On the selection of allowable stresses in the strength analysis of structures for cryogenic applications

Abstract

The methods developed at the Pisarenko Institute of Problems of Strength of the National Academy of Sciences of Ukraine for determining the allowable stresses for structural elements operating at cryogenic temperatures are considered. The methods make it possible to take into account the effect of the low-temperature hardening and the softening electromagnetic action on the value of allowable stresses.Introduction. By a mechanical strength of any component or structural element is meant their ability to withstand the action of various factors while maintaining the integrity and serviceability. The retention of the integrity and serviceability of the structure is governed by its ultimate state and is dependent on the stress-strain state of its elements, level and nature of loads, environment, etc. Depending on the type of the structure ultimate state (in terms of the load-carrying capacity, the development of excessive strains, the formation or opening of cracks), one or another ultimate state criterion is chosen. The maximum loads resulting in fracture or occurrence of residual strains, the residual strains, crack initiation, time or number of cycles to fracture, etc., can be chosen as the criteria. The onset of the ultimate state testifies to the fact that further operation of the given structural element is impossible. As noted in [1], "... such approach does not coincide completely with the commonly accepted one when the strength is limited by the fracture of the component. If the ultimate state of the object under study is known to us, and the criterion is chosen that represents the quantity describing the attainment of the ultimate state, namely, the force, stress, temperature, time, size (variation in the length or thickness) of the worn out layer or the crack length, then this criterion should be taken as a limiting value, using which the allowable value of this quantity with a certain safety margin should be chosen that will be dependent on the degree of accuracy of the estimated parameter adopted as a criterion of the ultimate state." For a number of structures operating at low and cryogenic temperatures, the onset of the ultimate state can be defined by the occurrence of residual strains and crack nucleation. Accordingly, in these cases, the criteria for its onset will be the following: the equality of the calculated equivalent stresses in the dangerous zone of the structure to the stresses of the onset of the material plastic flow (σ σ