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     

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.     

Mode I and Mode II delamination resistance and mechanical properties of woven glass/epoxy–PU IPN composites

The effect of polyurethane on the mechanical properties and Mode I and Mode II interlaminar fracture toughness of glass/epoxy composites were studied. Polyurethanes (PU) synthesized using polyols and toluene diisocyanate were employed as modifier for epoxy resin by forming interpenetrating polymer network. The PU/Epoxy IPN was used as matrix material for GFRP. PU modified epoxy composite laminates having varying PU contents were prepared. The effect of PU content on the mechanical properties like interlaminar fracture toughness (Mode I, G_1c and Mode II, G_IIc), tensile strength, flexural strength, and Izod impact strength were studied. The morphological studies were conducted on the fractured surface of the composite specimen by scanning electron microscopy (SEM). Tensile strength, flexural strength, and impact strength of PU‐modified epoxy composite laminates were found to increase inline with interlaminar fracture toughness (G_1c and G_IIc) with increasing PU content to a certain limit and then it was found to decrease with increase in PU content. It was observed that toughening of epoxy with PU increases the Mode I and Mode II delamination toughness up to 17 and 120% higher than that of untoughened composite specimen, respectively. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers     

Effect of hybridization on the mode II fracture toughness properties of flax/vinyl ester composites

In this study, flax fiber reinforced and flax/basalt hybridized vinyl ester composites were produced and their interlaminar fracture toughness (mode II) behavior was investigated using the three‐point bend end‐notched flexural (3ENF) testing. From the results, the average of the maximum values for each group of specimen obtained for critical strain energy release rate G _IIC and stress intensity factor K _II for flax/vinyl ester specimens were 1,940 J/m² and 134 kPam^0.5. Similarly, G _IIC and K _II values recorded for hybridized specimens were 2,173 J/m² and 178 kPam^0.5, respectively. The results for the flax/basalt hybridized composites exhibited an improved fracture toughness behavior compared to flax/vinyl ester composites without hybridization. The cohesive zone modeling (CZM) was also used to predict the delamination crack propagation in mode‐II in laminated composite structures. After the experimental study, the 3ENF specimens were modeled and simulated using ANSYS. The CZM/FEA results were in reasonable agreement with the experimental results. POLYM. COMPOS., 38:1732–1740, 2017. © 2015 Society of Plastics Engineers     

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.     

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     

“Ex situ” concept for toughening the RTMable BMI matrix composites,Part I: Improving the interlaminar fracture toughness

Aerospace‐grade bismaleimide matrix composites was toughened based on a novel ex situ resin transfer molding (RTM) technique using a special manufactured ES? carbon fabrics. The toughening mechanism and toughening effect by the technique are studied using thermoplastic PAEK as toughener. Mode I fracture toughness (G_IC) of the composites toughened by ex situ RTM technique increased up to three times higher than that of the control system, and Mode II fracture toughness (G_IIC) increased two times higher as well. The composite without toughening was denoted as control system. The microstructure revealed that a reaction‐induced phase decomposition and inversion happened in the interlaminar region, which resulted in a particles morphology that showed the thermosetting particles were surrounded with the PAEK phase. The plastic deformation and rupture of the continuous PAEK phase are responsible to the fracture toughness improvement. And the influence of PAEK concentration on toughness improvement was also investigated. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Effect of physical aging on the toughness of carbon fiber-reinforced poly(ether ether ketone) and poly(phenylene sulfide) composites. I

The effect of physical aging on the penetration impact toughness and Mode I interlaminar fracture toughness of continuous carbon fiber (C.F.) reinforced poly(ether ether ketone) (PEEK) and poly(phenylene sulfide) (PPS) composites has been investigated by using an instrumented falling weight impact (IFWI) technique and a double cantilever beam (DCB) test. Composite materials studied are aged below their glass transition temperature (T_g) at various periods. Initiation force and energy of damage, failure propagation energy, impact energy and ductility index (D.I.) are reported. The Mode I critical value of strain energy release rate (G_IC) of the unidirectional carbon fiber-reinforced PEEK (APC-2) composites is obtained. Results show that aging has a significant effect on the toughness of both composite materials. Energy absorbed during impact decreases with the increase of aging temperature and period. The PEEK/C.F. composites exhibit a higher retention of impact toughness than that of the PPS/C.F. composites after aging; however, the PPS/C.F. composites show a much higher ductility index. The Mode I fracture mechanism of the APC-2 composite is a combination of stable and unstable failure and shows a “stick-slip” behavior. Owing to the formation of a relative rigid structure, the fracture toughness (G_IC) of APC-2 decreased with the increase of aging temperature and period.     

