Effects of Hot-Water Aging on the Compression Properties of E-Glass/Epoxy Composites at Varying Strain Rates

Mechanics of Composite Materials - The effects of hot-water aging on the quasi-static and dynamic compression properties of unidirectional E-glass/epoxy laminates were investigated. E-glass/epoxy...     

Effect of surface modification of fibers with a polymer coating on the interlaminar shear strength of a composite and the translation of fiber strength in an F-12 aramid/epoxy composite vessel

The surface of aramid fibers was modified with a polymer coating — a surface treatment reagent containing epoxy resin. The resulting fibers were examined by using NOL tests, hydroburst tests, and the scanning electron microscopy. The modified fibers had a rougher surface than the untreated ones. The interlaminar shear strength of an aramid-fiber-reinforced epoxy composite was highest when the concentration of polymer coating system was 5%. The translation of fiber strength in an aramid/epoxy composite vessel was improved by 8%. The mechanism of the surface treatment of fibers in improving the mechanical properties of aramid/epoxy composites is discussed. Russian translation publeshed in Mekhanika Kompozitnykh Materialov, Vol. 42, No. 6, pp. 729–738, November–December, 2006.     

Numerical aspects on computational homogenization of epoxy/glass composites

Numerical aspects of two-scale modeling of epoxy/glass composites are presented. The homogenization process is carried out under consideration of periodic boundary constraints (PBC) of the representative volume element (RVE) due to the periodic structure of glassfiber reinforced epoxy systems. The introduction of artificial constraints for computing macro-stresses and macro-moduli is presented by giving the modified algorithmic treatment of a two-scale approach using PBC. The proposed algorithm is applied to an ISO 527 epoxy/glass test specimen. The results of computations considering or not considering interphases and interfaces within the composite are compared. (© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Dynamic compressive strength of epoxy composites

The strength of laminated and unidirectionally reinforced composite materials was investigated in conditions of dynamic uniaxial compression with a strain rate of 50–1000 sec^–1 using the split Hopkinson pressure bar method. It was shown that in conditions of dynamic compression, glass/epoxy, aramid/epoxy, and carbon/epoxy composites exhibit elastic-brittle behavior with anisotropy of the strength and elastic properties. The effect of the strain rate on the strength characteristics of fiberglass-reinforced plastics was demonstrated.Translated from Mekhanika Kompozitnykh Materialov, Vol. 31, No. 6, pp. 776–782, November–December, 1995.Presented at the Ninth International Conference on the Mechanics of Composite Materials (Riga, October, 1995).     

Computational model for reinforced concrete beams strengthened by epoxy bonded steel plates

This paper presents a nonlinear finite element model for the flexure-shear response of reinforced concrete (RC) beams strengthened externally by epoxy bonded steel plates. The model includes a special interface element to simulate the thin epoxy adhesive layer and which allows for the metamorphosis of failure mode from plate yielding to separation as the plate thickness t_p is increased. The numerical results show close correlation to experimental data available for an RC beam strengthened by plates of various thickness.     

The onset of mixed-mode intralaminar cracking in a cross-ply composite laminate

The intralaminar fracture toughness of a unidirectionally reinforced glass/epoxy composite is determined experimentally at several mode I and mode II loading ratios. The crack propagation criterion, expressed as a quadratic form in terms of single-mode stress intensity factors (alternatively, linear in terms of energy release rates), approximates the test results reasonably well. The mixed-mode cracking criterion obtained is used to predict the intralaminar crack on set in a cross-ply glass/epoxy composite under off-axis tensile loading. Translated from Mekhanika Kompozitnykh Materialov, Vol. 44, No. 6, pp. 785–794, November–December, 2008.     

