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This paper presents a study of the phenomenon of cycle-dependent strain accumulation (ratcheting), especially under multiaxial steady and cyclic loads. The practical significance of ratcheting has become more apparent in recent years in dealing with the design/analysis of pressure-retaining components and structures which may be subjected to certain (reversed) thermal or seismic (dynamic) cyclic loads. A review of experimental observations on the fatigue and ratcheting response of such components suggests that the likely mode of failure (under the noted load combinations) is different from the plastic collapse (as assumed for design basis); this provides an opportunity to reduce the over-conservative safety margin requirements. Based on a brief survey of prior material test data and theoretical evaluations of the ratcheting under multiaxial stress conditions, it is concluded that a proper accounting of the material strain hardening and yield behavior (the Bauschinger effect) under multiaxial stresses is needed and can serve a useful purpose.
The nonlinear strain hardening and the Bauschinger effect are taken into account in this work by using the full implementation of incremental plasticity based on the generalized kinematic-type hardening proposed previously by the author. It is shown that, under certain combined stress conditions, the incremental plasticity predicts continued strain accumulation in each (half) cycle even for a kinematic-type hardening assumption. A number of characteristic observations on ratcheting under multiaxial loads are also predicted.
The case of triaxial steady stress with imposed cyclic strain is analyzed in detail; this is similar to typical loads on a pipe section. For such a case the theory predicts conditions of (local) wall thinning and circumferential growth (in the pipe section) that is limited mainly by the structural considerations. For Type 304 stainless steel and A333 Grade 6 carbon steel a graphical representation is developed showing allowable load combinations for various rates of strain accumulation. A simple relation is proposed between the (steady) primary stress and the (cyclic) secondary stress for a specified total ratchet strain as a criterion which is discussed with reference to the ASME Code. The extent of over-conservatism associated with the neglect of material strain hardening and with the (change in) mode of failure can be better judged by means of the results presented here. The paper is concluded with a discussion of related issues and the future direction.
cyclic strain accumulation, nonlinear strain hardening, rachet assessment diagrams, multiaxial stresses, carbon steel, stainless steel, ASME Code
Consultant, S. Levy Incorporated, Campbell, CA