Estimation of Depth Direction of Crack by measuring the Residual Magnetic Vector Flux Density around Grains

Abstract

We suggests an algorithm to estimate the depth direction of fatigue cracks. This algorithm uses the distribution of residual magnetic vector flux density (RMVFD) on the specimen. Magnetic domains are the region with a magnetic moment in the same direction. A grain has several magnetic domains. Domain wall is the boundary of the magnetic domain. Magnetic energy stabilized when no external magnetic field applied. That is, the sum of the total magnetic moments is close to zero. It is because that the magnetic flux densities are canceling each other at both ends of the domain wall. The domain wall moves when an external magnetic field applied. As a result, the magnetic domain, which has the same direction with the external magnetic field, enlarged. This phenomenon is magnetization. The domain wall stops moving when it meets with factors that limit the domain wall movement (FLWM). Here FLWM are dislocations, defects, and crystal boundaries. The magnetic energy is unstable at the end of FLWM in the magnetic hysteresis phenomenon. Other end of FLWM has to have the opposite direction of RMVFD to stabilize the magnetic energy. Thus, there are opposite direction of RMVFD each other around crack or grain boundaries. Three dimensional RMVFD (3-RMVFD) can indicates crack existence and depth direction of crack. To verify this theorem, we prepared fatigue crack specimens with different depth direction. The three dimensional shapes of grains reconstructs at the crack. The RMVFD measured by scanning a 3-axis magnetic sensor. And the 3-RMVFD indicated in each grains around crack. The depth directions of crack and grains could estimated by using the 3-RMVFD.