Measurement of small cracks initiating from inclusions, Focused Ion Beam notches and drilled holes

The focus of this study is small crack initiation, growth and arrest from small defects in quenched and tempered steels. To study the initiation and growth threshold of microcracks from small defects a specialised test method was developed. Round bars were axially loaded at R = -1 and fatigue initiators were manufactured by drilling (Ø 50 µm) and Focused Ion Beam (FIB) milling (a = 10-50 µm). The specimens were fatigued at the range of the smooth specimen fatigue limit. The initiated microcracks were monitored to observe crack initiation; growth and arrest. The high correlation of the FIB small crack growth results with inclusion test results indicates that using a FIB notch to initiate small cracks is a good way to test small crack growth in high strength steels