No abstract

No abstract

The times to failure for several materials, both metals and plastics, are compared for static and cyclic loadings. Their similarity leads to the conclusion that the fracture is caused by the same elementary processes, with their thermofluctuatinn nature being described by the Zhurkov formula. IN TR ODUC TIONA large number of papers have been devoted to the study of fatigue under a cyclic load; their data having been summarized in numerous reviews, monographs, and symposia(1-9). The vast amount of experimental data accumulated in these studies is considered by various authors from different viewpoints with the purpose of elucidating the nature of fatigue. A new approach in the study of fatigue which has not yet been covered, however, is based on the consideration of the fatigue fracture process within the framework of the kinetic concept of fracture developed in the USSR by Zhurkov and his coworkers (1°'17), as well as other investigators (is). This paper presents the principal results of the investigation of fatigue utilizing this approach which was carried out at the A.F. Ioffe Physical Technical Institute of the USSR Academy of Sciences, mainly on polymers (19-2°) but also to some extent on pure metals (21). The works of other investigators considering fatigue within the same framework(22.23) are not covered here. CONCEPT OF FRACTUREThe kinetic concept considers fracture as a process developing in a body under load, rather than a critical event occuring at the moment a critical stress (the ultimate strength) has been reached. The elementary events responsible for the fracture process are thermofluctuation ruptures of interatomic bonds. An accumulation of interatomic bond ruptures then results in a macroscopic fracture of the specimen under test, viz. in a formation and growth of cavities and cracks and eventual breakdown of the specimen into parts. This time-dependent fracture process cannot be characterized by a critical stress. As a kinetic process, it can be characterized by either a rate, e.g. the number of bonds ruptured per unit time, the rate of crack growth, the rate of accumulation of damage or a reciprocal quantity, the time required to reach a given degree of damage or the total time-to-break of a specimen under load, i.e. the lifetime.The kinetic concept of fracture has evolved from a systematic study of the time-temperature dependence of strength for a broad range of materials, and has been directly confirmed recently in experilnents employing modern techniqu%s ~hich are capable of detecting individual events of interatomic bond rupture u~-r,). The empirical formula relating the lifetime, ~, with stress, G, and temperature, T, namely U o -"y~'~ ~" = ~o exp k, k~ -J

A B S T R A C T The growth rate of main cracks at different temperatures and under the static load has been investigated in aluminum and zinc foils. The observation of the crack growth during loading has been made by means of general microfilming as well as by electron microscope. It has been shown that the growth of main cracks occurs by means of the initiation, development, and coalescence of secondary microcracks which arise at the tip of the main one. The interaction and the growth of cracks of all dimensions, from submicroscopic and up to macroscopic, takes place at constant rate, without any steps. The rate depends exponentially upon the applied stress and the reciprocal test temperature; the activation energy of the crack growth process is near to that of sublimation for the metals.

By means of several methods (fractography, selective etching, electronography, absorption spectroscopy, optical microscopy in transmitted polarized light, measuring of the temperature dependence of conductivity, estimation of mechanical properties by microhardness, yield point and dimension of dislocation rosettes) the nature of damages created in LiF crystals by bombardment with heavy ions (carbon, chromium, and xenon) having high energy (up to 100 MeV) is studied. It follows from the results that the effect of implanted heavy ions on the defect structure is complicated. A sputtering or/and evaporation of LiF from the irradiated zone is discovered; there are strong damages including radiolysis effects inside this zone. Changes beyond the implantation zone are revealed, where the traces of the spreading shock wave and secondary ionizing radiation are evident.