Effect of ramp treatment on the cyclic response of coarse-grained copper

Abstract

Recent studies on the effects of different variables, both intrinsic and extrinsic to the material itself, on the cyclic response of polycrystalline copper have been encouraging in the drive to understand the phenomenon of localized deformation during fatigue. In studying the effect of two intrinsic material parameters (grain size and annealing texture) on the cyclic response of f c c polycrystals, several sets of workers [1-3] have reported that the cyclic response of coarse-grained metal shows much more pronounced cyclic hardening and higher stresses, at low and intermediate strain amplitudes, than those for fine-grained metal. This behaviour is associated, in copper, with the strongly defined hard (1 1 1 ) ( 10 0) fibre texture, inherited in the coarsegrained material after annealing at relatively high temperatures [1]. Llanes et al. [1] also showed that in large-grained, strong-textured copper specimens, double and multiple slip related structures are already promoted in a relatively large number of grains (those oriented to favour multiple slip) at plastic strain amplitudes as low as 10 .4 . This early multiple slip activation homogenizes the deformation and seems to be the main factor contributing to the very strong cyclic hardening and the 10w levels of localization of deformation observed in the cyclic stress-strain curve (CSSC) of coarse-grained copper. In terms of the effect of extrinsic variables, Llanes and co-workers [4-6] observed enhancement of strain localization in small-grained copper in association with the use of ramp-loading both as loading mode and as mechanical pretreatment, in agreement with previous results [7-12]. From these studies it is now well established that ramp-loading promotes a gradual substructure evolution which leads to: a much more homogeneous substructure, from grain to grain; localization of slip in primary systems; and formation of persistent slip bands (PSBs). In the CSSC of small-grained copper, and in support of the results of Wang and Laird [7], Llanes and Laird [4] observed that ramp-treated copper polycrystals show, under subsequent conventional strain control testing, lower cyclic hardening rates, and therefore a greater tendency to localize deformation, than nontreated samples. However, the fact that Llanes and Laird [4] were unable to obtain the clear plateau behaviour observed by Wang and Laird [7] in the CSSC of ramp-treated small-grained copper suggests that the observation of a plateau in the CSSC of