Experimental program for characterization of ultra low-cycle fatigue fracture in structural steel

Structural steel components experience cyclic inelastic strains when subjected to extreme loading events such as strong earthquakes. In such cases, the steel may experience ultra-low cycle fatigue (ULCF) fracture, associated with the growth and coalescence of microscopic voids in the material. A number of ULCF criteria have been formulated and used in the literature for the analysis of fracture under cyclic loading. The calibration of such criteria requires experimental test data covering a variety of stress states. To address this need, a testing program, involving both monotonic and cyclic loading protocols, was conducted on 60 circumferentially notched coupons extracted from rectangular steel tube sections made of ASTM A1085 steel. A relatively novel approach was used to achieve a wide range of stress states by subjecting the coupons to combinations of axial and torsional loading. The experimental tests are accompanied by finite element analyses to elucidate the local stress and deformation states at the crack initiation location of each coupon. To demonstrate the importance of the experimental tests for the reliable characterization of ULCF in structural steel, an example calibration is conducted for an ULCF criterion from the literature using the experimental results.