Micromechanical modelling and two-scale simulation of epoxy/glass composites with interphases and interfaces

Composite systems consisting of glass fibres and epoxy matrix with interphases and interfaces will be considered in the modelling approach. The interphase forms the transition zone between the epoxy matrix and the glass fibre. The interface is the layer between the glass fibre and the surrounding interphase. The macroscopic strength of the composite material is intrinsically related to the bond strength of the polymeric/solid interface and the micromechanical characteristics of the three phases (epoxy, glass and interphase). Homogenization is an appropriate methodology to link these two scales to predict the overall physical behaviour of the composite. The nonlinear behaviour of amorphous polymers, cohesive interface elements and the elastic behaviour of glass fibres as part of the considered composite material are presented, as well as a representative example to show the necessity of taking interface influences into account. (© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

The Effect of Surface Treatment of F-12 Aramid Fibers with Rare Earths on the Interlaminar Shear Strength of Aramid/Epoxy Composites

Solutions of a rare-earth modifier (RES) and the epoxy chloropropane (ECP) grafting modification method are used for the surface treatment of F-12 aramid fibers. The effects of RES concentration on the interlaminar shear strength (ILSS) of F-12 aramid fiber/epoxy composites are investigated in detail, and the fracture surfaces of ILSS specimens are analyzed by SEM. It is shown that the RES surface treatment is superior to the ECP grafting treatment in promoting the interfacial adhesion between aramid fibers and the epoxy matrix. However, the tensile strength of single fibers is almost unaffected by the RES treatment. The optimum ILSS is obtained at a 0.5 wt.% content of rare-earth elements.__________Russian translation published in Mekhanika Kompozitnykh Materialov, Vol. 41, No. 2, pp. 265–272, March–April, 2005.     

The bearing strength and failure behavior of bolted e-glass/epoxy composite joints

An experimental study was carried out to investigate the failure strength of a pinned-joint E-glass/epoxy composite plate. The main objective was to examine the influence of the preload moment, the edge distance to the pin diameter ratio, and the specimen width to pin diameter ratio on the strength. The load-carrying capacity of the pin-loaded hole could be changed by varying the specimen geometry and the preload moment. Guidelines for effective geometrical configurations and the preload moment for the mechanically pin-connected E-glass/epoxy composite plate were specified based on the ultimate bearing strength. Russian translation published in Mekhanika Kompozitnykh Materialov, Vol. 44, No. 4, pp. 569–590, July–August, 2008.     

A Progressive FE Failure Model for Laminates under Biaxial Loading

Mechanics of Composite Materials - A finite-element (FE) model simulating the elastic behavior of anisotropic glass/epoxy composite laminates subjected to a biaxial tensile loading is proposed. A...     

Numerical-experimental estimation of the mechanical properties of an epoxy nanocomposite

The possible reasons for the violation of additivity laws for the density and elastic modulus of polymer nanocomposites (epoxy resin filled with silicon oxide nanoparticles) are considered. The fact that each nanoparticle is surrounded by a distinctive boundary layer is used to describe this phenomenon. The thickness and density of the layer are determined by measuring the density and elastic modulus of a polymer with different volume fractions of filler. A model for determining the strength of an epoxy nanocomposite by using the theory of short fibers is proposed. This model allowed us to describe the nonmonotonic relationship between the ultimate strength of the epoxy nanocomposite and the volume fraction of filler. It is shown that the filling of epoxy resin with silicon oxide nanoparticles beyond 5 vol. % decreases the strength of the composite in comparison with that of pure epoxy resin.     

Comparison of Glass-Transition Temperatures for Epoxy Polymers Obtained by Methods of Thermal Analysis

Mechanics of Composite Materials - In this review, the characteristic temperatures for epoxy polymers and epoxy matrices of fiber-reinforced composites in the glass-transition ( α-transition)...     

Estimation of the Tensile Strength Reduction of a Composite Laminate with a Hole

The strength problem for a body with an artificial defect in the form of a hole is considered. To solve this problem, an approach is suggested according to which the local strength of a material in the area of stress concentration is assumed to depend on the size of this area. The scale of the problem is introduced via the ratio between the characteristic sizes of the deformed area and the characteristic length of the material. This approach is used to estimate the strength of composite laminates weakened by holes and notches. Expressions for the failure stress are obtained, which can be applied to isotropic and orthotropic laminates both in quasi-brittle failure and in failure associated with significant inelastic deformations. A comparison between the calculated _c values and the known experimental data for glass/epoxy and graphite/epoxy laminates is presented.     