Delamination characteristics of multi‐directional carbon fiber/epoxy composites under high pressure

It was shown in a previous study that for unidirectional (0‐deg) graphite/epoxy composites, the fracture toughness under hydrostatic pressure increased 38% as hydrostatic pressure increased from 0.1 MPa to 200 MPa. This work investigates the compressive delamination behavior of multi‐directional graphite/epoxy laminated composites subjected to various hydrostatic pressures. Compressive delamination tests were performed under four hydrostatic pressure levels: 0.1, 100, 200, and 300 MPa Eighty‐eight‐ply dog‐bone type specimens with a single delamination at the center of the specimen were used. The stacking sequence applied was [0°/±45°/90°]_lls. The compliance and fracture load were determined from load‐displacement curves as a function of hydrostatic pressure. The results show that the compliance decreases with increasing pressure while fracture load increases with increasing pressure. The compressive delamination toughness, G_c, was determined from the compliance method as a function of applied hydrostatic pressure. The results also show that G_c is significantly affected by hydrostatic pressure and increases from 2.11 kJ/m² to 3.04 kJ/m2 (44% increase) as hydrostatic pressure increased from 0.1 MPa to 300 MPa. Visual examination of the fractured surface revealed that the increase of G_c is due to the suppression of micro‐cracks With increasing pressure. It was also found from SEM examination of delaminated surface that the G_c increase is due to more epoxy adhering to the fibers and more plastic deformation of epoxy material as applied hydrostatic pressure increases.     

The fracture toughness of natural fibre- and glass fibre-reinforced SMC

The fracture toughness properties, in terms of stress intensity factor K_Ic and strain energy release rate G_Ic, of hemp fibre mat-reinforced sheet moulding compound (H-SMC) are measured using the compact tension (CT) method and compared with those of glass fibre-reinforced SMC (G-SMC). Three material parameters were considered for composite optimisation: fibre volume fraction, CaCO₃ filler content and hemp fibre surface treatments using either alkaline, silane or a combination of these two treatments. The highest fracture toughness for H-SMC composites was obtained at a fibre loading of around 30?vol.-%, while it was also shown that the fracture toughness properties of H-SMC are sensitive to mineral filler content. Surface treatment of the hemp fibres using a combined alkaline-silane treatment resulted in a significant improvement in fracture toughness of H-SMC composites. Optimised H-SMC composites exhibited fracture toughness properties similar to those of G-SMC at fibre contents of 20?vol.-%, with K_Ic values of around 6?MPa.m^?1/2.     

Mechanical properties of high temperature cyanate ester/BMI blend composites

A new Schiff base functionalized dicyanate ester was synthesized and the monomer was characterized by FTIR, ¹H-NMR, ¹³C-NMR and elemental analysis techniques. This prepared dicyanate ester with catalyst was then blended with BMI resin at different ratios by solution technique. The composites were made by impregnating the fibers with the blend solution followed by curing at various time-temperature schedules. The mechanical properties of the blend composites were tested. The fiber volume fraction of the composites were found to be in the range 41 ± 3%. The mechanical properties such as tensile modulus (32–35 GPa), flexural modulus (56–59 GPa) and Mode I fracture toughness (G_IC = 104–136 J/m²) and impact response (1,121–1,218 J/m) were found to increase with increasing cyanate ester content in the Cy/BMI blends. From the DMA study it was observed that as the cyanate content increases from 3 to 9% in the blend the tan δ value increases from 0.112 to 0.126 and the storage modulus decreases from 24,750 to 22,870 MPa indicating that the crosslink density of the blends decreases. The SEM analysis shows the absence of phase separation. Moisture absorption and chemical resistance of the blend composites increase with increasing cyanate content. POLYM. COMPOS., 2009. © 2009 Society of Plastics Engineers     

Fracture toughness and mechanical properties of poly(n-pentyl-n-alkylsilanes)

