Effect of thermoplastic veils on interlaminar fracture toughness of a glass fiber/vinyl ester composite

The effects of two thermoplastic micro‐veils, polyamide (PA) and polyethylene terephthalate (PET) veil, on the interlaminar fracture toughness of a glass fiber/vinyl ester (GF/VE) composite were investigated. The veils incorporated into the composite as interleaving materials were first characterized via scanning electron microscopy (SEM), differential scanning calorimetry (DSC), contact angle and tensile testing in order determine the best candidate as toughening agent for the GF/VE composite. Composite laminates were manufactured by vacuum‐assisted resin infusion process. Double cantilever beam (DCB) testing was performed to investigate the Mode I type interlaminar fracture toughness of the composites, which was characterized by critical strain energy release rate (G _IC). An increased G _IC was obtained by incorporating the PA veil, but it changed negligibly by the addition of the PET veil. The analysis of the composites fracture surface via SEM revealed increased fiber bridging between adjacent plies in the case of PA veil interleaved composites which played a key role in enhancing the Mode I interlaminar fracture toughness. However, the PET veil present in the interlaminar region did not take part in any energy absorbing mechanism during the delamination, thus keeping the G _IC of the composite unaltered. POLYM. COMPOS., 38:2501–2508, 2017. © 2015 Society of Plastics Engineers     

PET interleaving veils for improved fracture toughness of glass fibre/low‐styrene‐emission unsaturated polyester resin composites

The use of interleaved polyethylene terephthalate (PET) veils to increase the interlaminar fracture toughness of glass fiber‐reinforced, low‐styrene emission, unsaturated polyester resin composites, was investigated. PET, being chemically similar to the unsaturated polyester resin, was expected to exhibit good wetting and strong interaction with the matrix. Composite laminates were manufactured by hand lay‐up, with the veil content varying up to 7%. The effects of PET veils on the interlaminar shear strength, flexural strength, flexural modulus, glass transition temperature, damping parameters, and Mode‐I interlaminar fracture toughness of the composite were studied. The veils were found to enhance most of these properties, with only minor negative effects on flexural stiffness and T_g. The PET/resin bonding did indeed prove to be strong, but the enhancement of fracture toughness was not as much as expected, because of the weaker glass/resin interface providing an alternative crack propagation path. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 42877.     

The influence of matrix chemical structure on the mode I and II interlaminar fracture toughness of glass‐fiber/epoxy composites

The present paper is concerned with Mode I and Mode II delamination tests performed on three different glass fiber reinforced epoxy composites, chosen to obtain different final structures. The effect of crosshead speed on the fracture resistance of the composites was also analyzed. It was found that Mode I propagation values (G_IC) increase as the crosshead speed decreases, probably because of the increase of brittleness in the studied range. An Arrhenius type relation between G_IC and the glass transition temperature of the epoxy resin/amine system (T_g) was found. Mode II initiation values (G_IIC^init) and apparent shear strength (S_H) were found to increase with the decrease of T_g. The relation between matrix toughness and composite interlaminar fracture toughness was also considered. Finally, the G_IC propagation values were compared to the data available in literature for similar materials.     

The influence of cooling rate on the fracture properties of a glass reiforced/nylon fiber-metal laminate

