| It has recently been reported that sagging behavior in spring steels is a direct result of asymmetrical cyclic loading. Although previous work showed that cyclic creep and cyclic softening are the two main reasons for the sagging of mechanical springs, the mechanisms of sagging are still not well understood.; In order understand sagging mechanisms in commercial spring materials, both symmetrical and asymmetrical cyclic loading tests were carried out systematically on industrial pure iron and spheroidized AISI 1045 steel. The materials were chosen with the aim of simplifying the complicated microstructure effect on the cyclic deformation responses of commercial spring steels. In this work, slip band, cyclic deformation behavior, and the corresponding dislocation structure were studied. In addition, the effects of pre-strains on cyclic softening and cyclic creep were evaluated from the energy point of view.; Based upon the present study, it was found that fatigue slip bands showed much less multi-slip even at high cyclic peak stress levels and with a large amount of cumulative cyclic plastic strain. It has been experimentally proven that the Bauschinger effect caused by the pre-strain would become negligible after a certain, but small, number of cycles in cyclic deformation. Hence certain types of cyclic tests, such as the cyclic creep test, should be considered more realistic methods for evaluating sag behavior. Increasing pre-strain had the same effect as that of decreasing cyclic peak stress on the amount of recovered energy, and the unreleased prestrain energy is believed to assist the materials to enter their cyclic saturation stage early. There exists a transition phenomenon between cyclic hardening and softening for iron under symmetrical cyclic loading condition and this transition depends upon the cyclic stress amplitude. On the other hand, spheroidized 1045 steel showed strong cyclic softening through the entire cycling under both symmetrical and asymmetrical cyclic loading conditions, resulting in the reduction of the corresponding dislocation cell sizes. In addition to the effect on the total cumulative creep strain, mean stress was found to have a significant effect on the dislocation microstructures. |
