First crack strength of ferrocement in flexure

Methods based on classical lamination theory have been developed for calculating the bending stiffness and stresses in individual layers, as well as predicting the influence of the synergetic effect on the first crack strength in ferrocement. These methods take into account the arrangement and geometry of meshes and the mechanical properties of components.Riga Technical University, LV-1047 Latvia. Published in Mekhanika Kompozitnykh Materialov, Vol. 33, No. 4, pp. 489–495, July–August, 1997.     

External circular crack under arbitrary shear loading

An exact closed form solution in terms of elementary functions has been obtained to the governing integral equation of an external circular crack in a transversely isotropic elastic body. The crack is subjected to arbitrary tangential loading applied antisymmetrically to its faces. The recently discovered method of continuity solutions was used here. The solution to the governing integral equation gives the direct relationship between the tangential displacements of the crack faces and the applied loading. Now a complete solution to the problem, with formulae for the field of all stresses and displacements, is possible.     

On the determination of crack initiation directions under mixed mode loading

An important issue in the mechanics of multilayered structures is the knowledge of local stress fields. Light‐weight heterogeneous structures with crack‐ or notch‐like discontinuities possess great danger of stress concentration and possible crack initiation leading to failure. In the present study, computational procedure, namely the finite element method, has been used to analyze bimaterial medium with various notch opening angles under arbitrary loading conditions. The prediction of the direction of crack initiation is important for wedge type constructions to gather a first hand knowledge of a potential damage zones which may undermine the integrity of the structure as a whole. The application of computational procedure to predict potential directions of crack initiation gives the flexibility required for different structural configurations, lamina orientations and incorporation of various boundary conditions. Superposition of various external loads to simulate pure mode I, mode II and mixed mode cases can also be carried out with excellent results in computational procedure. All the analysis results are based on the hypothesis of Erdogan and Sih. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Influence of the synergistic effect on formation of the first crack in ferrocement

An empirical approach is proposed for assessing the influence of the synergistic effect on the increase in strength and the strain limit of a cement matrix at the moment of formation of the first crack. Riga Technical University, Riga, LV-1047, Latvia. Translated from Mekhanika Kompozitnykh Materialov, Vol. 32, No. 5, pp. 663–667, September–October, 1996.     

Predicting the transverse strength of unidirectional composites under combination loading

This article presents a mathematical model for predicting the transverse strength of unidirectional fiber composites subjected to combination transverse loading under different conditions. The behavior of the matrix is described by nonlinear physical equations consistent with the strain theory of plasticity for the active loading section. The fibers are assumed to be isotropic and elastic. The boundary-value problem of micromechanics that is formulated includes strength criteria for the matrix and fibers that mark the beginning of their possible failure. The modeling of the fracture process is taken farther through the use of a scheme that reduces the stiffness of the matrix and fibers in the failed regions in relation to the sign of the first invariant of the stress tensor. The method of local approximation is used together with the finite-element method to calculate the stress and strain fields in unidirectional composites with cylindrical fibers in a tetragonal layup. The model is used to study the behavior of an epoxy-based organic-fiber-reinforced plastic subjected to transverse loading in different simple paths — including simultaneous compressive and tensile loads, as well as transverse shear.Paper to be presented at the Ninth International Conference on the Mechanics of Composite Materials (Riga, October 1995).Translated from Mekhanika Kompozitnykh Materialov, Vol. 31, No. 4, pp. 473–481, July–August, 1995.     

Rapid crack growth in a thermoelastic solid under mixed-mode thermomechanical loading

A two-dimensional steady-sate analysis of semi-infinite brittlecrack growth at a constant subcritical rate in an unboundedfully-coupled thermoelastic solid under mixed-mode thermomechanicalloading is made. The loading consists of normal and shear tractionsand heat fluxes applied as point sources (line loads in theout-of-plane direction). A related problem is solved exactly in an integral transformspace, and robust asymptotic forms used to reduce the originalproblem to a set of integral equations. The equations are partiallycoupled and exhibit operators of both Cauchy and Abel types,yet can be solved analytically. The temperature change field at a distance from the moving crackedge is then constructed, and its dominant term is found tobe controlled by the imposed heat fluxes. The role of this termis, indeed, enhanced if the heat fluxes serve to render thecrack as a net heat source/sink for the solid, as opposed tobeing a transmitter of heat across its plane. More generally,the influence of the thermoelastic coupling on this field, aswell as other functions, is found to increase with crack speed.     

