Low-Cycle Fatigue and Cyclic Creep of Metals

Abstract

This paper deals with the effect of accumulated (oriented) plastic deformation (cyclic creep) on the life of metals under conditions of cyclic loading typically in the range of low-cycle fatigue. Specific features of cyclic creep of aluminium, titanium alloys and structural steels were investigated for the lives ranging from 0.5 to 2 x lo5 cycles to rupture as well as specific features of transition from quasistatic to fatigue fracture associated with the variation in the mode of deformation of metals. It is shown that under conditions of repeated stress-controlled loading in a wide range of low temperatures, the cyclic creep processes are the predominant ones determining life of metals over a wide range of cycles, and when calculating life under these conditions it is necessary to take into account the rate of these processes irrespective of the type of fracture: quasistatic or fatigue. An equation was obtained for calculating life under low-cycle loading conditions which takes into account the interrelationship between the life, the accumulated plastic strain, steddy-state cyclic creep rate and the resistance of the material to cyclic creep.

A Re‐evaluation of the Life to Rupture of Ductile Metals by Cyclic Plastic Strain

“…Similar behaviour in a wide variety of metals was found by Pisarenko et al [13] in nonreversing tension cycles. At lower cyclic stress ranges the lives were long and failure was by fatigue fracture.…”

“…However, if fatigue does intervene the ratchetting deformation curve would be prematurely terminated by the development of a fatigue crack. This effect may be seen in the results of Pisarenko et al [13]: the deformation curves referred to above are incomplete at lower stresses in that the tertiary and even part of the secondary phase is missing. If fatigue had not intervened the curve would be expected to have continued and the material to have failed at the end of the tertiary phase by the development of a neck.…”

“…An important point (which was noted by Benham [9] and Benham and Ford [lo]) is that the specimens of the same material which failed by necking had accumulated roughly similar magnitudes of strain. Pisarenko et al [13] also show that for the specimens which failed by "quasistatic fracture" the accumulated strain at fracture is nearly uniform. Benham [9] had argued that at lower stresses low cycle fatigue intervenes and causes failure.…”

“…Finally, we consider the results of Pisarenko et al [13] who performed push-pull tests on a variety of materials at room temperature, in liquid nitrogen (77 K) and in liquid helium (4.7 K). They found all the tested materials failed by either LCF or ratchetting (which they called quasistatic fracture) and reported results for a titanium alloy.…”