Interaction of non-metallic inclusions, microstructure, and fatigue loading with small crack growth in high-strength steels: Dissertation

The increased demand and requirements for high-strength steels drives the need to better understand and predict the fatigue endurance and crack growth challenges related to their use in critical machine components. Non-metallic inclusions or defects in the steel become increasingly important as the hardness or strength of the steel increases. The distribution and the ability to predict the largest inclusion that causes failure is crucial for the proper and successful design and production of the components. The extreme value distribution is effective in predicting the maximum inclusion in a volume of steel. The proper prediction and use of inclusion data gathered from polished specimen as well as differences in anisotropy are important to consider when gathering data for use in design and prediction of fatigue life or failure. The difference in non-metallic inclusions and the forging direction affects the distribution of the size of the inclusions as well as the fatigue endurance limit and its scatter of the steel. The extreme value distributions combined with the Murakami-Endo model are used as a design approach for fatigue failure for components with ultra-long fatigue lives and step loading. This design approach uses the master curve for Optically Dark Area (ODA) growth obtained by Murakami et al. and combines it with the prediction of the largest non-metallic inclusion along with the estimate of the fatigue life of the component. The initiation and growth of small cracks from inclusions as well as small Focused Ion Beam (FIB) notches behave in a similar manner and show a strong tendency to follow the local microstructure. The effect of the local microstructure on the small fatigue crack growth is studied using FIB milling to create cross-sections of the microstructure. This showed that the microstructure is also linked to the formation of ODA around non-metallic inclusions in ultra-long fatigue. The behaviour of a small crack growing from notches in high cycle fatigue is studied by using high-speed microscopy and Rumul fatigue testing machines. The test results show that small cracks initiate and grow quickly in the beginning of the fatigue life after which they propagate slowly at a stress intensity range lower than the large crack growth threshold until it is reached. Comparing different data results for different R-ratios shows that the parameter ?K+ works well to compare the crack growth rate of small cracks in the studied quenched and tempered steel. For crack arrest the ?K+ or Kmax thresholds are lower for cracks with higher compressive loading. Also test results showed that increasing only the compressive portion of loading can reinitiate arrested.