Changes in magnetic field intensities around fatigue crack tips of medium carbon low alloy steel (S45C, JIS)

•Magnetic fields around fatigue crack tips were observed using an SHPM.•The magnetic intensities decrease linearly with the maximum stress intensity factors.•Plastic deformation due to the crack growth causes the decrease in the intensity.•The decrease in soft specimens was larger than that in hard specimens.•This feature can be seen even when their stress intensity factors were comparable.

Damage of machine components occur when cracks form and continue to grow to a size large enough to cause fracture. In order to understand the crack propagation phenomena, non-destructive evaluation methods that can be correlated to measurements around the fatigue crack tips are necessary. In the present work, we developed a scanning Hall probe microscope (SHPM) equipped with a three-dimensional sensor and observed magnetic fields around fatigue cracks at room temperature in air while they were growing. In order to study the relation between plastic deformations and magnetic flux densities, both as-received non-heat-treated and heat-treated (quenched and tempered) specimens were used. Medium carbon low alloy steel specimens (S45C, JIS) were used in the experiments. The area around the crack tip was magnetized and the changes in the area were observed. The changes in magnetic fields in soft and hard specimens with the same intensity factors were compared. A strong correlation between the changes in the magnet fields and plastic deformation areas was found.