Long-time strength of high-elastic polymers

This article examines the question of the inadequacy of the Zhurkov equation for describing the laws of long-time strength of polymers, loading of which is accompanied by the development of high-elastic or induced-elastic deformations. On the basis of general considerations concerning the deformation dependence of the stress concentration coefficient and orientation hardening of polymers, a more general relation between static fatigue life and long-time strength is proposed.Mekhanika Polimerov, Vol. 1, No. 6, pp. 85–91, 1965     

Strengthening of glass-fiber-reinforced plastic under biaxial tension

Conclusions 1. The effect of axial tension, applied at the first stage of loading under biaxial tension, on the increase in the hoop strength of an oriented glass-fiber-reinforced plastic was observed. The increase in the hoop strength depends both on the value of the axial tensile load, and on the angle of winding of the fibers.2. The effect observed is in this case apparently independent of the condition of loading of the sample under biaxial tension.Institute of Mechanics, Academy of Sciences of the Armenian SSR, Erevan. Translated from Mekhanika Polimerov, No. 6, pp. 1025–1029, November–December, 1976.     

Long-time strength of isotropic polymers in plane stress state

A criterion of strength of isotropic polymers in the form of the critical work done by stress was postulated and shown, by statistical analysis, to be independent of the time and form of loading. The introduction of the time factor in the existing theories of strength of isotropic materials was postulated and experimentally verified with the aid of this criterion.Mekhanika Polimerov, Vol. 2, No. 6, pp. 837–844, 1966     

Long-time study of Poisson's ratio for polyethylene stressed in the small strains range

The time dependence of the Poisson's ratio for low-pressure polyethylene has been studied by a method involving superposition of time and temperature (in the short-time range) and by recording creep curves for the longitudinal and transverse strains (under prolonged loading). It is shown that is not constant, its value changing from approx. 1/3 for short loading times to approx. 1/2 under prolonged loading conditions.Mekhanika Polimerov, Vol. 1, No. 3, pp. 52–56, 1965     

Deformation criterion of the strength of plastics in biaxial compression

The deformation at break under uniaxial loading is taken as a criterion of the strength of an anisotropic glass-reinforced plastic in biaxial compression. Formulas are obtained for determining the breaking stresses and their limits of applicability are investigated. The calculated values of the breaking stresses are in agreement with the experimental values for both instantaneous and long-term loading.Mekhanika Polimerov, Vol. 4, No. 2, pp. 276–281, 1968     

Relations between the strain and stress tensors in successive biaxial tension

The manner of applying the load is one of the main factors governing the formation of the principal strains and their limiting values. Starting from the theory of local strains, relations between the strain and stress tensors are formulated for both simple and complex loading. It is shown that the principal strains associated with a given stress state vary depending upon the loading path.Mekhanika Polimerov, Vol. 1, No. 3, pp. 43–51, 1965     

Analysis of the carrying capacity of panels under biaxial tension with shear

A method is described applicable to the analysis of the carrying capacity of flat or curved panels, made of composite materials, under biaxial tension with shear. The design of a test apparatus is shown, and the errors in simulating a given load distribution on it are evaluated.     

Static strength and failure mechanism of CFRP under biaxial loadings

Tests of cross-ply composite tubes were performed under combined axial and torsional loading up to failure. Strength properties and failure mechanisms were evaluated with reference to the biaxiality ratio of the loading. The scattering of the biaxial strength data was analyzed using the Weibull distribution. The axial contraction of carbon fiber-reinforced plastic (CFRP) tubes under biaxial loading was investigated theoretically and experimentally. Artificial neural networks were introduced to predict the failure strength using the algorithm of the error back-propagation. The prediction was also made by the Tsai-Wu theory using the experimental data and by the combined optimized tensor-polynomial theory. A comparison shows that the artificial neural network has the smallest root-mean square (RMS) error of the three prediction methods. The prediction of the axial contraction of the tubes correlates well with the results of a linear variable differential transformer (LVDT) of the testing machine. From the phenomenological analysis of the failure and the fractographic observations of the fracture surface, three types of failure modes and microscopic failure were investigated, depending on the biaxiality ratio, and the corresponding failure mechanisms are suggested.Submitted to the Tenth International Conference on Mechanics of Composite Materials, Riga, April 20–23, 1998.Department of Mechanical Engineering, Pohang University of Science and Technology, Pohang 790–784, Korea. Published in Mekhanika Kompozitnykh Materialov, Vol. 34, No. 1, pp. 38–56, January–February, 1998.     

