METHOD FOR DETECTING STRESS AND DEFECT IN A METAL PIECE

Abstract: A sensor for a system that detects stresses and defects in a metal piece by employing the Barkhausen phenomenon enables static or dynamic testing of such metal pieces as hard steel in which only low levels of Barkhausen noise can be generated or which are configured in one of a large variety of shapes. The sensor includes apparatus that ensures that a uniform time-varying magnetic field can be produced in all the metal pieces of a variety of shapes. The sensor (10) can include an energizing coil assembly (14) which generates the magnetic field in the metal piece having a core constructed from such a material in which a low level of magnetic noise is generated as ferrite. The level of the magnetic noise gener­ ated in the ferrite core is so low that, even the magnetic noise is sensed by the sensing coil assembly (16), which receives the Barkhausen noise generated within the metal piece, that noise cannot interfere with the higher levels of Barkhausen noise generated in all ferromagnetic metals, including hard steel. The dynamic sensor (200) has apparatus, such as wheel assemblies (256), for maintaining an air-gap of a substantially constant depth between the specimen and the energizing (236) and sensing (248) cores. The dynamic sensor (300) can also include an energizing core of an energizing coil assembly (310) and which can be reoriented relative to the sensor (300) to determine the directional characteristics of stress and defects in the specimen.

Description: The present invention relates generally to stress and defect identification and, in particular, to a method employing magnetic fields to identify stress and defects in a metal piece. Stress and structural defects in a metal piece and certain other properties of the metal piece can be identified by creating a time-varying magnetic field within the metal piece and analysing the magnetic noise created in the metal piece by the magnetic field. In particular, one method employs a phenomenon known as the "Barkhausen effect" to identify stresses and defects in a metal piece. As is stated in column 2 of U.S. Patent No. 3,427,872 ("the Leep et al. patent"), the Barkhausen effect is "defined in Webster's New International Dictionary (3rd Edition) as "a series of abrupt changes or jumps in the magnetization of a substance when the magnetizing field is gradually altered". The abrupt jumps that occur in the magnetization as the intensity of the field is changed can be detected as electrical noise by a sensing coil disposed proximate or in contact with the metal piece. The noise carried by the electrical leads from the coil - commonly referred to as "Barkhausen noise" - can be fed through a suitable processing network and, if desired, to a speaker. The level of the Barkhausen noise that is generated at a location within the metal piece depends in part on the sense, magnitude and direction of the stress at that location and the microstructure of the metal. Accordingly, workers in the art have attempted to employ the Barkhausen effect and Barkhausen noise to identify stresses and defects in and some microstructural characteristics of a metal piece. A discussion of the Barkhausen effect can be found in columns 1 and 2 of the Leep et al. patent. Most systems which employ the Barkhausen effect to identify stresses and defects in a metal piece include an energizing coil assembly and a sensing coil assembly. The energizing coil is disposed proximate the location of the metal piece under examination and is energized with a periodically time-varying signal to induce in the metal piece a periodically time-varying magnetic field. The time-varying magnetic field in the metal piece causes abrupt jumps in the magnetization of the metal piece to occur. A sensing coil is disposed near or in contact with the metal piece near the same location and detects the abrupt jumps in the magnetization and converts those jumps to Barkhausen noise on the electrical conductor of the coil. The Barkhausen noise is then fed to circuitry, usually located externally of the sensor, which can process the noise in a variety of manners, depending upon the type of information to be obtained and displayed. Ultimately, the processed Barkhausen noise is fed to a device which displays it. A system for energizing the energizing coil assembly and processing the Barkhausen noise, and a description of displays of Barkhausen noise are described in the Leep et al. patent. Known Barkhausen sensors are suggested for use in either static or dynamic applications. Static testing involves making a relatively small number of inspections of a single piece or of each of a relatively small number of pieces. That limitation is imposed by the fact that most sensors used in static testing require physical contact between the energizing core of the energizing coil assembly and the test piece. Therefore, changing the location of inspections requires removing the sensor from the test piece and moving it into contact with the next inspection site to prevent rapid and undesirable wear of the energizing core that would result from moving the sensor from test site to test site while the core is