Plastic Void Growth Research Articles

EPOXY-SILICA NANOCOMPOSITE: CREEP RESISTANCE AND TOUGHENING MECHANISMS

In this article, the effect of modified nanosilica (m-nanosilica) on the mechanical and creep performance of epoxy resin were investigated. At the same time, the toughnening mechanisms of epoxy resin with m-nanosilica were also considered. The results show that the crack pinning and crack deflection mechanism contributes only a small part to nanocomposite, this due to the size of the nanosilica particles is such smaller than crack-opening displacement. The FE-SEM surface fracture shows evidence of the debonding of the nanoparticles from the polymer matrix and the expansion of the holes around the nanoparticles as well as the existence of the plastic deformation zone. Plastic deformation and plastic void growth are the main toughening mechanism in nanocomposite material.

Computational modeling of fracture in polymer nano‐composites undergoing large deformations via the graded‐interphase‐enhanced phase‐field fracture approach

AbstractThe phenomenon of fracture in polymer composites, polymer nano‐composites in particular, entails a multitude of intricate mechanisms including branching and merging of several cracks along with particle debonding and subsequent plastic void growth. This work involves an extension of the standard phase‐field fracture approach by means of the graded interphase concept in order to facilitate the modeling of a wide‐spectrum of fracture responses, which can be observed in experimental studies concerning the fracture behavior of polymer nano‐composites. Herein, motivated by the microstructure morphology of polymer nano‐composites, a continuous grading of the elastic and fracture properties is considered within a finite thickness interphase region around the filler particles. Appropriate tuning of the graded‐interphase model parameters enables to capture a variety of fracture responses, including cohesive failure between the filler particle and the matrix, as demonstrated via the presented numerical examples, which focus on polymer nano‐composites undergoing large deformations prior to fracture.

Fiber alignment and mode-mixity effects on fracture behavior of CNF/epoxy nanocomposite adhesive joints

In this study, the effect of carbon nanofiber (CNF) alignment was investigated on the enhancement of mixed mode fracture toughness of CNF/epoxy nanocomposite adhesive joints. CNFs were aligned in through-thickness direction inside epoxy using AC electric field. Asymmetric double cantilever beam (ADCB), Double cantilever beam (DCB), and end notched flexure (ENF) specimens were fabricated with pure epoxy, random, and aligned CNF/epoxy to investigate the mixed mode, mode-I, mode-II fracture response, respectively. Mixed mode and mode-II fracture toughness of 0.8 wt% aligned CNF/epoxy increased by 20% and 31.5%, respectively, compared to neat epoxy specimens (Gm,c = 960 ± 100 J/m2, GIIC = 2772 ± 116 J/m2). The mode-I fracture toughness of 0.8 wt% aligned CNF/epoxy was also increased by 195%, compared to the neat epoxy specimens (GIC = 300 ± 31 J/m2). Scanning electron micrograph analysis of fracture surfaces revealed that CNF pull-out, debonding and plastic void growth of epoxy are the key toughening mechanisms for all modes of fracture. These mechanisms were not so prevalent in the case of mixed mode and mode II fracture. Numerical modeling of all joints conducted using cohesive zone modeling matched satisfactorily with experimental findings.

Influence of ozone functionalization of carbon nanotubes on the fracture toughness improvement of polymer nanocomposites: A multiscale study

In this study, we developed a multiscale modeling framework for predicting the fracture toughness improvement of ozone functionalized carbon nanotubes (O-CNTs)/epoxy nanocomposites (NCs) due to debonding of O-CNT and plastic void growth. The proposed multiscale modeling framework was developed by combining molecular dynamics (MD), mean field (MF) homogenization and fracture mechanics theory. Based on this multiscale modeling framework, the contribution of O-CNTs to the interphase elastoplastic properties was characterized and the fracture toughness improvement was quantified through the degree of functionalization of O-CNTs at the microscale. The results demonstrated that increasing the functionalization degree of O-CNTs could effectively improve the elasticity properties of the interphase and fracture toughness of O-CNTs/epoxy NCs. We demonstrated that the interfacial fracture energy is more important compared to the elastoplastic properties of the interphase for improving the fracture toughness of O-CNTs/epoxy NCs. The multiscale modeling framework proposed in this study revealed the dependence of fracture toughness improvement on the functionalization degree of O-CNTs at the theoretical level, where a reference guide was provided for the rational design of composite structures in experiments.

