Study on fatigue crack initiation and propagation from forging defects
In this study, crack initiation and propagation under cyclic loading are first experimentally studied by testing specimens fabricated from a rotor material containing manufacturing defects. The latter represent clusters of non-metallic inclusion which size and location are examined by both ultrasonic testing (UT) and fractographic analyses. Three test specimens were extracted from a material block in such a way that the UT indications were located in the middle part of the cross-section. The specimens were then subjected to cyclic loading, applying tensile stresses with different magnitude and stress ratio. This test procedure produced beach marks on specimen fracture surfaces from which the crack initiation and propagation were backtraced. The experimental results suggest that a considerable number of cycles are required for a crack with a size corresponding to the UT indication to be formed. A numerical approach was then adopted for modeling damage accumulation and crack formation starting from a defect cluster. A material model was first calibrated to describe both the cyclic hardening behavior of the defect-free material and strain controlled low cycle fatigue tests. Subsequently, the model was applied to predict the damage evolution and crack formation at a defect group representative of that in one of the specimens studied.