Relaxation effects and the nature of damage accumulation in polymethyl methacrylate under static loads

The relation between the size of the specular zone on the fracture surfaces of PMMA specimens and the loading rate is investigated at various temperatures. It is shown that the rate and time dependences of the size of the specular zone have certain common characteristics determined by the kinetic nature of damage accumulation in the material. It is proposed to employ the Robertson method [8] for studying the crack growth kinetics in polymers. This makes it unnecessary to observe the growth of cracks in the specimen during testing.All-Union Correspondence Structural Engineering Institute, Moscow. Translated from Mekhanika Polimerov, No. 4, pp. 663–669, July–August, 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.     

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     

Creep and failure of polypropylene under low-frequency cyclic loadings

The results of tests on polypropylene specimens under conditions of low-frequency cyclic and static loading are compared. It is shown that as compared with static tests, there is an increase in longevity and a reduction in overall deformation under low-frequency cyclic loadings.     

The effect of microdamage on fatigue-crack growth

The theory of fatigue-crack growth, based on a synthesis of fracture mechanics and continuum mechanics of microdamage accumulation, is applied to the problem of crack growth under cyclic loading, taking into account the plastic deformations in the tip zones. The model of a thin plastic zone, which is a region of considerable plastic deformations at the crack tip, is supplemented by taking into account the effect of microdamage on the value of the specific fracture work and the limit stresses in the tip zone. Governing equations which describe fatigue-crack growth taking these factors into account are derived. The effect of the material characteristics and the load parameters on the growth rate and the distribution of microdamage in the tip zone and on its extensions is investigated by a computational experiment. Particular attention is given to the initial stage when crack growth may occur abruptly and the growth rate depends substantially on the initial conditions     

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.     

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.     

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.     

Modelling of interfacial damage and delamination of sprayed coatings with cohesive elements

Delamination of thermally sprayed corrosion protection coatings as a consequence of thermo-mechanical fatigue is investigated. This study focusses on the modelling of interfacial damage initiation and evolution under cyclic loading with the help of a cohesive zone model. The presented model features a slight non-linearity at unloading from the exponential “damage locus” as well as cyclic damage accumulation restricted to (re)loading conditions. Additionally, an endurance limit is introduced indicating the maximum sustainable traction for an infinite number of load cycles. The capability of the model is demonstrated. (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Cyclic Creep of Polymers and Polymer-Matrix Composites

The paper provides a state-of-the-art review of the current understanding regarding the long-term response of polymer-matrix composites subjected to cyclic loading conditions. Typically, under such conditions, the behavior of polymeric systems is characterized by much higher creep rates than those observed in the cases of static loading. In this paper, research accomplishments in the subject area are discussed. New experimental results are presented regarding the cyclic-creep response of a composite system consisting of a thin-film piezoelectric polymer polyvinylidene fluoride (PVDF) with thin metallic layers deposited on both surfaces of the polymer. This composite was tested under the conditions of tensile static stresses with superimposed sinusoidal oscillations. As a result, considerable acceleration of creep rates has been recorded as the mean stresses, vibration amplitudes, and frequencies of oscillations tended to increase. These effects were observed even within the linear viscoelastic deformation range at room temperature; however, the acceleration of cyclic-creep rates tended to decrease below the freezing temperature. In general, as indicated in the conclusion, the problem of cyclic creep in polymeric systems is far from being well understood and requires further studies.     

Fatigue and fracture of plasticized and stabilized polyamides

Polyamides have been tested in cyclic bending to determine the effect of certain plasticizers, plasticizing conditions, diffusion stabilization, and aging on the mechanism and energy of fatigue fracture. The fracture surfaces of the most typical plasticized specimens are analyzed. The optimal conditions of diffusion stabilization of polyamides with quinhydrone, giving a considerable improvement in antifatigue properties, are selected.Institute of the Physicotechnical Problems of Power Engineering, Academy of Sciences of the Lithuanian SSR, Kaunas. Translated from Mekhanika Polimerov, Vol. 4, No. 4, pp. 693–697, July–August, 1968.     

Interlaminar Crack Propagation Analysis of ENF Specimens Based on the Cohesive Zone Model北大核心CSCD

Based on the classical laminated plate theory and the cohesive zone model, a theoretical model for general delamination cracked laminates was established for crack propagation of pure mode Ⅱ ENF specimens. Compared with the conventional beam theory, the proposed model fully considered the softening process of the cohesive zone and introduced the nonlinear behavior of ENF specimens before failure. The predicted failure load is smaller than that under the beam theory and closer to the experimental data in literatures. Compared with the beam theory with only fracture toughness considered, the proposed model can simultaneously analyze the influences of the interface strength, the fracture toughness and the initial interface stiffness on the load-displacement curves in ENF tests. The results show that, the interface strength mainly affects the mechanical behavior of specimens before failure, but has no influence on crack propagation. The fracture toughness is the main parameter affecting crack propagation, and the initial interface stiffness only affects the linear elastic loading stage. The cohesive zone length increases with the fracture toughness and decreases with the interface strength. The effect of the interface strength on the cohesive zone length is more obvious than that of the fracture toughness. When the adhesive zone tip reaches the half length of the specimen, the adhesive zone length will decrease to a certain extent. Copyright ©2022 Applied Mathematics and Mechanics. All rights reserved.     