in contact with the piece. Dynamic testing is employed in situations where the entire surface of a single metal piece must be treated or where more than one location of each of a number of test pieces must be inspected. The sensor used in dynamic testing does not require physical contact between the energizing and sensing coil assemblies and the metal piece. Therefore, there can be constant relative movement between the metal pieces under examination and the sensor. Any system employing Barkhausen noise to locate stress and defects within a specimen depends, for proper functioning upon the phenomenon that the level of Barkhausen noise generated in a specimen changes significantly when the sensor is moved between a location where no stress or defects exist and a location where stress or a defect is located. Therefore, variations in the level of Barkhausen noise generated in the metal piece caused by the examining process itself must be minimized to the point where proper analysis of the Barkhausen noise is not prevented. Accordingly, the effects on the generated Barkhausen noise due to magnetic self-coupling within the Barkhausen sensor, inconsistent magnetic coupling between the sensor and the metal piece and variations in the shapes of metal pieces must be minimized. Magnetic self-coupling can occur between the energizing and sensing coils of the sensor. Often, it is necessary to examine a metal piece constructed of hard steel, in which only low levels of Barkhausen noise can be generated. The term "hard steel", as used herein, means steel having a Rockwell C hardness greater than about 50. Exemplary of such steels are (i) martensitic steels that have been either quenched or quenched and tempered; (ii) fine grained, high strength alloy steels; (iii) carburized steels. Generally, the cores of the energizing coil assemblies of conventional Barkhausen sensors are fabricated from iron or steel. A relatively high level of magnetic noise is generated within iron and steel as the magnetic field induced in the iron or steel is varied. Accordingly, a high level of magnetic noise is generated within an iron or steel energizing core, which can be sensed by the sensing coil assembly and interferes with the low level Barkhausen noise generated in a hard steel piece. Nonetheless, steel or iron has been used consistently as the material for energizing coil assembly cores - even for examining hard steel - for, generally, two reasons. First, the periodic signal used to energize the energizing coil assembly is of a rather low frequency. Electromagnetic devices which operate under low frequency commonly have cores constructed of iron or steel. Second, the relatively high noise generate in an iron or steel energizing core can interfere with Barkhausen noise generated in a test piece only if the energizing core and sensing coil assembly are located sufficiently proximate each other to permit electromagnetic coupling of the noise betwen the energizing core and sensing coil assembly. When conventional Barkhausen systems have been used to test hard steel for stress or defects, they have been used only in experimental settings and only on large pieces. Accordingly, the energizing core of the sensor has been large and, therefore, spaced a sufficient distance from the sensing coil assembly to prevent electromagnetic coupling from occurring. However, in commercial applications, metal pieces with small cross-sectional areas must be examined. Therefore, the sensor used to examine those metal pieces must be small and the distance between the energizing core and sensing coil assembly must be reduced from that of experimental sensors. Destructive electromagnetic coupling occurs between the energizing core and sensing coil assembly, which makes it impossible to distinguish the low level Barkhausen noise generated in a hard steel piece from the high noise generated in the iron or steel energizing core. Therefore, there exists a need for a Barkhausen sensor which generates lower intrinsic noise than conventional Barkhausen sensors, and that permits distinguishing the intrinsic sensor noise from the Barkhausen signal obtained from the metal piece. Proper magnetic coupling between the sensor and the metal piece can be achieved in several ways. A Barkhausen sensor having an energizing core that is adapted to contact the metal piece shall be referred to hereinafter as a "contact sensor". Since the magnitude of the magnetic field induced in a metal piece, and, accordingly, the level of the Barkhausen noise generated within the piece, by a contact sensor depends on the magnitude of the area of the core of the energizing coil assembly that is in contact with the metal piece, the same sensor will provide different readings for different magnitudes of contact area, making interpretation of the results difficult. Since conventional contact sensors have been employed only under experimental conditions, each sensor has been adapted for use with metal pieces of only one general shape. When used with that shape, a given constant magnitude of contact area is achieved. If the sensor were to be used in a commercial setting, the magnitude of the areas of the sensor and the metal pieces which