An Easy Understanding Of Magnetic Particle Inspection

By Author: Nicola Jackson
Total Articles: 40
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Magnetic particle inspection, abbreviated as MPI, is an inspection method that can detect surface and slightly sub- surface discontinuities in ferroelectric materials like iron, cobalt, nickel, and some of their alloys. It is classified as non- destructive testing or NDT which refers to a wide group of analysis techniques that can evaluate the properties of a material, system or component without causing any damage. The material to be tested is first magnetized wither by direct or indirect magnetization. Direct magnetization involves application of electric current through the test object which leads results in a magnetic field in the material. The electric current may be alternating current (AC) or some form of direct current (DC). The magnetic lines of force formed are perpendicular to the direction of the current. Whereas in indirect magnetization, electric current is not passed through the object, the magnetic field is applied from an external source. Let’s look at the type of currents used in details:
1. Alternating current (AC) - AC is mostly used to detect the surface discontinuities. It is not very ideal for ...
... detecting subsurface discontinuities because of what is known as ‘skin effect’ which the tendency of the current to run along the surface of the part. AC cannot penetrate deep enough to magnetize parts in the subsurface.

2. Full wave Direct Current (FWDC) - It is used to detect subsurface discontinuities in a material. It is also limited on very large cross sectional parts, regarding how effectively it can magnetize the part.

3. Half wave direct current (HWDC) - Also known as pulsating DC, HWDC works similar to FWDC but has can penetrate deeper than FWDC and can also detect surface continuities.

Various types of equipments are used to magnetize the parts. Some of the commonly used equipments are wet horizontal MPI machine, magnetic yoke and mobile power packs.
After the part is magnetized, it needs to be demagnetized. For this, equipments are needed that will work in an opposite way from the magnetizing equipments. The magnetization is generally done by a high current pulse that very quickly reaches a peak current and then turns off immediately, leaving the part magnetized. To demagnetize the part, current or magnetic field greater than the current or magnetic field used for magnetizing is used. Then the magnetic field or current is gradually reduced to zero, leaving the part demagnetized. Like magnetizing, demagnetizing can also be done in 3 ways:
1. AC demagnetizing
2. Reversing FWDC magnetizing
3. HWDC demagnetizing
Now we will see the steps involved in a MPI using a wet horizontal machine:
• The part to be tested in cleaned of oil and other contaminants.
• Calculations are done to calculate the amount of current needed to magnetize the part.
• The magnetizing pulse is then applied for 0.5 seconds during which operator washes the part by the particle and stops before the magnetic pulse is completed. If the operator fails to stop before the magnetic pulse is completed, the indications will wash away.
• The operator looks for indications when UV light is applied.
• The part is finally demagnetised.
If you live in Edmonton and are looking for performing an inspection for your industry, you needn’t worry. There are many good inspection companies that provide excellent services in all forms of inspections like liquid penetrant inspection, magnetic particle inspection in Edmonton.