Performance of plastic hinges in FRP-strengthened compressive steel tubes for different strain-hardening response

Plastic buckling of tubular columns has been attributed to rotational instability of plastic hinges. The present study aimed to characterize the plastic hinges for two different grades of strain- hardening, examined in mild- steel (MS) and stainless- steel (SS) tubes with un-strengthened- strengthened and strengthened conditions. At the primary stage, the formerly tested experimental specimens were simulated using full-scale- scale FE models considering nonlinear response of the materials, then to estimate the characteristics of the plastic hinges, a meso model was developed from the critical region of the tubes and the moment- rotation diagrams were depicted under pure bending conditions. By comparison of the relative rotation diagram obtained by the full-scale- scale models with the critical rotation under pure bending, the length and critical rotation of the plastic hinges under eccentric axial load were estimated. The stress and displacement diagrams indicated the mechanism of higher energy absorption in the strengthened tubes, compared to unstrengthened specimens, due to establishment of stable wrinkles along the tubes. The meso model showed that by increasing the critical rotation in the strengthened MS tube equal to 1450%, the energy absorption of the tube has been enhanced to 2100%, prior to collapse.