Fatigue analysis of a bucket wheel by using Linear Elastic Fracture Mechanics

An analysis by using the concepts of the Linear Elastic Fracture Mechanics (LEFM) was conducted in order to study the crack propagation in the welded joint of a bucket wheel that failed at service after approximately one year of operation. Some aspects related to the welded joint were considered in the analysis, such as the occurrence of residual stresses in the structure, the magnification factor of the stress intensity factor (SIF) due to the weld geometry and crack closure phenomenon. Stress intensity factors were numerically obtained for the evaluated cracks, considering tensile and shear loads. Walker equation was used for determining the propagation rate. A fatigue life consistent with the estimation of the number of cycles until the failure of the bucket wheel was obtained when considering the propagation of a preexisting crack in the structure in a field of residual tensile stresses across the thickness of the plate and along the weld bead. It was evaluated that fatigue life based on S-N curves, for a weld detail similar to the weld joint under study, was higher than the estimated number of cycles until the bucket wheel failure, indicating that some defect may have remained after manufacture of the wheel and compromised its useful life.