Ozone functionalized graphene nanoplatelets and triblock copolymer hybrids as nanoscale modifiers to enhance the mechanical performance of epoxy adhesives

In the present work, the mechanical and fracture performance of a newly developed high-performance nanocomposite adhesive was characterized experimentally. The nanocomposite adhesive was based on an epoxy resin, which was modified with 10 wt% of triblock copolymers (BCPs) including styrene-butadiene-methylmethacrylate (SBM) and methylmethacrylate-butylacrylate-methylmethacrylate (MAM), and ozone-functionalized graphene nanoplatelets (OZ-GNPs). The mechanical properties of the adhesive joints were systematically evaluated using lap shear, tensile-butt, and Mode-I fracture toughness tests. The lap shear strength and the ultimate shear strain of the 1.0 wt% OZ-GNP + SBM ternary nanocomposite epoxy adhesives obtained 91% and 97%, while 1.0 wt% OZ-GNP + MAM adhesives attained 64% and 63% improvement, respectively, compared to that of the pure epoxy. Consequently, the potential load capacity was improved by about 93% (1.0 wt% OZ-GNP + SBM) and 62% (1.0 wt% OZ-GNP + MAM) compared to that of pure epoxy joints. However, the tensile butt joint strength of the nanocomposite adhesives was slightly reduced due to the lower stiffness of the BCPs. The mode-I fracture toughness results for the 1.0 wt% OZ-GNP + SBM and 1.0 wt% OZ-GNP + MAM increased significantly by approximately 1000% and 200%, respectively, compared to the unmodified epoxy adhesive. Fractography using scanning electron microscopy showed the formation of BCP nanostructures, plastic deformation, crack deflection, debonding of the BCP, and subsequent plastic void growth, which all contributed to the increase in the fracture toughness of the nanocomposite adhesives. The extent of the plastic deformation zone was enhanced by the addition of OZ-GNPs.

On the mix-mode fracture of carbon fibre/epoxy composites interleaved with various thermoplastic veils

This work studied the mix mode-I/II fracture behaviour of an aerospace-grade carbon fibre/epoxy composite that was interlayer-toughened by Polyamide-12 (PA), Polyphenylene-sulphide (PPS), Polyimide (PI), Polyethersulfone (PES) and Polyethylenimine (PEI) fibres. During the laminate curing process, the PA fibres melted, the PPS and PI fibres kept in their original form and the PES and PEI fibres dissolved in the epoxy matrix. This resulted in different toughening mechanisms of the veils for the mix mode-I/II fracture of the laminates, which was studied using a cracked lap-shear test. The main toughening mechanisms were observed to be plastic deformation and failure of the thermoplastic resin for the meltable PA veils, thermoplastic fibre debonding and bridging for the intact PPS and PI veils, and thermoplastic particle debonding and plastic void growth for the dissolvable PES and PEI veils. The experimental results revealed that the fibre debonding and bridging mechanism was superior for toughness enhancement, followed by the thermoplastic particle debonding and plastic void growth mechanism. For instance, interleaving the PPS and PEI veils increased the mix-mode fracture propagation energy of the laminates by 345% and 171%, respectively. However, the toughening performance of the PA and PI veils was poor, since the crack mainly propagated at the vicinity around the interlayer/laminate interface.

