A study of very high cycle fatigue in high strength steels

Until 1970´s, fatigue properties of materials were usually studied and evaluated up to the HCF regime (N<106 cycles) beyond which failure was not expected. However, in the late decades of the 20th century proof of fatigue failure in the VHCF regime (N>106 cycles) was presented by different material scientists. This led to an aroused interest in developing a new testing technique where a very high number of loading cycles is achieved within reasonably short time. The ultrasonic fatigue testing system was developed for this purpose, testing materials in the VHCF regime, where specimens are loaded at 20 kHz reaching 1010 cycles in less than a week. In the ultrasonic fatigue testing system, an electric sinusoidal signal is generated and then converted to a mechanical sinusoidal vibration. The mechanical vibration is led through a magnifying horn to the specimen. The system vibrates at resonance frequency, hence all individual parts of the system are designed and dimensioned according to the system resonance frequency. Fatigue testing commonly implies testing for fatigue strength and testing fatigue crack growth. The staircase test method is used to determine a materials fatigue strength at a certain fatigue life. To measure the crack growth rate the crack is continuously monitored and measured and the stress intensity at the crack tip is computed for the growing crack. FEM has been used when the specimen geometry is complicated or when dynamic simulations are required. Results from fatigue tests in the VHCF regime have showed that interior non-metallic inclusions are the most common initiation sites. This led to further investigations on the inclusion type, shape and size and the effect they have on the fatigue strength. An inclusion initiated fatigue crack normally creates a fish-eye with a Fine Granular Area (FGA) around the inclusion. It has been confirmed that 90-99% of the total fatigue life is due to the formation of the FGA area. This study contains an assemblage of theories and models for, and results of, different VHCF tests