“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     

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 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     

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     

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     

Rubber modification of low temperature cure cyanate ester matrices and the performance in glass fabric composites

Low temperature cure cyanate ester resin systems were developed and modified with epoxy‐terminated butadiene acrylonitrile rubber (ETBN) and impregnated into woven glass fabric. Mode I and mode II interlaminar fracture toughness values of the cured laminates were evaluated as a function of rubber concentration. Mode I fracture toughness increased to almost twice that of the unmodified system, while mode II fracture toughness remained essentially unchanged. Composite samples were subjected to aging experiments in water and the absorption/desorption behavior was investigated as was the effect on thermal performance. The presence of rubber was found to reduce the rate of matrix deterioration but also caused a substantial increase in water uptake. It was found that although the addition of rubber to the matrices decreased the unconditioned (dry) T_g all specimens showed the same reduction in T_g, after equilibrium water absorption.     

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.     

Toughening of carbon fiber/thermoset composite by the morphology spectrum concept

Morphology spectrum toughening, a novel technique for toughening brittle carbon fiber/thermoset composites, was applied to a polyetherimide-modified carbon fiber/dicyanate composite. Composites were prepared by inserting a polyetherimide film between the prepregs of carbon fabric impregnated by dicyanate alone, and by controlling both the relative rates of dissolution of the polyetherimide and the polymerization of the dicyanate by adding zinc stearate catalyst. A composite with 13 wt% polyetherimide afforded a mode I interlaminar fracture toughness of 1.4 kJ/m², almost 1.8 times higher than that of a control composite containing same composition of polyetherimide. Two effects were observed. First, a thermoplastic continuous morphology formed in the center interply zone, where the concentration of polyetherimide was high, provided good fracture toughness. Second, a thermosetting continuous morphology formed in the fiber-rich zone, where the concentration of polyetherimide was low, maintained good interfacial shear strength between the fiber and the matrix.     

Crack-arrest study in mode II Delamination in composites

Procedures for measuring the crack initiation and arrest toughnesses in Mode II interlaminar fracture in composite materials were analyzed. Different techniques using flexural specimens were studied. The strain energy release rate, G, which is the energy available for crack propagation was calculated using simple beam theory. The calculation takes into account the transverse shear effect. Stable and unstable fractures are analyzed, and conditions required to measure the arrest toughness of interlaminar fracture are discussed. The methodology was applied to the measurement of fracture energy at the onset and arrest of delamination in glass/epoxy laminate.     

Sliding mode interlaminar fracture toughness of interply hybrid composite materials

To measure the sliding mode interlaminar fracture toughness of interply hybrid composites, end notched flexure (ENF) specimens with three different types of stacking sequence have been utilized. Finite element analysis is applied to separate the contribution from different modes on the strain energy release rate. In addition, the methods of beam theory, compliance, and compliance calibration to calculate the G_C values are compared. The effects of interface friction, crack length, and specimen width are also discussed. The results show that the crack growth in the three types of specimens is dominated by the sliding mode and the Mode II interlaminar fracture toughness can be approximated. The compliance method is not recommended for hybrid ENF specimens and the effects of interface friction can be neglected. To get rid of the edge effect, the specimen width must be carefully chosen, while the fracture toughness does increase with the initial crack length.     

Mode II interlaminar fracture behavior of carbon bead‐filled epoxy/glass fiber hybrid composite

To investigate the effect of including carbon beads on the mechanical properties of epoxy resin, the fracture toughness of carbon bead‐filled epoxy was earlier evaluated using a CT (compact tension) specimens and Mode I fracture toughness was observed. Based on those results, in this study, the Mode II interlaminar fracture toughness of carbon bead filled epoxy/glass fiber hybrid composites was evaluated using end notch flexure (ENF) specimens. The hybrid composites showed increased Mode II interlaminar fracture toughness. The optimal bead volume fraction was around 15%.     

Evaluation of interlaminar-toughened poly(etherlmide)-modified epoxy/carbon fiber composites

Amorphous poly(ether imide) has been used as interlaminar toughening particulate agent in laminated carbon fiber/epoxy composites. Mode I and Mode II delamination fracture toughness was characterized using the double cantilever beam (DCB) and end-notched flexure (ENF) specimens. The delamination surface was examined using a scanning electron microscopy (SEM) to investigate relationships between the morphology and properties. The results revealed that the PEI-modified composites exhibited a significantly increased fracture toughness, which increased with the PEI content. G_IC was improved from 165 to 540 J/m² (at 1 mm/min crosshead speed). G_IIC was improved more significantly from 290 to 1300 J/m². It is believed that these values could be further improved if the processing cycle were to be optimized.     

The effect of silicon carbide whiskers on the Mode I interlaminar fracture of carbon fiber composites

This paper reports on the Mode I interlaminar fracture toughness improvement of carbon fiber-epoxy composites as a result of incorporating SiC whiskers in the epoxy matrix. Five laminates of unidirectional carbon fiber-epoxy composites at different weight fractions of SiC whiskers were manufactured using hand layup vacuum bagging process. Optical and scanning electron microscopic analysis were conducted to give an insight into the fracture morphogoloy, failure mechanisms, and the energy dissipation mechanisms created by the presence of the whiskers in the composite. Experimental results showed that composites containing 5 wt% whiskers exhibited 67% increase in the crack initiation interlaminar fracture toughness G_IC, whereas it exhibited 55% increase in the maximum G_IC compared to pristine composite. The optical and SEM fractographs revealed a strong relation between the microstructure of the fractured surfaces and the energy release rate trend of the composites.     

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.     