The effect of varying cooling rate on the microstructure and resulting mechanical properties of a novel fiber-metal laminate (FML) based on a glass fiber-reinforced nylon composite has been investigated. Polished thin sections removed from plain glass fiber/nylon composites and their corresponding fiber-metal laminates indicated that the prevailing microstructure was strongly dependent on the rate of cooling from the melt. Mode I and Mode II interlaminar fracture tests on the plain glass fiber reinforced nylon laminates indicated that the values of G_Ic and G_IIc averaged approximately 1100 J/m² and 3700 J/m² respectively at all cooling rates. The degree of adhesion between the aluminum alloy and composite substrates was investigated using the single cantilever beam geometry. Here, the measured values of G_c were similar in magnitude to the Mode I interlaminar fracture energy of the composite, tending to increase slightly with increasing cooling rate. The tensile and flexural fracture properties of the plain composites and the fiber metal laminates were found to increase by between 10% and 20% as the cooling rate was increased by two orders of magnitude. This effect was attributed to over-aging of the aluminum alloy plies at elevated temperature during cooling. Finally, fiber metal laminates based on glass fiber/nylon composites were shown to exhibit an excellent resistance to low velocity impact loading. Damage, in the form of delamination, fiber fracture, matrix cracking in the composite plies, and plastic deformation and fracture in the aluminum layer, was observed under localized impact loading. Here, the fast-cooled fiber metal laminates offered superior post-impact mechanical properties at low and intermediate impact energies, yet very similar results under high impact energies.     

Impact fracture behavior of continuous glass fiber-reinforced nylon 6

The impact fracture toughness of nylon 6/continuous glass fiber composites at four levels of fiber content has been studied. The composites were produced by anionically polymerizing caprolactam within a glass mat using a vacuum injection technique. Application of linear elastic fracture mechanics to characterize the impact fracture toughness of the composites, using an energy approach (G_IC), has been found to be applicable provided that a correction is made for the size of the damage zone. The concept of J_c, fracture energy per unit ligament area, has also been applied to the composites and agreement between G_IC and J_c has been found to be reasonably satisfactory. The ratio of crack propagation energy to the total energy absorbed (ductility index) has also been calculated. The ductility index was found to be close to one for the composites, indicating that additional energy is involved in propagating the fracturing cracks probably due to fiber debonding and/or crack blunting and fiber pullout. Fractographic examination of the impact fracture surface confirmed the presence of these features.     

Mechanical Behavior and Fracture Toughness of Poly(L‐lactic acid)‐Natural Fiber Composites Modified with Hyperbranched Polymers

Summary: The use of hyperbranched polymers (HBP) with hydroxy functionality as modifiers for poly(L ‐lactic acid) (PLLA)‐flax fiber composites is presented. HBP concentrations were varied from 0 to 50% v/v and the static and dynamic tensile properties were investigated along with interlaminar fracture toughness. Upon addition of HBP, the tensile modulus and dynamic storage modulus (E′) both diminished, although a greater decline was noticed in the static modulus. The elongation of the composites with HBP showed a pronounced increase as large as 314% at 50% v/v HBP. The loss factor (tan δ) indicated a lowering of the glass transition temperature (T_g) due to a change in crystal morphology from large, mixed perfection spherulites to finer, smaller spherulites. The change in T_g could have also resulted from some of the HBP being miscible in the amorphous phase, which caused a plasticizing effect of the PLLA. The interlaminar fracture toughness measured as the critical strain energy release rate (G_IC) was significantly influenced by HBP. At 10% v/v HBP, G_IC was at least double that of the unmodified composite and a rise as great as 250% was achieved with 50% v/v. The main factor contributing to high fracture toughness in this study was better wetting of the fibers by the matrix when the HBP was present. With improved ductility of the matrix, it caused ductile tearing along the fiber‐matrix interface during crack propagation.

ESEM photograph of propagation region of the interlaminar fracture toughness specimens with 30% v/v of HBP.     

Synthesis and characterization of fluorinated poly(etherimide)s toughening for carbon fiber‐reinforced epoxy composites

Novel‐fluorinated poly(etherimide)s (FPEIs) with controlled molecular weights were synthesized and characterized, which were used to toughen epoxy resins (EP/FPEI) and carbon fiber‐reinforced epoxy composites (CF/EP/FPEI). Experimental results indicated that the FPEIs possessed outstanding solubility, thermal, and mechanical properties. The thermally cured EP/FPEI resin showed obviously improved toughness with impact strength of 21.1 kJ/m² and elongation at break of 4.6%, respectively. The EP/FPEI resin also showed outstanding mechanical strength with tensile strength of 91.5 MPa and flexural strength of 141.5 MPa, respectively. The mechanical moduli and thermal property of epoxy resins were not affected by blending with FPEIs. Furthermore, CF/EP/FPEI composite exhibited significantly improved toughness with Mode I interlaminar fracture toughness (G_IC) of 899.4 J/m² and Mode II interlaminar fracture toughness (G_IIC) of 1017.8 J/m², respectively. Flexural properties and interlaminar shear strength of the composite were slightly increased after toughening. POLYM. COMPOS., 2010. © 2009 Society of Plastics Engineers     