are in contact with each other would vary with the shape of the metal piece. Thus, the magnitude of the Barkhausen noise that indicates existence of a defect would vary with the shape of the metal piece and, accordingly, analysis of the signals received from the sensor would be unduly difficult. Therefore, there exists a need for a Barkhausen contact sensor which either provides constant contact area between the metal piece and the sensor, regardless of the shape of the metal piece, or electrically maintains a uniformly varying magnetic field among metal pieces of various shapes. Finally, the thickness of the metal piece itself can change the level of Barkhausen noise generated within the metal piece. Accordingly, examination of metal pieces of different thickness can require constant calibration of the system that analyzes the Barkhausen noise and makes it available for inspection. Because interpreting Barkhausen noise differently for different thicknesses of metal pieces is unduly inconvenient, it is common to normalize the system each time the thickness of the test piece changes by altering the magnitude of the current supplied to the energizing coil assembly. The proper current level can be determined in advance experimentally for different thicknesses of metal pieces. Known Barkhausen sensors have not successfully addressed the concerns described above. Because of its low intrinsic Barkhausen noise level, it has been proposed in Finland Patent No. 51873 to use ferrite as the core material for a Barkhausen noise sensor adapted to investigate flat metal pieces; however, in that proposal, a single coil serves as both the energizing coil and the sensing coil. The result of that arrangement is that the sensing coil tends to detect the intrinsic noise of the core material rather than the noise induced in the specimen. This is especially the case when measuring hard steels in which only low levels of Barkhausen noise can be generated. Accordingly that arrangement cannot be successfully used to detect Barkhausen noise in hard steels due to magnetic self-coupling occurring within the sensor. An article entitled "Measurement of Barkhausen Noise On Moving Steel Sheet With Non-Contact Sensor", published in Finland by Markku Pesonen, discloses a concept for dynamically measuring Barkhausen noise generated in a steel sheet. FIGS. 5.3(a) and (b) of the publication illustrate the sensors in diagrammatic form. FIG. 5.3(a) shows a DC sensor having energizing coils energized by a DC source and FIG. 5.3(b) shows an AC sensor having energizing coils energized from an AC source. Both sensors employ identical sets of energizing and sensing coils on both sides of a steel sheet to reduce the effect of vibration of the sheet. The DC sensor employs two sets of energizing coils, each set of which is energized by a DC source of a polarity opposite to that which energizes the remaining set. The DC sensor relies on movement of the sheet relative to the coils to generate a time varying magnetic field, which causes Barkhausen noise to be generated, in the sheet. Several problems are experienced through use of the DC sensor shown in FIG. 5.3(a). First, the DC sensor can be used to test only relatively soft metal pieces if the piece must be moved at a slow speed. At slow speeds, insufficient Barkhausen noise is generated within hard metal pieces. Examples of metal pieces which must be moved at a low rate of speed include odd shaped pieces which simply cannot be moved rapidly. Further, variations in the speed of movement of the metal sheet under observation change the "frequency" of the apparent time-varying field and introduces substantial variation in the generated Barkhausen noise that is not due to the presence or absence of stress or defects. Finally, the AC and DC sensors shown in FIGS. 5.3(a) and (b) are suitable only for inspection of flat, relatively thin metal sheets, which is a serious limitation on their use. Accordingly, the DC and AC sensors do not provide Barkhausen sensors that are practical for use during dynamic testing. US - A - 3783370 is concerned with reducing variations in a Barkhausen signal due to variations in the thickness of magnetic body being tested. The patent is directed to the use of gain control circuitry for varying the magnitude of a time varying current input to its energising electromagnetic to compensate for differences in thickness of the magnetic body being inspected. However, this reference is directed to inspecting magnetic bodies with a planar surface and does not disclose a method for examining a magnetic body with a non-planar surface. This patent also fails to address the problems of measuring Barkhausen noise in hard steels. FR - A - 2 141 854 is directed to examining the planar surface of a moving metal strip. Again, no reference is made to measuring Barkhausen noise in a body having a non-planar surface or to measuring Barkhausen noise in a hard steel. The IEEE transaction on Magnetics, volume MAG-9, No. 4 December 1973, pages 641-646 "Measurements of spectra of clustered Barkhausen transitions in technical steel", discloses a method for generating information relating to the presence or absence of stress and defects