The effects of main toughening mechanisms and functionally interphase properties on fracture energy and fatigue characteristics of nanocomposites containing various fillers

Nowadays, novel technologies like nanofillers-based nanocomposites constitute one of the most active research fields in the extremely high performance materials, caused by the fact that this nanomaterial enhances the properties of different polymers in a significant way. Using these materials in polymer structure could enhance the mechanical properties such as fracture energy and fatigue behavior, which caused by variable or cyclic loadings. In the present study, by using the hierarchical multi-scale modeling procedure the fracture toughness (resistance to cracking), fracture energy and fatigue behavior of nanofillers/epoxy nanocomposites with consideration the contributions of the toughening mechanisms of interfacial debonding and plastic void growth in dissipation energy were investigated and discussed. Afterwards, the effect of CNT/matrix interphase on the fracture toughness, fracture energy and fatigue behavior was evaluated by varying the parameters of Young’s modulus and thickness of interphase. Furthermore, analysis of multi-scale modeling strategy was utilized in order to assess the influences of interphase properties and interfacial fracture energy on major parameters such as the critical stress debonding stress and radial component of the stress concentration tensor. In addition, different micromechanical models such as Halpin-Tsai and Maxwell and models were utilized to determine the Young’s modulus of nanocomposite as functionally graded (FG) interphase properties. Fracture energy and fatigue behavior measurements indicated that the properties of interphase region have a crucial importance on the fracture toughness of CNTs/epoxy nanocomposites. In order to verify the model, comparisons were carried out between the results of model with those predicted by experiments. The consistency between results for most fillers show the good performance of the procedure which has been utilized in this study.

Improved mechanical performance and unique toughening mechanisms of UDM processed epoxy‐SiO2 nanocomposites

AbstractThis work demonstrates improved mechanical performance and unique toughening mechanisms of ultrasonic dual mixing (UDM) processed epoxy‐SiO2 nanocomposites. The loading of SiO2 nanoparticles in the epoxy matrix was varied from 1 to 5 vol% to produce the nanocomposites. The mechanical performance was evaluated by measuring the tensile and fracture properties of the nanocomposites. An optimal nanoparticle loading of 3 vol% in epoxy was found vital for improved performance of the nanocomposite. The optimal nanoparticle loading in epoxy lead to an increase of 18.45% in tensile strength and 120% increase in fracture toughness (GIC) compared to the neat epoxy. The fracture surface analysis revealed various toughening mechanisms including particle pull out, plastic void growth, plastic deformation, and crack deflection along with a unique mechanism of strong interfacial bonding between SiO2 particle and epoxy matrix.

Designing of high-performance epoxy adhesive with recycled polymers and silica nano particles (SNPs) in epoxy/carbon fiber composite-steel bonded joints: Mechanical properties, thermal stability and toughening mechanisms

In this study, the effect of hybrid reinforcement comprising silica nanoparticles (SNPs) and a blend of equal percentages of acrylamide-modified waste tire powder and polyethylene terephthalate (PET) on mechanical, thermal stability, and microstructure of the epoxy-phenolic adhesive was assessed using tensile test, TGA, and FESEM tests, respectively. To investigate the adhesion features, the formulated adhesive was applied in lap joint bonding of a stainless steel 316L to epoxy resin/carbon fiber composite. Based on the tensile test results, adding 10 wt.% modified waste tire powder- PET blend and 1 wt.% SNPs to the epoxy-phenolic adhesive increased the tensile strength, modulus, and the toughness of the dumbbell-shaped samples by 65.24%, 58.95% and 98.80% as compared with the neat epoxy-phenolic adhesive, respectively. Furthermore, the highest improvement of shear strength, modulus, and toughness in the steel-composite single lap joint was observed in the sample containing 10 wt.% modified waste tire powder-PET blend and 0.5 wt.% SNPs of 237.22%, 25.64%, and 279.66%. TGA results also indicated that the addition of 10 wt.% PET powder and 1 wt.% SNPs enhanced the initial decomposition temperature, maximum decomposition temperature, and residue char by 5.94°C, 3.64°C, and 59.45% respectively. The images of the fracture surface of the optimal samples in the tensile test showed that the debonding of nanoparticles, plastic void growth, and deviation from the main crack path are among the major mechanisms in the increase of the toughness of the samples.

