Shear properties and load–deflection response of cross–ply glass–epoxy composite short–beams subjected to three‐point‐bending tests,and the effect of moisture absorption

The extensive use of composites in aerospace, chemical, marine, and structural applications leads to exposure to humidity and water immersion. Hence, there is a need to study the effect of moisture absorption on the mechanical properties of composite materials, especially the matrix dominated properties, such as the interlaminar shear strength (ILSS). The horizontal shear test with a short‐beam specimen in three‐point‐bending is used as a general method of evaluation for the shear properties in fiber‐reinforced composites because of its simplicity. In this work, the ILSS of cross‐ply glass‐epoxy resin composites is determined in seven different fiber directions with short‐beam three‐point‐bending tests, before and after moisture conditioning. It is found that moisture absorption reduces ILSS and stiffness of the examined composites whereas it leads to larger failure deflections. It is also found that the direction of fibers strongly affects the load–deflection response and the ILSS of the dry and conditioned specimens. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Effects of polyhedral oligomeric silsesquioxane coatings on the interface and impact properties of carbon‐fiber/polyarylacetylene composites

Two kinds of polyhedral oligomeric silsesquioxane (POSS) coatings were used for the modification of the interface in carbon fiber (CF) reinforced polyarylacetylene (PAA) matrix composites. The effects of the organic–inorganic hybrid POSS coatings on the properties of the composites were studied with short‐beam‐bending, microdebonding, and impact tests. The interlaminar shear strength and interfacial shear strength showed that the POSS coatings resulted in an interfacial property improvement for the CF/PAA composites in comparison with the untreated ones. The impact‐test results implied that the impact properties of the POSS‐coating‐treated composites were improved. The stiffness of the interface created by the POSS coatings was larger than that of the fiber and matrix in the CF/PAA composites according to the force‐modulation‐mode atomic force microscopy test results. The rigid POSS interlayer in the composites enhanced the interfacial mechanical properties with a simultaneous improvement of the impact properties; this was an interesting phenomenon in the composite‐interface modification. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102:5202–5211, 2006     

Polyester composites reinforced with noncrimp stitched carbon fabrics: Mechanical characterization of composites and investigation on the interaction between polyester and carbon fiber

The primary purpose of the study is to investigate the anisotropic behavior of different noncrimp stitched fabric (NCF) reinforced polyester composites. Carbon fiber composite laminates were manufactured by vacuum infusion of polyester resin into two commonly used advanced noncrimp stitched carbon fabric types, unidirectional and biaxial carbon fabric. The effects of geometric variables on composite structural integrity and strength were illustrated. Hence, tensile and three‐point bending flexural tests were conducted up to failure on specimens strengthened with different layouts of fibrous plies in NCF. In this article an important practical problem in fibrous composites, interlaminar shear strength as measured in short beam shear tests, is discussed. The fabric composites were tested in three directions: at 0°, 45°, and 90°. Extensive photomicrographs of multilayered composites resulting from a variety of uniaxial loading conditions were presented. It was observed that broken fibers recede within the matrix in composites with weak interfacial bond. Another aim of the present work was to investigate the interaction between carbon fiber and polyester matrix. The experiments, in conjunction with scanning electron photomicrographs of fractured surfaces of composites, were interpreted in an attempt to explain the instability of polyester‐resin–carbon‐fiber interfaces. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 4554–4564, 2006     

Concentration effect of γ‐glycidoxypropyltrimethoxysilane on the mechanical properties of glass fiber–epoxy composites

In this study, glass fibers were modified using γ‐glycidoxypropyltrimethoxysilane of different concentrations to improve the interfacial adhesion at interfaces between fibers and matrix. Effects of γ‐glycidoxypropyltrimethoxysilane on mechanical properties and fracture behavior of glass fiber/epoxy composites were investigated experimentally. Mechanical properties of the composites have been investigated by tensile tests, short beam tests, and flexural tests. The short‐beam method was used to measure the interlaminar shear strength (ILSS) of laminates. The tensile and flexural properties of composites were characterized by tensile and three‐point bending tests, respectively. The fracture surfaces of the composites were observed with a scanning electron microscope. On comparing the results obtained for the different concentrations of silane solution, it was found that the 0.5% GPS silane treatment provided the best mechanical properties. The ILSS value of heat‐cleaned glass fiber reinforced composite is enhanced by ∼59% as a result of the glass fiber treatment with 0.5% γ‐GPS. Also, an improvement of about 37% in tensile strength, about 78% in flexural strength of the composite with the 0.5% γ‐GPS treatment of glass fibers was observed. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

