MAGNETIC NON DESTRUCTIVE TECHNIQUE

Magnetic Particle Inspection

A very efficient method for the detection of surface and near surface discontinuities in any ferromagnetic material.

It relies on the principle that a magnetic field will spread itself uniformly through a component unless disturbed by the presence of a flaw. The disturbance generates a locally stronger field, known as a leakage field. This will strongly attract any finely divided magnetic particles, either as an ink or dry powder, which are applied to the component. This produces an indication, making the flaw visible, Figure 2.

Figure 2. Magnetic particle inspection.

A number of factors can influence the production and quality of indications defect size depth of defect below surface magnetising current level of magnetisation relative orientation of defect and direction of magnetisation magnetic properties of test piece size and magnetic properties of detecting media

The orientation of the flaw with respect to the direction of the flow of magnetic flux greatly influences the likelihood of creating a leakage field. Ideally the flaw should be at 90° to the flux flow, but detection may still be possible at 45°. Components should be tested in at least two directions to ensure maximum detection of flaws.

Magnetic flux can be induced in a test piece by using a permanent or electromagnet passing an electrical current through the test piece passing an electric current through a coil which surrounds the test piece

For site conditions, or when dealing with large components, the simplest means of inducing a magnetic field is with a permanent or electromagnet. With such equipment it is normal practice to paint the test piece white and to use black magnetic ink particles suspended in paraffin for the greatest contrast, again two tests are required at 90 to each other for maximum certainty of detection of flaws.

The other methods are better suited to routine testing in laboratories or inspection departments. Modern equipment is much smaller and lighter than in former times. The limiting factor is now the ‘bed size’, i.e. the length between the head stocks which hold a component and allow the magnetising current to flow.

Advantages

•        Simple and easy to conduct

•        Will detect surface and near surface flaws

•        Can detect flaws filled with contaminants e.g. oxide or non metallic inclusions

•        Sensitivity of testing can be specified and checked

Disadvantages

•        Can only be applied to ferromagnetic materials

•        Will not detect deep internal flaws

•        High currents applied to component may cause damage

•         Components usually have to be demagnetised

MAGNETIC PARTICLE INSPECTION

Magnetic Particle Inspection "MPI" (also known as Magnetic Testing "MT") is used for the detection of surface and near-surface flaws (such as cracks) in ferromagnetic materials e.g. steel and iron.

A magnetic field can be applied to the item under test by either direct magnetization or indirect magnetization. 

• Direct magnetization is when an electric current is passed through the item under test and a subsequent magnetic field is formed within the material.
• Indirect magnetism is when no electric current is passed through the test object, but a magnetic field is applied from an outside source such as a 110v portable electromagnet or permanent magnet. 
  

The presence of a surface or near surface discontinuity causes the magnetic flux to distort or leak.  Fine magnetic particles (either dry or suspended in a carrier fluid) are applied to the surface of the item under test. These magnetic particles are attracted to any area of flux distortion or leakage thus creating a clear visible indication to the operator.

The more sensitive techniques of MPI utilise fluorescent magnetic particles which are viewed in a darkened area under a UV (black) light.

Services on offer from Venture: 

• 6000 Amp Magnaflux Bench Unit. 
• Fluorescent / Colour Contrast techniques in-house and on-site.
• Site services offering portable 110v coils, prods and electromagnets.
• Comprehensive 24hr in-house and on-site service.
• Approved procedures for control checks.
• Approved procedures to specific industry standard.
• Inspectors fully qualified to PCN & SNT.
• Approved Method Statements, Risk Assessments and COSHH Assessments including confined space entry work.