Impressive epoxy toughening by a structure-engineered core/shell polymer nanoparticle

Five polymer nanoparticles with different compositions and morphologies are prepared from methyl methacrylate, butyl acrylate and ethylene glycol dimethacrylate via aqueous emulsion polymerization. All the nanoparticles keep a discrete dispersion state when transferred into epoxy resin using a phase-transfer method. The addition of these nanoparticles almost has no impact on the cure and glass transition temperature of matrix resin. However, the results of mechanical tests demonstrate a clear correlation between nanoparticle structures and their effectiveness on toughening epoxy resins. Particularly, a soft core/rigid shell nanoparticle (B/M), represented by a 50/50 polybutyl acrylate core with a polymethyl methacrylate shell, shows an impressive toughening effect on the epoxy matrix. The addition of 10 wt% B/M gives 220% and 851% enhancement in critical stress intensity factor (KIC) and the critical strain energy release rate (GIC), respectively. Stress-induced whitening and cold drawing observed during tensile straining confirm the significant enhancement in fracture toughness. The microscopy observations show evidences of plastic void growth followed by the formation of plastic deformation, yielding and crazing, which absorb a significant amount of energy and inhibit crack propagation.

Mechanical properties and fracture behavior of high‐performance epoxy nanocomposites modified with block polymer and core–shell rubber particles

ABSTRACTThe mechanical properties, thermomechanical properties, and fracture mechanic properties of block‐copolymer (BCP), core–shell rubber (CSR) particles, and their hybrids in bulk epoxy/anhydride system were investigated at 23 °C. The results show that fracture toughness was increased by more than 268% for 10 wt % BCP, 200% for 12 wt % of CSR particles, and 100% for hybrid systems containing 3 wt % of each, BCP and CSR. The volume content of nanoparticles influences the final morphology and thus influences the tensile properties and fracture toughness of the modified systems. The toughening mechanisms induced by the BCP and CSR particles were identified as (1) localized plastic shear‐band yielding around the particles and (2) cavitation of the particles followed by plastic void growth in the epoxy polymer. These mechanisms were modeled using the Hsieh et al. approach and the values of GIc of the different modified systems were calculated. Excellent agreement was found between the predicted and the experimentally measured fracture energies. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020, 137, 48471.

Fluorinated polyimide with polyhedral oligomeric silsesquioxane aggregates: Toward low dielectric constant and high toughness

An amphiphilic alternating copolymer with hydrophobic polyhedral oligomeric silsesquioxane (POSS) segments and hydrophilic anhydride segments, which is able to self-assemble into nanoscale POSS aggregates, is introduced into fluorinated polyimide (FPI) films to polish up their toughness while lowering their dielectric constant. The dielectric constant of POSS/FPI composite films with 2.8 wt% amphiphilic alternating copolymer significantly decreases to 2.09 from 3.14 for pure FPI. Furthermore, a sharp brittle-ductile transition is observed in the tensile energy to break versus interparticle distance curve. When the interparticle distance is reduced to 215 nm, the elongation at break and tensile energy to break of POSS/FPI composite films increase to 27.5% and 23.5 MJ/m3 from 8.3% to 6.8 MJ/m3 respectively. The tensile-fractured surfaces of POSS/FPI composite films reveal that the toughening mechanism is mainly attributed to the prevention of crack propagation and the debonding of POSS aggregates with a plastic void growth.

