Patent ID: 7109702

Claim:
A non-destructive inspection method including: a step of magnetizing an inspection target by applying a first magnetostatic field to the target; a step of shutting off the first magnetostatic field and measuring transient change in a differential magnetic flux density of a first residual magnetic field passing through the magnetized target, the measuring being performed at a plurality of measurement positions; a step of obtaining a first time constant provided by a main time constant of the transient change for each of the measurement positions; a step of magnetizing the target by applying a second magnetostatic field to the target; a step of shutting off the second magnetostatic field and measuring transient change in a differential magnetic flux density of a second residual magnetic field passing through the magnetized target, the measuring being performed at each of the measurement positions; a step of obtaining a second time constant provided by a main time constant of the transient change for each of the measurement positions; and an information obtaining step of obtaining information about an internal structure of the target based on a difference between distribution of the first time constant and distribution of the second time constant at the measurement positions; wherein the information about the internal structure in the information obtaining step is obtained based on a ratio function which is derived from a distribution function of the first time constant with the measurement position as a variable and a distribution function of the second time constant with the measurement position as a variable.