Experimental investigation of the strength of anisotropic materials in biaxial and triaxial compression

It is proposed that the strengths in uniaxial and uniform biaxial compression in the direction of the axes of symmetry be used as parameters. The applicability of a strength criterion in the form of a fourth-degree polynomial to glass-reinforced plastics in biaxial compression in the plane of reinforcement and the transversal plane and to wood in triaxial compression has been experimentally confirmed.S. M. Kirov Leningrad Forest Engineering Academy. Translated from Mekhanika Polimerov, No. 6, pp. 991–996, November–December, 1973.     

Development of slits in polyethylene and polypropylene films stressed in uniaxial tension

Optical microscopy has been used to investigate the development of a slit in polyethylene and polypropylene films with spherulites of different types and sizes stressed in uniaxial tension. The angle between the slit and the direction of extension was 0, 45, or 90°. Slits at 45 and 90° initially develop through localization of slits by a stronger oriented polymer and subsequently as a result of failure of the oriented polymer at the tip of the slit. A longitudinal slit does not develop and does not affect the deformation of the film.Mekhanika Polimerov, Vol. 3, No. 1, pp. 95–98, 1967     

Structure formation in a crosslinked polyethylene melt crystallizing under uniaxial tension

The formation of supermolecular structure has been investigated in the case of medium-pressure polyethylene film subjected to a dose of 30 Mrad of n, radiation. The melt, prestretched at 160°C to various degrees of elongation , was crystallized at constant temperature. The results are interpreted on the basis of an analysis of the effect of the mechanical field on the growth kinetics and the orientation relative to the direction of extension of the individual crystallites; the first factor determines the shape and the second the sign of the spherulite.Institute of High-Molecular-Weight Compounds, Academy of Sciences of the USSR, Leningrad. Translated from Mekhanika Polimerov, No. 1, pp. 3–6, January–February, 1971.     

Calculation of the long-term strength of thin-walled polymeric tubes under biaxial loading

The problem on calculating the failure time of thin-walled polymer tubes in creep under internal pressure, internal pressure with tension, pure torsion, and torsion with tension is solved. The solution is constructed based on the concept of equivalent stresses. A mixed long-term failure criterion taking into account the sings of principal stresses is used as the equivalent stress. The calculation results are in a satisfactory agreement with experimental data.Translated from Mekhanika Kompozitnykh Materialov, Vol. 40, No. 6, pp. 811–826, November–December, 2004.     

Relation between the vectors E and S for a plastic material in succesive biaxial tension

The local strains theory is used to determine the values of the components of the vector E{E₁, E₂} in five-dimensional space for a material subjected to successive biaxial tension at degrees of nonlinearity n=3 and n=5. The relation between the vectors E and S and the tangents to the loading and strain trajectories is considered together with the relation between the moduli E of the strains for simple and complex loading.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 6, pp. 975–979, November–December, 1969.     

Experimental investigation of the strength of glass-reinforced plastics in biaxial compression in three planes of symmetry

Glass-reinforced plastics (GRP) with fiber (wound) and fabric (hot-molded textolites) reinforcement have been investigated in biaxial compression in the three planes of symmetry. The high strength in biaxial compression in the transverse plane, observed for all the GRP investigated, is explained. A method of investigating the tensile strength of the fiber components of the GRP from the results of testing a unidirectional composite in biaxial compression in the transverse plane is proposed. The possibility of a nondestructive estimation of the strength of GRP in biaxial compression at an arbitrary stress ratio is demonstrated.Translated from Mekhanika Polimerov, No. 1, pp. 63–72, January–February, 1976.