Evaluation of the particle geometry and interphase influence on the filler-matrix debonding process

Incorporation of rigid fillers into polymer matrices represents a widely used technique to obtain improved performance. The mechanical behavior finally obtained is closely related to a wide range of involved factors (filler type and size, internal structure, filler-matrix interaction, among others) and to activated dissipative mechanisms (debonding, plastic void growth, crazing, matrix yielding, etc). In this work, a debonding strength approach was applied for rigid particles (spherical, elliptical and fiber) surrounded by an interphase. The effect of interphase mechanical properties and thickness on the debonding process was investigated. The obtained results suggested a significant influence of the transition rigidity. In general, stiffer interphases promoted higher critical strength values. On the other hand, particle surrounded by a softer transition region displayed higher dissipated energy for all examined particle, except for spheres.

Toughening epoxy syntactic foams with milled carbon fibres: Mechanical properties and toughening mechanisms

Syntactic foams comprising hollow glass microspheres (GMS) in an epoxy matrix are critical materials for lightweight structures, being extensively used in marine and aerospace as cores for composite sandwich panels. They are buoyant and crush resistant, but their use is limited by their brittleness. Milled carbon fibres (MCF) were used to increase toughness, by introducing energy absorption mechanisms, to foams comprising ∼60 vol% GMS. Weight ratios of up to 40% MCF:GMS were used. The tensile modulus of the foams increased from 3.36 GPa to 5.41 GPa with the addition of 40% weight ratio of MCF. The tensile strength of the syntactic foam decreased with low loadings of MCF, but then recovers when more MCF particles are added, and the mechanisms responsible are explained for the first time. The fracture energy of the syntactic foam increased by 183%, from 182 J/m2 to 516 J/m2, due to the addition of 40% weight ratio of MCF. Toughening mechanisms were identified as crack deflection, debonding and subsequent plastic void growth, and fibre pull-out. Thus, the simple and cheap addition of MCF greatly increases the toughness of the syntactic foams, enabling lighter or more damage-resistant structures to be produced.

Interactions between HA/GO/epoxy resin nanocomposites: optimization, modeling and mechanical performance using central composite design and genetic algorithm

This study is focused on the role of hydroxyapatite (HA) nanoparticles and graphene oxide (GO) nanoplates on the flexural and compression properties of epoxy-based hybrid nanocomposites. In the first step, epoxy-based hybrid nanocomposites were reinforced by different HA nanoparticles and GO nanoplates up to 7 wt% and 0.5 wt%, respectively. Filler’s weight fractions that used as design parameters have been achieved by central composite design method in Minitab software. The experimental results showed different combinations of HA- and GO-enhanced mentioned mechanical properties in various states. In the second step, a statistical modeling has been done by response surface method (RSM), artificial neural network (ANN) and decision tree methods. Modeling results showed that ANN and decision tree methods have the best fitness. Finally, the mechanical properties are optimized by genetic algorithm. The optimum values are 21.54 MPa for flexural strength in 5.17 wt% HA and 0.38 wt% Go and 25.7 GPa for flexural modulus in 2.73 wt% HA and 0.195 wt% GO. Also, the optimum results for compression strength are 23.95 MPa in 7 wt% HA and 0.289 wt% GO, and also 690.5 MPa for compression modulus in 6.89 wt% HA and 0.007 wt% GO. Effective mechanisms of fillers have been analyzed by SEM and observed that debonding, crack path deflection, plastic void growth and pullout were dominant.

Mechanical properties of polymer grafted nanoparticle composites

Polymer nanocomposites have improved mechanical, optical and thermal properties compared to traditional thermoplastics. To study the impact of grafted brush–matrix interactions on the mechanical properties of nanoparticle filled polymers, uniaxial tensile testing with digital image correlation (DIC) was done on polystyrene (PS) grafted SiO2 in low molecular weight (MW) (N ∼ P), N = degree of polymerization of grafted brush, P = degree of polymerization of matrix, and high MW (2N = P) matrices at different loadings. The low matrix MW composites had higher strength than high MW composites at high loading but lower strength than the pure matrix. The high matrix MW composites had higher toughness at low loadings. SEM images of fracture surfaces revealed particle debonding and plastic void growth in the high toughness samples. The results show that the P/N ratio of polymer grafted nanoparticle composites is important for controlling the mechanical properties of polymer nanocomposites.