Comparison of mechanical properties of epoxy composites reinforced with stitched glass and carbon fabrics: Characterization of mechanical anisotropy in composites and investigation on the interaction between fiber and epoxy matrix

The primary purpose of the study is to evaluate and compare the mechanical properties of epoxy‐based composites having different fiber reinforcements. Glass and carbon fiber composite laminates were manufactured by vacuum infusion of epoxy resin into two commonly used noncrimp stitched fabric (NCF) types: unidirectional and biaxial fabrics. The effects of geometric variables on composite structural integrity and strength were illustrated. Hence, tensile and three‐point bending flexural tests were conducted up to failure on specimens strengthened with different layouts of fibrous plies in NCF. In this article, an important practical problem in fibrous composites, interlaminar shear strength as measured in short beam shear test, is discussed. The fabric composites were tested in three directions: at 0°, 45°, and 90°. In addition to the extensive efforts in elucidating the variation in the mechanical properties of noncrimp glass and carbon fabric reinforced laminates, the work presented here focuses, also, on the type of interactions that are established between fiber and epoxy matrix. The experiments, in conjunction with scanning electron photomicrographs of fractured surfaces of composites, were interpreted in an attempt to explain the failure mechanisms in the composite laminates broken in tension. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers     

Barrier properties for short‐fiber‐reinforced epoxy foams

The barrier properties of short‐fiber‐reinforced epoxy foam are characterized and compared with unreinforced epoxy foam in terms of moisture absorption, flammability properties, and impact properties. Compression and shear properties are also included to place in perspective the mechanical behavior of these materials. Compared with conventional epoxy foam, foam reinforced with aramid fibers exhibits higher moisture absorption and lower diffusion, while glass‐fiber‐reinforced foam is significantly stiffer and stronger. In addition, the polymeric foam composites studied present superior fire‐resistance compared with conventional epoxy foam systems. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 3266–3272, 2006     

Experimental characterization of traditional composites manufactured by vacuum‐assisted resin‐transfer molding

The primary purpose of this study is to investigate the anisotropic behavior of different glass‐fabric‐reinforced polyester composites. Two commonly used types of traditional glass fabrics, woven roving fabric and chopped strand mat, have been used. Composite laminates have been manufactured by the vacuum infusion of polyester resin into the fabrics. The effects of geometric variables on the composite structural integrity and strength are illustrated. Hence, tensile and three‐point‐bending flexural tests have been conducted at different off‐axial angles (0, 45, and 90°) with respect to the longitudinal direction. In this study, an important practical problem with fibrous composites, the interlaminar shear strength as measured in short‐beam shear tests, is discussed. The most significant result deduced from this investigation is the strong correlation between the changes in the interlaminar shear strength values and fiber orientation angle in the case of woven fabric laminates. Extensive photographs of fractured tensile specimens resulting from a variety of uniaxial loading conditions are presented. Another aim of this work is to investigate the interaction between the glass fiber and polyester matrix. The experiments, in conjunction with scanning electron photomicrographs of fractured surfaces of composites, are interpreted in an attempt to explain the interaction between the glass fiber and polyester. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Influence of two carbon plies on adhesion of unidirectional flax‐fibers reinforced epoxy composites

In this work, we undertook a comparative study of the dynamic dielectric analysis of two unidirectional epoxy composites: flax‐fiber‐reinforced epoxy and flax/carbon‐fiber‐reinforced epoxy (FCFRE). In both composites, three relaxation processes were identified. The first one is the water dipoles polarization imputed to the presence of polar water molecules in flax fiber. The second relaxation process associated with conductivity occurs as a result of the carriers charges diffusion noted for high temperature above glass transition and low frequencies. As for the third dielectric relaxation associated with the interfacial polarization effect is attributable to the accumulation of charges at the fibers/matrix interface. The presence of two carbon plies in the reinforcement gives rise to two interfacial polarization effects in the FCFRE composite. The analysis of the Maxwell–Wagner–Sillars and the water dipoles polarizations using the Havriliak–Negami model revealed that the presence of two plies of carbon can locally decrease the adhesion of flax fibers in the matrix. This analysis was supported by the thermal properties using a differential scanning calorimety and the mechanical properties using a short beam shear test. POLYM. COMPOS., 241–253, 2016. © 2014 Society of Plastics Engineers     

