Fatigue test series on butt welded plates, which have been carried out at the Research Center for Steel, Timber and Masonry, have been extended by low cycle fatigue tests between 10,000 to 40,000 load cycles. Different types of the butt welds, the influence of the base materials yield strength and the effect of misalignment and wall thickness are discussed with respect to the special behaviour in the lowcycle fatigue range within this article.
The growing demand for high load capacity in steel construction together with the need of decreasing total construction weight can be met by the use of high strength steels. Thereby, material cost and carbon footprint as well as weld volume can be reduced. Structures, for which high strength steel is preferentially used, are usually exposed to very high alternating loads. From a certain number of load cycles on, predominantly static loading can no longer be assumed. Due to the cyclic load behavior fatigue becomes the decisive factor for design. Low-cycle fatigue, that usually means a number of failure load cycles below 40.000, is barely investigated and especially for high strength steels there is little information available up to now.
In general, endurance is divided in three parts: low-cycle fatigue for number of load cycles below 40,000, high-cycle fatigue behavior for load cycles from 40,000 to about 10 million and very high-cycle fatigue for number of load cycles above 10 million (see Fig. 1). Another way to separate low-cycle from high-cycle fatigue for ductile materials is by means of deformation criterion (Gudehus and Zenner, 2000). AWS D1.1/D1.1M (2006) limits high-cycle fatigue by means of a recommended maximum load level, which is 60 percent of the yield strength of the base material for cyclic loading.