Enhanced fracture toughness of nanostructured carbon‐fiber reinforced poly(urethane‐isocyanurate) composites at low concentrations

Carbon fiber reinforced poly(urethane‐isocyanurate)‐nanosilica composites CF‐(PUI‐NS) were manufactured by means of the vacuum‐assisted resin transfer moulding technique (VARTM) at very low NS concentrations (0–4 wt%). The high strain to failure of the PUI matrix (>7%) affected tensile tests by CF reorientation. Both the tensile strength and strain to failure were highly dependent on its kinematics. CF(PUI‐NS) caused an increase of the static toughness with a maximum improvement of tensile strain to failure and modulus of +28.8% and +39% at 1 wt% and 2 wt% of NS, respectively. The interlaminar shear strength (G_IC) of the composites showed both a deterioration of ?12.9% and an improvement of +9.9% for NS concentrations of 1 wt% and 4 wt%, respectively. Regardless of the G_IC value, all of the composites prepared with NS presented secondary maxima of the force versus displacement plots, indicating a substantial arrest of the crack propagation velocity after delamination started. Fractographic analysis revealed several features, such as fiber pull‐out, bridging as well as river patterns whereas the composites prepared with NS behaved in a more ductile fashion due to the presence of river patterns and a reduced fiber pull‐out. POLYM. ENG. SCI., 58:1241–1250, 2018. © 2017 Society of Plastics Engineers     

Effect of graphene oxide on the interlaminar fracture toughness of carbon fiber/epoxy composites

Graphene oxide (GO) nanoparticles were introduced in the interlaminar region of carbon fiber–epoxy composites by dispersing it in a thermoplastic polymer carrier such as polyvinylpyrrolidone (PVP). Mode‐I fracture toughness (G_IC) was investigated using double cantilever beam testing to evaluate the effect of the GO on the delamination behavior of the composite. The GO content was varied from 0% to 7% by weight as a function of the PVP content. Improvement of ～100% in the Mode I fracture toughness (G_IC) was observed compared to composites with no GO. The optimum amount of nanoparticles for improving the interlaminar fracture toughness was found to be ～0.007% by weight of the composite. The increase in the value of flexural strength value was also observed. Scanning electron microscopy of fracture surfaces, X‐ray diffraction, and transmission electron microscopy, and reflectance Fourier transform infrared spectra, as well as Raman spectroscopy results, are presented to support the conclusions. POLYM. ENG. SCI., 59:1199–1208 2019. © 2019 Society of Plastics Engineers     

Enhancement of interlaminar fracture toughness of carbon fiber–epoxy composites using polyamide‐6,6 electrospun nanofibers

In this study, carbon fiber–epoxy composites are interleaved with electrospun polyamide‐6,6 (PA 66) nanofibers to improve their Mode‐I fracture toughness. These nanofibers are directly deposited onto carbon fabrics before composite manufacturing via vacuum infusion. Three‐point bending, tensile, compression, interlaminar shear strength, Charpy impact, and double cantilever beam tests are performed on the reference and PA 66 interleaved specimens to evaluate the effects of PA 66 nanofibers on the mechanical properties of composites. To investigate the effect of nanofiber areal weight density (AWD), nanointerlayers with various AWD are prepared by changing the electrospinning duration. It is found that the electrospun PA 66 nanofibers are very effective in improving Mode‐I toughness and impact resistance, compressive strength, flexural modulus, and strength of the composites. However, these nanofibers cause a decrease in the tensile strength of the composites. The glass‐transition temperature of the composites is not affected by the addition of PA 66 nanofibers. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45244.     