Molecular mobility in polymers under cyclic loading

Using the NMR method the authors have investigated thermal molecular movement in polymers under the conditions of fatigue tests. They have found that the increase in molecular mobility under cyclic loading (cycle frequency 1–10 Hz) is due only to general self-heating of the polymer. Heating of the specimens as a result of hysteresis heat release takes place uniformly throughout the volume.A. F. Ioffe Physicotechnical Institute, Academy of Sciences of the USSR, Leningrad. Translated from Mekhanika Polimerov, No. 6, pp. 1035–1038, November–December, 1972.     

Constitutive modeling of cyclic stress softening in filled elastomers

Quasi-static cyclic loading tests on filled rubber-like materials reveal a significant stress softening in the first cycle. The magnitude of this softening, widely known as the Mullins effect, reduces in the next cycles until it reaches a stabilized value referred to as hysteresis. In this contribution, we associate the hysteresis with the fracture of carbon bonds in the filler network. In order to calculate the cyclic energy dissipation, we modify the classic concept of network decomposition [1, 2] and add a new network considered to be responsible for the breakage and re-formation of carbon black aggregates during loading and unloading. The proposed model is in-line with a wide range of experimental observations. (© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Multiaxial constitutive modeling of aircraft engine materials

Based on a newly developed theory (Lu and Weng, Acta Mech., in press) the high temperature behavior of an aircraft engine material is studied under combined stress state. Both monotonic and cyclic deformations are examined to uncover its stress-strain response, as well as its cyclic hardening and strain-ratchetting characteristics. Under a biaxial loading it is disclosed that tensile cyclic hardening is greatly magnified with a superimposed lateral tension, whereas the strain-ratchetting process is led to an enhanced, unsettling state with a superimposed lateral compression. The biaxial transient and steady-state creep strains have also been calculated. The results suggest that while a superimposed lateral tension will inhibit the creep deformation, a lateral compression can greatly promote the inelastic flow. To reflect the practical service conditions of an aircraft engine, the theory is further applied to examine the effect of loading frequency on the development of inelastic strains under concurrent thermal and mechanical loading. It is found that a more frequently flying aircraft will have a greater accumulation of creep strains and, consequently, a greater possibility of material damage in its engine components over the same total flying time.     

Static and extending interface cracks with contact zones in anisotropic as well as in piezoelectric bimaterials

An interface crack with an electrically permeable and mechanically frictionless contact zone in a piezoelectric bimaterial under the action of a remote mixed mode mechanical loading as well as thermal and electrical fields is considered in the first part of this paper. By use of the matrix‐vector representations of thermal, mechanical and electrical fields via sectionally‐holomorphic functions the problems of linear relationships are formulated and solved exactly both for an electrically permeable and an electrically impermeable interface crack. For these cases the transcendental equations and clear analytical formulas are derived for the determination of the contact zone lengths and the associated fracture mechanical parameters. A plane strain problem for a crack with a frictionless contact zone at the leading crack tip extending stationary along an interface of two semi‐infinite anisotropic spaces with a subsonic speed under the action of various loading is considered in the second part of this paper. By introducing of a moving coordinate system connected with the crack tip and by using the formal similarity of static and propagating crack problems the combined Dirichlet‐Riemann boundary value problem is formulated and solved exactly for this case as well and a transcendental equation is obtained for the determination of the real contact zone length. It is found that the increase of the crack speed leads to an increase of the real contact zone length and the correspondent stress intensity factors which increase significantly for a quasi‐Rayleigh wave speed.     

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.     

Fractographic study of macrocrack propagation in oriented Kapron

The fracture surface of an oriented Kapron monofilament has been studied at electron-microscope and optical magnifications with the object of establishing the details of the micromechanism of crack propagation associated with the fracture of polymeric materials. Microparabolic figures can be observed in the specular zone of the fracture surface. This relief may be assumed to originate in the interaction of the main crack and the submicroscopic cracks present in loaded polymers. In the region of high main-crack velocities it is possible to observe a self-oscillatory motion of the tip of the main crack leading to the formation on the fracture surface of a system of bands parallel to the main crack front.Ioffe Physicotechnical Institute, Academy of Sciences of the USSR, Leningrad. Translated from Mekhanika Polimerov, No. 4, pp. 645–648, July–August, 1971.     

Mathematical Statement of Boundary Conditions for Problems of Three-Dimensional Deformation of Plates

We consider the problem of mathematical statement of boundary conditions on the end surfaces of elastic plates for a model that describes three-dimensional deformation of these bodies. We show that there exist five conditions that have a certain physical meaning and characterize the conditions of fixation of a plate or loading on its end surfaces. We illustrate the results obtained for a circular plate rigidly fixed on its ends and loaded over the lateral surfaces.