Properties of carbon fiber composite laminates fabricated by coresin film infusion process for different prepreg materials

A new cocured process called coresin film infusion (co‐RFI) process, which combines RFI process and prepreg/autoclave process, was introduced and four kinds of commercial carbon fiber prepreg material systems and a kind of resin film were applied to fabricate co‐RFI laminates. The compatibility between the resin film and the prepreg matrix and the application of co‐RFI process were investigated based on the resin flowability, glass transition temperature of cured resin, processing quality of laminate, and variation in resin modulus on cocured interphase region measured by nanoindentation. Furthermore, mode I (G_IC), mode II (G_IIC) delamination fracture toughness, and flexural strength and modulus were measured to evaluate the mechanical properties of cocured laminates with different prepreg materials. The experimental results show that thickness and fiber volume fraction of co‐RFI laminates with the four kinds of prepreg materials are similar to those of prepreg laminates and RFI laminate with acceptable differences. In addition, there are no obvious defects in co‐RFI laminates. Moreover, the reduced modulus of resin at cocured interface and glass transition temperature values of the mixed resin reflect good compatibility between prepreg matrix resin and RFI resin. The G_IC, G_IIC values, and flexural performances of cocured laminates lie between and even exceed those of prepreg laminates and RFI laminates, indicating no weakening effect in the cocured interface. Therefore, the co‐RFI process is believed to effectively fabricate composite with low cost and it can be applied using various prepreg systems. POLYM. COMPOS., 34:2008–2018, 2013. © 2013 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     

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.     

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     

“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     

Effects of stitch density and stitch thread thickness on mode II delamination properties of Vectran stitched composites

Abstract

Mode II delamination properties of Vectran stitched composites were investigated, and tabbed end notch flexural specimen testing was used to prevent premature failure. The effects of stitch density and stitch thread thickness were explored, and fibre compaction due to the stitching process was also verified. The results show that, in moderately stitched laminates (low stitch density), the improvement in G_IIC was negligible. Crack bridging by the stitch threads at the crack zone were mostly compensated for the effect of fibre compaction, which reduced the G_IIC values. Conversely, in densely stitched laminates (high stitch density), G_IIC values were improved significantly (2·4 times higher than those of unstitched laminates). The effects of stitch thread thickness appeared to be negligible in moderately stitched laminates. For densely stitched laminates, thicker stitch thread (500 denier) possessed G_IIC values that were 45·7% higher than thinner stitch thread (200 denier).     

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 matrix on the ballistic impact of aramid fabric composite laminates by armor piercing projectiles

Ballistic impact performance of aramid fiber fabric‐epoxy and aramid fiber fabric‐polypropylene (PP)‐based composite laminates has been studied against 7.62 mm armor piercing projectiles. Twaron® was used as aramid fiber fabric in the composites. Role of matrix on the damage pattern has been investigated by impacting the composites of different thickness with projectiles having different strike velocity (SV). Ballistic limit (BL) for each composite has been estimated through correlation of SV and residual velocity (RV) of the projectile by usual V₅₀ method. Ballistic limit was found to vary linearly with composite laminate thickness. Twaron®‐PP composites exhibited higher ballistic limit compared toequivalent thickness of Twaron®‐epoxy composites. Epoxy‐based composites exhibited localized damage mode compared to a global mode of failure in PP‐based composites. Scanning electron microscopy revealed that fibers in Twaron®‐epoxy composites failed largely by shear while tensile mode of failure was observed for Twaron®–PP composites. POLYM. COMPOS., 2012. © 2012 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.     