in a piece of planar sheet steel using the Barkhausen effect. Accordingly, there exists a need for a method for generating a Barkhausen signal in a piece of hard steel that minimizes magnetic coupling within the sensor. Further, there is a need for a method that provides proper coupling between the sensor and metal pieces of differing shapes and thicknesses. According to the present invention there is provided a method for generating information relating to the presence or absence of stress and defects in a piece of hard steel, having at least one nonplanar surface, comprising the steps of providing a Barkhausen noise sensor having a magnetizing core and a sensing coil assembly generating Barkhausen noise within said piece of steel, and sensing the Barkausen noise so generated to develop said information, characterised in that the magnetising core is composed of ferrite, the sensing coil assembly has a ferrite core and the Barkhausen noise is generated by effecting simultaneous contact by said magnetizing core and said core of said sensing coil assembly with said nonplanar surface of said piece of hard steel, said sensing coil assembly being positioned relative to said magnetizing core to minimize electromagnetic coupling between said magnetizing core and said sensing coil assembly so that intrinsic Barkhausen sensor noise is distinguishable from the Barkhausen noise generated within said piece of steel. The present invention provides a method that can be used to statically or dynamically detect stress and defects in metal pieces of a large variety of microstructures, compositions, sizes and shapes. The method is particularly useful when nondestructively inspecting relatively small pieces of hard steel, which represent a large proportion of metal pieces that must be tested in a commercial setting. Further, the problems of magnetic self-coupling and maintenance of proper magnetic coupling between the sensor and the test piece have been so minimized that only one set of criteria for determining defects from measured Barkhausen noise levels need be established and used for all shapes and sizes of metal pieces of the same composition and microstructure. The present invention provides a method of using a Barkhausen noise sensor that provides information related to the stress and defects in a metal piece by generating a magnetic field in the metal piece. The sensor includes an energizing coil assembly including an electrical conductor wound around a ferrite core. The energizing coil assembly is adapted to receive a periodic time-varying electrical signal which will induce in the metal piece, when the ferrite core is located proximate the metal piece, a time-varying magnetic field. The sensor also includes a sensing coil assembly including a second electrical coil which is so disposed relative to the energizing coil assembly and, when the ferrite core is located proximate the metal piece, to the metal piece as to permit the sensing coil assembly to detect noise generated within the metal piece by the induced magnetic field. The sensing coil is physically and electrically independent of the energizing coil assembly. For testing the ferrite core and the sensing coil assembly are adapted to make contact with the metal piece. Preferably, the magnetic field inducing apparatus includes a pair of contacts defined by the ferrite core. The contacts are so oriented in shape that uniform contact can be maintained between the ferrite core and the metal piece. The contacts can be rounded to permit a constant magnitude of contact area between the energizing coil assembly and metal pieces of a variety of shapes. Preferably, the sensor also includes apparatus associated with at least one of the energizing coil assemblies and the sensing coil assembly for ensuring that uniform contact is maintained between the metal piece and the energising and sensing coil assemblies. Preferably, the ensuring apparatus includes a device for urging the sensing coil assembly into contact with the metal piece when the energizing coil assembly is in contact with the metal piece and is an elastic, nonmagnetic member. Following is a detailed description of the invention, by way of example only, with reference to the accompanying drawings, in which:- FIGS. 1 and 2 are isometric views of a sensor; FIG. 3 is a front, sectional view of the sensor shown in FIGS. 1 and 2; FIG. 4 is a side, sectional view of the sensor shown in FIGS. 1 and 2; FIG. 5 is a front view of the sensor shown in FIGS. 1 and 2 being used to test the convex surface of a metal piece; FIG. 6 is a front view of the sensor shown in FIGS. 1 and 2 being used to test a tooth of a metal gear; FIG. 7 is a front view of the sensor shown in FIGS. 1 and 2 being used to test the root of a metal gear; and FIG. 8 is a side view of one of the converging-type contacts terminating in rounded edges in FIGS. 6 and 7. FIGS. 1 and 2 show a static sensor 10. Static sensor 10 is particularly adapted for testing where only a relatively small number of readings need to be taken since sensor 10 should be disengaged from one test site before it is moved to a different test site. Sensor 10 includes a housing 12 which contains an energizing coil