Effect of Pre-strain Mode and Loading Direction on Low-Cycle Fatigue Properties of Dual-Phase Sheet Steel

Abstract

This study investigated the effect of the loading direction and pre-strain mode on the low-cycle fatigue properties of hot-rolled dual-phase 780 sheet steel. Tensile and low-cycle fatigue tests were conducted at room temperature using plate type specimens that were pre-strained either by uniaxial deformation or plane strain deformation. The tensile properties of the pre-strained specimens were similar to each other, revealing slightly higher elongation in the specimen deformed by plane strain condition. However, the specimen pre-strained by plane strain condition had a much higher low-cycle fatigue life than that of the uniaxial pre-strained specimen or the as-received specimens. Furthermore, in various loading directions, the transverse direction specimens revealed outstanding fatigue properties compared to rolling direction and 45° of RD direction specimens. This difference in low-cycle fatigue property was interpreted in relation to the cyclic softening behavior and grain morphology, which varied greatly with the pre-strain condition. The behavior of low-cycle fatigue life was excellent in the plane pre-strained case where deflection, arrest and branching of fatigue crack propagation occurred most efficiently on account of microstructure change. The transverse direction specimen exhibited inefficient cyclic softening behavior.