Crack Formation Process in Galvanized Steel Under Dwell Fatigue

Abstract

The study investigated the dwell fatigue characteristics of hot-dip galvanized steel. Cyclic and dwell fatigue tests were conducted, their fatigue life was compared, and fracture surfaces were analyzed. When the cyclic maximum stress (σ_max) was the upper yield stress (σ_UYS), there was hardly a difference in fatigue life between cyclic and dwell fatigues. In σ_max=0.9 × σ_UYS, the fatigue life in dwell fatigue was shorter than that in cyclic fatigue. The cracks under dwell fatigue were generated in σ_max= σ_UYS before N=10 cycles. Their cracks did not grow until N=100,000 cycles. On the other hand, no cracks were observed on the specimen surface under cyclic fatigue before N=100,000 cycles. The formation of cracks on the surface of the galvanized layer under cyclic fatigue was remarkably delayed compared to that under dwell fatigue, regardless of the applied stresses in this study. Therefore, dwell fatigue mode debases the surface of the hot-dip galvanized steel. The applied stress affected the crack morphology on the specimen surface. In σ_max= σ_UYS, the large cracks were observed at the grain boundary triple junctions. In σ_max=0.9 × σ_UYS, not only the cracks at triple junctions of grain boundary but also some cavities along the grain boundaries were detected. Their defects were often reported under creep deformation. The cavities seemed to adjoin each other and coalesce. In the stress relaxation testing, the hot-dip galvanized steel exhibited creep behavior. The decrease in the fatigue life under dwell fatigue would be due to the creep phenomena.