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.     

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.     

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.     

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.     

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.     

Evaluation of layered composites by laser optoacoustic transducers

An ultrasonic method for evaluation of composite structures by laser-excited wide-band acoustic pulses is proposed. Two types of laser optoacoustic transducers, in transmission and reflection modes, are used in the spectral range of 0.1–10 MHz. In the first case, the frequency dependence of the attenuation factor of ultrasound is measured. In the reflection mode, the optoacoustic transducer allows us to obtain the time-dependent shape and spectrum of the backscattered acoustic signal with one-sided access to a specimen or product. Specimens of a layered graphite-epoxy composite of different porosity (up to 1.2%) are studied. In the frequency range of 1–5 MHz, the attenuation factor of ultrasound measured in the transmission mode is the greater the higher the porosity of the specimen. The noise part of the backscattered acoustic pulse is found to increase with increasing porosity of the specimens. A 1% growth in porosity causes a threefold increase in the noise power. The results obtained confirm the advantages of the laser optoacoustic method for evaluation of composite materials and products during their manufacturing and operation.International Laser Center, Lomonosov Moscow State University, Moscow, Russia. Translated from Mekhanika Kompozitnykh Materialov, Vol. 35, No. 1, pp. 125–134, January–February, 1999.     

Fatigue strength of glass fiber/epoxy angle-ply laminates with different matrix ductility

Conclusion Angle ply laminates made up of glass fiber/epoxy plies have a good static strength for laminate angles up to about 45 °. This is due to the limitation of transverse strain in the plies due to constraints exerted by neighboring plies. At laminate angles of 50 ° and above the transverse and shear strains in the plies are not sufficiently constrained and cause failure in the matrix material. The constraint effect is present in fatigue but since the polymeric matrix material is sensitive to fatigue loading fatigue failure will occur at much lower strain levels than in static loading. This effect cannot be offset by increasing the ductility of the matrix material. Thus, strain to failure under fatigue loading will be only a small fraction of the static strain to failure for angle ply laminates with ply angles up to around 45 ° where the static strength is due to the constraint effect.Published in Mekhanika Kompozitnykh Materialov, No. 5, pp. 632–638, September–October, 1992.     

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.     

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.     

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     

Modeling the influence of mineral content and porosity on ultrasound parameters in bone by using synthetic phantoms

Model composite media – 10×15×80 mm³ bone tissue phantoms based on an epoxy resin with fillers—were made to study the influence of porosity and mineral content on ultrasound velocity and attenuation. The pores were simulated by 1 mm³ particles of a soft rubber, while the mineral content was imitated by a mineral residue of natural bone obtained by burning and grinding. The porosity and mineral content were varied in the range of 0–70% by volume with a step of 10%. The velocity, attenuation, and prevalent frequency of ultrasound were measured by the pulse transition method, using transducers with nominal frequencies 0.1, 0.2, 0.5, and 1.0 MHz. It was experimentally found that the ultrasound velocity decreased nearly exponentially with growth in porosity, while the velocity dispersion was negligible at frequencies >0.2 MHz; the ultrasound attenuation increased linearly with growth in porosity and strongly depended on the frequency; the velocity increased nonlinearly with growth in mineral content above 40%; the attenuation did not exhibit a distinct dependence on the mineral content; the porosity provoked a shift in the prevalent frequency of transducers, tending to the common value of 0.2 MHz, while the mineral content did not excite similar changes. The complex measurement of velocity, frequency-dependent attenvation, and prevenlent frequency of ultrasound is proposed in ultrasonic diagnostics of bone for more precise determination of the influence of the porosity and the degree of mineralization on the bone condition.Published in Mekhanika Kompozitnykh Materialov, Vol. 35, No. 2, pp. 211–220, March–April, 1999.     

Comparison of concrete slabs pretensioned with composite bars and steel wires

A test program was set up to study AFRP as prestressing reinforcement for concrete slabs. The paper describes short-term, long-term and cyclic loading tests on ten slabs. The structural behavior of the pretensioned slabs, under these loading conditions, is investigated. Finally, it is concluded that the use of AFRP as prestressing reinforcement can offer promising perspectives.Presented at the Ninth International Conference on the Mechanics of Composite Materials. Riga. October, 1995.University of Ghent. Department of Structural Engineering, Magnel Laboratory for Concrete Research, Technologiepark-Zwijnaarde 9, 9052 Ghent (Belgium). Published in Mechanika Kompozitnykh Materialov, Vol. 32, No. 2, pp. 149–157, March–April. 1996.     

