Deformations and stresses around submicroscopic cracks in loaded polymers

Structure deformations upon loading of oriented films of polycaproamide (Kapron) or polypropylene when submicroscopic cracks (dimensions of hundreds of Ångstroms) have appeared in the samples in high concentration (up to 2 · 10¹⁵ cm^–3) have been studied by x-ray diffraction methods at low and large angles. It has been established that the appearance of submicro-cracks causes relieving of the regions adjacent to them along the loading axis (extent about 1000 Å) and an increase in stress in the lateral zone relative to the crack, which is manifested in an intensification of the stretching of these zones.A. F. Ioffe Physicotechnical Institute, Academy of Sciences of the USSR, Leningrad. Translated from Mekhanika Polimerov, No. 1, pp. 51–58, January–February, 1972.     

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.     

Effect of submicroscopic crack formation and relaxation phenomena in polymers

The development of submicrocracks and the changes taking place in their concentration during loading and unloading cycles were studied by the low-angle x-ray scattering method in oriented amorphous-crystalline polymers (capron and polypropylene). The deformation () of the polymers was compared with the changes in submicrocrack concentration (N_cr).A. F. Ioffe Physicotechnical Institute, Academy of Sciences of the USSR, Leningrad. Translated from Mekhanika Polimerov, No. 3, pp. 387–392, May–June, 1970.     

Investigation of self-healing of cracks in polymers

A method is worked out for studying the process of self-healing of cracks in polymers. The criterion of self-healing of a crack — the degree of self-healing — is found, and its temperature dependence is determined. It is shown that a change of the relaxation spectrum of a polymer affects the process of self-healing occurring at the tip of a crack.Deceased.L. Ya. Karpov Physicochemical Scientific-Research Institute. Translated from Mekhanika Polimerov, No. 2, pp. 271–275, March–April, 1970.     

Investigation of self-healing of cracks in polymers

The effect of the molecular weight of a polymer and the environment on self-healing of cracks in polyvinyl acetate was investigated. It was confirmed that any change of the relaxation spectrum of the polymer affects the self-healing process occurring at the crack tip.See [1] for report 1.Deceased.L. Ya. Karpov Scientific-Research Physicochemical Institute, Moscow. Translated from Mekhanika Polimerov, No. 3, pp. 445–448, May–June, 1970.     

Investigation of the self-healing of cracks in polymers

The effect of the medium and the thickness of the polymer film on the self-healing of cracks in polyvinyl acetate has been investigated. The medium in which the specimen "recovers" may either facilitate or impede the self-diffusion of macromolecules and other structural units in the contact zone formed by the closing of the edges of the crack. The "recovery" coefficient decreases when healing takes place in the presence of the vapor of a surface-active agent and increases under conditions such that water molecules are desorbed from the surfaces of the crack; transition from a thick to a thin layer of polymer slows the self-healing process as a result of the reduced mobility of the kinetic units.For communication 2 see [2].Deceased.Karpov Scientific-Research Physicochemical Institute, Moscow. Translated from Mekhanika Polimerov, No. 6, pp. 1110–1113, November–December, 1970.     

Estimation of the loaded chain fraction in oriented polymers

The relation between the structure coefficient in the expression for the long-term strength of solids under load and the loaded chain fraction is examined on the basis of the thermal-fluctuation theory.Leningrad Branch, All-Union Scientific-Research Institute of Synthetic Fibers. Translated from Mekhanika Polimerov, No. 1, pp. 3–7, January–February, 1973.     

Supramolecular structure and incipient cracks in three-dimensional stitched amorphous polymers

The electron-microscopic method was used to study the structure of unsaturated polyester films. The initial submicrocracks developing in the loaded films were studies by the low-angle x-ray diffraction method. It is shown that for these polymers a typical feature is the globular structure with a globule diameter of 10³–3·10³ Å, and the transverse dimension of the submicrocracks developing under the action of a load practically coincides with the globule diameter.     

Comparison of the shapes of macro- and submicrocracks in loaded polymers

Small-angle x-ray scattering has been used to estimate the longitudinal (L) and transverse (L) dimensions of the submicrocracks formed in loaded polymers. The growth under load of macrocracks, initiated in the same polymers by a slit or a circular hole, has also been observed. The ratios L/L for the macro- and submicrocracks are found to coincide.A. F. Ioffe Physico-Technical Institute, Academy of Sciences of the USSR, Leningrad. Translated from Mekhanika Polimerov, No. 6, pp. 1013–1018, November–December, 1971.     

