by William C. Chedister*

 

As technology in general advances, many benefits can be derived in the transfer of these advances to specific applications. NDE also benefits from current technologies advances. This article describes improvements in rapid-pulsed DC probe, different UV illumination and use of dual light particles that enhance remote magnetic particle inspection. The modifications have simplified field inspections and enable instrumentation to be battery powered. G.P. Singh Associate Contributing Editor

The use of magnetic particle inspection in remote applications (applications which require the use of a handheld electromagnetic probe) is widespread for critical detection of minute defects in ferromagnetic materials. The scope of remote inspection includes the evaluation of structures such as tanks, bridges, power plants, refineries, pipelines, and petrochemical processing facilities, especially for welding inspection. Magnetic particle inspection is a relatively uncomplicated technique that provides a reliable surface inspection at relatively low cost in a small amount of time.

While the actual inspection process itself is quite attractive, the logistics of setting up the inspection can be a little more cumbersome. The most common AC and DC yokes can be fairly heavy and require the use of 110 V AC electrical power. If the use of fluorescent particles is required, some sort of ultraviolet lamp, also electrically powered, must be used. When the inspection must be performed in a location away from an electrical outlet there is no other choice than to run a considerable length of extension cords, provide some sort of generator, or both. Furthermore, conventional yellow green fluorescent powders require that the workplace have less than 22 lx (2 ftc) of visible light. This dark atmosphere can be another significant inconvenience to the inspector who must haul around and work with all the aforementioned equipment in a confined area.

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Manufacturers of equipment and material for remote MT have responded to the inspection industry's needs to make this critical process more "inspector friendly."

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Manufacturers of equipment and material for remote MT have responded to the inspection industry’s needs to make this critical process more "inspector friendly." One example is the greater use of water bath particles when applicable for wet method inspections, especially where the use of aerosol cans is required (Shreve and Chedister, 1995). The new generation materials replace oil fluids with conditioned water and hydrocarbon propellants with carbon dioxide.

More recent innovations include a new technology in probe design, the adaptation of another source of UV illumination, and the development and acceptance of more attractive and useful magnetic particles. The technical aspects of each of these developments are described below.

 

Electromagnetic Probes: AC, DC and Pulsed DC
It has long been established as common knowledge that AC yokes are more proficient at locating surface discontinuities with magnetic particle testing than their DC counterparts. DC yokes are found to be more effective for finding discontinuities that may be slightly below the metal surface as well as those on the surface. Magnetic particle inspection is essentially considered to be a surface NDT technique, with subsurface inspection generally approached with ultrasonic, radiographic, or other NDT methods. Therefore it is advantageous for the magnetic field used for magnetic particle inspection to be focused at the inspection surface.

The strength requirements for the two different types of yokes, measured on the basis of simple lifting power at a specified leg spacing, are also different. For instance, ASTM E-709: A Standard Guide for Magnetic Particle Examination, requires that, while the AC probe must lift a 4.5 kg (10 lb) certified test weight, the DC probe must lift a 13.6 to 22.6 kg (30 to 50 lb) certified test weight.

Now a third type of probe, one using rapid pulsed direct current (PDC), has been developed. The beauty of this technology is that it allows near AC probe type performance while using a lightweight DC battery as its power source (see the section below for more information on the use of rechargeable batteries). How does PDC compare to the conventional AC and DC probes, and why are there such differences in performance and requirements in the first place?

The answer to these questions lie in a phenomenon known as "skin effect," which applies to the metal surface exposed to a steady or rapidly changing magnetic field. This explanation is based upon Lenz’s Law which states:

If the path of a moving charged particle (such as an electron) is changed in any way by a magnetic field, this change is always of such a nature as to generate a new magnetic field which directly opposes the one which caused the change.

An AC probe operating at 60 Hz generates both the inspection field and eddy currents. These eddy currents prevent the inspection field from moving into the surface being inspected. How the eddy currents generated by the oscillating field move around in the surface is dependent upon the shape of the surface. The depth of their penetration is determined by the parameters of drive frequency, the electrical conductivity of the surface, and the value of the magnetic permeability that they scan.

