Magnetic Particle Testing
What makes Magnetic Particle testing work is that when you magnetize the component it creates one big magnet. Any obstructions on the surface of the part will usually cause a concentrated flux leakage field as the magnetism attempts to jump the flaw and go around it. This is the key that makes Magnetic Particle work. The flaw creates a concentrated north and south pole in the same location attracting the magnetic shaving used for Magnetic Particle Training, Testing & Inspection. If you look at the end of a bar that is magnetized correctly you will not see particles accumulating. The accumulation of particles at the end of a bar is a sign of the process being done wrong which is caused by over magnetizing the part. The indications are essentially non-relevant indications caused by an inexperienced operator.
All of the magnetic induction processes may be used with different currents. The most popular are AC and HWAC. AC is primarily utilized for surface crack detection and HWAC/DC can be used for surface and just below the surface.
Yoke, Coil, Cables, Prods all have an effective region. During Magnetic Particle you should know that effective region and ensure that you observe it with at least a 10% overlap. Your internal procedure should cover that for the applicable equipment used at your facility. In order to perform Magnetic Particle Inspection per a code you usually need your own written internal procedure that covers your specific equipment. This procedure should be approved by an NDT Level III qualified & certified per your qualification and certification procedure.
Particles are usually used in two primary forms Dry and Wet Fluorescent. Each has its advantages and both are misused with much frequency. Dry red powder should be puffed on in cloud form. The cloud should cover the entire area that you are observing ensuring complete magnetic coverage of the area which you are evaluating. A common mistake seen in the field is the technician assuming the field extends much further than it actually does. This can be verified by asking the technician to draw you an imaginary line of the effective region of the magnetism. Then place a pie gage or QQI on the imaginary line furthest from the equipment. If the lines on your pie gage shows then the technician is in fact getting adequate coverage. If they are unable to show the lines then your technician probably needs some re-training. Please note pie gages and QQs are meant to be used notches down.
In service wet particles used on a bench or a circulating system are usually misused by not ensuring proper concentration and lack of performing the settling test. Spray can application is usually misused by not spraying enough of the particles during energizing of the magnetic field. A quick an easy way to ensure optimum usage or the particle is to use a QQI or Pie gage (notches down) for verification purposes.