Non-destructive inspection device and method utilizing a magnetic field and sensor coil array

1. A non-destructive inspection device comprising:

an exciting pole including a magnetic flux exciting surface for exciting a magnetic flux to form a magnetic field in an inspection target;

a recovering pole including a magnetic flux recovering surface for recovering the magnetic flux excited from the magnetic flux exciting surface; and

a coil ray including a plurality of loop coils though which the magnetic flux excited from the magnetic flux exciting surface passes prior to reaching the inspection target, the coil array facing the magnetic flux exciting surface,

wherein the magnetic flux exciting surface includes a first edge located closer to the recovering pole and a second edge located farther from the recovering pole than the first edge, and

wherein the coil array extends along the first and second edges and the plurality of loop coils are offset toward the recovering pole to be located closer to the first edge than to the second edge.

2. The non-destructive inspection device according to claim 1 , wherein the plurality of loop coils are disposed within a region between a center of the magnetic flux exciting surface as viewed widthwise thereof and the first edge of the magnetic flux exciting surface.

3. The non-destructive inspection device according to claim 1 , wherein a distance of separation between the magnetic flux exciting surface and the magnetic flux recovering surface is 1.0 to 2.0 mm.

4. The non-destructive inspection device according to claim 1 , wherein the magnetic flux exciting surface and the magnetic flux recovering surface are congruent in shape.

5. The non-destructive inspection device according to claim 1 , wherein the magnetic flux exciting surface is entirely equal in width to a trunk portion of the exciting pole.

6. The non-destructive inspection device according to claim 1 , wherein the loop coils of the coil array constitute a plurality of coil rows, each coil row aligned parallel to the first and second edges.

7. A non-destructive inspection method carried out using a non-destructive inspection device comprising: an exciting pole including a magnetic flux exciting surface for exciting a magnetic flux to form a magnetic field in an inspection target; a recovering pole including a magnetic flux recovering surface for recovering the magnetic flux excited from the magnetic flux exciting surface; and a coil array including a plurality of loop coils though which the magnetic flux excited from the magnetic flux exciting surface passes prior to reaching the inspection target, the coil array facing the magnetic flux exciting surface; wherein the magnetic flux exciting surface includes a first edge located closer to the recovering pole and a second edge located farther from the recovering pole than the first edge; and wherein the coil array extends alone the first and second edges and the plurality of loop coils are offset toward the recovering pole to be located closer to the first edge than to the second edge; the method comprising:

a step of magnetizing the inspection target by applying a first static magnetic field to the inspection target;

a step of shutting off the first static magnetic field and measuring transient change in a differential magnetic flux density at a plurality of positions in a first residual magnetic field passing through the magnetized inspection target;

a step of determining a first time constant by a main time constant of the transient change at the respective measurement positions;

a step of magnetizing the inspection target by applying a second static magnetic field to the inspection target;

a step of shutting off the second static magnetic field and measuring the transient change of the differential magnetic flux density at the respective measurement positions in a second residual magnetic field passing through the magnetized inspection target;

a step of determining a second time constant by the main time constant of the transient change at the respective measurement positions; and

an information acquisition step of obtaining information relating to the internal structure of the inspection target, on the basis of the differences between the distribution of the first time constant and the distribution of the second time constant, at the measurement positions.

8. The non-destructive inspection method according to claim 7 , wherein the inspection target is a spot weld section in a joined member formed by spot welding of two metal plates, and information relating to the shape of the nugget section contained in the spot weld section, is obtained in the information acquisition step.