Lecture
Micro-bending beams and micro-tensile specimen, machined by focused ion beam technique in the f.c.c. nickel based superalloy CMSX-4, are used to study size effects like stress gradients during fatigue bending tests or strain gradient plasticity effects during micro-tension. One focus is the influence of the typically used notching of micro-specimens instead of having a standard fatigue starter crack in macro specimens for a static fracture toughness measurement. Therefore, transcrystalline stage-I fatigue cracks initiated at a notch are used to characterize the small-scale fatigue behavior of a ductile material and to act as starter cracks for a static elastic and elastic-plastic fracture toughness evaluation. da/dN-data was gathered by crack extension observation by insitu fatigue in the SEM and a combination of experimental stiffness measurement and FEM-simulation (compliance method). A dislocation pile-up at the neutral axis of a bending beam resulting in a significant Bauschinger effect was identified and its strong effect on the fatigue and fracture behavior has been investigated. As result, a severe and unexpected size effect has been found for the classical static fracture toughness parameters.
