Limiting states in calculations on cyclic strength and life estimation

Abstract

The paper discusses the connection between limiting strength state and safety. The standard condition for strength is examined and a possible approach is presented for transferring from the rigid standard limiting state to a soft one as regards safety in a probabilistic formulation.The Russian Federation Law On Technical Regulation [1] envisages all forms of engineering activity and the use of technical components, with evaluation in terms of risk, and decision on use only with an acceptable degree of risk, for which it is obligatory to perform probabilistic estimates of component strengths on manufacture and also during use.Many researches have been done on aspects of strength.Strength studies for defect-free metal in the main are based on standard tests, in which visible failures in the continuity of the material are formed:• initial formation in a specimen of a small surface crack;• the attainment of a visible crack of a size comparable with the least dimension of the specimen; and • complete separation of the crack specimen into parts. These forms of local damage are determined organoleptically in current standardization documents and correspond to the following:• limiting state on cyclic strength (formation of a macrocrack of size 0.5-2.0 mm [2]); • the limiting state in viability (development of a local crack to a preset size); and • limiting state on safety (for example, failure of major equipment in an object). A new criterion is proposed along with these limiting states for complicated multielement structures: integrity, for which the limiting state has not yet been specified.Consider the transition from the most rigid limiting state concerned with the formation of a macrocrack to the mildest limiting state for safety (for example, for reactor installations in nuclear power stations, failure of the body of the reactor with a probability not more than 10 -7 per reactor and year [3]).One uses the time characteristics of the strength in choosing major dimensions for components. The temporary or static strength of the metal is estimated by comparison with the limiting strength, yield point, and other characteristics.It is unrealistic to perform preliminary tests on the metal for all the variety of static states of stress and possible modes of cyclic loading, so one needs logically nonconflicting and experimentally tested hypotheses on which one can estimate the limiting strength and working life of real structures with various loads.To estimate strength in single or static loading, there are several theories or hypotheses on strength that are used for various types of construction and which have experimentally confirmed and accepted values for the limiting states corresponding to the failure stresses of standard specimens.