Thermodynamic nature of exponential empirical time dependences of strength characteristics and the failure of materials. Report 1. Creep and long-term strength

It is shown that the correlation between two parameters describing the power law of creep or the long-term-strength curve, which can be demonstrated on the basis of numerous experimental data, is governed by the thermodynamics of the processes of steady-state creep or creep-induced failure. Analytical expressions describing this correlation are obtained, and values of the parameters entering into this relationship are explained on the assumption of the logarithmic character of the dependence of the activation energy of the process on deforming stress. An independent method of evaluating the cohesive energy of the material from results of creep tests is proposed. A simple formula is presented for the a priori estimation of the slope of the long-term strength curve from the material's melting point.The paper is written as an outgrowth of a study performed by the authors, which was published in the journal Problemy Prochnosti, No. 6, 3–9 (194).Translated from Problemy Prochnosti, No. 2, pp. 5–24, February, 1996.     

Refining the high-temperature strength characteristics of composite materials under bending

On the basis of the complete equation of bending, we establish the refined mechanical characteristics of a carbon-filled plastic under bending as functions of temperature by taking into account the effect of rotation of reactions in the supports. We also show that friction in the supports affects the fracture process in specimen subjected to bending. Institute for Problems of Strength, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 1, pp. 139–141, January–February, 1999.     

Experimental equipment for evaluation of the strength characteristics of ceramics materials

We describe the design and operating characteristics of a complex of experimental equipment developed at the Special Design and Technological Office of the Institute of Problems of Strength of the National Academy of Sciences of Ukraine for determination of the characteristics of strength, elasticity, and crack resistance of ceramics and other brittle nonmetallic materials and investigation of fracture processes in these materials within the temperature range of 20–1450°C. The equipment developed enables one to decrease the measurement errors significantly and increase both the efficiency of the tests and the amount of accumulated data. Institute of Problems of Strength, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 5, pp. 117–126, September–October, 1999.     

An empirical approach to obtaining bounds on the failure probability through stress/strength interference

This paper proposes a new empirical approach to computing the unreliability bounds based upon the subinterval probabilities of the stress and strength in the interference region. It is demonstrated by the numerical examples that in most cases the unreliability bounds so obtained not only will include the exact value but also are narrower than the existing ones. The average of the lower and the upper bound is shown to serve as a good point estimate of the unreliability. It is also shown that the approach is practical since the number of subintervals required for an acceptable accuracy is small. In some cases a rather acceptable accuracy is achieved when the number of intervals is around ten.     

Reliability optimization of a redundant system with failure dependencies

In a multi-component system, the failure of one component can reduce the system reliability in two aspects: loss of the reliability contribution of this failed component, and the reconfiguration of the system, e.g., the redistribution of the system loading. The system reconfiguration can be triggered by the component failures as well as by adding redundancies. Hence, dependency is essential for the design of a multi-component system.In this paper, we study the design of a redundant system with the consideration of a specific kind of failure dependency, i.e., the redundant dependency. The dependence function is introduced to quantify the redundant dependency. With the dependence function, the redundant dependencies are further classified as independence, weak, linear, and strong dependencies. In addition, this classification is useful in that it facilitates the optimization resolution of the system design. Finally, an example is presented to illustrate the concept of redundant dependency and its application in system design. This paper thus conveys the significance of failure dependencies in the reliability optimization of systems.     

A method for investigating the strength characteristics of sheet materials under impulse loading

We propose a method for the determination of the strength characteristics of metallic and nonmetallic materials under conditions of repeated impulse loading. By using this method, one can apply impulse loading to the tested objects both directly from an inductor and by hydroimpact, i.e., transferring impact pulses via a contact liquid. The method of electric pulses used as a basis of the suggested method of loading enables us to realize gradual control over the power of pulses and the frequency of their repetition. Translated from Problemy Prochnosti, No. 5, pp. 122–125, September–October, 1997.     

A method for investigating the strength characteristics of sheet materials under impulse loading

We propose a method for the determination of the strength characteristics of metallic and nonmetallic materials under conditions of repeated impulse loading. By using this method, one can apply impulse loading to the tested objects both directly from an inductor and by hydroimpact, i.e., transferring impact pulses via a contact liquid. The method of electric pulses used as a basis of the suggested method of loading enables us to realize gradual control over the power of pulses and the frequency of their repetition.     

Assessment of damage level in materials by the scatter of elastic characteristics and static strength

The parameters of scatter of elastic characteristics and strength of an aluminum alloy and carbon steel are determined from the results of large-batch tests under identical static deformation conditions. The material degradation is demonstrated to occur in a stagewise manner associated with peculiarities and nature of structural damages accumulated during various deformation stages, which is responsible for nonlinearity of the damage accumulation law. A good correlation has been found between the Weibull homogeneity coefficient and the maximum probability density on lognormal distribution and Weibull distribution curves. Emphasis is put on the advantages of the LM hardness method in terms of information it provides about the integral pattern of material degradation over its lifetime. __________ Translated from Problemy Prochnosti, No. 2, pp. 5–14, March–April, 2006.