A composite model for municipal solid waste

The mechanical behaviour of municipal solid waste is very complex due to the internal structure and differs considerably from ordinary materials. The fundamental idea of the presented mechanical constitutive model is the split of the material into two solid phases, justified by experimental observations, where fibrous particles signi.cantly influence the composite behaviour. Constitutive laws are independently introduced for each phase. A .nite deformation theory is applied, whereas the concept of elastoplasticity allows reproducing the highly non-linear stress-strain relations. A creep law describes time-depending deformation under compression loads. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

On the theory of anisotropic creep of unidirectional glass-reinforced plastics

The basic relations of the theory of anisotropic creep of unidirectional glass-reinforced plastics (GRPs) in a three-dimensional stress state are found using a model of a material with inhomogeneous structure and the rheological properties of the resin. These relations are suitable for investigating the stress state of GRPs for resin stresses not exceeding certain values. The phenomena of simple creep and stress relaxation are studied with reference to a unidirectional GRP with an epoxy-maleic resin matrix.Mekhanika Polimerov, Vol. 1, No. 2, pp. 64–69, 1965     

A non-linear thermo-viscoelastic rheological model based on fractional derivatives for high temperature creep in concrete

In this paper, a novel non-linear thermo-viscoelastic rheological model based on fractional derivatives for high temperature creep in concrete is proposed. The rheological model consists of a linear springpot unit placed in series with a second springpot used for non-linear creep which activates under high stress and temperature. The model parameters which include the dynamic viscosities of the springpots and the fractional exponent are calibrated using existing experimental data of basic creep strain in concrete under constant stress and temperatures for various aggregate types. The power law form of the naturally resulting creep compliance allows an accurate representation of experimental data with the use of only a few model parameters. Furthermore, the variable-order fractional differential stress-strain equation provides a compact method for analytical and numerical modelling of basic creep under conditions of time-varying stress and temperature. In addition, applications of the proposed model to determine axial deformations in columns and transverse deflections in beams under constant and varying temperatures are demonstrated.     

Creep buckling of glass-reinforced plastic cyclindrical shells

The buckling modes of cylindrical shells made of a polymeric composite with creep properties are considered. The initial imperfections of the shell are characterized by a Fourier series. The time dependence of a large number of harmonics of the Fourier series is investigated by means of nonlinear equations of the Timoshenko type. It is established that some are damped, while others grow at an ever-increasing rate. It is shown that in the course of time the effective shape of the deflections is transformed and at the critical moment the shell buckles in a mode that differs from the shape of the initial deflections.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 4, pp. 697–703, July–August, 1971.     

Investigation of the temperature strain relaxation of urethane elastomers by the opticomechanical method and infrared spectroscopy

The data of an experimental investigation of polyester- and polyether-based elastomers are presented. The elastomers were investigated in creep while increasing the temperature in steps of 20°C. Infrared absorption spectra were obtained for a series of loads and temperatures and the variation of the total orientation of the molecular structure (birefringence) and the growth of the deformation with time were recorded. An attempt is made to interpret the changes in the rates of orientation of the molecular structure by varying the concentration of certain characteristic chemical bonds.Translated from Mekhanika Polimerov, No. 3, pp. 437–441, May–June, 1975.     

Post-Buckling Range of Plates in Axial Compression with Uncertain Initial Geometric Imperfections

The method of reliable solutions alias the worst scenario method is applied to the problem of von Karman equations with uncertain initial deflection. Assuming two-mode initial and total deflections and using Galerkin approximations, the analysis leads to a system of two nonlinear algebraic equations with one or two uncertain parameters-amplitudes of initial deflections. Numerical examples involve (i) minimization of lower buckling loads and (ii) maximization of the maximal mean reduced stress.     

