Investigation of internal stresses in glass-reinforced plastics

The strength properties of and internal stresses in epoxy and epoxyphenol resins and GRPs based on them are investigated using an optical method of determining internal stresses. The GRPs had tape and fabric reinforcement. Compared with the internal stresses in unplasticized specimens, the stresses in pure resin films and in GRPs based on plasticized resins are found to be smaller. It is shown that the distribution of internal stresses in GRPs is anisotropic. The highest internal stresses are observed in tape-reinforced GRPs in a direction normal to the fibers. Glass reinforcement in two directions at right angles reduces the internal stresses in GRPs as compared with pure resin films. In both reinforced and unreinforced films, the internal stresses depend on the curing conditions.Mekhanika polimerov, Vol. 1, No. 1, pp. 82–88, 1965     

Creep and long-time strength of unidirectional reinforced plastics in compression

An attempt is made to predict the creep and long-time strength of unidirectional reinforced plastics in compression in the direction of the reinforcement from the properties of the components. The reinforcement is assumed to be elastic, while the resin is described by a Boltzmann-Volterraintegral equation with fractional-exponential Rabotnov kernel. Experimentally obtained creep and long-time strength curves are presented for unidirectional reinforced plastics.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 5, pp. 830–835, September–October, 1970.     

Simple loading of a polymer material with nonlinear creep

Nonlinear tensor relations between strain, stress, and time are examined for a memory-type medium using degenerate kernels. The material parameters are determined from creep tests in a simple state of stress. Expressions for the strain associated with a complex state of stress and simple loading, found on the basis of the local strains theory, are in satisfactory agreement with the experimental data obtained for specimens of high-density polyethylene.Mekhanika Polimerov, Vol. 3, No. 2, pp. 236–242, 1967     

Numerical simulation of rheological processes in hardening plastics under stress control

The paper concerns the simulation of rheological processes in hardening plastics (resins) under stress control. It is assumed that the resins work in the glassy state, under normal conditions, and the rheological processes are quasi-static and isothermal. The reduced stress levels do not exceed 30% of the instantaneous tensile strength. A resin is modelled as a homogeneous, isotropic, linearly viscoelastic material. The HWKK/H rheological model, developed recently by the author, is used. Short-term, medium-term, and long-term shear strain components are considered and described by one fractional and two normal exponential functions as the stress history (memory) functions. A novel algorithm for the numerical simulation of rheological processes in resins has been developed, which is unified for all stress history functions in the HWKK/H model. The algorithm employs the Boltzmann superposition principle, a virtual table for the classic creep process, and a high-rank Gaussian quadrature. The stress function is approximated with a stair case function. The constitutive equations governing the HWKK/H model are trans formed into an algebraic form suitable for algorithmization. The problem of quasi-exact calculation of the double-improper integral resulting from the fractional exponential function is solved effectively. The algorithm has been tested successfully on selected loading programs of unidirectional tension of epoxide. Russian translation published in Mekhanika Kompozitnykh Materialov, Vol. 43, No. 2, pp. 201–212, March–April, 2007.     

Creep of unidirectional glass-reinforced plastics based on new types of fibers

Theoretical and experimental compressive creep relations for a unidirectional glass-reinforced plastic based on hollow fibers are presented. The agreement between the theoretical and experimental data is shown to be satisfactory. The effect of fiber capillarity on the creep process is unimportant.All-Union Scientific-Research Institute of Glass-Reinforced Plastics and Fiber Glass, Moscow; Institute of Mechanics of Machines and Polymeric Materials, Academy of Sciences of the Georgian SSR, Tbilisi. Translated from Mekhanika Polimerov, No. 4, pp. 747–750, July–August, 1971.     

A rheological model of polymers and glass-reinforced plastics

Network polymers and the corresponding glass-reinforced plastics are investigated for a homogeneous uniaxial state of stress and constant temperature. A physical law relating the high-elastic strain and the stress in explicit form with once-determined structural constants is obtained for the damped (bounded) creep regime. The theoretical solutions are compared with the experimental data for a glass-reinforced plastic based on unsaturated polyester resin and glass mat reinforcement.Institute of Engineering Mechanics, Bulgarian Academy of Sciences, Sofia. Translated from Mekhanika Polimerov, No. 5, pp. 851–857, September–October, 1971.     

Water Resistance of Polyester Polymer Concrete

The results of experimental investigation of polyester resin and polymer concrete at a long-term (four-year) exposure to water and air with 98% RH are presented. The polymer concrete was composed of a polyester resin as a binder, lime flour, quartz sand, and granite chips. The resin content in concrete was 20 wt.%. The features of sorption properties of the materials investigated are discussed. Data on the water effect on the compressive strength in short-term loading are reported. The creep tests of virgin polymer-concrete specimens were carried out for five years at different stress levels from 0.11 to 0.44 of the short-term prismatic strength. The effect of moisture on the creep behavior was also studied.     

