWCNTs, the KIc of epoxy resins is significantly increased by 97.3%, exhibiting much higher than the linear superposition of the corresponding epoxy resin/PEO‐PPO‐PEO and epoxy resin/MWCNTs composites. Meanwhile, the decrease in other properties is retarded. This is because PEO‐PPO‐PEO block copolymer can enhance the interactions between MWCNTs and epoxy resins in the epoxy resin/PEO‐PPO‐PEO/MWCNT composites, thereby promoting the dispersion of MWCNTs in the epoxy resins. Consequently, the fracture toughness of epoxy resins is efficiently boosted by the superposition of the pull‐out and crack bridging of well‐dispersed MWCNTs and the epoxy matrix shear yielding triggered by the strengthened interactions between PEO‐PPO‐PEO block copolymers and MWCNTs. This work supplies an easy and efficient means to solve the dispersion of nanoparticles, which can prominently advance the comprehensive properties of epoxy resins.

Nano-silica reinforced epoxy resin/nano-rubber composite material with a balance of stiffness and toughness

In the electronics and aerospace industries, epoxy resins are generally regarded as economical and efficient adhesives and have a high status. However, epoxy resins are highly crosslinked polymers and are very brittle adhesives where they are prone to fast crack propagation under dynamic loads. Therefore, it is very necessary to enhance the toughness of epoxy resin adhesives. Nano-rubber has been proved to be an important toughening agent for epoxy resin, which can significantly improve the fracture toughness of epoxy resin. However, increasing the toughness of epoxy resin by adding nanomaterials is often accompanied by decreasing the strength and stiffness of resin. Therefore, in this work, rigid nano-silica particles were added to improve the rigidity and tensile strength reduction caused by the addition of rubber particles. And further increase the toughness of the epoxy resin to obtain an epoxy adhesive with balanced stiffness-toughness. As a result, it can be found that the addition of silica particles can significantly improve the decrease in stiffness caused by the addition of rubber particles. For example, Young’s modulus and tensile strength are increased by 28%, and 23%, respectively, with 4% silica is added based on rubber particles. Through the single lap shear experiment, it is found that the shear strength of the epoxy/RnP/silica composite adhesive has increased, which further proves that the addition of nano-silica particles can increase the stiffness of the epoxy composite. The dynamic mechanical analysis experiment found that after adding nano-silica particles, the storage modulus of epoxy composites increased, which also shows that adding nano-silica particles can improve the stiffness of epoxy composites. Scanning electron microscopy analysis was performed to study the reinforcement mechanism of epoxy/RnP/silica composite materials. The thermal stability of epoxy composites was characterized by Dynamic mechanical analysis and thermogravimetric analysis.

Phosphorus and bromine modified epoxy resin with enhanced cryogenic mechanical properties and liquid oxygen compatibility simultaneously

Herein, epoxy resins for composite liquid oxygen (LOX) fuel tanks are modified with phosphorous flame retardant (DOPO) and brominated epoxy resin (EX48). With the incorporation of DOPO and EX48, the processing properties requirements for the hot-melt prepreg are contented with the significantly enhanced viscosity of the modified epoxy resin. At the same time, the tensile performance, flexural performance and fracture toughness of epoxy resin at room temperature and 90 K are improved with the introduction of DOPO and EX48. In addition, the epoxy resin of EP/P7.7/Br40 offers a highest limited oxygen index of 33.5% and a lowest LOX impact sensitivity of 5% with the eliminated LOX incompatible reactions including explosion and combustion. Consideration of the excellent cryogenic mechanical properties and LOX compatibility, combined with ideal viscosity for hot-melt prepreg preparation, the epoxy resin modified with DOPO and EX48 simultaneously is attractive as the polymer matrix for composite LOX fuel tanks.

The Effect of Single-Walled Carbon Nanotube (SWCNT) Concentration on the Mechanical and Rheological Behavior of Epoxy Matrix

The improvement of Mode I fracture toughness of epoxy by the addition of single-walled carbon nanotubes (SWCNTs) is considered. To prepare nanocomposites, chemical-vapor-deposition-grown SWCNTs noncovalently functionalized with an ethoxylated alcohol was used as the additive and a diglycidyl ether of bisphenol-A-based epoxy as the matrix material. The SWCNTs were dispersed in the epoxy matrix via a mechanical stirrer and a 3-roll mill. The effect of their concentration (0.0125, 0.025, 0.05, 0.1, 0.3, and 0.5 wt.%) on the mechanical properties of the nanocomposites was investigated, and the optimum concentration was determined. Mode I fracture toughness (single-edge-notch 3-point bending) and tensile tests were carried out on neat epoxy and SWCNT-reinforced epoxy nanocomposites. The fractured surfaces of fracture toughness and tensile test specimens were examined by the SEM to reveal the effect of SWCNTs on their failure modes.

