Correlation between fractographic aspects and stress intensity factor in very high cycle fatigue

Abstract

Very high cycle fatigue (VHCF) tests carried out at high frequency made it possible for one to evaluate the behavior of materials, particularly steels, for fatigue lives exceeding 106 - 107 cycles in relatively short time intervals compared to conventional fatigue tests. This reduction in the test duration is due to the ultrasonic frequencies, usually situated in the range 15 - 30 kHz, provided by the test machines. In the VHCF regime, the fatigue crack tends to start internally or in subsurface from internal defects, intrinsic to the material. The evolution of fatigue cracks occurs in different stages: crack initiation, crack growth within the fish-eye, crack growth outside the fish-eye, and then final fracture. In addition, another phenomenon that may occur inside the fish-eye and nearby the initial defect, is known as fine granular area (FGA). VHCF tests of a crankshaft steel were performed at a frequency of 20 kHz and loading ratio of -1. The S-N curve indicates an increase in fatigue life as the applied stress decreases. The fracture surfaces analyzed have also indicated the presence of fish-eye which includes an FGA region. Aspects referring to fish-eye and FGA are presented and related to stress intensity factor (SIF).