Changes in magnetic flux density around fatigue crack tips

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 the cracks, which eventually grow to cause failures. However, at present, it is not easy to distinguish, in the early stages of growth, the cracks that will grow fast and cause failure. We hypothesized that it may be possible to distinguish them by comparing changes in the magnetic flux density around the tips of those cracks that grew to failure. In order to measure these changes in magnetic flux density, we developed a scanning Hall probe microscope and observed the fatigue cracks growing from artificial slits in soft bearing steels. Note that we did not magnetize the specimens artificially but succeeded to measure the changes in magnetic flux density during the fatigue tests. We also compared the changes in magnetic flux density around crack tips, which grew under different loads, and found that there is a strong correlation between the magnetic flux density, crack growth and stress intensity factors. In order to understand this, we looked into the relation between stress field, residual strain and magnetic flux density, and concluded that the changes in magnetic flux density are caused not only by the residual strain occurring around the crack tips but also by the increase in the elastic stress.