Influence of microstructural barriers on small fatigue crack growth in mild steel

Increasing the microstructural resistance of metallic materials to small crack growth is a major task of today’s materials science. In this regard, grain boundaries and precipitates are well known to decelerate small cracks, but a quantitative understanding of the blocking effect is still missing. This is because crack deceleration is influenced by many parameters: cyclic load, crack length, distance to obstacles, orientations of grains and obstacles. Even the examination of a huge number of small cracks would not be sufficient to identify the effect of these parameters independently, especially since fatigue crack growth is a 3D problem and investigations of the sub surface orientation of cracks and grain boundaries are scarce. The technique of artificial crack initiation by a focused ion beam combined with focused ion beam tomography was used for the first time to get a quantitative understanding of the mechanisms which determine the resistance of boundaries against crack propagation. As the experiments were performed on nickel based super alloys only, now this technique was used to investigate how cracks interact with boundaries in mild steel. The behaviour here is completely different which is due to the pencil glide behaviour of dislocations. However, as a main result an orientation limit was found where cracks cannot cross a grain boundary.