Failure of solid polymers under pulse tension

Results of tests on specimens of polymethyl methacrylate (PMMC) and epoxy resin ÉD-20 MT GFA which were obtained with the help of a dynamic polarization set-up are presented. It is shown that for the entire stage of propagation of the crack from the instant it starts until it stops, the crack length increases spasmodically. Fracture surfaces were fractographically analyzed with a view to establish the interdependence between the spasmodic character of crack growth and the formation and development of microfailures ahead of the crack front. It is observed that occurrence and development of secondary microcracks cannot appear as jumps of the main crack due to the chaotic formation of the microcracks and triviality of their dimensions in comparison with the magnitude of the jumps.Translated from Problemy Prochnosti, No. 7, pp. 14–17, July, 1992.     

The role of unconscious sets formed on the basis of the perception of concrete visual stimuli and of illusory representations in conscious cognitive activity]

The cognitive set can be formed in healthy humans (75 subjects have been examined) on the basis of the Müller-Lyer's visual illusion. The effect of this set is expressed as a contrast illusion similar to the effect of the set, which is formed on the basis of the real size of visual stimuli (pictures of circles). Time of the simple motor reaction to the probe stimulus (light spot) depends on the factor of set stability. In subjects with the stable set the reaction time and its variability are substantially longer than in persons with the unstable set. Analysis of correctness of perception of the visual stimuli and the reaction time shows that the set which has been formed at the unconscious level leads to a certain rigidity in cognitive activity. Thus, factor of lability/stability of sets is an important feature of human cognitive activity.     

Determination of effective surface energy of dynamic failure

A method is presented for determining the effective surface energy under dynamic loading. It is based on combining the effect of a valuewise different static tensile load and similar tensile pulses on identical specimens. It is shown that the method is insensitive to the sharpness of the originally made notch. This facilitates the form of the moving crack front to be controlled and eliminates the need to set up a fatigue crack. Graphs of variation in time of the SIF and length and velocity of the crack are given.Translated from Problemy Prochnosti, No. 7, pp. 10–14, July, 1992.     

A Model of Quasibrittle Fracture Caused by Pulsed Loading

We study the stress-strain state formed in a plate containing a stationary crack of finite length subjected to pulsed loading, develop a model of quasibrittle fracture caused by loads of this type, and deduce analytic expressions for the stress-strain state near the tip of the stationary crack in the plate. The proposed model admits rigorous physical justification and enables one to describe the development of brittle and quasibrittle fracture processes in materials with cracklike defects under dynamic and static loads within the framework of the same approach. The results of numerical simulation are in good qualitative agreement with the experimentally established regularities of the behavior of the critical parameters of fracture processes.     

Micromechanics of fracture of solid polymers in dynamic loading

The results of investigation of the influence of temperature on the micromechanics of fracture ofpofymethyl methacrylate and epoxy materials in dynamic loading are presented, A method was developed for determination of the value characterizing the formation and development of microcracks in front of the front of the main crack. Anomalous temperature relationships of crack length and the density of microfractures in polymethyl methacrylate were established. The influence of the processes of stress concentration, local inelastic deformation, and thermofluctuation rupture of polymer chains on the micromechanics of fracture of polymers is considered. It is shown that a change in temperature may lead to a qualitative change in the processes of microfractures. The formation of microcracks in front of the front of a propagating crack may transform into outstripping of it by the leading microcracks and the reverse. It is established that a thermofluctuation character is characteristic of fracture of polymers in intense short-term loading.Translated from Problemy Prochnosti, No. 2, pp. 80–85, February, 1990.     

Two-level fracture criterion for materials

A phenomenological model of dynamic fracture of solids and a two-level fracture criterion, which takes into account increase in the degree of damage of materials on loading, are proposed. Within the framework of a model of elastoplastic solid, solutions of a three-dimensional problem of the dynamic fracture of rock on its interaction with the cutting tool have been obtained by a numerical method. The effect of the criterion on the development of the fracture process and the dependence of cutting force on time are considered.     

Effect of contact conditions on limiting state parameters,elasticity moduli and failure modes of rock specimens under compression

The study is devoted to effect of contact friction on failure, ultimate stress and moduli of elasticity in rock specimens under compression. It is found that dependence of the listed characteristics on friction coefficient is incremental. The author reveals zones of full contact and slide.     

The psychophysiological criteria for assessing the excitable type of psychopathy

In the excitable psychopaths and persons with some psychopathic traits (Leonard's "accentuated personality") the study was made of a cognitive function of discriminating microintervals and event-related potentials to visual stimuli. A qualitative difference in the accomplishment of the cognitive task between the two groups was found. Disturbances of the cognitive function, learning deficiency and reduction of the P300 wave provide grounds for referring this "antisocial personality disorder" to pathology of the limbic system of the brain.