Local stresses and material damping in very high cycle fatigue

The increased interest in the very high cycle fatigue (VHCF) properties of materials has resulted in a broadened use of ultrasonic fatigue testing. The accuracy of the obtained fatigue data depends to a large extent on the estimate of the actual stresses acting in the ultrasonic test specimen. Finite element modeling (FEM) is used in this paper to study local stresses at the fatigue initiating defects during ultrasonic fatigue testing of an AISI H11 tool steel. Material damping was included in the calculations, which was found to reduce the magnitude of the stresses during 20 kHz ultrasonic fatigue testing. By combining local stresses with the material damping effect, the actual stresses operating in the region of the fatigue initiating defect were established. In the very long life regime microstructural defects, such as slag inclusions, often serve as fatigue crack origins. Locally amplified stresses enable fatigue crack formation and growth. Therefore, in this work the relationship between actual stresses, the defect geometry and fatigue life is examined.