Boundary integral equations for an interface linear crack under harmonic loading

The present study is devoted to application of boundary integral equations to the problem of a linear crack located on the bimaterial interface under time-harmonic loading. Using the Somigliana dynamic identity the system of boundary integral equations for displacements and tractions at the interface is derived. For the numerical solution the collocation method with piecewise constant approximation on each linear continuous boundary elements is used. The distributions of the displacements are computed for different values of the frequency of the incident tension-compression wave. Results are compared with static ones.     

Deformation of cylindrical composite shells under combined dynamic loading

Conclusions A procedure has been shown for calculating the stress-strain state of cylindrical multilayer shells made from composite materials under the combined action of dynamic axial compression and dynamic external pressure, as well as with different variants of combined loading with static and dynamic forces. An investigation has been made of the effect on the mode of the buckled shell surface of the ratio of the application rate of dynamic loads; ranges of loading rates have been established in which stresses predominate caused either by axial compression or external pressure. It has been shown that, as a result of preliminary static loading, a marked change occurs in the initial imperfections of the shell mode which affects subsequent dynamic buckling. To calculate the time when the first defect occurs and its location in the shell body, a procedure has been devised for layer-by-layer strength analysis employing a tensor-polynomial criterion. It was demonstrated that the level of preliminary static loading noticeably affects the time until the first failure of the layer, not only a reduction of this time being possible with an increase in the static loads, but also an increase in it.We should also point out the work in [10] where it is shown that it is possible to weaken the susceptibility of the shell to initial imperfections when internal pressure is applied.Translated from Mekhanika Kompozitnykh Materialov, No. 3, pp. 461–473, May–June, 1981.     

Distribution of the strength deviations of plastics under multiple pulse loading

Logarithmically normal dependences, similar to the corresponding graphs for cyclic loading at low rates, have been obtained for the distribution of the number of cycles to failure.Translated from Mekhanika Polimerov, No. 3, pp. 560–562, May–June, 1971.     

Plastic strains under combined loading in tension and torsion

On the basis of the local strains theory the compliance tensor is determined at characteristic points of the combined loading path (axial load with subsequent torsion and vice versa) for plastic and nonlinearly elastic materials.Mekhanika Polimerov, Vol. 2, No. 1, pp. 20–26, 1966     

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.     

Estimation of the long-term strength of polymer materials under arbitrary loading histories

A nonlinear version of the phenomenological theory of long-term strength of polymer materials (viscoelastic bodies) is proposed. It is based on the introduction of a function accounting for the damage accumulation connected with changes in the load intensity. The form of this function may be determined from the results of testing the material with a load changing with time in a certain way, for instance, periodically. As a parameter, the function contains the rate of the changing load or the frequency for periodic loads. For a quasi-isotropic material, the basic relationships of the theory proposed are generalized to the case of combined stresses. The durability (failure time) calculations of the material based on this theory are compared with experimental data for a number of polymer and composite materials in a wide range of loading modes. Translated from Mekhanika Kompozitnykh Materialov, Vol. 35, No. 5, pp. 585–594, September–October, 1999.     

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.     

Form of strength criterion in isotropic materials

An examination is made of the staged form of the generalized criterion of strength in isotropic media. The proposed variant for the criterion is constructed on the basis of the oriented character of the damage around the dangerous point. In this case it is proposed that the damage in some particular direction is effected with a certain probability. An example of its application is given. The process of loading the material is taken to be isothermic and short-term.