18th World Conference on Non destructive Testing, 16-20 April 2012, Durban, South Africa Magnetic Particle Inspection: Characterisation of the magnetic field for various magnetization techniques Ephraim MOTUKISI1 More info about this article: http://www.ndt.net/?id=12825 1 Southern African Institute of Welding; Johannesburg, Gauteng, South Africa motukisie@saiw.co.za; www.saiw.co.za Abstract Magnetic particle inspection is one of the most commonly used non destructive testing methods for detecting surface and limited subsurface indications in ferromagnetic materials. The magnetic field provides the means by which indications are formed and detected. It is thus essential that the magnetic field be optimized, not only in magnitude, but also in direction when detection of specific indications is required. For example, too low a field strength would not result in adequate particle build-up and would thus reduce the probability of detection for small indications. This paper discusses the magnitude, direction and special distribution of the magnetic field for various different magnetization techniques utilized. Special attention is given to the correct usage of various field detection instruments. Keywords: Magnetic Particle Testing (MPI/MPT), field strength, field direction, Nondestructive Test (NDT) Measurement of magnetic field characteristics for MPI 1. Introduction Magnetic Particle Inspection, a surface method used to inspect Ferromagnetic Materials, is one of the most commonly used methods in NDT. In order for the test method to be used effectively, it is essential that the strength of the magnetic field be appropriate for the task. If the strength of magnetic field is too low, the particle build-up identifying a defect may be inadequate and a defect may go undetected. If the magnetic field is excessive there can be a heavy accumulation of particles around superficial irregularities and real indications may be masked (furring), especially those indications that stem from in-service failure mechanisms, which result in tight cracks. This paper discusses some of the factors, which influence the quality of test results including:1. The importance of determining the direction of the induced magnetic field when affected by residual stresses. 2. The use of instruments correctly to measure magnetic field characteristics. 3. Using reference samples for optimizing test parameters. 2. Determining the direction of induced magnetic field 2.1 Equipment commonly used to verify magnetic field in MPI 1. 2. 3. 4. Gauss/Tesla Meter (Field Strength Meter) Pie Gages Shims Gages Reference samples 2.2 Common errors 1. Incorrect placement of Field Strength Meter Probe 2. Using instruments incorrectly to verify magnetic field 3. Making assumptions that the magnetic field is in the desired direction 2.3 Tesla meter Fig. 1 Tesla meter connected with a Tangential Probe Fig. 2 Tesla meter Probes [1] It has become a norm to place the instrument’s probe on the surface of the material to be inspected and record the readings. The direction of the probe and area of placement, are important things to consider when verifying magnetic field. Fig. 3 Probe output versus Flux Angle [2] A B C The plane of the probe must be perpendicular to the surface of the part at the location of measurement to within 5°. See figure 3A. The direction and magnitude of the tangential field on the part surface can be determined by placing the Hall-Effect Tangential-Field Probe on the part surface in the area of interest. The direction of the field can be determined during the application of the magnetizing field by rotating tangential field probe while in contact with the part until the highest field reading is obtained on the instrument. The orientation of the probe, when the highest field is obtained, will indicate the field direction at that point. Various specifications call for the use of different Field Strength Meter values in particular applications. [3] Fig. 4 A Correct placement of the probe Fig. 4 B Probe placed at an angle X Fig. 4 C Hall Effect sensor not perpendicular X 2.4 Pie gages Fig. 5 Pie gage Copper side Triangular segments on the opposite side Pie gages are disks of high permeability material divided into triangular segments separated by known gaps. The gaps are typically filled with a nonmagnetic material, to protect the integrity of the gap and to strengthen the disk. The testing surface is coated with a nonmagnetic (copper) layer. This device is used to determine the approximate orientation and, to a limited extent, indicate the adequacy of field strength. However, they do not measure the internal field strength of the test object. It merely detects external fields in the vicinity of test object. The presence of multiple gaps at different orientations helps reveal the approximate orientation of the magnetic flux. Slots perpendicular to the flux lines produce the distinct indications. Slots parallel to the flux lines produce little or nothing (see figure 6). [4] Fig. 6 Orientation of magnetic flux Pie gage used in Head Shot with slot direction perpendicular to flux lines. Copper side must face up. Magnetic field direction in