Polymer fatigue from the standpoint of the kinetic theory of fracture

A comparison of the static and cyclic lives of various polymers shows that, whatever the loading regime, fracture may be regarded from the standpoint of the thermal-fluctuation theory. Under comparable test conditions the lifetimes in different loading regimes coincide. The experimentally observed cases of a reduced cyclic as compared with static life do not conflict with the kinetic theory of fracture and can be attributed to hysteresis heating effects or to differences in the structural changes that take place in the test material in different loading regimes.From the standpoint of the kinetic theory, fracture is regarded as a certain process that develops in a body under load and not as a critical event that occurs when a critical stress — ultimate strength — is reached.A. F. Ioffe Physicotechnical Institute, Academy of Sciences of the USSR, Leningrad. Translated from Mekhanika Polimerov, No. 1, pp. 70–96, January–February, 1969.     

Nondestructive evaluation of fatigue-induced changes in the structure of composites by an ultrasonic method using a laser

Studies were conducted to nondestructively evaluate the damage to a glass-fiber-reinforced plastic during cyclic and static loading. The evaluation was made by an ultrasonic method employing a laser. In both the unloaded and loaded specimens, the ultrasound attenuation spectrum has a resonance peak attributable to the periodic nature of the structure of the composite. This peak is shifted to the low-frequency region during static loading, due to a decrease in the elastic modulus. The spectra obtained after cyclic loading have no resonance peaks, due to attenuation of the ultrasound over a broad range of frequencies by a large number of fatigue cracks. Additional static loading results in concentration of the cracks near the boundary between the glass fibers and the polymer matrix, which leads to the formation of a resonance peak in the high-frequency region of the spectrum.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. 3, pp. 405–410, May–June, 1995.     

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.     

Comparison of the long-term strengths of polymers under uniaxial alternating and pulsating loads

The laws of fracture of PMMA and PS under uniaxial alternating and pulsating cyclic loading have been investigated at several loading frequencies and two test temperatures. An analysis of the dependence of the long-term strength on stress, heating kinetics and the form of the specimen fracture surface has shown that transition from cyclic tension to more complex alternating loading in the tension-compression regime does not change the basic nature of the cyclic fatigue effect and is reflected only in an increase in the local stress concentration and heating.A. F. Ioffe Physicotechnical Institute, Academy of Sciences of the USSR, Leningrad. Translated from Mekhanika Polimerov, No. 3, pp. 483–488, May–June, 1972.     

Moisture and low temperature impact on the properties of a polyether glass-reinforced plastic

The cyclic action of water and the combined action of water and low temperatures on the physical-mechanical properties of a polyether glass-reinforced plastic are studied. For experimental investigations, methods of sorption, thermogravimetry, and mechanical testing in static bending were used. It was found that the most aggressive external factor for the glass-reinforced plastic was the cyclic action of water, but not the change in its phase state at cooling. The process of sorption-desorption of the glass-reinforced plastic is irreversible.Presented at the 10th International Conference on the Mechanics of Composite Materials (Riga, April 20–23, 1998).Institute of Non-Metallic Materials, Siberian Branch of the Russian Academy of Sciences, Yakutsk, Russia. Translated from Mekhanika Kompozitnykh Materialov, Vol. 34, No. 4, pp. 519–524, July–August, 1998.     

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.     

Role of heating in connection with the reduced life of cyclically stressed polymers

An investigation of the effect of heating connected with cyclic deformation on the life of three polymer materials has shown that the measured heating cannot account for the experimentally observed reduction in fatigue life as compared with static loading.Mekhanika Polimerov, Vol. 4, No. 3, pp. 483–488, 1968     

Polymer fractography and fracture kinetics

On the basis of an analysis of the fractographic data for two polymeric materials (polymethyl-methacrylate and polycaprolactam) it is shown that it is possible to differentiate the effects of local heating and structural changes at the end of the main crack on the reduced lifetime of polymers under cyclic as compared with static loading. A method is proposed for estimating the values of the local temperature and the structure-sensitive coefficient in the cyclic lifetime equation.For communication 2 see [3].Ioffe Physicotechnical Institute, Academy of Sciences of the USSR, Leningrad. Translated from Mekhanika Polimerov, No. 5, pp. 869–874, September–October, 1971.     

Dynamic properties of certain unvulcanized rubbers in the high-elastic and fluid states

The dynamic properties of a series of unvulcanized rubbers (cis-polybutadienes, Na-polybutadiene, cis-polyisoprene, butyl rubber, polyisobutylene, and ethylene-propylene copolymer) have been investigated in a low-frequency dynamic testing machine and a frequency rheometer on the frequency range from 5 · 10^–2 to 4.5 · 10³ Hz at temperatures of 25 and 60°C. At these temperatures the mechanical loss factor is the most sensitive criterion of transitions from one physical state of the polymer to another. For all the specimens investigated on the experimental range of angular frequencies the modulus of the complex dynamic viscosity and the effective viscosity determined under static conditions coincide, assuming the equivalence of angular frequencies and shear rates.A. V. Topchiev Institute of Petrochemical Synthesis, Academy of Sciences of the USSR, Moscow. Translated from Mekhanika Polimerov, No. 4, pp. 698–703, July–August, 1969.     

