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.     

Problems in the prediction of the long-term strength of polymer materials. A review

Conclusions As in the case of the selection of the simplified equations used in the theory of nonlinear viscoelasticity, variants of the equations of the temperature-time dependence of the strength of polymer materials are constructed on the basis of some similarity condition of the experimental lifetime curves. Principles according to which it would be possible to make a prior assignment of a material to some class on the basis of similarity criteria only using, for example, information on chemical composition and structure without testing for long-term strength are lacking. The equations presented in this review may be used for predicting the lifetime only when the nature of the resistance to long-term destruction over time in a given temperature range exactly corresponds to the assumptions inherent in some equation. Thus, the selection of the boundaries of the temperature ranges, in which we may assume uniformity for the deformation and destruction mechanisms leading in the final analysis to identical types of destruction, becomes most significant. Hence, the parametrical methods have a series of advantages since they do not place rigid limitations on the nature of the long-term strength curves at a given temperature.The temperature-time analogy method is the most favored method for predicting long-term strength since this method, without negating other approaches, is their generalization. The use of this method is most promising for cases of a complex stressed state and complex stress pathways. In this case, the temperature -time shift function is most conveniently introduced into the surface strength functionals.Translated from Mekhanika Kompozitnykh Materialov, No. 4, pp. 694–704, July–August, 1979.     

Comparison of the temperature-time dependences of polymer strength under static and cyclic loading

A comparison of the temperature-time dependences of the strength of highly oriented fibers under static and cyclic loading shows that in the region of high temperatures and low loading frequencies the static and cyclic specimen lives always coincide. The discrepancy between the static and cyclic lives observed in the region of low temperatures and higher loading frequencies is related with differences in the structural changes in the different loading regimes. These changes are relaxational in nature.The experimental data were reported at the 14th and 16th All-Union Conferences on High-Molecular Compounds (1964 and 1966).A. F. Ioffe Physicotechnical Institute, Leningrad. Translated from Mekhanika Polimerov, Vol. 4, No. 4, pp. 648–655, July–August, 1968.     

Effect of the starting structure of a polymer on the time dependence of its strength under cyclic loading

The stress dependence of the logarithm of the cyclic life for different modifications of the same polymer is represented by a fan of straight lines that converge upon extrapolation to zero stress at a single point log A_cy, which differs from the analogous extrapolation point log A_st. The possible causes of this effect are discussed.Ioffe Physico-Technical Institute, AS USSR, Leningrad. Translated from Mekhanika Polimerov, Vol. 4, No. 6, pp. 1008–1013, November–December, 1968.     

Long-Term Strength Criteria for Some Polymer Materials under a Plane Stress State

The problem of the choice and experimental justification of long-term failure criteria for isotropic polymer materials in creep under a plane stress state is considered. The criteria are defined by a linear two-parameter interpolation relating two stress-tensor invariants which limit the interpolation range with respect to the conditions of viscous and brittle failure and take into account the signs of principal stresses. The base experiment for determining material constants includes standard tests on long-term strength in uniaxial tension and a test on long-term strength under a plane stress state. The failure criteria have been approved in constructing unified long-term strength diagrams for thin-walled tubular specimens made of rigid polyvinylchloride and high-density polyethylene under the action of internal pressure, pressure with axial tension, torsion, and axial tension with torsion.     

Nonaxisymmetric thermal stress state of laminated rotational bodies of orthotropic materials under nonisothermic loading

A procedure for numerical investigation of nonaxisymmetric temperature fields and the elastic stress-strain state of laminated rotational bodies of cylindrically and rectilinearly orthotropic materials under nonisothermal loading is proposed. The deformation of orthotropic materials is described by the equations of anisotropic elasticity theory. The equations of state are written in the form of Hookes law for homogeneous materials, with additional terms which take into account the thermal deformation, changes in the mechanical properties of materials in the circumferential direction, and their dependence on temperature. A semianalytic finite-element method in combination with the method of successive approximations is used. An algorithm for numerical solution of the corresponding nonlinear boundary problem is elaborated, which is realized as a package of applied FORTRAN programs. Some numerical results are presented.Translated from Mekhanika Kompozitnykh Materialov, Vol. 40, No. 6, pp. 731–752, November–December, 2004.     

The ultimate state of polymeric materials and laminated and fibrous composites under asymmetric high-cycle loading

The prediction of the high-cycle fatigue strength of polymeric and composite materials in asymmetric loading is considered. The problem is solved on the basis of a nonlinear model of ultimate state allowing us to describe all typical forms of the diagrams of ultimate stresses. The material constants of the model are determined from the results of fatigue tests in symmetric reversed cycling, in a single fatigue test with the minimum stress equal to zero, and in a short-term strength test. The fatigue strength characteristics of some polymers, glass-fiber laminates, glass-fiber-reinforced plastics, organic-fiber-reinforced plastics, and wood laminates in asymmetric tension-compression, bending, and torsion have been calculated and approved experimentally. __________ Translated from Mekhanika Kompozitnykh Materialov, Vol. 44, No. 1, pp. 87–102, January–February, 2008.     

Adhesive ability of a heat-resistant double-chain polymer and the strength of CFRP based on it

The adhesive ability of a heat-resistant polyiminoquinazolindione (PIQD) binder, based on a double-chain polymer, and the physicomechanical characteristics of unidirectional CFRPs made with it are investigated. It is shown that, at room temperature, the strength of model adhesive joints (PIQD-steel wire) and of the CFRPs in shear and bending is rather low — about half of that of similar specimens based on an epoxy binder. At the same time, all their mechanical characteristics, to a large measure (50%), are retained at temperatures up to 450°C, which considerably exceeds the heat resistance of all polymer matrices used at the present time. The elastic modulus of the CFRPs in bending practically remains the same up to 450°C. __________ Translated from Mekhanika Kompozitnykh Materialov, Vol. 44, No. 4, pp. 535–546, July–August, 2008.     

The stress field problem for a pre-stressed plate-strip with finite length under the action of arbitrary time-harmonic forces

In the framework of the three-dimensional linearized theory of elastodynamics the finite element modeling of the stress field problem for the pre-stressed plate-strip with finite length resting on a rigid foundation under the action of inclined linearly located time-harmonic forces is developed. The numerical results involving the normal stress acting on the interface plane of the plate-strip and the rigid foundation are presented. Moreover, the dependencies between this stress, the frequency of the arbitrary inclined linearly located external force and the initial stretching of the plate-strip are analyzed.     

Description of the elastic deformation and degradation of elastic properties of dispersedly failing isotropic materials

A model is put forward for describing the elastic deformation of a quasi-homogeneous isotropic material capable of accumulating scattered microdamages during loading, which eventually leads to its total failure. The degree of damage of the material at a point is characterized by a centrosymmetric scalar function on the unit sphere, named the damage function, whose values depend on a dimensionless equivalent stress. This function is approximated by a fourth-rank tensor, which is used to construct a constitutive relation between stresses and strains in a differential form. By way of example, the elastic deformation of concrete and the degradation of its linearly elastic properties are described, and the basic three-dimensional sections of the corresponding strength surface are constructed. __________ Translated from Mekhanika Kompozitnykh Materialov, Vol. 42, No. 2, pp. 193–208, March–April, 2006.