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.     

Effect of anisotropy on the cyclic deformation and heating of SVAM-type glass-reinforced plastics

The effect of anisotropy of the mechanical properties on the cyclic deformation and heating of 1:1 and 5:1 SVAM-type glass laminates has been investigated for symmetrical tension-compression and pulsating tension and compression. It is shown that on the main part of the fatigue curve, depending on the anisotropy and the cyclic stress level, the temperature may be stabilized. At the same time, fracture is invariably accompanied, under fixed experimental conditions, by a rise in temperature to a certain value that depends only on the anisotropy property of the glass-reinforced plastic, the strain also reaching critical values that are determined by the cycle asymmetry and the anisotropy property of the material.Institute of Mathematics and Mechanics, Academy of Sciences of the Armenian SSR, Erevan. Translated from Mekhanika Polimerov, No. 5, pp. 898–903, September–October, 1971.     

Investigation of the relief of the fracture surfaces of epoxy compositions subjected to cyclic loading

Epoxy compositions containing various amounts of plasticizer have been fractured by cyclic loading under conditions of circular cantilever bending. The relief of the fracture surfaces is discussed.Design-Technological Scientific Research Institute, Tallinn. Translated from Mekhanika Polimerov, No. 6, pp. 1014–1017, November–December, 1969.     

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     

Fatigue strength of glass-reinforced plastics (GRP) in shear in the plane of reinforcement

The results of an investigation of the fatigue strength of three grades of glass-reinforced plastic are described. It is shown that cyclic loading with gradually increasing stress amplitude can be used to determine the fatigue strength at a given ambient temperature and loading frequency.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 5, pp. 928–931, September–October, 1975.     

Damping properties of soft viscoelastic materials for certain combinations of a planar stress distribution

The results of experimental studies of the energy scattering by soft, viscoelastic materials with certain combinations of a planar stress distribution, established by the superposition of a cyclic shear on tension-compression, are given. Special graphs were used for evaluating the damping properties of the materials, i.e., lines of equal damping, which enabled the equivalent uniaxial stress to be found for each combination of planar stress distribution, which produced the same specific scattering of energy. It is shown that the use of the superposition principle for calculating the mechanical losses in a planar stress distribution from experimental data obtained in uniaxial cyclic loading leads to large errors.Volgogradskii Polytechnic Institute. Translated from Mekhanika Polimerov, No. 6, pp. 999–1005, November–December, 1972.     

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     

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.     

Fatigue and fracture of stabilized polycaprolactam

We have investigated the effect of certain antioxidants and structure-forming agents on the mechanism and work of fatigue fracture of polycaprolactam under cyclic loading. We have analyzed the super-molecular structures and fracture surfaces of the most typical stabilized specimens and offer an explanation of the fatigue hardening mechanism in stabilized polycaprolactam.Mekhanika Polimerov, Vol. 3, No. 4, pp. 713–718, 1967     

Determination of fatigue life in bending with rotation

The fracture and stress redistribution processes in a specimen stressed in cyclic bending with rotation are considered. The calculations are based on the assumption of linear damage accumulation and a linear dependence of the modulus of elasticity on the damage; the fatigue strength curve for cyclic tension-compression is employed. The calculated damage accumulation and crack penetration times are compared. Specific calculations are made for the case of PMMA fatigue under isothermal conditions.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 5, pp. 875–880, September–October, 1971.     

Evolution of the Stress–Strain State of a Three-Dimensional Plate Weakened by a Central Crack under Impulsive Tension

A method is developed to describe the formation of the stress–strain state in the vicinity of the tip of a stationary crack in a three-dimensional plate under dynamic loading. The energy model used to describe the formation of the stress concentration zone around the crack tip is modified to take into account the transient character of the loading process and the influence of the free surfaces of the plate on the stress–strain state of the central part of the sample. The method is useful for describing static and dynamic brittle fracture from a unified point of view.     