Effects of temperature and moisture on the mechanical properties of polyester resin in tension

The effect of environment on the physical and mechanical properties of composite materials in some cases is determined by the environmental sensitivity of the binder. The results of experimental investigation of the deformability and strength of polyester resin, widely used as a binder in composites, upon the action of stationary and quasi-stationary loads, temperatures, and moisture are presented. The ranges of acceptable values of these services factors are determined. The elastic modulus and tensile strength of the material are obtained from quasi-static tests. The viscoelastic behavior of the resin is investigated in creep tests. From the results of a short-term experiment with stepwise loading up to failure, it is found that the creep of specimens with a moisture content of 0.14% can be described by a linear viscoelastic model for stresses up to 20 MPa (two thirds of the strength). The action of single loading impulses is summarized according to the Boltzmann superposition principle. The temperature and absorbed moisture are considered as factors accelerating the relaxation processes in the material. The creep activation under the action of these factors is described using the methods of time-temperature and time-moisture equivalence. The results of short-term creep tests allow us to determine the relaxation characteristics of the material in stationary conditions. The long-term creep under close-to-service conditions is predicted using these data and quite good agreement with the control test is obtained. The sensitivity of the material characteristics (strength, elastic modulus, and creep strain) to the action of temperature and moisture is estimated. The creep strain is most sensitive to the action of environmental factors. For a fully saturated material (moisture content 1.64 wt.%), after one hour creep, this strain four times exceeds that of a dry one. The same growth in deformability is caused by an 18°C increase in temperature. A quantitative comparison of the characteristics of polyester and epoxy resins allows us to choose the binder for composites and to estimate the expected environmental effect. Presented at the 11th International Conference on Mechanics of Composite Materials (Riga, June 11–15, 2000). Translated from Mekhanika Kompozitnykh Materialov, Vol. 36, No. 3, pp. 395–406, May–June, 2000.     

Stability of nonlinear viscoelastic systems under stochastic excitation

The dynamic behaviour of viscoelastic system with due account of finite deflections but under condition of small strains is described by the system of nonlinear integro-differential equations. On an example of a thin plate subjected to loads, which are assumed as random wide-band stationary noises and applied in the plate plane, the stability of nonlinear systems is considered. The stability in a case of finite deflections of the plate is considered as stability with respect to statistical moments of perturbations and almost sure stability. For the solution of the problem, a numerical method is offered, which is based on the statistical simulation of input stochastic stationary processes, which are assumed in the form of Gaussian ”colored” noises, and on the numerical solution of integro-differential or differential equations. The conclusion about the stability of the considered system is made on the basis of Lyapunov exponents. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

饱和多孔弹性Timoshenko悬臂梁的动、静力弯曲

在经典单相Timoshenko梁变形和孔隙流体仅沿饱和多孔弹性梁轴向运动的假定下,基于不可压饱和多孔介质的三维Gurtin型变分原理,首先建立了饱和多孔弹性Timoshenko悬臂梁动力响应的一维数学模型．在若干特殊情形下,该模型可分别退化为饱和多孔弹性梁的Euler-Bernoulli模型、Rayleigh模型和Shear模型等．其次,利用Laplace变换,分析了固定端不可渗透、自由端可渗透的饱和多孔弹性Timoshenko悬臂梁在自由端阶梯载荷作用下的动静力响应,给出了梁自由端处挠度随时间的响应曲线,考察了固相与流相相互作用系数、梁长细比等参数对悬臂梁动静力行为的影响．结果表明:饱和多孔弹性梁的拟静态挠度具有与粘弹性梁挠度类似的蠕变特征．在动力响应中,随着梁长细比的增大,自由端挠度的振动周期和幅值增大,且趋于稳态值的时间增长,而随着两相相互作用系数的增大,梁挠度振动衰减加快,并最终趋于经典单相弹性Timoshenko梁的静态挠度．     

Modelling the viscoelasticity of cell shape changes

In the theory of linear viscoelasticity, creep and relaxation functions are the important delay kernels in the stress-strain relations. Quite recently, a creep function power law has been obtained experimentally for an isolated living cell (on the scale of the cell). The corresponding form of the relaxation function is derived using the interconversion formula. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Creep in materials subject to aging

A strain-hardening theory is used in an attempt to describe the creep in aging materials. Samples of commercial celluloid are tested for uniaxial creep under tension and the results are compared with the corresponding theoretical relations. The latter are found to be in fairly good agreement with the test results.Mekhanika polimerov, Vol. 1, No. 1, pp. 61–64, 1965     

Torsional mode of buckling in nonlinear creep with allowance for the difference in loading and unloading laws

The torsional mode of creep buckling of a thin-walled element of material with nonlinear creep is investigated. The local strains theory is used for the physical relations.Mekhanika Polimerov, Vol. 3, No. 1, pp. 142–145, 1967     

A new correlation method for the aerodynamic service loads determination of a rigid wing based on CFD analysis

