FORMATION OF STRESS / STRAIN CYCLESFOR ANALYTICAL ASSESSMENTOF FATIGUE CRACK INITIATION AND GROWTH

摘要

This paper discusses standard techniques for setting up cycles of stresses, strains and stress intensity factors(SIF) for use in analysing the fatigue characteristics of crack-free components or the fatigue crack growth ifcrack-like flaws are present. A number of improved techniques are proposed.An enhanced procedure for analytical description of true metal stress-strain curves, covering plastic effects, ispresented. This procedure involves standard physical and mechanical properties of the metal in question, such asultimate stress, yield stress and elasticity modulus.It is emphasized that the currently practiced rain-flow method of design cycle formation, which is effective foran actual (truly known) cyclic loading history, is not suitable for a projected (anticipated) history, as it leaves out ofaccount possible variations in the sequence of operating conditions.Improved techniques for establishing design stress/strain and SIF cycles are described, which make allowancefor the most unfavourable sequence of events in the projected loading history.The paper points to a basic difference in the methods of design cycle formation, employed in assessment of thecurrent condition of a component (with the actual history accounted for) and in estimation of the residual lifetimeor life extension (for a projected history).