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.     

Quantitative modeling of quasibrittle fracture. Application to the problem of interaction and coalescence of cracks in polymers

Conclusions The results of qualitative and quantitative analysis of the interaction and coalescence of the cracks indicate that the role of these effects in accelerating quasibrittle fracture cannot be controlling. The largest reduction of the strength and endurance of the brittle, bodies is associated with the formation of a single crack of the largest dimension. The appearance of a large number of smaller cracks has only a slight effect on the further reduction of strength and increases the effective cracking resistance.Presented at the Sixth All-Union Conference on the Mechanics of Polymer and Composite Materials (Riga, November, 1986).Translated from Mekhanika Kompozitnykh Materialov, No. 2, pp. 209–217, March–April, 1987.     

Accumulation of submicroscopic defects in fluoroplastics during their viscous fracture

The character of the accumulation of submicrocracks in fluoroplastics 4, 4D, and 4DM during a moderate-duration cold stretching was studied by x-ray diffraction patterns and mechanical methods. It was shown that during viscous fracture of fluoroplastics, an active accumulation of submicrodefects takes place, which substantially depends on the previous thermal history of the samples. Irreversible submicrofractures accumulate in annealed samples not only at the opening of the main crack, but also throughout the entire volume of the sample. Qualitative correlations have been established between the crystallinity and the concentration of the submicrocracks, on the one hand, and the yield point and the ultimate strength of the polymers, on the other. The degree of damage before the fracture of the fluoroplastic samples was evaluated phenomenologically.     

Interface cracks in anisotropic composites

The linear model equations of elasticity often give rise to oscillatory solutions in some vicinity of interface crack fronts. In this paper we apply the Wiener–Hopf method which yields the asymptotic behaviour of the elastic fields and, in addition, criteria to prevent oscillatory solutions. The exponents of the asymptotic expansions are found as eigenvalues of the symbol of corresponding boundary pseudodifferential equations. The method works for three‐dimensional anisotropic bodies and we demonstrate it for the example of two anisotropic bodies, one of which is bounded and the other one is its exterior complement. The common boundary is a smooth surface. On one part of this surface, called the interface, the bodies are bonded, while on the complementary part there is a crack. By applying the potential method, the problem is reduced to an equivalent system of Boundary Pseudodifferential Equations (BPE) on the interface with the stress vector as the unknown. The BPEs are defined via Poincaré–Steklov operators. We prove the unique solvability of these BPEs and obtain the full asymptotic expansion of the solution near the crack front. As a special case we consider the interface crack between two different isotropic materials and derive an explicit criterion which prevents oscillatory solutions. Copyright © 1999 John Wiley & Sons, Ltd.     

Growth of arterial cracks in polymethyl methacrylate plates containing "silver" cracks

Conclusions 1. We have studied the influence of previously formed surface SC of various orientations on the force and kinetic characteristics of a slowly growing arterial FC. The subject of investigation was industrial glass based on polymethyl methacrylate. Steady growth of the FC was achieved in our experiments by applying torque in order to split lamellar samples.2. We have found that for no mutual disposition of the two kinds of cracks do surface SC ease the conditions of FC growth. On the contrary, in a number of cases the presence of SC having linear dimensions of only 3–5% of the front length interrupt the steady growth of the FC, reduce its average velocity, and sometimes stop it altogether. For the growth of the FC to be renewed the load has to be augmented, so that the limiting specific work of fracture W_lim increases by 10–40%. The growth of an FC is opposed most strongly by SC lying at right angles to it.3. The results of our experiments indicate comparatively easy fracture of the SC, and also show that FC prefer to develop along earlier-formed SC, subject to appropriate mutual orientation.4. By virtue of the presence of elastic interactions between different sections, any action applied to a small part of the leading edge (front) will alter the conditions of material fracture along the whole FC front.Institute of High-Molecular-Weight Compounds, Academy of Sciences of the USSR, Leningrad. Translated from Mekhanika Polimerov, No. 4, pp. 609–615, July–August, 1976.