Creep of polyester resin PN-3 under moisture-absorption conditions

A procedure is described for testing samples for creep in a climatic chamber of the Feutron 3001 type. Results of an experimental study of forward and back creep of polyester resin PN-3 under simultaneous moistening of samples in an air medium with elevated humidity are discussed. It is shown that the rate of the creep process depends greatly on the humidity state of the material.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 4, pp. 579–584, July–August, 1972.     

Equations of three-dimensional deformation of linear viscoelastic media

An equation is proposed relating the stress and strain tensors in time for the three-dimensional deformation of linear viscoelastic media whose properties are determined by four parameters. A method is indicated for determining these parameters based on simple creep or relaxation tests. Values of the parameters are given for certain polymer materials. For such materials, there is an upper limit of the stress state below which an equilibrium limit state is possible. A creep kernel is proposed for the case of no equilibrium limit state, and a resolvent constructed with account for the variation of stress and strain in time.Mekhanika Polimerov, Vol. 1, No. 4, pp. 35–42, 1965     

Methods of taking into account the nonlinearity of the deformation properties of the material in calculating thick-walled composite rings

The results of an investigation of the deformation properties of unidirectional composites are presented, and engineering models which take into account the effect of the nonlinearity of the properties in the radial direction on the state of stress of rings under pressure are proposed. The relations obtained are checked theoretically — by means of a refined calculation based on the nonlinear theory of elasticity — and experimentally — by loading rings with internal and external pressure by means of rubber cuffs in specially designed apparatus.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 1, pp. 73–84, January–February, 1976.     

Estimation of limit strains in disk-type flywheels made of a compliant elastomeric matrix composite undergoing radial creep

Fiber reinforced elastomeric matrix composites (EMCs) offer several potential advantages for construction of rotors for flywheel energy storage systems. One potential advantage, for safety considerations, is the existence of maximum stresses near the outside radius of thick circumferentially wound EMC disks, which could lead to a desirable self-arresting failure mode at ultimate speeds. Certain unidirectionally reinforced EMCs, however, have been noted to creep readily under the influence of stress transverse to the fibers. In this paper, stress redistribution in a spinning thick disk made of a circumferentially filament wound EMC material on a small rigid hub has been analyzed with the assumption of total radial stress relaxation due to radial creep. It is shown that, following complete relaxation, the circumferential strains and stresses are maximized at the outside radius of the disk. Importantly, the radial tensile strains are three times greater than the circumferential strains at any given radius. Therefore, a unidirectional EMC material system that can safely endure transverse tensile creep strains of at least three times the elastic longitudinal strain capacity of the same material is likely to maintain the theoretically safe failure mode despite complete radial stress relaxation. Submitted to the 11th International Conference on Mechanics of Composite Materials (Riga, June 11–15, 2000). Translated from Mekhanika Kompozitnykh Materialov, Vol. 31, No. 1, pp. 87–94, January–February, 2000.     

Effects of Moisture and Stresses on the Structure and Properties of Polyester Resin

The results of a complex study of structural changes in a cured Norpol 440 polyester resin under the action of damp environment and mechanical loading are presented. A considerable effect of absorbed moisture on the structure and some characteristics of the material is revealed by using thermophysical methods and X-ray diffractometry. The joint effect of moisture and mechanical stress is estimated by investigating the creep in stationary and nonstationary moisture conditions. The anisotropy of the material structure formed during creep is evaluated from the results of dilatometric measurements. It is found that the degree of anisotropy of the material after creep accompanied by moisture sorption is higher than that after creep in the conditions of moisture equilibrium with atmosphere. It is established that the aftercure and relaxation of the residual creep deformation come to an end at heating to 80-85°C. At a further rise in temperature and repeated heating, changes in the material structure are not observed.     

Creep and Long-Term Strength of Press-Powder Composites

The mechanical state of a press-powder filler for pyroautomatics system units of aerospace equipment during their long-term storage is investigated. With the use of the endochronic approach, the rheological properties of a dispersedly reinforced composite are simulated based on the conceptions of linear and nonlinear creep. Analytical expressions of constitutive relations of creep and strength for the composites are obtained. __________ Translated from Mekhanika Kompozitnykh Materialov, Vol. 41, No. 6, pp. 803–818, November–December, 2005.     

Comparative Studies on the Mechanical Properties of a Thermoset Polymer in Tension and Compression

Results of an experimental investigation into the mechanical properties of a polyester resin in tension and compression are reported. Features of the stress-strain curves obtained are discussed. Data on the elastic modulus, Poisson ratio, and volume strains are obtained. The results of creep behavior of the material in tension and compression are also presented. It is found that the time-dependent creep obeys a power law, but the nonlinear stress dependence can be described by using the hyperbolic sine function. The effect of load type (tension or compression) on the nonlinearity of the creep is analyzed. __________ Translated from Mekhanika Kompozitnykh Materialov, Vol. 41, No. 5, pp. 633–650, September–October, 2005.     