PDC presents a different situation. The PDC field is basically a rapid "on/off" phenomenon. The periodic pulse pattern generates a series of higher harmonics which have decreasing amplitude. The successive harmonics penetrate less into the surface and effectively provide more magnetic field activity at the surface.

Greater intensity and frequency of the change in applied magnetic field gives rise to a greater degree of resistance for the magnetic field to flow deeper in the metal. Both the AC and PDC fields concentrate the magnetic field to the inspection surface. It is difficult to ascertain the subtle differences between these two in terms of their effectiveness for magnetic particle inspection.

A conventional nonpulsing DC field does not induce a big difference in resistance between surface and subsurface in the metal, nor does it have a frequency. As a result the magnetic field flows more evenly through the metal body and is not concentrated at the surface as are the other two magnetic field types.

 

Lightweight UV Lamp Technology
The most conventional type of lamp used to provide ultraviolet illumination for NDT is the 150 W mercury vapor lamp. This is generally chosen because of its output power and efficiency. An alternative used less is a 50 W incandescent unit which can be operated from the same type of 12 V DC battery that operates the PDC probe. As with the probe, use of the readily portable and rechargeable 12 V DC battery allows the user to inspect areas that otherwise may be inconveniently located if heavier equipment and/or power umbilical cords are required.

The portable incandescent lamp offers other advantages to the 110 V AC powered lamps. While it has less illuminating power than the higher wattage AC lamps the 50 W lamps meet output requirements. Incandescent lamps can be thought of as "instant on" type units so the need for "warm up" time is virtually eliminated. While the lamps can heat up considerably and are limited for a single continuous use, the heat build-up is much less significant than that of the mercury vapor bulb.

Bulbs are inexpensive and easily replaced when necessary. By virtue of their construction, incandescent bulbs are little affected by the proximity of strong magnetic fields, such as those that can be generated by a yoke. Strong local magnetic fields can disrupt the arc inside a mercury vapor bulb. Such disruptions are known to cause the lamp to black out. Any black out requires time for the lamp to cool off and repeat the warm up cycle before it can be returned to use.

Besides the elimination of the necessary power packs and electrical cords associated with the 110 V AC lamps, the 50 W lamp itself is considerably smaller and lighter.

 

Battery Power for the Probe and Lamp
Any discussion of battery powered remote equipment must address battery life and rechargeability. Batteries for the probe and UV lamp are fairly conventional 12 DC lead acid batteries and can be used for both pieces of hardware. There are essentially two batteries available, depending on how much weight the inspector prefers to carry and how long a charge needs to be maintained. The battery nominally rated at 7 ampere-hours (A-h) weighs about 30 kg (6.5 lbs). This weight is about half of the larger 14 A-h battery. The smaller battery and PDC probe together weigh about as much as a conventional heavy duty probe.

The amount of working time per charge depends mostly on the amperage draw of the probe and lamp being used. A significant feature on the PDC probe is a battery level light emitting diode (LED). This LED must light up when the probe is turned on to operate. Failure of the LED to light indicates that the battery’s voltage level is too low and that it must be recharged.

The diagonal lines in Figure 1 display the battery duration time as a function of discharge amperage. There are two lines; one is for a 12 DC battery rated at 6.5 A-h and the other is for a battery rated at 12 A-h. For practical purposes the battery life is proportional to the A-h rating; that is, a battery with twice the A-h rating will last about twice as long, all other factors remaining constant. Bear in mind that the time depicted by the curves is a measure of actual operating time for either the probe or the lamp. Nominal amperage draw for the probe is 1.25 A and for the lamp is 5 A.

Figure 1 - Discharging current and discharge duration time for 6.5 A-h (left) and 12.0 A-h (right) lead-acid batteries. Data based on average values at 77 °F (25 °C). Battery data courtesy of Panasonic Battery Sales Group.

"Dual Light" UV/Visible and Visible Particle Indications
In the most recent years there has been a renewed interest in the so called "dual light" powders for magnetic particle inspection. These powders are detectable in visible light as well as fluorescent under long wave (365 nm) UV radiation. Especially effective are particles in the red color range (Chedister, 1994). Conventional yellow green fluorescent materials are notorious for losing their visible characteristics in the presence of visible light.

The advantage of the red visible UV powder is that it provides a much more striking indication in visible light than conventional red dry method powders. In addition it can be highlighted with the use of a UV lamp, even when working in a non-darkened atmosphere. The phenomena of fluorescence and visible color have been addressed previously (Chedister, 1993). Comparisons between conventional red dry method powder and the UV visible powder appear in Table 1.

The data were generated under a conventional methodology simulating sunlight conditions in a controlled manner. The designation D10 degree means first that daylight was simulated with the use of a 6,500 K illumination source. Secondly, the 10 degree designation refers to the size of the field observed per the 1964 CIE supplementary standard observer (Billmeyer and Saltzman, 1981). Figure 2 is a graphical presentation of the data.

Figure 2 - Comparison of color stimuli under D10 degree conditions . Red, green, and blue are sensed by the "cones" in the human eye. UV/visible powder color demonstrations stronger values in each of these hues than the conventional red dry method powder color. Measurements taken on a Macbeth 7000 spectrophotometer. Courtesy Don W. Parker COLORWRIGHT, Inc.

A second means of comparing the two different red powders is to compare their reflectance. An object, in this case the powder, absorbs light energy from some wavelengths of the light source spectrum and reflects energy from the others. The combination of the absorption and reflectance result in the characteristic color of the object. Figure 3 displays the relative reflectance of the two red powders and demonstrates that the UV visible powder reflects almost four times as much energy in the red range of the visible spectrum (about 650-700 nm wavelengths).

Figure 3 - This graph represents the greater amount of reflectance for the UV/visible particles as a function of wavelength. Data courtesy of Don W. Parker COLORWRIGHT, Inc.

The Benefit for the User
The value of the three items - the PDC probe, 12 V 50 W lamp, and UV visible particles - is in the benefits that they can offer field magnetic particle inspection. This system is especially attractive when the inspection area is in a location without accessible electrical power, or where the need for hauling around cords and equipment may affect inspector safety and performance.

The PDC probe offers a degree of performance for surface inspection that compares to the conventional AC probes but does not require 110 V AC power. The lamp provides UV illumination, either on its own or as a supplement to ambient light for the UV visible powders, also without the need for 110 V AC power. The UV visible powder can be used as an alternative to conventional materials, and even the typical yellow green fluorescent materials, because of its unique ability to provide distinct indications in visible ambient light areas that can be highlighted with UV irradiance.

In applications where this equipment is determined to be effective, the inspector can now expend a considerably lesser degree of time and energy transporting equipment to the test site. This can be especially advantageous when crawling through narrow passages and confined spaces.

 

References
Billmeyer, Fred W., Jr., and Max Saltzman, Principals of Color Technology, 1981. John Wiley and Sons, Inc., New York, NY.

Chedister, William C., "Evaluation of Fluorescent Magnetic Particle Indications," Materials Evaluation, Vol. 51, No. 9, Sep. 1993, p 976.

Chedister, William C., "Vision and the Detection of Magnetic Particle Indications," Materials Evaluation, Vol. 52, No. 8, Aug. 1994, p 935.

Shreve, Deborah A., and William C. Chedister, "The Evolution of Magnetic Particle Materials Follows Industry Needs," Materials Evaluation, Vol. 53, No. 8, Aug. 1995, p 883.

 

Acknowledgments
Dr. Roderic K. Stanley, NDE Information Consultants, Houston, Texas, for information on electromagnetic field behavior.

PDC probe information based on B310PD Contour Probe Yoke, courtesy Parker Research Corporation, Dunedin, Florida.

50 Watt UV lamp information based on CH50/12 Lamp, courtesy Spectronics Corporation, Westbury, New York.

Visible Dry-Method MT powder Dusting Powder #63 and UV/Visible MT powder Mi-Glow #600, courtesy Circle Systems, Incorporated, Hinckley, Ilinois.

 

* Circle Systems, Incorporated, 479 W. Lincoln, Hinckley, IL 60520-1228; (815) 286-3271; fax (815) 286-3352.

Copyright © 1997 by the American Society for Nondestructive Testing, Inc. All rights reserved.

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