Material properties and membrane residual stresses of S690 high strength steel welded I-sections after exposure to elevated temperatures

This paper reports a thorough experimental investigation into the material properties and membrane residual stresses of S690 high strength steel welded I-sections after exposure to seven levels of elevated temperatures ranging from 30 degrees C (room temperature) to 950 degrees C. The experimental programme included heating, soaking and cooling of S690 high strength steel coupons and welded I-section specimens as well as post-fire material tensile coupon tests and membrane residual stress measurements, with the experimental rigs, procedures and results fully reported. The key post-fire material properties were then carefully analysed together with the test data collected from the existing literature, and a new set of retention factor curves of simple multi-linear shapes was proposed and shown to result in accurate predictions of post-fire yield and ultimate stresses for S690 high strength steel after exposed to elevated temperatures. Regarding post-fire membrane residual stresses, the measured distribution pattern and peak amplitudes in S690 high strength steel welded I-section after exposed to an elevated temperature of 300 degrees C generally remained unchanged in comparison with those in S690 high strength steel welded I-section at room temperature. However, for higher elevated temperatures ranging from 600 degrees C to 950 degrees C, the peak values of both compressive and tensile membrane residual stresses dramatically decreased, and moreover the discrepancy between the peak compressive and tensile membrane residual stress values became smaller and the transition regions (where the peak tensile residual stresses are changed to the peak compressive residual stresses) became narrower; this can be attributed to the fact that prominent elastic strain redistribution and residual stress relaxation of steel starts from around 500 degrees C-600 degrees C. A membrane residual stress predictive model was proposed for S690 high strength steel welded I-sections after exposed to elevated temperatures, and shown to well represent the measured membrane residual stress patterns and amplitudes over the full temperature range from 30 degrees C to 950 degrees C.