Room- and Low-Temperature Deformation of Multilayered Fiberglass Plastics Reinforced with a Fabric of Sateen Weave

The regularities of elastic deformation of multilayered fiberglass plastics reinforced with a fabric of sateen weave are studied. The effect of cooling to 77 K on the averaged elastic characteristics of the orthotropic material is analyzed. The efficiency of mathematical modeling in calculating the stiffness and compliance parameters of the woven composites based on the geometry and mechanical properties of their constituents is investigated.     

The Effect of an Active Diluent on the Properties of Epoxy Resin and Unidirectional Carbon-Fiber-Reinforced Plastics

The influence of an active diluent on the properties of an epoxy matrix and carbon-fiber-reinforced plastics (CFRP) is investigated. The physicomechanical properties of an ED-20 epoxy resin modified with diglycidyl ether of diethylene glycol (DEG-1), the adhesion strength at the epoxy matrix–steel wire interface, and the mechanical properties of unidirectional CFRP are determined. The concentration of DEG-1 was varied from 0 to 50 wt.%. The properties of the matrix, the interface, and the composites are compared. It is stated that the matrix strength affects the strength of unidirectional CFRP in bending and not their strength in tension, compression, and shear. The latter fact seems somewhat unexpected. The interlaminar fracture toughness of the composites investigated correlates with the ultimate elongation of the binder. A comparison between the concentration dependences of adhesion strength and the strength of CFRP shows that the matrices utilized provide such a high interfacial strength that the strength of CFRP no longer depends on the adhesion of its constituents.     

Stress-Strain State in the Zone of Load Transfer in a Composite Specimen under Uniaxial Tension

The stress-strain state in the zone of load transfer in a uniaxially stretched specimen made of a unidirectional epoxy carbon-fiber-reinforced plastic (CFRP) is investigated. A parametric analysis of the influence of geometric and mechanical characteristics of the specimen on its stress-strain state is performed by means of finite-element modeling. The parameters allowing us to significantly reduce the dangerous concentration of transverse and tangential stresses are revealed. The mechanical tensile characteristics of a high-strength pultruded unidirectional CFRP are determined experimentally, and the size effect of its strength is estimated.     

Analysis of the adhesive strength variance in expoxy matrix-boron fiber joints

A new approach to the problem of adhesive joint failure based on the study of the scale effect of the adhesive strength variance is applied to an epoxy matrix-boron fiber system. The shear adhesive strengh is measured using the pull-out technique. It is found that the variance D practically does not depend on the joint area S, whereas the adhesive strength markedly decreases with increase in S. It is noted that the scatter of the values of by far exceeds the experimental errors. The –S and D–S curves obtained behave similarly to the corresponding curves for EDT-10-steel wire joints. The independence of D from S confirms the concept that the failure of fiber-polymer joints in pull-out tests starts near the end of the joint at which the external force is applied. An analysis of the adhesive strength distribution curves within sufficiently narrow (S=0.1 mm²) intervals of S values also confirms this concept.Presented at the 11th International Conference on Mechanics of Composite Materials (Riga, June 11–15, 2000).Semenov Institute of Chemical Physics, Russian Academy of Sciences, Moscow, Russia. Translated from Mekhanika Kompozitnykh Materialov, Vol. 36, No. 4, pp. 435–444, 2000.     

Deformation of dispersely failing unidirectional composites

An anisotropic medium is considered in which, upon loading, scattered microdamages accumulate giving rise to nonlinear and residual strains. The damage at a point of the medium is characterized by a scalar function on a unit sphere, referred to as the damage function. This function is approximated by a fourth-rank tensor used for specifying the relation between the increments of strains and stresses. The calculation dependences are presented in detail for a unidirectional composite, which is taken to be a homogeneous transversely isotropic medium. Determination of the unknown constants is illustrated by the example of an actual fiberglass plastic. Institute of Polymer Mechanics, University of Latvia, Riga, LV-1006, Latvia. Translated from Mekhanika Kompozitnykh Materialov, Vol. 35, No. 5, pp. 561–574, September–October, 1999.     

Properties of unidirectional GFRPs based on an epoxy resin modified with polysulphone or an epoxyurethane oligomer

The mechanical properties of unidirectional GFRPs based on an ED-22 epoxy resin were investigated. The resin was modified with a PSK-1 polysulphone or a PEF-3a epoxyurethane oligomer. Triethanolaminotitanate or diaminodiphenilsulphone was used as a hardener. The modification did not improve the mechanical properties of GFRPs in quasi-static loading; but in a low-speed impact loading, the shear strength of epoxypolysulphone GFRPs with 20 wt.% PSK-1 increased by 20–25%. For all the GFRPs investigated, the shear strength grew linearly with the logarithm of loading rate. The introduction of the modifiers increased the fracture toughness considerably: by 100 and 70% for GFRPs modified with 20 wt.% PSK-1 and 50 wt.% PEF-3a, respectively. __________ Translated from Mekhanika Kompozitnykh Materialov, Vol. 42, No. 6, pp.739–758, November–December, 2006.     