Effectiveness of using hollow fibers to improve the rigidity of glass-reinforced plastics

Calculated and experimental relations between the modulus of elasticity and rigidity of unidirectional glass-reinforced plastics (GRP) and the capillarity coefficient of the hollow fiber reinforcement are presented. It is established that in calculating the flexural rigidity ratio of hollow- and solid-fiber GRP specimens of equal weight it is not permissible to neglect the mass and modulus of elasticity of the resin. The potentialities of hollow fibers as reinforcement for GRP shells subjected to external pressure are demonstrated.All-Union Scientific-Research Institute of Glass-Reinforced Plastics and Glass Fibers, Moscow Region. Translated from Mekhanika Polimerov, Vol. 4, No. 4, pp. 672–676, July–August, 1968.     

Dynamic properties of composite laminates and panels with highly contrasting directions of anisotropy

An asymptotic analysis of the dynamic stress-strain state of a thin laminar packet of anisotropic layers is presented. The statement is nonclassical, since in layer materials the ratio of elastic moduli in the longitudinal and transverse directions can generate small parameters comparable to the relative half-thickness of the packet, as, for example, in high-strength unidirectional composites. Alternation of strong load-carrying layers and a relatively soft filler with a similar difference in the elastic moduli between the layers is also allowed. The averaged two-dimensional equations and the total stress tensor in the layers are determined. The results are classified with respect to the types of anisotropy and the indices of differences in the elastic moduli. It is shown that first-approximation models lead to kinematic relations similar to those of the theories of high-order shear strains. Institute of Problems of Mechanics, Russian Academy of Sciences, Moscow, Russia. Translated from Mekhanika Kompozitnykh Materialov, Vol. 35, No. 5, pp. 605–614, September–October, 1999.     

Characteristics of the creep and repeat creep deformation process for polymers in uniaxial tension. Part 2

Physical relations for approximating the creep deformation and recovery have been worked out for a material in the physically nonlinear state with a nonmonotonic θ (t) dependence. It has been shown experimentally that for filled low-density polyethylene and fluoroplastic there is no time similarity for the processes of longitudinal and transverse deformation, deformation under primary and repeat loading, and creep deformation and recovery.     

Questions relating to the design of multilayer glass-reinforced-plastic systems in compression

The stability and states of stress and strain of five-layer orthogonally reinforced and unidirectional systems loaded in compression are investigated. The calculations are based on the system of differential-difference equilibrium equations for a multilayer composite in a complex state of stress [1, 2].All-Union Scientific-Research Institute of Glass-Reinforced Plastics and Glass Fiber, Moscow Region. Translated from Mekhanika Polimerov, No. 6, pp. 1019–1028, November–December, 1973.     

Modeling of primary and secondary creep for a wide stress range

Many materials exhibit a stress range dependent creep behavior. The power–law creep observed for a certain stress range changes to the viscous type creep if the stress value decreases. Recently published experimental data for advanced heat resistant steels indicate that the high creep exponent (in the range 5–12 for power–law behaviour) may decrease to the low value of approximately 1 within the stress range relevant for engineering structures. The aim of this paper is to confirm the necessity of the assumption of the stress range dependent power–law–viscous creep transition for the solution of stress relaxation problems affected by creep behavior at elevated temperatures. A constitutive model for the minimum creep rate is introduced to describe both the linear and the power law creep depending upon the stress level. The proposed constitutive model includes a strain hardening function to describe the primary creep stage. To demonstrate the existence of the linear creep behaviour in the low stress range of application area and the influence of the primary creep behaviour on relaxation, several solutions of a uniaxial stress relaxation problem are presented for the loading values relevant to engineering applications. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structural-statistical analysis of the strain properties of laminated reinforced plastics

Conclusion The structural relations derived here make it possible to analytically predict the main statistical characteristics of the strain properties of an LRP with allowance for the mutual correlation of the random parameters of the structure. Use of the proposed relations is valid for studying combination loading (bending and a plane stress state), as well as when the structure of the composite is unbalanced. We evaluated the scale effect associated with the presence of multiple layers and determined the impact of this effect on the scatter of the strain properties of the LRP for different types of deformation. We also proposed a method of determining the actual statistical characteristics of layers indirectly — on the basis of experimental testing of multilayered unidirectional plastics. One promising prospect is the use of these results to formulate and solve problems involving optimization of the structure of LRP's on the basis of criteria ensuring the composite's reliability and dimensional stability. Finally, the results can also serve as a basis for establishing scientifically substantiated safety factors for stiffness.Translated from Mekhanika Kompozitnykh Materialov, No. 6, pp. 995–1001, November–December, 1990.