Toughening and Healing of CFRPs by Diels–Alder-Based Nano-Modified Resin through Melt Electro-Writing Process Technique

In the current study, a novel approach in terms of the incorporation of self-healing agent (SHA) into unidirectional (UD) carbon fiber reinforced plastics (CFRPs) has been demonstrated. More precisely, Diels–Alder (DA) mechanism-based resin (Bis-maleimide type) containing or not four layered graphene nanoplatelets (GNPs) at the amount of 1 wt% was integrated locally in the mid-thickness area of CFRPs by melt electro-writing process (MEP). Based on that, CFRPs containing or not SHA were fabricated and further tested under Mode I interlaminar fracture toughness experiments. According to experimental results, modified CFRPs exhibited a considerable enhancement in the interlaminar fracture toughness properties (peak load (P_max) and fracture toughness energy I (G_IC) values). After Mode I interlaminar fracture toughness testing, the damaged samples followed the healing process and then were tested again under identical experimental conditions. The repeating of the tests revealed moderate healing efficiency (H.E.) since part of the interlaminar fracture toughness properties were restored. Furthermore, three-point bending (3PB) experiments were conducted, with the aim of assessing the effect of the incorporated SHA on the in-plane mechanical properties of the final CFRPs. Finally, optical microscopy (OM) examinations were performed to investigate the activated/involved damage mechanisms.     

Mode I interlaminar fracture toughness of Nylon 66 nanofibrilmat interleaved carbon/epoxy laminates

Carbon/epoxy laminates interleaved with laboratory scale electrospun Nylon 66 nanofibrilmat and spunbonded nonwoven mats were investigated. The effect of the nanoscale fibers on the fracture toughness of the composite under pure Mode I loading was evaluated. It was shown that the nanofibrilmat is responsible for a major interlaminar fracture toughness improvement, as high as 255–322%, compared to a noninterleaved carbon/epoxy reference laminate. We further studied the improvement mechanism of the electrospun nanofibrilmat compared to a commercial spunbonded nonwoven Nylon 66 mat. A combination of two interlayer fracture mechanisms responsible for the toughness improvement is suggested: the first is related to the high energy dissipated by bridged thermoplastic nanofibers and the second is attributed to the generation of a plastic zone near the crack tip. The interlaminar fracture mechanisms of both electrospun nanofibrilmat and the nonwoven mat interleaving was analyzed and discussed. POLYM. COMPOS., 2011. © 2011 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 fiber orientation on Mode I fracture toughness of CFRP

The effect of fiber orientations on fracture toughness of carbon fiber reinforced plastics (CFRP) in Mode I loading was investigated using double cantilever beam (DCB) specimens, based on mesoscopic mechanics. Mesoscopic interlaminar fracture toughness of 0//0 interphase of CFRP was evaluated with mesoscopic finite element models using experimental data. The fracture surface roughness was observed by confocal laser scanning microscopy. Then the mesoscopic interlaminar fracture toughness of CFRP was correlated with the fracture surface roughness. Additionally, the change of the Mode I macroscopic fracture toughness of CFRP was experimentally measured with changing the numbers of 0 and ±θ layers of DCB specimens. The correlation between the fracture toughness of 0//0 and θ//?θ interphases was discussed and a novel procedure was proposed to predict the macroscopic fracture toughness of θ//?θ interphase using finite element method (FEM). The fracture toughness of θ//?θ interphase analyzed by FEM was finally compared with the experimental results to verify the proposed prediction procedure. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Influence of cyanate content on the morphology and properties of epoxy resins with phenolphthalein poly(ether ketone)

Phenolphthalein poly(ether ketone) (PEK‐C) was blended with the diglycidyl ether of bisphenol A epoxy resin and bisphenol A dicyanate ester. The effect of cyanate content on cure behaviors, thermal and mechanical properties of PEK‐C/epoxy/cyanate mixtures was investigated. As results, the increase of cyanate content slightly hindered the cure reaction of the mixtures. Fourier transform infrared results indicated that the curing reaction of the cured mixtures was complete. When the cyanate ester content increased, the flexural properties and T_g values were enhanced, and the initial thermal decomposition temperature was reduced. A significant improvement in fracture toughness was obtained when the cyanate group in the mixtures was excessive. The fracture toughness can be well explained by SEM observations. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Numerical and experimental evaluation of the Mode III interlaminar fracture toughness of composite materials

The Crack Rail Shear (CRS) specimen is a proposed test method to characterize the interlaminar Mode III critical strain energy release rate (G_IIIc) of continuous fiber-reinforced composite materials. The specimen utilizes the two rail shear test fixture and contains embedded Kapton film between designated plies to provide a starter crack for subsequent fracture testing. Analytical expressions for specimen compliance and G_III are based upon Strength of Materials (SM) principles. The model identifies important material and geometric parameters and provides a simple data reduction scheme. A quasi-three-dimensional, linear elastic finite element stress analysis verifies the purity of the Mode III fracture state and identifies admissible crack lengths to be used in the experimental study. A fully three-dimensional linear elastic finite element analysis of the CRS is employed to investigate the influence of edge effects on the fracture state for the finite length sample. Results based upon a uniform crack extension indicate a small region of mixed mode behavior at traction free edges which decay to a pure Model III fracture state in the interior of the sample. Furthermore, the G_III distribution along the crack front decreases at the free edges from a maximum plateau region in the interior. The three-dimensional analysis allows edge effects to be minimized by selecting appropriate specimen lengths. Compliance and strain energy release rates are in good agreement with the SM model. An experimental program was performed to measure G_IIIc of two graphite epoxy systems. G_IIIc results for AS4/3501-6 were found to be 1.6 times the Mode II fracture toughness, while IM7/8551-7 exhibited equivalent Mode II and Mode III fracture toughnesses. Mode III fracture surfaces revealed microstructural deformations characteristic of Mode II fracture.