Method of primers in calculating the probability of destruction of a unidirectional composite material in tension

A method of primers is elaborated which allows one to calculate the distribution function of durability of a composite material in tension in the reinforcement direction. Integral and differential equations for calculating the probabilities of formation of primers and destruction of a material caused by their formation are presented. Distribution functions of material strength for the parameter of Weibull distribution of fiber strength on the interval 2.1 ≤ β _f ≤ 50.1 are calculated. From the functions, the average values and root-mean-square deviations of material strength are found. The results obtained agree well with calculations by using the structural-imitation simulation. The distribution functions of material strength with a high precision are approximated by the three-parameter Weibull distributions. The distribution parameters are approximated by the linear functions of ln (β_f). __________ Translated from Mekhanika Kompozitnykh Materialov, Vol. 43, No. 6, pp. 823–838, November–December, 2007.     

Influence of the concentration of carbon nanotubes (CNT) on the thermophysical properties of epoxy/CNT nanocomposites at low temperatures

The influence of the content of carbon nanofillers (multi-and single-wall nanotubes) on the thermophysical properties of epoxy nanocomposites was investigated on the temperature range from −150 to 150°C. A “plateau” was found to exist in the concentration dependence of thermal conductivity on the concentration interval from 0.1 to 1.0 wt.% carbon nanotubes (CNTs). The thermal conductivity of the CNT composites exceeded that of pure epoxy resin by about 40%. A further increase in CNT content de creased the conductivity, owing to increasing interfaces between the two phases and the additional thermal resistance caused by phonon scattering on them. It is found that the temperature interval of transition of the composite from a glassy to a viscoelastic state greatly depends on the filler type and concentration. There exists a critical concentration at which a drop in the glass-transition temperature by 30% can be observed. The reason is the undercure of binder as a result of interaction between CNTs and epoxy macromolecules, which reduces the cross-linking density of structure of the polymer. Translated from Mekhanika Kompozitnykh Materialov, Vol. 44, No. 5, pp. 697–708, September–October, 2008.     

Micromechanical Analysis of Inelastic Deformation of Unidirectional Fibrous Composites under Multiaxial and Shear Loadings

An algorithm is proposed for numerically solving nonlinear 3D problems of micromechanics of a unidirectionally reinforced composite with a regular structure. For the matrix, equations of the deformation theory of plasticity and relations of reduced rigidity in its failure zones are used, whereas the fibers are elastic and indestructible. According to the method of local approximation, fields of microstresses and microstrains are determined in a structural fragment containing nine periodic cells. Boundary conditions of the fragment correspond to an arbitrary combination of longitudinal, transverse, and shear microstresses occurring in the central part of the fragment. The solution to the nonlinear 3D problem is sought by the method of superposition with an iterational refinement based on the successive solution of an antiplane problem and a problem on a generalized plain strain state of the structural segment. Special features of the iteration procedure are considered. The calculated deformation diagrams and ultimate strengths of a unidirectional glass-epoxy composite are presented for several loading trajectories.     

Deformation and strength of hybrid composites reinforced with unidirectional fibers. 1. Numerical models

The hybrid composite consists of n(n > 2) jointly working phases. We define the thermomechanical characteristics and strength of composites by filling and reinforcing materials thermomechanical characteristics and strength basing on the suggestion that thin and strong fibre reinforced composite is quasiuniform, and there is a continuous contact between the filling medium and reinforcing fibers. The development of a mathematical model of the design under consideration has been based on following assumptions: 1) for irreversible processes, the classical thermodynamic postulates are valid, and they are introduced as functions of state of internal energy and entropy; 2) for a solitary volume of materials, internal energy is assumed to be proportional to the volume fraction of the j-th phase v_j; 3) for the material pressure limit conditions just before the essential damage, it is suggested that: a) the whole composite as well as the components are steady, i.e. Drukker's postulate is valid; b) the deformation law associated with the corresponding strength surface is valid, and c) small values of increases in plastic deformation play the leading role. The strength of unidirectionally reinforced hybrid monolayers is predicted by using a linear programming code.Presented at the Ninth International Conference on the Mechanics of Composite Materials (Riga, October, 1995).Translated from Mekhanika Kompozitnykh Materialov, Vol. 31, No. 2, pp. 186–192, March–April, 1995.The studies were carried out with financial support of the International Scientific Fund founded by G. Soros.     

