Short crack behavior at deep and microstructurally shallow notches

Usually, fatigue crack propagation is experimentally characterized using cracks emanating from deep, through-specimen-width notches that average the crack's propagation behavior over many microstructural features along the crack front. Such a deep and wide crack does not accurately represent the reality in most tools, where cracks typically emanate from small microstructural features, such as carbides or shallow notches at the surface. Therefore, the general applicability of conventionally obtained fatigue data on application-relevant short cracks was uncertain. Furthermore, by averaging local properties, the effect of microstructural features on the propagation behavior remained hidden. The current work addresses the open question of the validity of short crack fatigue data obtained on deep notches using a novel method to monitor the propagation of application-relevant, microstructurally small, and shallow cracks. The technique enables in situ measurement of microstructurally small cracks emanating from an artificial defect of application-relevant size near a small number of microstructural features by using the alternating-current potential drop method. The technique is demonstrated on the example of µm-sized semi-elliptical notches introduced via focused ion beam milling. Measured potential increases were correlated to fatigue crack extensions for the utilized material, signal current, and signal frequency combination. The short crack propagation behavior of microstructurally small cracks resembled that observed for short cracks in single-edge notched bending specimens. The results indicate that the short crack propagation behavior obtained on physically short but macroscopically deep cracks is also valid for application-relevant defect sizes for high-strength materials.