Observation of magnetic flux density around fatigue crack tips in bearing steel using an SHPM with a three-dimensional small-gap probe

Fatigue failure of steel occurs when cracks form in a component and continue to grow to a size large enough to cause failure. In order to understand the strength of a steel component it is important to locate these cracks. We developed a scanning Hall probe microscope (SHPM) equipped with a GaAs film sensor and observed fatigue cracks at room temperature in air while they were growing. In this paper we report two important findings regarding the magnetic field properties: a decrease in the magnetic flux density at the early fatigue stage and the polarization of the magnetic field ahead of the fatigue cracks occurring during crack growth.