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The Usage of Endpieces to Create the Length to Diameter Ratio

by S.L. Weatherly*

 

Standards are necessary and useful for industrial production. But once a standard has been issued, it is often challenged by the users who try to extend its limits. Sometimes this results in misinterpretations and abuse. This article reports a good example of such abuse and may be helpful to return to a good practice. Dietmar Henning Tutorial Projects Editor

Having had the opportunity to witness and review numerous magnetic particle procedures from suppliers all over the United States, I have continually noted a misunderstanding in the usage of endpieces to create a good length to diameter (L/D) ratio for longitudinal magnetization. The problem is that the usage of endpieces to create a good L/D ratio has led some to use this arrangement to magnetize more parts quickly without understanding the consequences.

 

Specifications
As stated in ASTM E 1444-05, Appendix X5.2, these formulas "hold only if the L/D is greater than 2 and less than 15. If L/D is less than 2, pole pieces (pieces of ferromagnetic material with the same diameter as the part being examined) may be placed on one or each end to effectively increase the L/D to 2 or greater" (ASTM International, 2005). Some specifications have the length to diameter ratio set at 3. Whether the ratio is set at 2 or 3 is not the issue, however. The problem is that many magnetic particle inspectors are stacking parts not solely to create a proper length to diameter ratio but to increase the number of parts that they can test with a single coil shot. This procedure can be and is frequently misapplied (Figure 1).

 

(a)

(b)

Figure 1 - The usage of endpieces in magnetic particle testing: (a) correct usage to obtain proper length to diameter ratio; (b) incorrect usage attempting to increase the number of parts tested.

 

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The problem is that the magnetic field diminishes rapidly in the stacked parts that are beyond the coil width.

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A Sample Technique
An example of this is when a technician receives 20 parts for magnetic particle testing. The parts are "sleeves" that are 19 mm (0.75 in.) in diameter and 25.4 mm (1 in.) in length with a wall thickness of 1.3 mm (0.05 in.). The technique developed for these parts is a circular central conductor shot with 10 or all 20 of the parts placed on the central conductor and magnetized at the same time. This is an acceptable technique for the circular shot, saving time and money. The problem is in the next shot, when the technician applies a field in the opposite direction. For the longitudinal coil shot, the technician takes approximately 10 of these parts and stacks them up against each other, frequently on the same shaft used for the central conductor shot. This is when the misinterpretation of the specification occurs. The magnetic particle inspector claims to be creating a proper length to diameter ratio for longitudinal testing. The technician now has 10 parts with 25.4 mm (1 in.) in length stacked to create a length to diameter ratio of 10.

Is this a good technique? The answer is no, this was not the intention of the specification section that allowed pole pieces to create a proper length to diameter ratio. A serious problem occurs when parts are stacked and some of the stacked parts extend beyond the width of the coil wall. The problem is that the magnetic field diminishes rapidly in the stacked parts that are beyond the coil width. This can be proven by placing several standard, notched shims, known as quantitative quality indicators, on small, multiple stacked parts. Place a quantitative quality indicator on one of the stacked parts within the cross section of the coil and then place another on the end of the stacked pieces. Notice that as the stacked parts extend more than 25 or 51 mm (1 or 2 in.) beyond the coil width, the image on the quantitative quality indicator diminishes as the longitudinal field is not transferred to the outer pieces.

 

Conclusion
The specification allowing the use of endpieces in magnetic particle testing was not written to increase the number of parts an inspector can test with a single coil shot, but to create a proper length to diameter ratio. Whether through improper instruction or just years of misunderstanding, the practice of stacking numerous parts during a coil shot continues to occur quite often, with an insufficient field, or no field, being developed in the outside pieces.

Therefore, when adding endpieces to create a proper length to diameter ratio for longitudinal magnetization, one must remember to create and verify a good field in the endpieces. This will determine the number of pieces that can safely be placed end to end.

My desire is to help instruct technicians in performing a good test. I hope this article aids in clarifying the use of endpieces to establish a length to diameter ratio for usage with longitudinal magnetization.

 

References
ASTM International, ASTM E 1444-05: Standard Practice for Magnetic Particle Examination, West Conshohocken, Pennsylvania, ASTM International, 2005.

 

* The Boeing Company, 2401 E. Wardlow Rd., Mail Code C1 054-0023, Long Beach, CA 90807; (562) 982-7073; e-mail <stanley.l.weatherly@boeing.com>.

 

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