Environmental degradation of carbon nanotube-modified composite laminates: a study of electrical resistivity

The environmental durability of carbon nanotube (CNT)-modified carbon-fibre-reinforced polymers (CFRPs) is investigated. The key problem of these new-generation composites is the modification of their polymer matrix with nanoscaled fillers. It was recently demonstrated that the damage tolerance of these materials, as manifested by their fracture toughness, impact properties, and fatigue life, can be improved by adding CNTs at weight fractions as low as 0.5%. This improvement is mainly attributed to the incorporation of an additional interfacial area between the CNTs and the matrix, which is active at the nanoscale. However, this additional interface could have a negative effect on the environmental durability of the aforementioned systems, since it is well known that the moisture absorption ability of a matrix is enhanced by the presence of multiple interfaces, which serve as an ingress route to water. To examine this problem, CNT-modified CFRPs were exposed to hydrothermal loadings. At specified intervals, the composites were weighted, and the water uptake vs. time was recorded for both the modified and a reference systems. The electrical conductivity of the composites was registered at the same time intervals. After the environmental exposure, the interlaminar shear properties of the conditioned composite systems were measured and compared with those of unmodified composites, as well as with the shear properties of unexposed laminates. Russian translation published in Mekhanika Kompozitnykh Materialov, Vol. 45, No. 1, pp. 31–48, January–February, 2009.     

有限变形下的等效应力和等效应变问题

重点讨论了在有限变形条件下，弹塑性理论中的等效应力、等效应变是否仍然成立．选择了平面应力和平面应变下的单向压缩应力状态，对这一问题进行了探讨．在这两种应力状态下，在众多的应力应变描述中，对数应变与旋转Kirchhoff应力得到的应力应变曲线与实验数据符合良好．     

Comparative micromechanical analysis of nonelastic behavior of unidirectional plastics with tetragonal and hexagonal structures

A mathematical model for determining the effective elastic properties and describing the processes of inelastic deformation and damage accumulation of unidirectional fiber-reinforced composites with tetragonal and hexagonal structures is developed. A comparative analysis of the effective elastic moduli of glass, boron, organic, and carbon unidirectional plastics shows that, if the fiber volume fraction does not exceed 0.5, the effective elastic properties calculated by the models presented give closely related results. The calculation results for nonlinear fields of deformation and failure are presented and the limiting strength surfaces of fibrous glass plastics with hexagonal and tetragonal structures are obtained for different transverse loading paths. It is found that the structure of a composite affects significantly its strength properties.Submitted to the 11th International Conference on Mechanics of Composite Materials (Riga, June 11–15, 2000).Perm' State Technical University, Perm', Russia. Translated from Mekhanika Kompozitnykh Materialov, Vol. 36, No. 4, pp. 455–464, July–August, 2000.     

Calculation and experimental study of a unidirectional rubber-cord composite in tension and compression

Calculation and experimental study of transverse tension and compression of a rubber-cord composite with a unidirectional scheme of reinforcement is presented. The calculation results for components of the composite and for the composite as a whole are compared with the experimental data. The loss of stability of the deformed rubber-cord composite in transverse tension is analyzed.Translated from Mekhanika Kompozitnykh Materialov, Vol. 35, No. 3, pp. 325–334, 1999.     

Strength of Unidirectional Composites under High Hydrostatic Pressures

The dependence of the strength properties of two unidirectional fibrous composites on high (up to 500 MPa) hydrostatic pressure has been studied experimentally. Ring specimens of epoxy carbon- and glass-fiber-reinforced plastics were tested in tension using half-disk devices. The tensile strength in the reinforcement direction increased with increase in the pressure up to 300 MPa. However, at a further increase in the pressure, this strength decreased. It was found that the failure mode of unidirectional composites depends on the magnitude of hydrostatic pressure. The failure shapes differed in the location of the failure zone and in the relative extent of longitudinal cracks in the specimens. At atmospheric pressure, the failure zone covered practically the whole volume of the specimens. With increased pressure, the failure zone became localized. At the highest pressures investigated, the failure was accompanied by the formation of a single crack across the reinforcement direction.     

Boundary-value problem in the correlative approximation of the method of quasi-periodic components for a unidirectional fiber composite

The boundary-value problem in the correlative approximation of the method of quasi-periodic components and a numerical algorithm based on the boundary element method for determining the nonuniform stress fields in the matrix of a unidirectional fiber composite with a disordered structure are considered. The numerical results and analysis of the probability density function, for example, for normal stresses at some points of the interface of absolutely rigid fibers of the composite are presented. Perm State Technical University, Russia. Translated from Mekhanika Kompozytnykh Materialov, Vol. 35, No. 5, pp. 629–642, September–October, 1999.