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: