Electricity: measuring and testing – Magnetic – With means to create magnetic field to test material
Reexamination Certificate
1999-01-19
2001-05-15
Oda, Christine K. (Department: 2862)
Electricity: measuring and testing
Magnetic
With means to create magnetic field to test material
C324S240000, C324S232000
Reexamination Certificate
active
06232774
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of examining an internal structure, such as a welding state and an internal defect, of a magnetic body like a steel plate in a non-destructive manner, as well as to an apparatus for the same.
2. Description of the Related Art
Spot welding is generally used in the automobile industry and for assembly of various metal thin plate products. The typical procedure of spot welding pinches layers of a metal base material between the appropriately formed ends of electrodes. The process then concentrates the electric current and the pressing force on a relatively narrow portion and thereby locally heat the relatively narrow portion, while applying the pressure to the relatively narrow portion with the electrodes. This is a sort of resistance welding.
The spot welded part generally has a sectional structure shown in FIG.
19
. The surface of the welded part is concaved (indentation) by the pressure to be lower than the surrounding non-welded part. The dimension of the indentation is referred to as the indentation diameter. The welded part includes a nugget (deposit), which is the center of the welded part, and a circumferential contact-bonded portion. The nugget is formed by solidification of the fused metal. The contact-bonded portion is formed by bonding of the surfaces of the metal layers which are in closed contact with each other. The dimension of the nugget is referred to as the nugget diameter. The dimension of the sum of the nugget and the contact-bonded portion (that is, the actually joint part) is referred to as the joint diameter. Since the spot welding welds the layers of the metal base material at one point, it is often followed by an examination to check whether the welding strength is sufficient or not.
One effective method of measuring the welding strength in a non-destructive manner measures the nugget (deposit) diameter of the welded part to determine the welded strength. A known method of measuring the nugget diameter applies an ac magnetic field, which is produced by a coil through which a high-frequency electric current flows, to the spot welded part and measures a resulting variation in inductance of the coil to determine the nugget diameter. The conventional method takes advantage of the characteristic that the permeability is different between the nugget and the non-nugget portion, and observes a variation in permeability as a variation in inductance to determine the nugget diameter.
The indentation has a less thickness, and the indentation diameter thereby affects the structural strength. The nugget diameter and the joint diameter, on the other hand, affect the joint strength. In order to determine the welding strength of the spot welded part with high accuracy, it is desirable to determine all the indentation diameter, the nugget diameter, and the joint diameter.
The conventional method, however, measures only the nugget diameter and does not determine the other structural characteristic values, such as the indentation diameter and the joint diameter. There has been a strong demand or the technique that measures a variety of internal structures of not only the spot welded part but a general magnetic body in a non-destructive manner.
SUMMARY OF THE INVENTION
The object of the present invention is thus to provide a method of measuring a variety of internal structures of a magnetic body in a non-destructive manner and an apparatus for the same.
At least part of the above and the other related objects is attained by a method of examining an internal structure of a target magnetic body, the method comprising the steps of: (a) applying a magnetostatic field to a target magnetic body to magnetize the target magnetic body; (b) cutting the magnetostatic field off and subsequently measuring a transient variation in a differential magnetic flux density at a plurality of positions in the vicinity of the target magnetic body; (c) determining a time constant of the transient variation in the differential magnetic flux density at the plurality of positions; and (d) determining a specific characteristic value relating to the internal structure of the target magnetic body, based on a distribution of the time constant over the plurality of positions.
The term ‘internal structure’ is used here in a wide sense that includes not only a mechanical structure but magnetic properties, a chemical composition, and the like.
The time constant of the transient variation in differential magnetic flux density is affected by the inductance of a space having a magnetic field and the degree of resistance to the change of a given magnetic flux in the space having the magnetic flux, as well as by the inductance of the internal portion of the magnetic body, through which the magnetic flux passes, and the electrical resistance of the magnetic body. The inductance of a space having a magnetic field, the degree of resistance to the change of a given magnetic flux in the space having the magnetic flux, the inductance of an internal portion of the magnetic body through which the magnetic flux passes, and the electrical resistance of the magnetic body have distributions reflecting the internal structure of the magnetic body. The method of the present invention can thus specify the internal structure of the magnetic body, based on a change of the time constant of the transient variation in differential magnetic flux density. This arrangement enables a variety of internal structures of the magnetic body to be measured in a non-destructive manner.
In accordance with one preferable application of the method, the step (c) includes the step of: assuming the transient variation in the differential magnetic flux density to be a combination of a transient variation in a first differential magnetic flux density corresponding to an attenuation of a first magnetic flux density caused by the magnetostatic field and a transient variation in a second differential magnetic flux density corresponding to an attenuation of a second magnetic flux density caused by an eddy current, which is induced by the attenuation of the first magnetic flux density, and determining at least one of a first time constant defining the transient variation in the first differential magnetic flux density and a second time constant defining the transient variation in the second differential magnetic flux density.
The first time constant and the second time constant are related to different characteristic values, which respectively represent internal structures of the target magnetic body. This means that the different characteristic values, which represent the different internal structures of the target magnetic body, are specified from the first time constant and the second time constant.
In accordance with one preferable structure of the method, the target magnetic body is a plate assembly obtained by joining two metal plates by spot welding, and the step (d) includes the step of: determining a shape of a nugget portion of a spot welded part, based on a distribution of the first time constant.
The nugget portion of the spot welded part experienced a change of the metal configuration during the welding. The first time constant is varied with the change in the chemical composition inside the magnetic body. The shape of the nugget portion can thus be specified from the distribution of the first time constant.
In accordance with another preferable structure of the method, the target magnetic body is a plate assembly obtained by joining two metal plates by spot welding, and the step (d) includes the step of: determining a shape of a joint portion of a spot welded part, based on a distribution of the second time constant.
The magnetic path of the magnetic flux changes at a certain position in the vicinity of a joint end (that is, a boundary of the joint portion) and outside the joint portion of the spot welded part. Concretely, the magnetic path of the magnetic flux goes around the side of the joint portion. There is accordingly a position where the magn
Beyer Weaver & Thomas LLP
Magnegraph Co., Ltd.
Oda Christine K.
Zaveri Subhash
LandOfFree
Method and apparatus for measuring internal structure of a... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for measuring internal structure of a..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for measuring internal structure of a... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2472089