Electrical-Mechanical Model of Electrical Breakdown of Epoxy-Impregnated-Paper Insulated Tubular Busbar With Bubble Defects

Insulated tubular busbar is a new type of current-carrying equipment with excellent performance. However, the research on this equipment is insufficient because of the short application and the lack of technical digestion, which has resulted in many accidents. In this paper, the full-scale model of epoxy-impregnated-paper insulated tubular busbar with bubble defects was designed and produced, its electrical breakdown process was investigated by withstand voltage test. In the experiment, the failure mode of breakdown caused by gas expansion was found for the first time. The research showed that the electrical-mechanical breakdown process could be divided into two stages: gas increase stage and breakdown threshold stage. The research also obtained the characteristics and the change criterions of the two stages. The gas in the bubble defects generated from epoxy decomposition which caused by partial discharge. When the gas pressure exceeds the critical fracture toughness of the bubble defects wall, the unstable crack extension occurs, therefore, improving the ability about resistance to partial discharge of epoxy resin or improving the fracture toughness of epoxy resin by modifying would enhance the breakdown performance of insulated tubular busbar. The result also has reference significance for other epoxy-impregnated-paper insulated electrical equipment.

Bacterial cellulose filled epoxy resin-based green composites: fabrication and characterization

ABSTRACT The bacterial cellulose (BC) was extracted from nata-de-coco and used as green reinforcement material for epoxy resin. In order to enhance the chemical affinity of BC and epoxy resin the surface of BC was decorated with silane coupling agent. Many techniques were also used to assist the dispersion of BC in epoxy resin such as: mechanical stirrer, grinding and ultrasonication. The green composite of BC in epoxy resin was obtained from hot curing processing using methylhexahydrophthalic anhydride as hardener. The characteristics of green composite such as mechanical properties, thermal stability, fracture toughness and fracture surface were studied in detail. The silane coupling agent with epoxide group can be participated in curing reaction with epoxy resin and helped to enhance the mechanical properties and fracture toughness of epoxy resin. Micro crack, stopping the crack, and changing the crack pathway were considered as main mechanisms of improvement of fracture energy.

Epoxidized soybean oil grafted with CTBN as a novel toughener for improving the fracture toughness and mechanical properties of epoxy resin

We prepared epoxidized soybean oil (ESO) grafted with carboxyl-terminated poly(acrylonitrile-co-butadiene) (CTBN) (ESO-g-CTBN) by a ring-opening reaction between the epoxide group and the carboxyl group. The structural features of the resulting product were determined using modern analytical techniques, such as Fourier transform infrared (FTIR) spectroscopy, proton nuclear magnetic resonance (1H NMR) spectroscopy, and gel permeation chromatography (GPC). The ESO-g-CTBN was applied as a toughener for an epoxy resin-based composite that was fabricated by blending the epoxy resin with diethylenetriamine (DETA) as the curing agent. The main aim of this procedure is to simultaneously improve the mechanical properties and fracture toughness of a bisphenol A-based epoxy resin. When 15 phr of ESO-g-CTBN was added to the EP/DETA mixture, the resin fracture toughness (KIC) and tensile strength increased from 0.65 to 1.09 MPa m1/2 and from 34.42 to 42.55 MPa, respectively. The ESO-g-CTBN existed in the EP matrix as a separate phase and induced an increase in the KIC via stopping crack growth or changing the crack direction. The epoxydized soybean oil grafted with CTBN (ESO-g-CTBN) was synthesized from ring opening reaction between epoxide group and carboxyl group. The ESO-g-CTBN help to improve the fracture toughness of epoxy resin by the mechanism as shown in the figure.

Significant enhancement of fracture toughness and mechanical properties of epoxy resin using CTBN‐grafted epoxidized linseed oil

ABSTRACTCarboxyl‐terminated poly(acrylonitrile‐co‐butadiene) (CTBN)‐grafted epoxidized linseed oil (ELO) (CTBN‐g‐ELO) was synthesized and used as an effective toughener to simultaneously enhance the mechanical properties and fracture toughness of epoxy resin (EP). The ELO was fabricated from linseed oil via epoxidation processing. The characteristics of the ELO and CTBN‐g‐ELO, such as the average molecular weight and chemical structure, were determined using gel permeation chromatography, proton nuclear magnetic resonance, and Fourier transform infrared spectroscopy. The effects of the CTBN‐g‐ELO loading on the characteristics of the EP were investigated in detail. The test results indicated that by adding 15 phr CTBN‐g‐ELO, the tensile strength, impact strength, and critical stress intensity factor (KIC) were significantly increased, by approximately 23.62, 91.8, and 33.8%, respectively, compared with pristine EP. The glass‐transition temperature (Tg) and storage modulus, which were examined via dynamic mechanical thermal analysis and differential scanning calorimetry, respectively, exhibited decreasing trends. Scanning electron microscopy revealed that the CTBN‐g‐ELO existed as spherical particles in the EP, helping to stop the crack growth and change the crack growth directions. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci.2020,137, 48276.

