Reasons for the lowering of the lifetime of polymers under cyclic loading

A model is proposed which explains the lowering of the lifetime of polymers on transition from static to cyclic loading by the interaction of local stresses from an external load with residual stresses which arose after preceding loading cycles. The results predicted on the basis of the model have been checked experimentally on polymethyl methacrylate, and a good agreement has been shown.M. I. Kalinin Leningrad Polytechnic Institute. Translated from Mekhanika Polimerov, No. 2, pp. 279–283, March–April, 1976.     

Behavior of the supermolecular structure of oriented amorphous-crystalline polymers under cyclic loading

Equipment has been developed for investigating small-angle X-ray diffraction in various phases of the loading period in cyclically extended polymers. The behavior of the large-period structure of oriented polycaprolactam (kapron) films in cyclic extension has been studied. The results are compared with the data on the behavior of the same structure in static tests.Mekhanika Polimerov, Vol. 4, No. 2, pp. 195–199, 1968     

Deformation of polymers under friction loading

A study has been made of the laws of distribution of elastic and residual deformations in the surface layers of polymers subjected to friction. The dependence of these deformations on normal pressure, sliding velocity, and duration of loading has been determined. A relation between deformation and antifriction characteristics has been established, and the relative effect of normal and tangential loads on surface-layer deformation determined.Mekhanika Polimerov, Vol. 3, No. 3, pp. 539–543, 1967     

Volume strains of polymers under short-time loading

The effect of the magnitude of the stress, the type of stress, and the previous loading history on the nonlinear deformation characteristics, the lateral strain coefficient, and the volume strains of two crystalline polymers has been investigated.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 5, pp. 828–833, September–October, 1974.     

Lifetime of amorphous polymers under intermittent loading

The lifetimes of two types of amorphous polymers (polyvinyl chloride and plasticized epoxy resin) under intermittent loading have been investigated. It is established that the lifetime, determined without allowance for recovery time, is less than that under constant load. Under periodic loading at constant stress the lifetime varies with the loading regime. For each of the investigated polymers a lifetime minimum is observed at a certain duration of the loading and recovery periods.Higher Chemical Technology Institute, Sofia. Translated from Mikhanika Polimerov, No. 5, pp. 912–916, September–October, 1971.     

Behavior of rigid plastics under cyclic loading

Research progress on the dynamic fatigue of plastics is briefly reviewed. Attention is concentrated on the problems of damage accumulation and self-heating. The effect of various factors on the fatigue of plastics is considered. The possibility of predicting the cyclic life-time from the results of long-time static strength tests is examined. The prospects for the construction of a theory of fatigue strength in complex states of stress are weighed.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, Vol. 5, No. 1, pp. 97–107, January–February, 1969.     

Accumulation of submicroscopic damage in polymers under simple loading

The accumulation of microdamage in specimens of polystyrene, Ftorlon, and high-pressure polyethylene loaded continuously to failure has been investigated. It is shown that at small specimen lifetimes the residual damage accumulates primarily in the fracture zone. Outside the fracture zone appreciable damage accumulation is observed at specimen lifetimes close to the long-term strength. The ultimate strain at various lifetimes is determined. It is shown that the strain corresponding to failure depends on the fraction of irreversible damage accumulated and is largely determined by the resistance of the polymers to fracture.Plastopolimer Scientific-Production Association, Leningrad. Translated from Mekhanika Polimerov, No. 3, pp. 519–524, May–June, 1972.     

Self-heating and failure of plastics under cyclic loading

It has been shown that plastics heated by cyclic deformation have two characteristic temperatures—a critical temperature T_hc, at which the most heavily stressed part of the test piece is intensely heated, and a temperature T_h, at which the test piece fails. The values T_hc and T_h are determined not only by the physicomechanical properties of the material, but also by the state of stress and strain, by the scale factor, and by the heat transfer conditions. It has likewise been shown that the form of the fatigue fracture surface of plastics is determined by the type of deformation and the temperature T_h.Mekhanika Polimerov, Vol. 3, No. 3, pp. 483–492, 1967     

Viscoelastic-viscoplastic-damage modeling of thermoplastics under long-term cyclic loading

Thermoplastics tend to creep under mechanical loads and recover deformation during unloading. A viscoelastic-viscoplastic-damage model is presented to accommodate the behavior of a thermoplastic under cyclic loading-unloading. Finite element simulations may be computationally expensive in case of many loading cycles. A cycle jump method is proposed to mitigate parts of the cost. It can efficiently approximate large parts of the cyclic calculations by extrapolating internal variables over several cycles at once. Hence, only a fraction of the original computational effort remains. (© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Creep and damage accumulation in orthotropic composites under cyclic loading

Experimental results and theoretical prediction of the response of glassfiber-reinforced polyester under quasi-static, static (creep), and cyclic (fatigue) loading are presented. The nonlinear strain component at static loading and the strain amplitude rate at cyclic off-axis loading of an orthotropic composite are shown to follow the associated flow rule with a single-parameter quadratic potential function. The influence of fatigue damage on deformation is considerable due to the reduction in the elastic modulus of the composite and is apparently negligible with respect to its effect on the parameters of the creep kernel.Presented at the 10th International Conference on the Mechanics of Composite Materials (Riga, April 20–23, 1998).Translated from Mekhanika Kompozitnykh Materialov, Vol. 34, No. 4, pp. 447–460, July–August, 1998.     