Effect of deposited carbon nanotubes on interlaminar properties of carbon fiber‐reinforced epoxy composites using a developed spraying processing

Carbon fiber‐reinforced epoxy composites, with incorporated carboxylic multiwall carbon nanotubes (CNTs), were prepared using vacuum‐assisted resin infusion (VARI) molding, and the in‐plane and out‐of‐plane properties, including mode‐I (G_Ic) and mode‐II (G_IIc) interlaminar fracture toughness, interlaminar shear strength (ILSS), tensile, and flexural properties were measured. A novel spraying technique, which sprays a kind of epoxy resin E20 with high viscosity after spraying the CNTs, was adopted to deposit the CNTs on the surface of carbon fiber fabric. The E20 was used to anchor CNTs on the fabric surface, avoiding that the deposited CNTs were removed by the infusing resin during VARI process. The spraying processing, including spraying amount and spraying sequence, was optimized based on the distribution of CNTs on the fibers. After that, three composite specimen groups were fabricated using different carbon fiber fabrics, including as‐received, CNT‐deposited with E20, and CNT‐deposited without E20. The effects of CNTs on the processing quality and mechanical properties of carbon fiber‐reinforced polymer composites were studied. The experimental results show that all studied laminates have uniform thickness with designed values and no obvious defects form inside the laminates. Compared with the composite without CNTs, depositing CNTs with E20 increases by 24% in the average propagation G_Ic, by 11% in the propagation G_IIc and by 12% in the ILSS, while it preserves the in‐plane mechanical properties, However, depositing CNTs without E20 reduces interlaminar fracture toughness. These phenomena are attributed to the differences in the distribution of CNTs and the fiber/matrix interfacial bonding for different spraying processing. POLYM. COMPOS., 2013. © 2012 Society of Plastics Engineers     

Simultaneous effects of silanized coal fly ash and nano/micro glass fiber on fracture toughness and mechanical properties of carbon fiber‐reinforced vinyl ester resin composites

In this study, the simultaneous effects of both silanized coal fly ash (S‐CFA) and nano/micro glass fiber (nGF) on fracture toughness and mechanical properties of vinyl ester (VE) resin filled with carbon fiber‐based composite materials were investigated. The CFA was treated with (3‐trimethoxysilyl) propyl methacrylate to introduce the methacryloxy groups into the surface of CFA, and was confirmed by using FTIR technique. The nGF and S‐CFA with different weight ratios were well mixed with VE resin by using of high‐speed mechanical stirrer, and ultrasonic technique before using as matrices for fabrication of carbon fiber‐based composite materials via sheet molding compound (SMC) method and hot curing processing. Many characteristics of both cured VE resin composites and carbon fiber‐based composite were examined such as mechanical properties, fracture toughness, and morphology. The results showed that by adding of both 0.1 wt% nGF and 1 wt% S‐CFA into VE resin the tensile strength, tensile modulus, flexural strength, K_IC, impact strength as well as the Mode I interlaminar fracture toughness (G_IC) of VE composites and carbon fiber based composites get optimum values and increased about 61.39%; 39.83%; 36.21%; 103.1%; 81.79%; 48.61%, respectively when compared with pristine materials. POLYM. ENG. SCI., 59:584–591, 2019. © 2018 Society of Plastics Engineers     

Fracture toughness of particle filled fiber reinforced polyester composites

Fracture behavior of polyester composite systems, polyester mortar and glass fiber reinforced polyester mortar, was investigated in mode I fracture using single edge notched beams with varying notch depth. The beams were loaded in four-point bending. Influence of polymer content on the flexural and fracture behavior of polyester composites at room temperature was studied using a uniform Ottawa 20–30 sand. The polymer content was varied between 10 and 18% of the total weight of the composite. The flexural strength of the polyester mortar systems increase with increase in polymer content while the flexural modulus goes through a maximum. The critical stress intensity factor (K_IC) for the optimum polyester mortar (14%) was determined by two methods including a method based on crack mouth opening displacement. The K_IC for polyester mortar is linearly related to the flexural strength. Polyester mortar (18%) reinforced with 4% glass fibers was also investigated, and crack growth resistance curve (K_R) was developed with crack extension (Δa). A model has been proposed to represent the fracture toughness with change in crack length, K_R - Δa relationship, of fiber reinforced polyester composite.     