Effect of biaxial orientation on the fracture kinetics of polymethyl methacrylate

Biaxially oriented PMM is shown to have important advantages as a structural material over unoriented PMM owing to a difference in fracture kinetics. Under identical conditions primary cracks appear later in the oriented PMM, their growth is impeded, and the rate of crack propagation in the avalanche stage of failure is much lower. This accounts for the greater resistance of biaxially oriented PMM to stress raisers and its higher fracture energy under biaxial loading.All-Union Scientific-Research Institute of Aviation Materials, Moscow. Translated from Mekhanika Polimerov, Vol. 5, No. 2, pp. 274–281, March–April, 1969.     

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.     

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.     

A method of determining the functional dependence between stress and deformation under isothermal loading of a reinforced plate

We propose an approximate method for specifying the functional dependence among the stress, deformation, temperature, and time under arbitrary laws of loading and heating of a reinforced plate in a uniaxially stressed state. The method is justified experimentally in tests of the uniaxial extension of models under isothermic and nonisothermic loading with stress and temperature varying at a constant rate.Translated fromMatematicheskie Metody i Fiziko-Mekhanicheskie Polya, Issue 27, 1988, pp. 75–77.     

Nonsteady-state damage accumulation processes in composite flanges under cyclic loads

We consider two typical flanges: for a powerplant casing and for the casing of suspensions with a sound-absorbing duct. We model the behavior of flanges under the action of cyclic external loads based on solution of the nonlinear axisymmetric problem of elasticity theory. In order to take into account fatigue fracture of the layers of the structure, we use maximum stress criteria, a tensor damage function, the rule of linear summation of damage, and reduction schemes for the deformation characteristics of the layers, describing different fracture mechanisms. We indicate the possible nature of the development of fracture zones in composite flanges. We have established the spare load-bearing capacity of the flanges after fracture begins.Report presented at the Tenth International Conference on Mechanics of Composite Materials (Riga, April 20–23, 1998).Perm State Technical University, Russia. Translated from Mekhanika Kompozitnykh Materialov, Vol. 33, No. 5, pp. 636–643, September–October, 1997.     

Hereditary theory of creep and low-cycle fatigue of polymeric materials

The results of an investigation of the creep of polymethyl methacrylate under alternating loading and recovery are presented. The experiments ended in the failure of the specimen. The creep data obtained are described on the basis of the nonlinear Boltzmann-Persoz theory and Bryzgalin's theory of hereditary recovery. Korabel'nikov's result, according to which for alternating loading and recovery the total time under load up to failure is less than the static life of the polymer, is confirmed.Institute of Problems of Mechanics, Academy of Sciences of the USSR, Moscow; Scientific-Research Institute of Mechanics, Lomonosov Moscow State University. Translated from Mekhanika Polimerov, No. 4, pp. 615–621, July–August, 1970.     

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.     

Microstress fields and the mechanical properties of disordered fiber composites

Conclusion The effective elastic moduli and Poisson's ratios and the mean characteristics of the stress fields in the components of unidirectional fiber composites with a stochastic structure are nearly the same as the corresponding values calculated for a regular model of the composite. Relatively small increase (up to 6%) is seen in the transverse shear moduli with the transition from a regular structure to a stochastic structure. In the latter, there is a substantial increase in the stress concentration factor. Here, the difference between the stochastic structure and the regular structure increases with an increase in fiber stiffness and is particularly great (with a difference of two to three orders of magnitude) in the case of shear loading. The probability of the occurrence of microscopic fracture in the binder of the investigated materials is higher in transverse tension, but the difference from the results obtained for the regular models is more significant in the case of shear loading. Microscopic fracture nuclei will be formed in the matrix of the composite with the stochastic structure at considerably lower macroscopic stresses than are required for the regular structure.Translated from Mekhanika Kompozitnykh Materialov, No. 5, pp. 860–865, September–October, 1990.     

Resistance of glass-reinforced plastics to long-term shear failure

The time dependencies of the strength of glass-reinforced plastics in shear under steady, continuous and alternating loads are examined. Failure under steady and continuous loading is depicted by means of an exponential model of the build-up of damage.Moscow Aviation Technological Institute. Translated from Mekhanika Polimerov, No. 2, pp. 295–302, March–April, 1970.