Bending properties of the multilayer‐connected biaxial weft knitted fabrics‐reinforced composites made with carbon fibers

This article presents an experimental study of bending properties of multilayer‐connected biaxial weft knitted (MBWK) fabrics‐reinforced composites made with carbon fibers. Three types of composites are used in bending test, which are three‐layer‐connected biaxial weft knitted fabric‐reinforced composite, four‐layer‐connected biaxial weft knitted fabric‐reinforced composite and five‐layer‐connected biaxial weft knitted fabric‐reinforced composite. Two‐way ANOVA analyzing method was used to deal with whether the carbon fiber volume fraction and the cutting direction have significant effect on the bending strength of the MBWK fabrics‐reinforced composites. Failure analysis is also available by means of samples debris examination to identify the failure mode. POLYM. COMPOS., 36:2291–2302, 2015. © 2014 Society of Plastics Engineers     

Experimental characterization and morphology investigation of composites based on high‐density and low‐density polyethylene reinforced with non‐crimp‐stitched glass fabrics

In this study, the effects of matrix material on mechanical properties were investigated in glass fiber reinforced high‐density and low‐density polyethylene composites. Also, in order to compare the fiber configuration effect on anisotropic behavior, unidirectional and biaxial glass fabrics were used as reinforcement material. Composite laminates were manufactured via the compression molding technique. Tensile and three‐point bending flexural tests were conducted up to failure on specimens cut out in different directions. Extensive fracture photomicrographs were presented for observing the failure modes (e.g. delamination) of the composites resulting from a variety of loading conditions. In addition, Scanning electron micrographs of postfractured surfaces of composites were interpreted in an attempt to explain the failure mechanisms (adhesive or cohesive failure) of the composites. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers     

Study on long fiber–reinforced thermoplastic composites prepared by in situ solid‐state polycondensation

Long glass fiber–reinforced thermoplastic composites were prepared by a new process, in situ solid‐state polycondensation (INSITU SSP). In this process reinforcing continuous fibers were impregnated by the oligomer of PET melt, and then the impregnated continuous fibers were cut to a desired length (designated prepreg); finally, the prepreg was in situ polymerized in the solid state to form the high molecular weight matrix. SEM, FTIR spectra, short‐beam shear stress test, flexural strength test, impact strength test, and the intrinsic viscosity measurement were used to investigate the wetting and interfacial adhesion, the mechanical properties of the composite, and the molecular weight of matrix resin in the composite. The results showed that the molecular weight of PET in the matrix resin and mechanical properties could be adjusted by controlling the SSP time and that the high level of interfacial adhesion between reinforcing fibers and matrix resin could be achieved by this novel INSITU SSP process, which are attributed to the good wetting of reinforcing fibers with low molecular weight oligomer melt as the impregnation fluid, the in situ formation of chemical grafting of oligomer chains onto the reinforcing fiber surface, and the in situ formation of the high molecular weight PET chains in the interphase regions. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 91:3959–3965, 2004     

Inspection of geometry influence and fiber orientation to characteristic value for short fiber reinforced ceramic matrix composite under bending load

For use of short fiber reinforced ceramics the knowledge of the influence of coupon geometry on the failure mode and the determined resistance in bending tests is necessary. In contrast to the continuous fiber reinforced ceramic matrix composites (CMC), for short fiber reinforced CMC there are only few studies and no standard on consideration of size effects. In the present work, the influence of coupon geometry and test conditions on the average value and distribution of flexural strength has been investigated. Two short fiber CMCs with different fiber length were examined under four point bending load. Moreover, the relationship between fiber orientation in the loaded area, failure location and measured flexural strength was investigated through high resolution X-ray computer tomography (μCT) and SEM. The presented outputs will be proposed to a future standard for bending test of short fiber reinforced CMC materials with different fiber length.     

