Three-dimensional observations of magnetic flux density around fatigue crack tips of bearing steels

Fatigue failure of steel occurs when small cracks form in a component and then continue to grow to a size large enough to cause failure. In order to understand the strength of steel components it is important to find these cracks. However, at present, it is not easy to distinguish the cracks that will grow fast and cause failure. We developed a three-dimensional scanning Hall probe microscope (3D-SHPM) and observed fatigue cracks at room temperature while they were growing. Four-point-bending fatigue tests were carried out using pre-cracked specimens (JIS-SUJ2, bearing steel). We observed the two-dimensional magnetic flux density distributions around the crack tips and found that there is a strong correlation between the changes in the magnetic flux densities and the crack growth. In order to understand this, we looked into all the three components of the magnetic flux densities, and found that they shape an arched bridge around a crack. We also found that the magnetic flux density moves in front of the crack tip along the crack growth direction.