Mechanical evaluation of polylactic acid (PLA) based composites reinforced with different calcium phosphates

In the present work, the mechanical performance of polylactic acid (PLA) based composites reinforced with hydroxyapatite (HA) or β-tricalcium phosphate (β-TCP) was investigated. The polymer was melt compounded with 1 and 2.5 wt% of particles by using an intensive mixer or a twin screw extruder. Morphological, thermal and rheological studies were performed to analyze the composites internal structure and filler-matrix interaction. The mechanical behavior was investigated through uniaxial tensile and quasi-static fracture tests. The different characterization techniques evidenced a better filler dispersion for composites obtained by extrusion independently of the filler used. A relatively weak filler-matrix interaction was revealed from morphological observations and rheological measurements. In addition, thermal analysis evidenced similar crystalline structure for all of the investigated materials. In general, uniaxial tensile parameters displayed almost constant values independently of the filler content or compounding method. Particularly, extruded composites with 2.5 wt% filler exhibited slightly increased ductility respect to neat PLA which was related to improved filler dispersion. The PLA matrix displayed load-displacement curves with ductile instability in quasi-static fracture tests. On the other hand, the composites with 2.5 wt% filler exhibited an increased stable crack growth followed by ductile instability. The fracture process was quantitatively described by means of critical stress intensity factor (KIQ) and strain energy release rate at propagation (GCP) parameters. The extruded composites with 2.5 wt% filler displayed improved propagation fracture toughness. Based on fractured surfaces analysis this enhanced behavior, not largely reported for untreated rigid fillers, was attributed to the effective activation of the toughening mechanisms of particle debonding and subsequent plastic void growth.

Fracture behaviour of rubber- and silica nanoparticle-toughened glass fibre composites under static and fatigue loading

The crosslinked polymers used in fibre composites are very brittle, and require toughening for structural applications. Research over many years has increased the fracture energy, but the fatigue resistance of these toughened polymers is very poor, limiting the optimisation of structures. This work reports the first successful use of hybrid toughening to increase both the quasi-static interlaminar fracture energy, GIC, and the fatigue threshold strain-energy release-rate, Gth. Amine-cured epoxy glass-fibre composites were toughened using carboxyl-terminated butadiene-acrylonitrile (CTBN) which forms micron-sized rubber particles and 20 nm-diameter silica nanoparticles. The toughening mechanisms were identified as cavitation of rubber particles and debonding for the silica nanoparticles, followed by plastic void growth. The CTBN greatly increases GIC, and the nanoparticles increase Gth. Combining both particles as a hybrid has a synergistic effect on the fatigue resistance. This demonstrates the effectiveness of hybrid toughening, enabling the design of optimised composites by combining micro- and nanoparticles.

Mechanical Properties of Glass Bead-Modified Polymer Composite

This present work investigated the adhesive effect of the material properties and fracture toughness of a thermosetting epoxy polymer modified with micron-sized glass beads. Various surface treatments were used to modify the surfaces of the glass beads in order to consider the effect of the glass bead particle/epoxy adhesion on the material properties and toughness of the polymer-based composites. Adding glass beads into polymer resulted in an increase in the modulus. The maximum value of the modulus was obtained for the polymer composite modified with well-bonded glass beads. The addition of the glass beads to polymers leads to a significant improvement in the fracture toughness. The particle/matrix adhesion strongly affects the toughness of the glass bead-modified polymers. The coupling agent silane treatment and release agent Frekote treatment caused well-bonded particles and poorly-bonded particles with the epoxy matrix. Both of the surface treatments result in a significant increase in the toughening performance. Toughening mechanisms of plastic void growth and crack deflection caused by the addition of the glass beads were found on the fracture surfaces of the tested specimens.

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.