The mechanical properties and fracture toughness of thin films of a series of poly(n-pentyl-n-alkylsilanes) were investigated. Poly(n-butyl-n-pentylsilane) is the strongest of these polymers with an elastic modulus of 2.96 × 10⁸ Pa and a fracture strain of 85% at 25°C. The hexagonal mesophases of these polymers generally show elastic moduli on the order of 10⁷ Pa and are often quite extensible. A J-integral analysis of the ductile tearing of thin films of poly(n-butyl-n-pentylsilane) and poly(n-propyl-n-pentylsilane) using an Instron tensile testing machine and specimens in the single edge notch (SEN) geometry yielded plane stress J_1c (critical value of J for fracture initiation) of 1745 J/m² and 205 J/m², respectively. Both values are significantly higher than the plane stress G_1c (critical energy release rate) value of 109 J/m² obtained for poly(di-n-hexylsilane) with a residual stress analysis using the same apparatus and testing procedure.     

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     

Bis[4‐(4‐maleimidephen‐oxy)phenyl]propane/N,N‐4,4′‐bismaleimidodiphenylmethyene blend modified with diallyl bisphenol A

In this article, 2,2′‐bis[4‐(4‐maleimidephen‐oxy)phenyl)]propane (BMPP) resin and N,N‐4,4′‐bismaleimidodiphenylmethyene (BDM) resin blends were modified by diallyl bisphenol A (DABPA). The effects of the mole concentration of BMPP on mechanical properties, fracture toughness, and heat resistance of the modified resins were investigated. Scanning electron microscopy was used to study the microstructure of the fractured modified resins. The introduction of BMPP resin improves the fracture toughness and impact strength of the cured resins, whose thermal stabilities are hardly affected. Dynamic mechanical analysis shows that the modified resins can maintain good mechanical properties at 270.0°C, and their glass transition temperatures (T_g) are above 280.0°C. When the mole ratio of BDM : BMPP is 2 : 1(Code 3), the cured resin performs excellent thermal stability and mechanical property. Its T_g is 298°C, and the Charpy impact strength is 20.46 KJ/m². The plane strain critical stress intensity factor (K_IC) is 1.21 MPa·m^0.5 and the plane strain critical strain energy release rate (G_IC) is 295.64 J/m². Compared with that of BDM/DABPA system, the K_IC and G_IC values of Code 3 are improved by 34.07% and 68.10%, respectively, which show that the modified resin presented good fracture toughness. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40395.     

Fracture behavior of coated/uncoated graphite-epoxy composites

The static delamination behavior of graphite/epoxy composite specimens subjected to mode I tensile opening (using UDCB

1 Uniform double cantilever beam.

specimens), and pure mode II shear loading (using ENF

2 End-notched flexural.

specimens) were studied. The graphite epoxy composites for the study were made from commercially treated fibers, with and without an electropolymerized interlayer. The mode I fracture energy (G_IC) was found to be significantly higher (more than 50 percent) for the coated fibers. However, this improvement was accompanied by a high reduction (more than 3 times) in the mode II fracture energy (G_IIC). This effect is apparently related to poor adhesion between the interlayer and the epoxy resin, which may be corrected by use of a “top layer” of appropriate composition to form chemical bonds between the phases. The fracture toughness (K_IC) of composites made with commercially treated fibers was also evaluated, using double side-notched specimens.     

Structural composites formed by reaction injection moulding Interlaminar fracture properties of glass fibre mat-copoly(urea/isocyanurate) resin composites

Abstract

Structural (SRIM) composites, comprising up to 40% by volume of random continuous glass fibres in a specially developed copoly(urea/isocyanurate) (PUrI) matrix, have been formed via reaction injection moulding (RIM). The two stage polymerisation process of the PUrI matrix provided low initial viscosity during mould filling followed by a 'snap cure' to give tough composite materials in <30 s. Characterisation by DMTA confirmed the two phase morphology of the rubber toughened glassy matrix. Mode I and mode II interlaminar fracture tests, carried out in accordance with the ESIS protocol, gave values of G _Ic and G _IIc in the ranges 1·4-2·8 kJ m^-2 and 3·3-5·0 kJ m^{- 2}, respectively, and were an order of magnitude greater than those determined for unidirectional carbon fibre-epoxy composites. The G _Ic values for the SRIM composites are a factor of 2-3 greater than that (0·8 kJ m^-2) for the unreinforced PUrI matrix and show significant variation due to extensive fibre bridging during crack propagation.     