Theoretical model of the anisotropy of low-cycle deformability of glass-fiber reinforced plastics

Conclusion A theoretical model is suggested for the anisotropy of low-cycle deformability of glass plastics determined by the anisotropy of the deformational properties of the material in short-term static tension (compression). The effect of the anisotropy of cyclic loss of strength of the deformational properties is taken into account by the introduction of the characterizing function which is taken to be independent of the mechanical properties of the material and of the conditions of low-cycle loading.Translated from Mekhanika Kompozitnykh Materialov, No. 3, pp. 437–442, May–June, 1985.     

Fracture of a glass-reinforced plastic in cyclic tension-compression

Uniaxial tension-compression tests have revealed the existence of a correlation between the static and cyclic deformation characteristics of a glass-reinforced plastic. The possibility of calculating the cyclic lifetime from the static test data using Bailey's rule and Zhurkov's equation for the time dependence of the strength has been investigated. A method of allowing for the particular characteristics of the fatigue fracture of a reinforced material, based on the correlation between the static and cyclic deformation, is proposed and is found to lead to a considerable improvement in the convergence of the theoretical and actual cyclic lifetimes.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 4, pp. 654–662, July–August, 1971.     

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     

Kinetic theory of strength as a scientific basis for predicting the lifetime of polymers under load

The possibility of using advances in the kinetic theory of strength for predicting the performance of polymers is analyzed. It is shown that the theory provides a scientific basis for predicting the life of polymers under load not only in uniaxial tension at constant stress, when the basic lifetime equation with constant coefficients ₀, U₀, and holds, but also in the presence of various complications — in the region of small stresses and high temperatures, in the presence of structural changes during service, in complex loading regimes (including cyclic loading) and complex states of stress, and, moreover, when the stressed polymer is exposed to radiations or aggressive media.Presented at the All-Union Symposium on Predicting Service Properties of Polymeric Materials, Tbilisi, 1970.Ioffe Physicotechnical Institute, Academy of Sciences of the USSR, Leningrad. Translated from Mekhanika Polimerov No. 1, pp. 98–112, January–February, 1971.     

Nature of low temperature relaxation of polyformaldehyde

The viscosity of high and low crystallinity samples of homopolymer and copolymer polyformaldehyde were studied in the low temperature region at subaudible frequencies. On the basis of the data received, suggestions regarding the low temperature relaxation of polyformaldehyde are put forward.T. G. Shevchenko State University, Kiev. Translated from Mekhanika Polimerov, No. 4, pp. 723–725, July–August, 1972.     

Growth of arterial cracks in polymers subject to static and fatigue loading

The growth kinetics of artificial and natural cracks (the former arising from notches) are studied in viscose, di- and triacetyl cellulose, nitrocellulose, polymethylmethacrylate, polystyrene, polypropylene, and carpon films by a micro-motion-picture method over a wide temperature range under both static and cyclic (fatigue) loading. In all the cellulose materials studied, the time required to form the nuclei of visible cracks is much shorter than the total life of the sample. The initial rate of crack growth depends exponentially on the applied stress and test temperature, so that the equation for the growth rate of an arterial crack is analogous to the general life equation. The different ways in which stress and test temperature affect the parameters of the equation describing the kinetics of crack growth enable us to distinguish the effect of local heating and that of the relaxation processes in the fatigue problem.A. F. Ioffe Physicotechnical Institute, Academy of Sciences of the USSR, Leningrad. Translated from Mekhanika Polimerov, No. 2, pp. 253–265, March–April, 1970.     

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.     

Static and dynamic elasticity of some thermotropic liquid-crystalline copolyesters

Two series of thermotropic liquid crystalline copolyesters were studied in static and dynamic tensile deformation. Static loading generally does not lead to any peculiar phenomena: the stress-strain diagrams are linear or convex, except one copolyester fibre, the stress-strain diagram of which is concave at high temperatures. On the contrary, in dynamic loading the elasticity modulus increased during deformation for all the samples investigated. So it can be assumed that formation of a more rigid and stable structure takes place due to the dynamic deformation.Published in Mekhanika Kompozitnykh Materialov, Vol. 33, No. 5, pp. 704–709, September–October, 1997.     

Change of the physicomechanical properties of fibrous polymer composites exposed to moisture and low temperatures

We investigated the cyclic effect of water and the simultaneous effect of water and low temperatures on the properties of fibrous polymer composites. Methods of sorption, thermogravimetry, and mechanical tensile, bending, and interfacial shear tests were used. It was established that for carbon plastic and organite the most aggressive external factor is water, and the vulnerable link is the matrix and the reinforcing fiber. For boron plastic the aggressive factors are water and the joint effect of water and low temperatures; the vulnerable link is the fiber—matrix interface.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. 501–508, July–August, 1995.     

An experimental study of the deformation of multilayer cylindrical shells

Experimental results are presented on the deformation of two-layer and four-layer reinforced shells subject to static loading. The strains on the outside and inside surfaces are unequal at points remote from the region of the edge effect when the shell is subject to an internal pressure and an axial force.P. I. Baranov Central Institute for Aviation Engine Design. Translated from Mekhanika Polimerov, No. 5, pp. 943–944, September–October, 1973.     

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