III Международная студенческая научная конференция Студенческий научный форум - 2011

In assessing the reliability of strength design must address the long-term strength (or static fatigue) - the decline of strength of materials over time under the load. The phenomenon of long-term strength inherent in all materials, but essentially manifested dangerous when changes in physical and mechanical properties of the material with time (creep, corrosive media, etc.). If there are currently a large number of physical relationships creep theory, the theory: describing the interactions of structural elements with aggressive media, in their infancy. These theories also in accordance with the accepted model of the material - a model of a continuous medium - used a phenomenological approach, i.e internal microstructure of the material is not studied. This approach allows us to construct mathematical equations, which even in the absence of clarity in fact occurring in the material complex internal processes that adequately reflect their external manifestations in the range of variation of process parameters, which conducted an experimental study of the mechanical properties of the material.    To determine the conditions of structural strength during the period of operation necessary to know the strength characteristics of the material design. The basis for the construction of various theories of long-term strength is the long-term strength curve that relates the time of destruction with the value of a permanent as well. Effect of long-term strength is manifested in the exhaustion of the resource strength with time, which can be explained damage accumulation during deformation, which lead to the appearance of cracks to fracture even at relatively small deformations.

As is known, for example, the physical process of destruction of materials can be divided into three stages: 1 - stage scattered destruction, during which the accumulation of microdamage, ending with the formation of macroscopic cracks; 2 - stage developed cracks; 3 - stage destruction (intensive growth of main crack). In engineering calculations on the strength of structural elements that do not have significant defects in the form of surface sinks, voids, foreign inclusions, etc., usually based on the assumption that the appearance main crack is equivalent to the total destruction, and therefore for the ultimate resistance of the material considered only the first stage - the process of accumulation of scattered damages. The accumulation of damage has an impact on physical and mechanical properties of the material so the degree of accumulated damage can be introduced into the equation of state. In the simplest case to describe the extent of accumulated damage parameter is introduced.

In the initial state - absence of damage. In this paper, some results of studies in the calculation of long-term strength of structural elements of the nonlinear viscoelastic materials under various operating conditions on the basis of determine the extent of accumulated damage. It solved at least two problems: the need to describe the change in the stress-strain state of the material in time and build strength criteria for structures made of this material.