A new correlation method for the aerodynamic service loads determination of a rigid wing based on CFD analysis is presented. All flight conditions can be handled by the proposed method. The derived correlation equations are achieved by considering a training fighter aircraft as a prototype. Each wing of aircraft is divided into thirty three parts in the span wise direction. Extensive numerical solutions have been attempted by varying a number of parameters that directly affect the wings aerodynamic loads, such as Mach numbers, angle of attack, control surfaces deflections and etc. For each set of input parameters, the corresponding aerodynamic loads applied to different wing parts are calculated. The resulted loads and the corresponding input parameters are incorporated into a linear regression method in order to develop the appropriate correlation equations. The outputs of the developed equations are the aerodynamic loads at each part of the wing based on the independent variables, which are the above mentioned input parameters. The validity of the developed equations is shown by comparing the loads obtained from the latter equations with the corresponding ones calculated through numerical analysis for different flight conditions. The correlation equations can now be used to calculate the aerodynamic loads at each part for any set of arbitrary values assigned to the input parameters.     

由压痕蠕变试验确定材料的蠕变性能参数

研究由平头压痕蠕变试验来确定受压材料蠕变性能参数的可行性。利用有限元蠕变分析确定在定压痕应力下的压痕蠕变率,重点放在稳定压痕蠕变率和受压材料蠕变性能参数的关系上。详细地研究了压头形状、大小和宏观约束对压痕蠕变响应的影响:当压头的尺寸和受压材料为同一数量级时,宏观约束将有十分明显的影响。提出两种方法来由压痕蠕变试验来确定受压材料蠕变性能参数,并给出了算例,结果有利于准确认识平头压痕蠕变试验,从而拓宽其应用范围。     

Acoustic creep of glass-fiber-reinforced plastic

A method of investigating acoustic creep of polymer composites developing under the effect of static tensile forces and small amplitudes of dynamic stresses with a frequency of 20 kHz acting coaxially with the static loads is considered. Results of investigating acoustic creep of glass-fabric-reinforced plastic are presented. It is shown that the presence of a high-frequency stress component regularly accelerates relaxation processes and reduces considerably the deformation resistance of the material. Generalized curves simulating the long-time static creep of reinforced plastic are constructed by the method of temperature-time, stress-time, and vibration-time analogies. The results of prediction are compared with the control experiment.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, Vol. 9, No. 3, pp. 397–404, May–June, 1973.     

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.     

Creep equations for orthotropic material based on the theory of local deformations

By introducing a time factor into the local deformation function, the theory of local deformations can be used to unite the theories of plasticity and creep. The local deformation function is taken as a rheological dependence in the form of an integral equation, and it is assumed to vary in a way that depends on the direction relative to the principal isotropic axes. Thus, equations are obtained for an orthotropic material with nonlinear creep. The relations obtained also allow for the variation of Poisson's ratio with time. It is shown that special cases of these expressions were previously used to describe the creep of glass-reinforced plastics at low stresses.Mekhanika polimerov, Vol. 1, No. 1, pp. 44–49, 1965     

Analysis of nonlinear polymer creep

The relaxation properties of polyethylene are analyzed. The nonlinear time-dependent stress-strain relations and the creep and relaxation equations are obtained from the experimental creep data. The analysis is based on an appropriate variant of the nonlinear memory theory with singular functions whose parameters, together with the modulus of elasticity, are determined by the method described in [1].Moscow. Translated from Mekhanika Polimerov, No. 3, pp. 410–414, May–June, 1969.     

Mechanical properties of polyester polymer-concrete

The properties of a polymer-concrete composed of polyester matrix and locally available rock aggregate are investigated. The formula of the concrete is found by an experimental-calculation approach in such a way as to attain a closer packing of the aggregate particles on the one hand, and to ensure the needed processing characteristics (placeability) of the mix on the other. It is shown experimentally that the material obtained has a rather high compression strength. Under prolonged compression loads, the polymer-concrete exhibits a noticeable creep behavior with a linear relation between the creep strains and stresses. After the action of half the ultimate load over 3000 h, the total strains exceed the instantaneous ones by 2.0 to 2.2 times. The accumulation of irreversible strains is also observed; however, their contribution to the total strain is small. It is found that the stress-strain relation can be represented by the equation of linear hereditary creep theory.