Fatigue life and cyclic material behavior of butt-welded high-strength steels in the LCF regime

Fatigue life approach methods based on general local concepts for welded joints assign universal material properties to geometrically well-defined weld toes or weld roots, in order to evaluate local stresses and strains, ignoring any metallurgical discrepancies. In this study, analysis of the main influences on these concepts, linear damage accumulation and cyclic material behavior, of the high-strength and ultra-high-strength steels S960QL, S960M and S1100QL was performed for the base material and for butt-welded specimens. Results of stress-controlled tests under constant amplitude loading (CAL) and variable amplitude loading (VAL) show that a linear damage accumulation with a damage sum of D-al = 0.5 is conservative. Effects of the geometrical and metallurgical notch on the cyclic material behavior are shown by strain controlled testing of both base material and welded specimens. Cycles to crack initiation, presented in strain-life curves, are consequently reduced in the as-welded state. Cyclic stress-strain curves show a cyclic softening of the base material as well as of the welded joints. Furthermore, the advantages of the ultra-high-strength steel S1100QL disappear in welded connections.