Applicability of linear cumulative damage rule to TMCP steel under variable strains

This study systematically evaluates the applicability of the linear cumulative damage rule for predicting fatigue life under various strain histories across a wide strain-amplitude range encompassing both the low-cycle fatigue and extremely low-cycle fatigue regimes. To this end, two high-performance thermo-mechanically controlled process (TMCP) steels increasingly used in Japanese high-rise steel buildings were investigated through a series of strain-controlled fatigue tests conducted over a strain-amplitude range of 1%–12%. The loading programs included variable-strain-amplitude loading, offset constant-strain-amplitude loading with a nonzero mean strain, gradually increasing or decreasing strain-amplitude loading, and random loading histories. In addition, damage-initiation life, fatigue life defined on the basis of strength reduction, and total fracture life were examined. The experimental results indicate that, under variable-amplitude cyclic loading with relatively large strain amplitudes, the influence of loading sequence is limited. Consequently, the fatigue life of these steels in both the low-cycle fatigue and extremely low-cycle fatigue regimes can be evaluated appropriately using the linear cumulative damage rule. In steel building structures, crack initiation or fracture may occur after only a limited number of loading cycles. From this perspective, the demonstrated validity of the linear cumulative damage rule at the material level provides a practically useful basis for simplified fatigue-life assessment and engineering design of steel structures.