Long-term strain ageing effects on low-carbon structural steels

This paper experimentally investigates the long-term strain ageing effects on the stress-strain behaviour of low-carbon structural steels manufactured in both China and Japan. In the experimental programme, the effects of pre-strain levels (ranging from 2% to 8%) and ageing durations (ranging from 7 to 180 days) on yield strength, ultimate strength, yield-to-ultimate strength ratio, and ductility were assessed. The results revealed that increasing the pre-strain level to 8% resulted in enhancements of 54% and 7% in the yield and ultimate tensile strengths, respectively, accompained by a 48.4% reduction in ductility, as indicated by the ultimate strain. Moreover, extending the ageing duration to 90 days led to further increases in the yield and ultimate strengths by up to 6% and 4%, respectively. Quantitative analysis also revealed that strain ageing treatments can affect the yield-to-ultimate strength ratio, with high pre-strain levels causing the ratio to exceed the limit value of 0.85 specified in current design codes. Although the influence of strain ageing on the strength ratio and ductility was generally consistent across the four categories of structural steels, minor variations were observed in the effect of strain ageing on the yield and ultimate strengths. Based on the test results obtained from the present study and supplementary database collected from the literature, a modified quad-linear stress-strain model considering strain ageing effects (QLMSA) was proposed, accounting for the pre-strain effects on the stress-strain characteristics of low carbon structural steels. The proposed models have been shown to accurately predict the long-term strain ageing effects on the stress-strain behaviour of low-carbon structural steels with nominal yield strengths ranging from 235 MPa to 355 MPa.