Fracture of adhesive joints and laminated composites

The mechanisms of resin controlled failure in adhesive joints and composites (delamination and transverse cracking) are examined. An in-situ failure model based on the fracture mechanics principles is applied here to describe the failure processes involved. The model centers on the crack tip plastic zone developed in the thin resin layer between the fibers or the adherends. The plastic zone in the resin layer is heavily influenced by a dominant slow varying stress distribution, approximated to be r^?m/2 dependent with m ? 1 (r is the distance from the crack tip). The adhesive or composite fracture toughness G*_IC can then be expressed as a function of several resin properties of comparable importance: modulus E, yield stress σ_y, resin G_IC and residual stress. The relative significance of the resin properties on the adhesive or composite fracture is discussed. The effects of temperature, loading rate, and resin toughening on such failure as a result of the corresponding variations in resin properties are also addressed.     

Physical and mechanical properties of kenaf/flax hybrid composites

This research investigates the physical and mechanical properties of hybrid composites made of epoxy reinforced by kenaf and flax natural fibers to investigate the hybridization influences of the composites. Pure and hybrid composites were fabricated using bi-directional kenaf and flax fabrics at different stacking sequences utilizing the vacuum-assisted resin infusion method. The pure and hybrid composites' physical properties, such as density, fiber volume fraction (FVF), water absorption capacity, and dimensional stability, were measured. The tests of tensile, flexural, interlaminar shear and fracture toughness (Mode II) were examined to determine the mechanical properties. The results revealed that density remained unchanged for the hybrid compared to pure kenaf/epoxy composites. The tensile, flexural, and interlaminar shear performance of flax/epoxy composite is improved by an increment of kenaf FVF in hybrid composites. The stacking sequence significantly affected the mechanical properties of hybrid composites. The highest tensile strength (59.8 MPa) was obtained for FK2 (alternative sequence of flax and kenaf fibers). However, FK3 (flax fiber located on the outer surfaces) had the highest interlaminar shear strength (12.5 MPa) and fracture toughness (3302.3 J/m²) among all tested hybrid composites. The highest water resistance was achieved for FK5 with the lowest thickness swelling.     

Studies on the mechanical and mechanical interfacial properties of carbon–carbon composites impregnated with an oxidation inhibitor

In this work, the relationships between work of adhesion and fracture toughness parameters, such as work of fracture (W_f), the critical stress intensity factor (K_IC), and the specific fracture energy (G_IC), of carbon–carbon composites (C/C composites) were investigated. The impact properties of the composites were also studied in the context of differentiating between the initiation and propagation energies for failure behavior. Composites consisting of different contents of the oxidation inhibitor MoSi₂ displayed an increase of the work of adhesion between the fibers and the matrix, which improved both the fracture toughness and impact properties of the composites. The 12 wt% MoSi₂ composites exhibited the highest mechanical and mechanical interfacial properties. This was probably due to the improvement of the London dispersive component, W_A^L, of the work of adhesion, resulting in an increase in the interfacial adhesion force among the fibers, filler, and matrix in this system.     

Determination of initiation value of mode I interlaminar fracture toughness of unidirectional polymer composites

The use of interlaminar fracture tests to measure the delamination resistance of unidirectional composite laminates is now widespread. However, because of the frequent occurrence of fiber bridging and multiple cracking during the tests, it leads to artificially high values of delamination resistance, which will not represent the behavior of the laminates. Initiation fracture from the crack starter, on the other hand, does not involve bridging, and should be more representative of the delamination resistance of the composite laminates. Since there is some uncertainty involved in determining the initiation value of delamination resistance in mode I tests in the literature, a power law of the form G_IC= A · Δ a^b (where G_IC is mode I interlaminar fracture toughness and Δ a is delamination growth) is presented in this paper to determine initiation value of mode I interlaminar fracture toughness. It is found that initiation values of the mode I interlaminar fracture toughness. G_ICini, can be defined as the G_IC value at which 1 mm of delamination from the crack starter has occurred. Examples of initiation values determined by this method are given for both carbon fiber reinforced thermoplastic and thermosetting polymers.