Improved fracture toughness of epoxy resin reinforced with polyamide 6/graphene oxide nanocomposites prepared via in situ polymerization

In this work, a facile in situ polymerization method was developed for the preparation of polyamide 6 (PA6) chains grafted graphene oxides (GO). Based on ethanolic solution precipitation method, the powders of PA6, PA6/GO-0.5 and PA6/GO-1.0 were fabricated and dispersed uniformly into epoxy resins at varying loadings. The surface modification of GO and mechanical properties of epoxy composites were investigated systematically. The mechanical tests indicate that, without impairing tensile properties and glass transition temperature, the fracture toughness of PA6/GO-1.0 particles reinforced epoxy composites exhibits a 52.6% increase compared with neat epoxy at 1.5 wt% loading. Based on adequate fractographic study, the excellent toughening effect of PA6/GO-1.0 can be attributed to the better particle/matrix interface, rougher interface with small convex structures and more energy consumption of particle fracturing, which are resulted from the existence of PA6 chains grafted GO nanosheets. Moreover, the addition of GO and PA6 is 0.015 and 1.5 wt% respectively for the maximum fracture toughness value in this work. The dosage is very small compared with epoxy reinforced with GO or PA6 separately, giving an economically feasible method for the toughening of epoxy resin.

The effect of reactive diluent on mechanical properties and microstructure of epoxy resins

The effects of an aliphatic reactive diluent on the viscosity, gel time, chemical resistance, mechanical properties, fracture toughness, microstructure and relaxations of two different epoxy resins, i.e., a bifunctional and a multifunctional epoxy, were investigated. The gel time was not significantly affected by the diluent. The modulus and ultimate strength gradually declined, while the ductility steadily increased as the diluent content was increased. The failure mode of epoxy samples under flexural test changed from brittle to much more ductile upon the progressive incorporation of diluent into the epoxy resins. The strain at break of bifunctional resin improved by 75%, while that of multifunctional resin enhanced by more than 24%. The dynamic mechanical thermal analysis (DMTA) showed that the introduction of reactive diluent into different epoxy resins reduced the storage modulus at the glassy region and significantly broadened the glass transition region of the epoxy matrices with approximately no change in the glass transition temperature. The DMTA results also indicated increased microstructural heterogeneities upon the addition of diluent to the epoxy resins as evidenced by atomic force microscopy. The fracture toughness of epoxy resins enhanced with diluent content, about 20% and 29% increase for bifunctional and multifunctional resins, respectively. This was attributed to the improved material’s ductility and activation of plastic deformation in the epoxy resins upon the addition of reactive diluent.

Epoxy‐functionalized polysiloxane reinforced epoxy resin for cryogenic application

ABSTRACTThe poor cryogenic mechanical properties of epoxy resins restrict their extensive application in cryogenic engineering fields. In this study, a newly synthesized epoxy‐functionalized polysiloxane (PSE) is used to improve the cryogenic mechanical properties of bisphenol‐F epoxy resin. The Fourier transform infrared spectra and nuclear magnetic resonance confirm the formation of epoxy‐functionalized –Si–O–Si– molecular chain. The surface free energy test results show that the PSE has a better compatibility with epoxy resin. The mechanical test results show that the cryogenic tensile strength, failure strain, fracture toughness, and impact strength of epoxy resin is improved significantly by adding the suitable amounts of PSE. Compared to the neat epoxy resin, the maximum tensile strength (196.92 MPa, an improvement of 11.2%), failure strain (2.97%, an improvement of 33.8%), fracture toughness (3.05 MPa·m1/2, an improvement of 30.7%) and impact strength (40.55 kJ m−2, an improvement of 14.8%) at cryogenic temperature (90 K) is obtained by incorporating 10 wt % PSE into the neat epoxy resin. Moreover, the results also indicated that the tensile strength, Young's modulus, and fracture toughness of epoxy resin with the same PSE content at 90 K are higher than that at room temperature (RT). © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci.2019,136, 46930.

Tensile, Quasistatic and Dynamic Fracture Properties of Nano-Al₂O₃-Modified Epoxy Resin.

Epoxy resin, modified with different particle sizes (50 nm, 100 nm, 200 nm) and contents (1 wt %, 3 wt %, 5 wt %, 7 wt %) was manufactured. The mechanical behaviors of tensile, quasistatic fracture and dynamic fracture under SHPB (split Hopkinson pressure bar) loading were investigated. The dynamic fracture behaviors of the composites were evaluated by 2D-DIC (digital image correlation) and the strain gauge technique, and the fracture surface was examined by SEM (scanning electron microscope). According to the results, the tensile modulus and strength significantly increased for epoxy resin modified with 5 wt % Al2O3 of 50 nm. The quasistatic fracture toughness of modified epoxy resin increased with the particle content. However, the fracture toughness of epoxy resin modified with high content fillers decreased for particle agglomeration that existed in epoxy resin. The crack propagation velocity can be decreased for epoxy resin modified with particles under dynamic loading. The dynamic initiation fracture toughness of modified epoxy resin increases with both particle size and content, but when the fillers have a high content, the particle size effects are weak. For the composite under dynamic loading conditions, the toughening mechanism is also affected by particle size.