Comparison between the ENF and 4ENF fracture characterization tests to evaluate GIIC of bonded aluminium joints

ABSTRACT

Adhesively bonded joints have been increasingly used in structural applications over mechanical joints. Cohesive Zone Modelling (CZM) is the most widespread technique to predict the strength of these joints, and it uses the tensile fracture toughness (G_IC) and the shear fracture toughness (G_IIC). Different fracture characterization methods are available for shear loadings, among which the End-Notched Flexure (ENF) is undoubtedly the most popular. The 4-Point End-Notched Flexure (4ENF) is also available. This work consists of a detailed comparison between the ENF and 4ENF tests for the experimental estimation of G_IIC of bonded aluminium joints. Three adhesives were used: a strong and brittle (Araldite® AV138), a less strong but with intermediate ductility (Araldite® 2015) and a highly ductile (SikaForce®7752). Different data reduction methods were tested, and the comparison included the load-displacement (P-δ) curves, resistance curves (R-curves) and measured G_IIC. It was found that the ENF test presents a simpler setup and has a higher availability of reliable data reduction methods, one of these not requiring measuring the crack length (a) during its growth. For the 4ENF test, only one test method proved to be accurate, and the test geometry revealed to be highly affected by friction effects.     

Evaluation of Mode-II Fracture Energy of Adhesive Joints with Different Bond Thickness

A study on the mode-II edge-sliding fracture behaviour of aluminium-adhesive joints was carried out. Compact pure shear (CPS) adhesive joints of different bond thickness were produced using a rubber-modified epoxy resin as the adhesive. An analytical model was developed to calculate the stress distribution along the bond line of the joint. A crack-closure technique was used to evaluate the mode-II strain energy release rate. G _II, as a function of the adhesive bond thickness. The results indicated that for a given applied load, G _II increased gradually with the bond thickness. A finite element model (FEM) was also developed to evaluate the stress state along the bond line and the strain energy release rate of the CPS specimens. Consistent results were obtained between the theoretical model and finite element analysis. Scanning electron micrographs of the fracture surface illustrated a mainly interfacial fracture path between the adherends and the adhesive for all adhesive joint specimens. The critical fracture load increased very rapidly with bond thickness in the range 0.02 mm to 0.1 mm but remained constant thereafter. However, the mode-II critical fracture energy rose more gradually as the bond thickness was increased.     

Variable temperature cure polyetherimide epoxy-based prepreg systems

This work identifies the necessary attributes of variable temperature cure epoxybased prepreg systems as they relate to high performance prepreg systems capable for composite repair. Model polyetherimide epoxy blend resins were developed and hot-melt impregnated into woven carbon fabric and compared with a commercial prepreg system. It was found that when the PEI content was increased from 0 to 14 wt% in the base resin of the prepregs, the G_IC and G_IIC fracture toughness increased by over 70%. The fracture toughness was found to be similar when the model prepreg was cured at either 121°C or 177°C, a result of only a 9% difference in conversion and complete phase separation of the PEI at both cure temperatures. Void content in vacuum cured laminates were found to decrease as the PEI content was increased because of a large quantity of resin in the interstitial areas between the longitudinal and transverse tows. A comparison of the model and commercial prepreg system demonstrated many similarities and some significant differences. For example, the commercial prepreg had a 15% difference in conversion when cured at 121°C versus 177°C and very little PEI phase separation after both cure cycles. As a result, a significant difference in G_IIC for the commercial prepreg was observed for the two cure temperatures.     

Simultaneous enhancement of electrical conductivity and interlaminar shear strength of CF/EP composites through MWCNTs doped thermoplastic polyurethane film interleaves

Carbon fiber-reinforced epoxy (CF/EP) composites have been widely used in aerospace industry, while poor electrical conductivity and interlaminar shear fracture toughness could reduce their safety as structural components in use. In this work, we achieved simultaneous improvement in electrical conductivity and interlaminar shear strength through interleaved multi-walled carbon nanotubes (MWCNTs) doped thermoplastic polyurethane (TPU) conductive thin films (CTFs), which were prepared by a solution casting method. The experimental results showed that the electrical conductivity of the laminates increased by about 13 and 16 times in the transverse and thickness directions with only about 1 wt % MWCNTs content in the laminates. The end-notch flexure (ENF) tests showed that the mode II interlaminar fracture toughness (G_IIC) of composites with 10 wt % MWCNTs CTF interleaf shows a significant increase of about 106%. The enhancement mechanism was further explored through microscopic morphological observation. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47988.     

