Effects of pre-fatigue damages on ultimate strength of steel columns: From material to structure

Abstract

Progressive deterioration in the mechanical properties of materials with increasing service period is commonly observed, but it is not well addressed how the ultimate strength of steel components vary with respect to prior fatigue damages. In the present work the influence of pre-fatigue damage on the residual mechanical properties of Q345 steel was investigated. Various pre-fatigue damages were induced by subjecting the specimens to different loading cycles, and subsequent tensile tests were performed to obtain the residual mechanical properties. A stress-strain relation model accounting for the pre-fatigue damage was developed based on the material tests. Ultimate bending strengths of thin-walled steel columns were numerically studied using the developed stress-strain relation model, in which 480 cases of columns with unstiffened and stiffened box-sections, and circular-section were analyzed. The effects of the design parameters and the pre-damage levels on the ultimate strength were investigated, and practical prediction formulae were proposed. Results indicate that the fatigued material exhibits considerable difference with respect to the damage free material. The pre-fatigue loading leads to a reduction in yield stress, ultimate stress and ductility. The yield plateau gets shorter and even disappears when the pre-damage reaches up to 0.6. The ultimate strength of the thin-walled stub-columns decreases apparently with the increase of the pre-fatigue damage level. The predictions of the proposed formulae for different cross-section types agree very well with the numerical results.