assembly 14 and a sensing coil assembly 16. Energizing coil assembly 14 includes an electrical conductor 18 which is wound around ferrite base 20. A pair of ferrite legs 22 and 24 are disposed within fittings 26 and 28, respectively, of housing 12. Preferably, base 20 and legs 22 and 24 are constructed, for instance, from such a manganese oxide based ferrite material as that sold under the trademarks PHILIPS 3C6 or 3C8 by Philips A.G. Seats 30 and 32 are adapted to be in contact with ends 34 and 36, respectively, of ferrite base 20. Legs 22 and 24 define rounded contacts 38 and 40, respectively, which are adapted to contact the metal piece under examination. Legs 22 and 24 are held by friction within fittings 26 and 28 and can be removed to reshape contacts 38 and 40 and then replaced within fittings 26 and 28. Conductor 18 receives a periodically time-varying signal from any suitable conventional source. When contacts 38 and 40 are in contact with a metal piece, the periodic electrical signal induces a magnetic field in ferrite base 20 and ferrite legs 22 and 24, which causes the metal piece to become magnetized between contacts 38 and 40 of legs 22 and 24. The varying magnetic field generated by energizing coil assembly 14 causes abrupt jumps in the magnetization of the metal piece, known as the Barkhausen effect, to occur. The rounded contacts 38 and 40 of legs 22 and 24 permit point contact to be established between legs 22 and 24 and metal pieces of a variety of shapes. Sensing coil assembly 16 includes an electric coil 42 disposed within container 12 and wound around a ferrite sensing core 44. Sensing core 44 is secured to an elastic silicone member 50, which is held by friction within fitting 46 of case 12. Contact 48 of core 44 is rounded to permit point contact between sensor 10 and metal pieces of a variety of shapes. Core 44 can be removed from fitting 46, to reshape contact 48 of core 44, and subsequently replaced. Member 50 urges core 44 away from passage base 52 and into contact with the metal piece under examination, but cannot expel core 44 completely from within fitting 46. The abrupt magnetic jumps occurring in the metal piece due to the magnetic field established by energizing coil assembly 14 induce electrical Barkhausen noise in coil 42 of sensing coil assembly 16 when contact 48 is in contact with the metal piece. The Barkhausen noise induced in coil 42 can be processed and displayed in any known desired fashion. FIGS. 6, 7 and 8 show sensor 100, which is identical to sensor 10, with the exception that core legs 22 and 24 and core 44 of sensor 100 define sections each of which includes two opposed converging sides defining rounded contact surfaces 112, 114 and 116, respectively. Contacts 112,114 and 116, facilitate testing the teeth and roots of metal gears, as is shown in FIGS. 6 and 7. To use sensor 10, contacts 38, 40 and 48 are brought into contact with the metal piece. A button 80, located on the exterior of case 12, is depressed to begin a test cycle. Depression of button 80 causes a predetermined number of cycles of the energizing electrical signal to be imposed across energizing coil assembly 14. Energizing coil assembly 14 causes a periodically time-varying magnetic field to be established in the metal piece. Sensing coil assembly 16 detects the magnetization jumps generated within the metal piece and converts them to an electrical signal. A conventional averaging circuit averages the level of the Barkhausen noise generated during the test cycle and provides the averages signal to circuitry which processes and displays the noise in any desired known fashion. Generally, sensor 100 is used in the same manner as that employed with sensor 10. The length of the test cycle should be chosen to ensure that a statistically sound value of Barkhausen noise is obtained. An indicator light can be provided to indicate that a test cycle is in progress.

Claims: A method for generating information relating to the presence or absence of stress and defects in a piece of hard steel, having at least one non-planar surface, comprising the steps of providing a Barkhausen noise sensor (10) having a magnetizing core (20, 22, 24) and a sensing coil assembly (16) and (44) generating Barkhausen noise within said piece of steel and sensing the Barkausen noise so generated to develop said information, characterised in that the magnetizing core (20, 22, 24) is composed of ferrite, the sensing coil assembly (16) has a ferrite core (44) and the Barkhausen noise is generated by effecting simultaneous contact by said magnetizing core (20, 22, 24) and said core (44) of said sensing coil assembly (16) with said non-planar surface of said piece of hard steel, said sensing coil assembly (16) being positioned relative to said magnetizing core (20, 22, 24) to minimize electromagnetic coupling between said magnetizing core (20, 22, 24) and said sensing coil assembly (16) so that intrinsic Barkhausen sensor noise is distinguishable from the Barkhausen noise generated within said piece of steel.

Inventors: TIITTO, SEPPO I.
Applicants: AMERICAN STRESS TECHNOLOGIES INC.
Application year: 1984
Main classification: G01R33
IPC codes: G01N27, G01R33, G01L1