Microstructural changes of filled rubber-like materials under cyclic loading

The concept of network decomposition has vastly been used in many micro-mechanical models describing stress-softening of rubber-like materials in the first loading cycle as for example the Mullins-effect. Nevertheless, this concept highly simplifies the rubber matrix by decomposing it into two parallel networks that have no interactions with each other. In order to take into account the interactions between these networks some further extensions to the network decomposition concept seem necessary. To this end, a third network is introduced and the concept of rubber cell is proposed and developed. Using this concept, the influence of the filler network on elasticity of the two networks can be predicted by considering the cell as a RVE. (© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

On the simulation of soils under rapid cyclic loading conditions

When simulating soils, which are subjected to dynamic loadings conditions, base on a convenient soil model, special attention has to be paid to the approximation of the real problem by a suitable initial-boundary-value problem (IBVP). In this regard, the semi-infinite half-space is split into a near-field, which is, in general, the domain of interest, and a far-field. However, truncating the half-space at the near field, which is often sufficient in quasi-static simulations, introduces artificial boundaries at which, in dynamic analysis, the incident waves are reflected back into the domain of interest. Several methods have been proposed in the literature to overcome this problem. The present contribution proceeds from a coupling approach, where the near and the far field are discretised with finite and infinite elements, respectively, and where additionally an energy-absorbing layer is introduced at the interface. The aim of the present contribution is to perform a parametric study associated with a biphasic soil model based on the Theory of Porous Media (TPM). Therein, numerical simulations are carried out in order to judge the energy-absorbing capabilities of the present far-field treatment under different parametric conditions. (© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Fracture mechanics analysis of debond growth in a single-fiber composite under cyclic loading

A model is developed to analyze the growth of a fiber/matrix debond along a broken fiber interface in a single-fiber composite subjected to tension-tension fatigue. The Paris law expressed in terms of debond growth and strain energy release rates is used. An analytical solution for the Mode II energy release rate G _II is obtained for long debonds, where the interface crack growth is self-similar. For short debonds, the interface crack interacts with the fiber break, and therefore a FEM modeling in combination with the virtual crack closure technique was performed to calculate the increase in G _II . Finally, the calculated G _II dependences are summarized in simple expressions that are used to simulate the debond growth in fatigue. A parametric study of the effect of Paris law parameters on the debond growth is performed.     

Instability prevention at the modeling of large elasto-plastic strains under cyclic loading

The cyclic instability phenomenon is investigated at the modeling of large elasto-plastic strains. The possible causes of the cyclic instability and conditions ensuring cyclical stability of elasto-plastic models are analyzed for the case of large strains. Among the possible causes of the cyclic instability the following are considered: the method of strain decomposition on elastic and plastic parts; the constitutive law for the elastic deformation (hypo- and hyper-elasticity); the constitutive equation for the plastic deformation; the constitutive relation for the plastic spin; kinematic hardening law, in particular, the type of the objective rate in the generalized Prager's law. Predictions of 50 various models of the elasto-plastic material have been compared in order to find the causes of the cyclic instability. Two test problems are considered: cyclic simple shear, combined cyclic simple shear and tension-compression. Results of numerical experiments for the various material models are presented and discussed. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Comparison of the reliefs of fracture surfaces in PMMA under static and cyclic loading

Investigations of the relief of fracture surfaces in PMMA with the help of optical and interference microscopes have shown that there are no qualitative differences in the appearance of the fracture surfaces obtained under static and cyclic loading conditions. Quantitative studies have established that the size of the specular zone increases linearly with increase in the logarithm of the lifetime at a given test temperature. Other conditions being equal, in cyclic tests the specular zone is larger than in static tests. The results obtained are considered to be further confirmation of the validity of the theory that under any loading conditions fracture is based on the same thermo-fluctuation mechanism.Mekhanika Polimerov, Vol. 2, No. 1, pp. 52–59, 1966     

Polycaprolactam hardening during cyclic loading

It has been shown that two competitive processes occur during the fatigue of polycaprolactam under symmetrical tension—compression: the buildup of defects—weakening—and the hardening of the material as a consequence of the removal of the plasticizer and structural changes.Kaunas Polytechnic Institute. Translated from Mekhanika Polimerov, No. 5, pp. 815–819, September–October, 1973.     

Fatigue strength and structure changes of polyethylene-oxide films under static and cyclic loading

Conclusions The behavior of the PEO films under long-term loadings show certain special features reflected mainly in the unusual form of the fatigue curve and the presence of three types of creep curves. The most significant factors affecting the fatigue characteristics of the PEO films are the molecular mass and the degree of crystallinity — when these factors increase the fatigue resistance also usually increases. To evaluate suitability of PEO films for service it is promising to use the cyclic bending test. This loading mode characteristic of practice makes it possible to carry out tests at low strain levels and cumulate considerable damage. Examination by the x ray diffraction method showed that in cyclically bent PEO films not only the amorphous but also crystalline structures are damaged.Translated from Mekhanika Kompozitnykh Materialov, Vol. 3, No. 3, pp. 404–412, May–June, 1994.     

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.