Effect of pre-strain and mean stress on cyclic plastic deformation response of iron-based alloys

In order to understand the mechanism of sag behavior of engineering materials, systematic symmetrical and asymmetrical cyclic loading tests, rather than stress relaxation and the Bauschinger effect test, were carried out with two model materials, namely industrial pure iron and spheroidized mid-carbon steel, under different pre-strain conditions. It was found that the applied mean stress had a significant effect on the amount of cumulative cyclic creep strain and it could also strongly extend the secondary creep stage. It was also found that the Bauschinger effect caused by pre-strain would become negligible in cyclic deformation after a certain, but small number of cycles. This indicates that Bauschinger effect test cannot thoroughly reflect the nature of sagging behavior. Finally, both increase in the pre-strain level, and decrease in the cyclic peak stress level resulted in reduced amount of energy that could be recovered by cyclic deformation from the total energy accommodated during pre-plastic straining. On the other hand, it is such unrecovered energy that would make the material enter a stable stage of dislocation substructure earlier during cycling.