Introduction to Magnetic Testing
Magnetic testing is an inspection method that uses magnetic fields and small magnetic particles (i.e., iron filings) to detect flaws in components. The component being inspected must be made of a ferromagnetic material. In theory, magnetic testing (MT) is a relatively simple concept, combining two nondestructive testing methods: magnetic flux leakage testing and visual testing. The method is used to inspect various product forms including castings, forgings, and weldments.
Basic Principles
This type of test involves the usage of magnets, either permanent or temporary. All magnets have two regions called magnetic poles – North pole and South pole – with magnetism producing a definite chain of organized and balanced pattern of invisible lines of flux around it. These lines of flux collectively form the magnet’s “magnetic field.”
– North Pole: where a magnetic line of force exits the magnet
– South pole: where a line of force enters the magnet
Magnetic Field Behavior
When a magnet is broken in the center of its length, it results in two complete magnets with magnetic poles on each end. If the magnet is just cracked but not broken completely in two, a north and south pole will form at each edge of the crack. The magnetic field exits the north pole and reenters at the south pole.
The magnetic field spreads out when it encounters the small air gap created by the crack because air cannot support as much magnetic field per unit volume as the magnet. When the field spreads out, it appears to leak out of the material and is called a flux leakage field.
Inspection Process
1. If iron particles are sprinkled on a cracked magnet, they will be attracted to and cluster not only at the poles at the ends of the magnet but also at the poles at the edges of the crack.
2. This cluster of particles is much easier to see than the actual crack, forming the basis for magnetic particle inspection.
3. The inspection process involves:
– Magnetizing the component
– Applying iron particles (dry or wet suspended form)
– Detecting particle clusters at flux leakage fields
Advantages
– Sensitive to both surface & near-surface discontinuities
– Gives immediate indication of defects
– Quick and relatively uncomplicated
Limitations
– Restricted to ferromagnetic material
– Can’t be used in thick painted materials
– Materials that are rough, irregular in shape, very small, exceptionally thin or not homogeneous can cause spurious indications
Course Information
The Magnetic course combined with Penetrant course consists of:
– 6 days for Training (theory & Practicum)
– 2 days for exams
[Read more for course outline and syllabus]