Multiwall carbon nanotube/epoxy composites produced by a masterbatch process

A masterbatch process based on a minicalander (three-roller mill) and a vacuum dissolver was developed in order to produce multiwall carbon nanotube/epoxy composites with loading fractions of 0.5, 1.0, and 2.0 wt.%. TEM and SEM analyses were performed to investigate the dispersion results. A contrast imaging in the SEM backscattering mode revealed a homogeneous distribution of carbon nanotubes in the whole volume of the material. Furthermore, an interesting correlation was found to exist between the network structure formed by the nanotubes in the epoxy matrix and the appearance of fracture surface of the nanocomposites. Furthermore, the nanocomposites exhibited an electrical conductivity in the regime of some 10⁻² S/m. Russian translation published in Mekhanika Kompozitnykh Materialov, Vol. 42, No. 5, pp. 567–582, September–October, 2006.     

The effect of C<Subscript>60</Subscript> fullerene on the toughening of epoxy compositions

The effect of C₆₀ fullerene on the mechanical properties of epoxy resins has been investigated. It is found that this filler affects the tensile modulus and tensile strength of epoxy compositions only slightly, but their impact strength at a 0.01–0.12 wt.% content of C₆₀ increases by about 100–200%. A molecular mechanism of the toughening effect of C₆₀ on epoxy resins is suggested.     

Interaction at the Interface of Phosphorus-Containing Carbon Fibers and Diglycidil Ether of Bisphenol-A

Two-component blends of phosphorus-containing carbon fibers (PCF) and diglycidyl ether of bisphenol-A (DGEBA) are investigated. It is found that PCF are better wetted by the epoxy oligomer considered than unmodified carbon fibers. It is stated that the equilibrium work of adhesion of the epoxy oligomer to PCF increases considerably. Heating the two-component blends is accompanied by conversion of epoxy groups and formation of a gel-fraction nonextractable from the fiber surface. The investigation results indicate that chemical bonds are formed at the fiber-oligomer interface, which causes grafting of the DGEBA immediately to the surface of PCF without the use of intermediate compositions usually employed in such cases. It is shown that a transition layer is formed whose morphology differs from that of the fibers and polymer in the blend volume.     

Effect of hydrostatic pressure on the water absorption of epoxy polymers

The water absorption of epoxy polymers at hydrostatic pressures up to 1000 kgf/cm² has been studied. It has been shown that pressure very definitely reduces the water absorption value but essentially does not change the diffusion coefficient of water into the polymers under study. An explanation of the data obtained is given.     

Rheological and dynamic thermomechanical propert ies of epoxy composites reinforced with single- and multiwalled carbon nanotubes

An epoxy resin (Epon 828) was filled with single- and multiwalled carbon nanotubes (SWCNT and MWCNT) in two steps by using the high shear mixing and ultrasonication techniques. The melt flow of the composites was characterized in a plate/plate rheometer. The thermomechanical properties of the composites were determined in dynamic mechanical analysis tests performed at various frequencies and temperatures. It was found that the incorporation of SWCNT or MWCNT increased the viscosity and stiffness of epoxy above its glass-transition temperature. The time-temperature superposition principle was employed to estimate the storage modulus of the composites as a function of frequency (f = 10^–33–10³ Hz) in the form of master curves.     

Macromechanical Characteristics Deduced from Three-Point Flexure Tests on Unidirectional Carbon/Epoxy Composites

Effects of different factors on the measured values of flexural modulus and flexural strength of unidirectional carbon/epoxy composites are investigated. This allows a correct interpretation of the results of flexural macromechanical properties obtained in three-point flexure tests. The shear deformation, the displacement of neutral axis from the mid-depth of the beam, and the nonuniform distribution of stresses in bent coupons are discussed.