Effect of cooling rate and crack propagation direction on the mode 1 interlaminar fracture toughness of biaxial noncrimp warp‐knitted fabric composites made of glass/PP commingled yarn

The mode 1 interlaminar fracture toughness of biaxial (±45°) noncrimp warp‐knitted fabric composites made of glass/PP commingled yarn was investigated. The crack propagation along the warp and weft directions, respectively, was considered for the composites cooled at two different rates during laminate molding. The interlaminar fracture toughness was characterized by determining the critical strain energy release rate (G_IC) of initiation and propagation measured from the double cantilever beam tests. In the case of a slow cooling rate (1°C/min), most specimens possess pure interlaminar crack propagation and direction‐independence characteristics. Nevertheless, the high‐cooled (10°C/min) specimens fractured in both directions suffer extensive intraply damage (crack branching, debonding, and bridging of 45°‐oriented interfacial yarns) and knit thread breakage, leading to G_IC of propagation two times higher than that of the slow‐cooled specimens, and the clear difference in the G_IC values of initiation between the two directions may be due to the contribution of the knit thread breakage to the fracture energy. POLYM. COMPOS., 2008 © 2007 Society of Plastics Engineers     

Study on the effect of woven sisal fiber mat on mechanical and viscoelastic properties of petroleum based epoxy and bioresin modified toughened epoxy network

Sisal fiber reinforced biocomposites are developed using both unmodified petrol based epoxy and bioresin modified epoxy as base matrix. Two bioresins, epoxidized soybean oil and epoxy methyl soyate (EMS) are used to modify the epoxy matrix for effective toughening and subsequently two layers of sisal fiber mat are incorporated to improve the mechanical and thermomechanical properties. Higher strength and modulus of the EMS modified epoxy composites reveals good interfacial bonding of matrix with the fibers. Fracture toughness parameters K_IC and G_IC are determined and found to be enhanced significantly. Notched impact strength is found to be higher for unmodified epoxy composite, whereas elongation at break is found to be much higher for modified epoxy blend. Dynamic mechanical analysis shows an improvement in the storage modulus for bioresin toughened composites on the account stiffness imparted by fibers. Loss modulus is found to be higher for EMS modified epoxy composite because of strong fiber–matrix interfacial bonding. Loss tangent curves show a strong influence of bioresin on damping behavior of epoxy composite. Strong fiber–matrix interface is found in modified epoxy composite by scanning electron microscopic analysis. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42699.     

Effect of cloisite 30B clay and sisal fiber on dynamic mechanical and fracture behavior of unsaturated polyester toughened epoxy network

This study examined the dynamic mechanical properties of sisal fiber reinforced unsaturated polyester (UP) toughened epoxy nanocomposites. The chemical structures changes in Epoxy, UP and UP toughened epoxy (Epoxy/UP) systems were characterized by Proton Nuclear magnetic resonance (¹HNMR) spectroscopy. The morphological alterations of the nanocomposites were analyzed by X‐ray diffraction (XRD) and transmission electron microscopy (TEM). The untreated, chemically treated fibers, nanoclays, and the fiber reinforced Epoxy/UP nanocomposites were confirmed by FTIR spectrometer. The obtained mechanical results showed that alkali‐silane treated fibers improve the tensile strength (96%) and flexural strength (60%) of the Epoxy/UP nanocomposite than that of Epoxy/UP blend due to the strong interfacial bonding between the sisal fiber and matrix. The fracture toughness (K_IC) and fracture energy (G_IC) of treated sisal fiber reinforced DGEBA/UP/C30B nanocomposites found to be higher than that of untreated sisal fiber nanocomposites. The dynamic mechanical analysis (DMA) reveals that the fiber reinforced Epoxy/UP nanocomposites contains 30 wt% treated fiber and 1 wt% nanoclays, exhibits the highest storage modulus and better glass transition temperature (T_g) among the other kind of systems. The surface morphology of the fibers, fractured surface of the resins and composites were confirmed by scanning electron microscope (SEM). POLYM. COMPOS., 37:2832–2846, 2016. © 2015 Society of Plastics Engineers     

