Preliminary Experimental Investigation of Loading Sequence Effects on Low-Cycle Bending Fatigue

Bending fatigue, as one of the impact factors to reduce a metal's service life, could play a critical role in fatigue life estimation of aerospace components and civil infrastructures, such as in machine gears, splined shafts, and wind-turbine blades, for example. However, the loading effects related to fatigue behavior mainly focus on uniaxial or multiaxial loading conditions, in which the tensile or compressive stress is uniformly distributed. Therefore, bending behavior, including the random tensile/compressive stress and shear stress distribution, may lead to different results. In this paper, experimental investigations are performed on the low-cycle bending fatigue of 1018 steel under different loading effects. Constant deflection tests are carried out to determine the specimen fatigue cycles under different deflections. Three types of variable deflection tests, namely, two-step, periodic overload, and combined step, are conducted to analyze the effects of loading amplitude and sequence with the linear damage rule. The final bending cracks are characterized by scanning electron microscopy (SEM) to further explore crack initiation and propagation during the bending test. The mechanical test results show that a change in flexural strain causes a reduction in fatigue life, showing that damage accumulation and crack propagation accelerate during the bending test. Further, a drop in flexural change from high to low can cause more damage than a rise in flexural strain. Microscale observation shows a complex crack pattern of the primary crack growth path. In addition to the cracks initiating from the specimen boundary, several cracks can be observed forming on the edge of the primary crack, which results from resistance during the propagation of the primary crack.