Non-destructive inspection method

1. 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.

2. The method according to claim 1 , wherein the measurement positions are in a row facing the target.

3. The method according to claim 1 , wherein the target is a spot weld section in a jointed plate member made by spot welding two sheet metals.

4. The method according to claim 3 , wherein the information obtained in the information obtaining step includes information about a shape of nugget section included in the spot weld section.

5. A non-destructive inspection method comprising:

a scanning step including a cycle of magnetizing a target by applying a magnetostatic field to the target, shutting off the magnetostatic field to measure transient change in a differential magnetic flux density of a residual magnetic field which passes through the magnetized target at a plurality of measurement positions, and obtaining a main time constant of the transient change for each of the measurement positions, the cycle being repeatedly performed for each of a plurality of magnetostatic fields of different magnetic flux densities;

an analyzing step of analyzing a change that the main time constant undergoes at each measurement position as the plurality of magnetostatic fields are changed in the scanning step; and

an information obtaining step of obtaining information about an internal structure of the target based on an analysis result obtained by the analyzing step;

wherein in the analyzing step, at each of the measurement positions, a magnetic flux density of a critical magnetostatic field is determined, for which field the change of the main time constant during the scanning step with respect to changes in the magnetostatic field achieves a maximum value, and wherein in the information obtaining step, information about the internal structure of the target is obtained based on a distribution function of the critical magnetostatic field with the measurement positions as a variable.

6. The method according to claim 5 , wherein the measurement positions are in a row facing the target.

7. The method according to claim 5 , wherein the target is a spot weld section in a jointed plate member made by spot welding two sheet metals.

8. The method according to claim 7 , wherein the information about the internal structure obtained in the information obtaining step comprises information about a shape of a nugget section included in the spot weld section.