Effect of Large Alternating Strains of Steel Beams

Eleven 4 by 4 M 13.0 structural steel cantilever beams were subjected to cyclic reversed loading. The maximum strain at the clamped end was carefully controlled and varied between ± 1.0% and 2.5%. In all instances, except for 1% controlling strain, the strains induced in the distorted flanges caused early formation of cracks that finally led to the fracture of the beam. In the experiments the number of cycles to cause fracture varied from 16 to 650. A drastic drop in the low cycle fatigue endurance occurred in the beams as the controlling strain was increased. An analogous relationship was observed between the number of cycles causing the initiation of local buckling of flanges and controlling strain. Based on these experiments it is concluded that in low cycle fatigue of structural steel members of large strains, local buckling of flanges is of the utmost importance. The problem of preventing local buckling of elements is considerably more important than that which concerns the low fatigue endurance of the material itself.