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Magnetic Particle Inspection (MPI) is used to detect cracks, laps, seams, inclusions & other discontinuities on or near the surface of ferromagnetic material. MPI Yoke is portable equipment that is essentially used to magnetize a specimen longitudinally and detect surface cracking in any ferromagnetic material. It is essential to know the prerequisites of using a yoke for an operator and therefore below are some important parameters that are to be considered.
Weight of the Yoke:
Yokes are essentially used for portable/on-site inspections and thus if it is heavy, it can slow down the inspection and cause fatigue to the operator. It is important that the weight of the yoke should be equally distributed as if it is concentrated at the bottom then the operator may get injured while using it at different angles. The lightweight also ensures that a single operator can perform the inspection. The ideal weight of an AC Yoke, AC/DC & the Battery-Operated yoke would be between 3-5 Kg (6.5-11lbs) and for permanent magnetic Yoke between 2-3 Kg (4.5-6.5lbs).
Design and Ergonomics:
The design of the yoke should be operator friendly, with the correct positioning of Switch, Cable, and Grip so that he/she can work comfortably. One should be able to hold the yoke with one hand with a proper grip and it should reach all orientations. Before selecting the Yoke, the availability of articulated Legs and leg spacing of 3 to 8 inches should be considered.
Lifting Power:
Most of the yokes today meet ASTM requirements for magnetizing power. But have you checked the lifting power of your yoke?
| According to ASTM E1444/E1444M – 12 & E709-08 ,AC & permanent magnetic yoke shall have a lifting force of at least 10lbs (4.5kgs) with 2 to 6 inches (50-150mm) spacing between legs. DC shall be used for lifting force of at least 40lbs (18.1kgs) with 2-4 inch (50-100mm) spacing between legs or 50lbs (22.5kgs) with 4-6inch (50-150mm) spacing. Practically you will have to lift the weight at the same leg spacing which you are using during the inspection. |  |
Technical Specification of the Yoke:
According to standard EN ISO 9934-3, minimum requirements for a yoke are duty time should be greater than or equal to 10%, current ON time is greater than or equal to 5 sec, the surface temperature in the handle should be less than or equal to 40°C and the tangential field strength is greater than 2 kA/m (RMS).
Utilization of Currents in Defect Detection:
ALTERNATING CURRENT (AC): It is most widely used for conducting magnetic particle testing. AC has a little penetrating power and provides the best detection of the surface crack due to the skin effect. It is not effective for subsurface as the magnetic flux stays preferentially close to the surface following the contours giving a significant amount of flux leakage around the crack to form the necessary attractive poles for the particles to migrate resulting in bold indications.
DIRECT CURRENT (DC): Single phase AC can be rectified to produce halfwave alternating current commonly called Halfwave Direct Current (HWDC). HWDC is generally used for the detection of surface and sub surface discontinuities and FWDC is used for the detection of surface discontinuities because of the high penetration power. HWDC means that the reverse polarity or negative portion of the sine curve is eliminated as below due to which there is a small amount of skin effect and there is a penetration in some amount, hence can be used for both surface and sub-surface defects.
So, one must decide the type of Yoke as per the defects he/she wants to see.
Which kind of Yoke is best suited for your inspection:
Let us look at the type of yokes and their respective application as below:
Permanent Magnetic Yokes – As these yokes do not need any kind of power supply, they can be mainly used at remote locations where a source of power supply is not available at the site. Considering the weight of the yoke, it is difficult to carry it.
AC/DC or AC Electromagnetic Yoke– As this yoke needs electricity for operating, it can be used where we have proper power supply facility available, it can be indoor or outdoor. Also, one can switch between AC/DC to detect surface and sub-surface defects.
Battery Operated Yoke – Since it comes with a battery, it is portable and great for job sites and field inspections.
Now that you know how to choose the correct yoke, so next time while choosing a yoke we hope you would remember these points.
International Standards and References:
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