Electricity: measuring and testing – Magnetic – Stress in material measurement
Patent
1987-10-15
1990-06-05
Strecker, Gerard R.
Electricity: measuring and testing
Magnetic
Stress in material measurement
324227, 324232, 324239, 324240, 324243, 73779, 7386269, G01B 724, G01N 2780, G01N 2782, G01R 3312
Patent
active
049317300
DESCRIPTION:
BRIEF SUMMARY
The present invention concerns a method and apparatus for non-destructive materials testing and more particularly a method and apparatus for magnetostructural materials investigations of diamagnetic, paramagnetic, ferromagnetic and ferrimagnetic materials.
Several methods for non-destructive testing of materials are at present known in the art, like radiography by means of X-rays, gamma rays or particle radiation, ultrasonic testing, acoustic emission testing, eddycurrent testing etc. These methods are applied in a number of circumstances, typically for instance in non-destructive testing of structural materials. The results obtained by such methods are usually based on indirect measuring methods, and although these may be quite straightforward, their interpretation is based on empirical procedures and related to materials' properties and defects by means of a calibration against specimens and thus liable to errors when the test specimens have properties and defects not accounted for by empirically obtained calibration data. Moreover, signal
oise ratios are often too poor to allow for a more precise structural analysis, for instance of crystalline properties of the materials, lattice defects, dislocations, stress fields and the like.
In prior art there are known methods based on measuring the magnetic properties of the materials to be tested, and devices have been developed and used to this end. Most of these methods are based on magnetizing the materials and recording either the magnetization curve commonly known as the B-H-graph, or the hysteresis loop. This allows the determination of parameters as magnetic remanence or the coercive force, which may be related to mechanical properties of the materials by empirical calibration. Remanence and coercive force may for instance be related to the hardness of a material, as they to some extent are dependent on grain structure, which determines the hardness of the specific material. They may also be correlated with the tensile strength of the material to a fairly accurate degree. Small discontinuities or jumps may show up in the magnetization curve and the size and number of these jumps, which are known as the Barkhausen effect, may be measured and analyzed to show the existence of defects, cracks and voids in the material. As well known to those skilled in the art, the Barkhausen effect is due to the movement of the so-called domain (Bloch) walls in a ferromagnetic material, a movement which is strongly affected by lattice defects, dislocations, precipitations, inclusions, cracks and voids, thus giving an indication of stress fields and grain properties. These phenomena all contribute to observe the discontinuities of the magnetization curve. Recording and analyzing the Barkhausen effect may thus furnish important information about the properties of materials. Another method of magnetic testing relies on applying a magnetic flux to a material and recording the remanent flux pattern of the material. Defects in the material may then show up as a distortion of the flux pattern. The use of magnetic measurement, for instance measuring the change of the magnetic field strength, has been applied to thickness testing of material, e.g. in metallurgical industry, where control of the thickness of rolled or extruded products is wanted.
Magnetic methods as mentioned above are for instance stated and disclosed in DE-OS No. 27 36 477, which discloses the detection of defects in magnetic materials, based on an analysis of noise signals generated by the Barkhausen effect, EP 96 078 disclosing on-line hardness testing of steel sheet by means of measuring remanence, GC Patent No. 1 266 248, which discloses determination of the carbon content of iron alloys by means of a hardness measurement based on recording the coercive force and U.S. Pat. No. 4,495,465 which discloses the use of magnetic flux in non-destructive testing by detecting a variation of the flux pattern indicating a variation in reluctance and thus the occurrence of defects.
Magnetic testing methods as disclosed in
REFERENCES:
patent: 2098064 (1937-11-01), Pfaffenberger
patent: 3742357 (1973-06-01), Kubo et al.
patent: 3825819 (1974-07-01), Hansen et al.
patent: 4495465 (1985-01-01), Tomaiuolo et al.
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Hartai Julius
Lekven Nils C.
Olsen Terje
Dam Patent A/S
Edmonds Warren S.
Strecker Gerard R.
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