Glass fiber‐reinforced polyamide composites toughened with ABS and EPR‐g‐MA

A series of glass fiber‐reinforced rubber‐toughened nylon 6 composites was prepared. The mechanical properties and morphology of the composites toughened with ABS were investigated and compared with composites toughened with EPR‐g‐MA. A study of the mechanical properties showed that the balance of the impact strength and stiffness for both types of systems can be significantly improved by proper incorporation of glass fibers into toughened nylon 6. The differences between these two types of rubber‐toughened composites are significant at a high rubber content. However, the ductility of both composites toughened with rubber was significantly lower than that of blends without glass fiber. The relationships between rubber content, nylon 6 molecular weight, compatibilizer, processing, and mechanical properties are discussed. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 484–497, 2001     

Mode I interlaminar fracture behavior of 2D woven ceramic matrix composites

Interlaminar fracture properties of melt-infiltrated woven SiC/SiC ceramic matrix composites were investigated using traditional and wedge-loaded double cantilever beam methods. The two methods produced comparable G_IC results for some specimens. The difference in boundary conditions between the two methods appeared to influence the crack propagation path. The DCB method, having free-end boundary condition, allowed more interaction between the crack and the composite microstructure than the wedge method did. The effect of fiber tow layout sequence had an effect on the interlaminar properties. Higher toughness was observed for the orientation where crack propagation occurs between planes with more transverse tows. Jump-arrest phenomenon was found to have higher significance on the rising R-curve behavior than fiber bridging.     

The influence of interphase on nylon‐6/nano‐SiO2 composite materials obtained from in situ polymerization

Three commercially available silane, titanate and aluminate based coupling agents were used to pretreat nano‐SiO₂ for the preparation of nylon‐6/nano–SiO₂ composites via in situ polymerization. The interphases formed in different composite systems and their influence on material properties were investigated. Results indicated that the interfacial interactions differed between composite systems, whereas rigidity and toughness of composites were all improved by addition of pretreated silicas at an optimal content of 4.3 wt%. The presence of pretreated silicas did not have a distinct influence in the non‐isothermal crystallization behaviour of the nylon matrix. The composites containing pretreated silicas had slightly higher dynamic viscosities and superior storage moduli at high frequency, compared with neat nylon‐6. Copyright © 2003 Society of Chemical Industry     

Mode‐I intralaminar fracture toughness of silica particles‐filled glass woven fabric‐reinforced vinyl ester composites

Glass woven fabric reinforced vinyl ester (GV) composites filled with different weight proportions of silica particles were fabricated by hand lay up technique followed by oven curing. The plane strain Mode‐I Intralaminar fracture toughness, K_IC of the silica filled GV composites has been studied and the experimental results were compared with those of unfilled GV composites. The findings of the experiments showed that the fracture toughness has improved by the addition of silica particles up to 6 weight % with marginal increase of tensile properties. The silica filled and unfilled GV composites showed brittle fracture, with maximum toughness for 6 weight % silica particles. The morphology of fracture surfaces was examined by using SEM. Pulled and fractured fibers are observed on the fracture surface of GV composites evidencing fiber bridging but not in the silica filled GV composites. POLYM. COMPOS., 2012. © 2012 Society of Plastics Engineers