Small scale fatigue crack growth and fracture: a case study in the nickelbase superalloy CMSX-4Wednesday (26.09.2018) 17:30 - 17:45 S1/01 - A03 Part of:
Fracture phenomena at small scales have been under discussion in the recent past. Intensive investigations have been made regarding the simplifications made for static fracture toughness evaluations at small scales such as the use of notches cut with a focused ion beam (FIB) as initial failures. Literature reports that J-integral based evaluation methods of microspecimens with such FIB cut notches result in valid fracture toughness evaluations. It also reports a change in fracture toughness at small scales, which is ascribed to the influence of size effects.
However, one challenge in small scale fracture analysis is testing highly ductile materials, where the assumptions for the lower bounds of the J-integral based evaluation methods are violated. For this purpose, we performed fatigue crack growth experiments as well as static fracture toughness measurements on microcantilevers made from the ductile nickelbase superalloy CMSX4. We tested the differences between FIB cut notches and fatigue precracks as initial failures for the static fracture toughness evaluations. The fracture toughness of microcantilevers has been compared to that of macroscopic bending beams. The capability of a ductile material to grow fatigue cracks in micron sized specimens has been tested. Thereby, the influence of size effects and grain boundaries on the fatigue crack growth behavior has been investigated as well.
Our experiments show differences between notches and fatigue cracks as initial failures, indicating that the use of fatigue precracks is inevitable for a fracture toughness evaluation even at small scales. However, a strong deviation between microscopic and macroscopic fracture toughness values can still be measured. Finally, the fatigue crack growth experiments show a distinct interaction of the cracks with the neutral axis of the bending beams and with grain boundaries. Overall, it shows that care has to be taken when fatigue cracks and fracture phenomena are investigated at small scales.