Mode-l Fracture Behaviour of Adhesive Joints. Part I. Relationship Between Fracture Energy and Bond Thickness

The fracture properties of adhesive joints of aluminium were investigated using a rubber-modified tough epoxy resin system (G_IC = 2.76 kJ/m²) as adhesive material. Compact tension (CT) adhesive joints were manufactured for a wide range of bond thickness t (from 0.05mm to 10mm) and fracture tests conducted under static load. Scanning electron microscopy (SEM) was used to examine the fracture surface morphology. A large deformation elastic- plastic finite element model was developed to evaluate the J-integral value for different bond thickness. The fracture energy, J_c , was found to be highly dependent on the bond thickness and was lower than that of the bulk adhesive. As the bond thickness was increased J_c also increased, though not monotonically, towards the fracture energy of the bulk adhesive. This result was caused by the complicated interactions between the stress and strain fields, plastic deformation of the adhesive around the crack tip, constraint from the adherends and the failure path. It was shown that values of J_c as a function of bond thickness correlated well with the variation of plastic zone height. Scanning electron micrographs from the fracture surfaces of the CT adhesive joints illustrated that the failure path was mainly cohesive through the centre-plane of the adhesive layer. Brittle fracture mechanisms were observed for thin bonds (0.04mm < t< 0.5 mm) but tough fracture mechanisms were identified for thick bonds (t > 1 mm).     

Theoretical Analysis for Calculation of the Through Thickness Effective Constants for Orthotropic Thick Filament Wound Tubes

This paper presents a method for determining the theoretical values for all the elastic constants needed for three-dimensional stresses of angle-ply laminates and filament wound tube from the properties of unidirectional fiber reinforced epoxy resin. The layers were arranged symmetrically about the mid-surface of the laminate. The stress in the longitudinal axis is assumed to be constant throughout the thickness of the laminate. The results for the effective elastic constants versus filament winding angle from θ = 0° to θ = 90° for glass/epoxy composite are presented. The results show that the through thickness elastic modulus E _z did not change significantly with the winding angle. The E _x , E _y , and G _xy vary significantly with the winding angle of the tube θ. Out of plane shear modulus G _xz and G _yz also did not vary significantly with the winding angle of the tube θ.     

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.     

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     

Fracture toughness of adhesive joints. III. Temperature and rate dependencies of mode II fracture toughness and adhesive shear strength

Adhesive strength shows temperature and rate dependencies that reflect viscoelastic properties of an adhesive. Similarly, a critical strain energy release rate is expected to show temperature and rate dependencies because deformation and fracture of the adhesive occur at the time of the measurement of the strain energy release rate, which is a kind of fracture mechanical parameter for adhesive joints. In this study, the critical strain energy release rate (G_IIC) of a plane-shear mode was measured over a wide range of temperatures and rates, and then a master curve was obtained by applying the temperature-rate superposition principle to the obtained data. The relation between G_IIC and adhesive shear strength was also investigated. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 63: 835–841, 1997.     

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.     

Influence of rheological properties of adhesive polymer on strain energy release rate of mode II and adhesive shear strength

It is well known that adhesive strength shows temperature and rate dependencies reflecting viscoelastic properties of an adhesive used. Specifically, a mechanical relaxation mechanism around the glass transition temperature (T_g) has a strong effect on the adhesive strength, which involves deformation of the adhesive layer. In addition, it is very interesting to know how viscoelastic properties of the adhesive affect the value of strain energy release rate since deformation and failure of the adhesive occur at the measurement of strain energy release rate for adhesive joints. In this study, adhesive tensile strength and strain energy release rate (G_IIC) in plain-shearing mode were measured under a constant experimental condition using adhesives consisting of two types of epoxy resins; the influence of viscoelastic properties on these two values was investigated, and we discuss the relationship between the adhesive shear strength and G_IIC. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 525–536, 1997