and around the article 2.5 Shims Gages (Indicators) Shim indicators are thin foils of high permeability material containing well-controlled notch discontinuities (see fig. 7). Frequently, multiple shims are used at different locations and different orientations on the article to examine the magnetic field distribution. Fig. 7 Shim Gages [1] One popular version of the shim indicator is a strip of high permeability magnetic material containing three slots of different widths. The strip is placed in contact with the testing surface and shares flux with the test object. For the purposes of producing test indications, the slots in the strip act as if they were cracks in the test object. Shims are sometimes called paste-on discontinuities because they must be attached to the test object with pressure sensitive tape. Shims are used most often during the development of test procedures, where they help indicate the relative strength and direction of a magnetic field for a particular test configuration. Because shims are often made of high permeability foils, they are generally small and flexible enough to fit into complex test object geometries to help determine the adequacy of field strength in these critical areas. Once the field distribution is found adequate, the testing procedure is recorded and the components are tested with the parameters established by the shims. [4] Fig. 8 Shim gage used in a coil shot [1] 2.6 Reference samples DISTORTION OF FIELD DUE TO SHAPE. If there is an upset section along the length of the article as shown in Fig. 9, the field tends to flow out into the upset portion, but does not do so uniformly. The larger the relative diameters of the upset portion, the farther will the field in this section depart from a strictly longitudinal direction. When attempting to magnetize a part of irregular shape for the first time, such an analysis of the probable path of the field should be made. As the shape of the part becomes more complex, the problem becomes correspondingly more difficult. Sometimes separate coil magnetization must be applied to various projections of the part to ensure proper field direction at all locations. A satisfactory procedure can usually be worked out for the most complicated shapes after some experimentation. In predicting the direction, the field will take when magnetizing with a coil, it is well to remember that flux lines always must close upon themselves to form a complete circuit, and that tend to follow the path of lowest total reluctance. Further, because of the very low reluctance of iron as compared to air, they will follow the iron path as far as possible. CIRCULAR FIELDS. In predicting or determining the distribution and intensity of fields produced by passing current through the part or through a central conductor threaded through an opening in the part, a different set of rules applies. Some of these rules are simple, and when applied to relatively simple shapes, they enable the operator to carry out the inspection with the assurance that he has the correct fields. However, often in complicated cross-sections, cut-and-try methods must still be resorted to. [5] Fig 9 Complex shape shaft South Pole Fig. 10a Steeples on a rotor body North Pole Fig. 10b A 20mm crack was found in a steeple groove Phased Array Ultrasonic Inspection was the method used to find a crack in figure 10b. The article’s configuration made the sizing of the crack difficult, so, Eddy Current Inspection was the method used to estimate the length of the flaw. A visual image of the crack was obtained using Magnetic Particle inspection and a dental mirror. 3. Conclusions It is easy to omit one fundamental stride when using the above-mentioned instruments. Proper understanding or usage of these gages must, be inculcated in NDT personnel to make optimum use of this NDT method in order to get indications that are sought. Different codes give specific instructions on the usage of these instruments to quantify field adequacy. We can also conclude that articles of complex shape and dimensions are not only difficult to inspect with Magnetic Particle Testing, but exhibit equal difficulty in measuring field adequacy used prior to inspection. A combination of different NDT Methods is essential, and when verifying the presence of an indication in complex shape articles. No NDT method stands alone. They all work together to eliminate the chances of missing detrimental indications that could otherwise be missed with one method. Acknowledgements For compilation of this paper, I feel a deep sense of gratitude to my colleagues at Southern African Institute of Welding who shared their experience with me. References 1. NDT Resource Centre web site, http://www.ndt-ed.org, Introduction to Magnetic Particle Inspection. 2. Pacific Scientific-OECO, Gauss/Tesla meter Instruction Manual UN-01-229 Rev. D; ECO 12745, Model 5060, pp 3-19, Figure 3-14. 3. ASME section v, Article 7, Appendix A, pp 153 A-750, 2011a 4. Nondestructive Testing Handbook, Second Edition, Volume Six Magnetic Particle Testing, pp 345-346, 1989. 5. C. E. Betz, ‘Principles of Magnetic Particle Testing’, First Edition, February 1, 1967.