Rheological analysis of vapor‐grown carbon nanofiber‐reinforced polyethylene composites

The melt rheological analysis of high‐density polyethylene reinforced with vapor‐grown carbon nanofibers (VGCNFs) was performed on an oscillatory rheometer. The influence of frequency, temperature, and nanofiber concentration (up to 30 wt %) on the rheological properties of composites was investigated. Specifically, the viscosity increase is accompanied by an increase in the elastic melt properties, represented by the storage modulus G′, which is much higher than the increase in the loss modulus G″. The composites and pure PE exhibit a typical shear thinning behavior as complex viscosity decreases rapidly with the increase of shearing frequency. The shear thinning behavior is much more pronounced for the composites with high fiber concentration. The rheological threshold value for this system was found to be around 10 wt % of VGCNF. The damping factor was reduced significantly by the inclusion of nanofibers into the matrix. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 155–162, 2004     

Self‐healing and interfacially toughened carbon fibre‐epoxy composites based on electrospun core–shell nanofibres

Dual components of a self‐healing epoxy system comprising a low viscosity epoxy resin, along with its amine based curing agent, were separately encapsulated in a polyacrylonitrile shell via coaxial electrospinning. These nanofiber layers were then incorporated between sheets of carbon fiber fabric during the wet layup process followed by vacuum‐assisted resin transfer molding to fabricate self‐healing carbon fiber composites. Mechanical analysis of the nanofiber toughened composites demonstrated an 11% improvement in tensile strength, 19% increase in short beam shear strength, 14% greater flexural strength, and a 4% gain in impact energy absorption compared to the control composite without nanofibers. Three point bending tests affirmed the spontaneous, room temperature healing characteristics of the nanofiber containing composites, with a 96% recovery in flexural strength observed 24 h after the initial bending fracture, and a 102% recovery recorded 24 h after the successive bending fracture. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44956.     

Experimental study on the mechanical behavior and failure mechanism of 3d MWK carbon/epoxy composites under quasi‐static loading

Quasi‐static tensile, out‐of compression, in‐plane compression, three‐point‐bending and shear tests were conducted to reveal the mechanical behavior and failure mechanisms of three‐dimensional (3D) multiaxial warp‐knitted (MWK) carbon/epoxy composites. The characterization of the failure process and deformation analysis is supported by high‐speed camera system and Digital Image Correlation. The results show that tensile, bending, out‐of‐plane compression, in‐plane compression stress–strain response exhibit obvious linear elastic feature and brittle fracture characteristics, whereas the shear response exhibits a distinct nonlinear behavior and gradual damage process. Meanwhile, 3D MWK carbon/epoxy composites have good mechanical properties, which can be widely used in the fields of engineering. In addition, the failure for tension behaves as interlayer delaminating, 90/+45/−45° interface debonding and tensile breakage of 0° fibers; the damage for out‐of‐plane compression is mainly interlaminar shear dislocation together with local buckling and shear fracture of fibers; the failure pattern for in‐plane compression is 90° fiber separating along fiber/matrix interface as well as 0/+45/−45° fiber shear fracture in the shear plane. The main failure for bending is fiber/matrix interface debonding and fibers tearing on the compression surface, 0° fibers breakage on the tension surface as well as fiber layers delaminating. Although the shear behavior is characterized by a gradually growing shear matrix damage, 90/+45/−45° interface debonding, +45/−45° fibers shear fracture, and final 0° fiber compression failure. POLYM. COMPOS., 37:3486–3498, 2016. © 2015 Society of Plastics Engineers     

Tensile stress relaxation of short‐coir‐fiber‐reinforced natural rubber composites

The stress relaxation behavior of natural rubber (NR) and its composites reinforced with short coir fibers under tension was analyzed. The rate of stress relaxation was a measure of the increase in the entropy of the compounds: the higher the rate was, the greater the entropy was. At lower strain levels, the relaxation mechanism of NR was independent of strain level. However, the rate of relaxation increased with the strain level. Also, the strain level influenced the rate of stress relaxation considerably in the coir‐reinforced NR composites. However, the relaxation mechanisms of both the unfilled compound and the composite were influenced by the strain rate. The rate of relaxation was influenced by fiber loading and fiber orientation. From the rate of stress relaxation, we found that fiber–rubber adhesion was best in the composite containing fibers subjected to a chemical treatment with alkali, toluene diisocyanate, and NR solutions along with a hexaresorcinol system as a bonding agent. In this study, the stress relaxation curves could not be viewed as segments with varying slopes; however, a multitude of inflection points were observed on the curves. Hence, we propose neither a two‐step nor three‐step mechanism for the coir‐fiber‐reinforced NR composites as reported for some other systems. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 96–104, 2004     

Tribological behavior of short‐fiber‐reinforced polyimide composites under dry‐sliding and water‐lubricated conditions