An estimation of reliability of unidirectional composites by catastrophe theory

In the present work, interest is centered on the theory of fracture and practical approaches to reliability estimation of unidirectional composite materials which are based on it. Fracture mechanics is considered not as a theory of macrocracks but as mechanics of fracture mechanisms of the composites with allowance made for probability estimation. A model of composite material with fibers eliminated from the carrying scheme and parameters specific to the stress-strain-damage state (SSDS) was considered. According to the model, during loading, self-accelerated energy rise is accounted for by structural damages growth — the catastrophe takes place. The model allows us to calculate the critical stress. The composites redistribute forces from overloading zones to neighboring ones by microstructural deformations and damages. It is the effect of reservation of carrying ability. Due to this effect, low-value probability of fracture is sufficiently less for the composite than for homogeneous materials. The approach allows us to evaluate the reliability function for both static loading and fatigue.Presented at the Ninth International Conference on the Mechanics of Composite Materials, Riga, October, 1995.Computing Center of the Russian Academy of Sciences, Krasnoyarsk, Russia. Published in Mekhanika Kompozitnykh Materialov, Vol. 32, No. 4, pp. 539–548, July–August, 1996.     

Investigation of stress intensity factors near interacting cracks in linear viscoelastic plates

Stress intensity factors as a function of time near interacting cracks during creep in plates made of linear viscoelastic materials are investigated. The interaction of collinear as well as parallel cracks disposed on different levels perpendicular to the tensile stress was studied. It was found that during interaction of collinear cracks, the stress intensity factor K₁(t) grew in creep and varied mainly due to the change in the distance between the tips of interacting cracks. There was practically no increase in the length of the crack. During interaction of parallel cracks, the value of K₁(t) decreased slightly in time, which was also due to an increase in the distance between the tips of the cracks in creep.Institute of Mechanics, Lomonosov Moscow State University, Moscow, Russia. Translated from Mekhanika Kompozitnykh Materialov, Vol. 34, No. 5, pp. 643–650, September–October, 1998.     

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     

The effect of overloads on the residual strength and life of laminated GRP

The effect of a short duration cyclic overload on the residual life and strength of laminated glass-fiber reinforced polyester is studied. A uniaxial tensile fatigue loading with the stress ratio 0.1 is considered. The residual life of the composite decreases due to the overload, while the residual strength is almost unaffected. A reasonable agreement of experimental data with the prediction by a residual strength model and by Miner's rule is observed.Translated from Mekhanika Kompozitnykh Materialov, Vol. 35, No. 5, pp. 701–706, May–June, 1999.     

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     

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.     

Experimental investigation of the strength of structural plastics in multiple pulsed bending

Strip-beams of molded fabric-reinforced plastic, unidirectional wound glass-reinforced plastic, and polystyrene and syntactic foam have been tested in pulse bending in a modified electrodynamic testing device. The main stages of the failure process and the nature of the relation between the peak load and the number of cycles to failure have been established. For all the materials tested these relations are similar to the cyclic strength curves obtained at low loading rates. It is shown that for glass-reinforced plastics there is a correlation between the various stages of the failure process.Translated from Mekhanika Polimerov, No. 2, pp. 337–341, March–April, 1971.     

Fracture of plates,the compression of which is accompanied by cracking

The appearance of fracture shapes, analogous to fractures in the shape of a "Chinese lantern" in rectangular plates has been discussed, the compression of which is accompanied by cracking. It has been shown that one or two cracks can appear when rectangular hinged plates are subjected to compression. An arbitrary number of cracks can be formed when a cylindrical shell is subjected to compression.Paper presented at the 9th International Conference on the Mechanics of Composite Materials, Riga, October (1994).Translated from Mekhanika Kompozitnykh Materialov, Vol. 31, No. 3, pp. 326–329, May–June, 1995.