Polyimide composites reinforced with short‐cut fibers such as carbon, glass, and quartz fibers were fabricated by the polymerization of monomer reactants process. The mechanical properties of the composites with different fiber contents were evaluated. The friction and wear properties of the polyimide and its composites were investigated under dry‐sliding and water‐lubricated conditions. The results indicated that the short‐carbon‐fiber‐reinforced polyimide composites had better tensile and flexural strengths and improved tribological properties in comparison with glass‐fiber‐ and quartz‐fiber‐reinforced polyimide composites. The incorporation of short carbon fibers into the polyimide contributed to decreases in the friction coefficient and wear rate under both dry and water‐lubricated conditions and especially under water lubrication because of the boundary lubrication effect of water. The polyimide and its composites were characterized by plastic deformation, microcracking, and spalling under both dry and water‐lubricated conditions, which were significantly abated under the water‐lubricated condition. The glass and quartz fibers were easily abraded and broken; the broken fibers transferred to the mating metal surface and increased the surface roughness of mating stainless steel, which led to the wear rate increasing for the glass‐fiber‐ and quartz‐fiber‐reinforced polyimide composites. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2008     

A gradient structure formed in injection‐molded polycarbonate in situ hybrid composites and its corresponding performances

Three polycarbonate (PC) composites that were reinforced, respectively, with liquid crystalline polymer (LCP), glass fibers, and both of them were prepared by a single injection‐molding process. The role of LCP in improving the processibility of the composites was characterized by torque measurement test. The transitions of LCP morphology in two‐ and three‐component composites were investigated by using polarizing optical microscopy and scanning electron microscopy. The micrographs showed a skin–core gradient structure in all three systems investigated, and the addition of glass fiber to the PC/LCP blend affected the morphological transition and content distribution of dispersed LCP phase through the thickness of the injection‐molded samples. These results were correlated well with the measurements of tensile mechanical properties and dynamic mechanical analysis. How to fully use the dispersed LCP phase in PC in situ hybrid composites was discussed for the thickness change of core layer and the heterogeneous distribution of more LCP in the core. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 625–634, 2004     

Investigating the acoustical properties of carbon fiber‐, glass fiber‐, and hemp fiber‐reinforced polyester composites

Wood is one of the main materials used for making musical instruments due to its outstanding acoustical properties. Despite such unique properties, its inferior mechanical properties, moisture sensitivity, and time‐ and cost‐consuming procedure for making instruments in comparison with other materials (e.g., composites) are always considered as its disadvantages in making musical instruments. In this study, the acoustic parameters of three different polyester composites separately reinforced by carbon fiber, glass fiber, and hemp fiber are investigated and are also compared with those obtained for three different types of wood specimens called poplar, walnut, and beech wood, which have been extensively used in making Iranian traditional musical instruments. The acoustical properties such as acoustic coefficient, sound quality factor, and acoustic conversion factor were examined using some non‐destructive tests based on longitudinal and flexural free vibration and also forced vibration methods. Furthermore, the water absorption of these polymeric composites was compared with that of the wood samples. The results reveal that the glass fiber‐reinforced composites could be used as a suitable alternative for some types of wood in musical applications while the carbon fiber‐reinforced composites are high performance materials to be substituted with wood in making musical instruments showing exceptional acoustical properties. POLYM. COMPOS., 35:2103–2111, 2014. © 2014 Society of Plastics Engineers     

Temperature effect on graphene‐filled interface between glass–carbon hybrid fibers and epoxy resin characterized by fiber‐bundle pull‐out test

The overall mechanical performance of glass–carbon hybrid fibers reinforced epoxy composites depends heavily upon fiber–matrix interfacial properties and the service temperatures. Fiber‐bundle pull‐out tests of glass (GF) and/or carbon fiber (CF) reinforced epoxy composites were carried out at room and elevated temperatures. Graphene nanoplatelets were added in the interfacial region to investigate their influence on the interfacial shear strength (IFSS). Results show that IFSS of specimens with fiber‐bundle number ratio of GF:CF = 1:2 is the largest among the hybrid composites, and a positive hybridization effect is found at elevated temperatures. IFSS of all the specimens decreases with the increasing of test temperatures, while the toughness shows a contrary tendency. As verified by scanning electron microscopy observations, graphene nanoplatelets on fiber surface could enhance the IFSS of pure glass/carbon and hybrid fibers reinforced epoxy composites at higher temperatures significantly. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46263.