Magnetic Particle Testing (MT)

Magnetic Particle Testing (MT)

Magnetic Particle Testing (MT or MPT), also referred to as Magnetic Particle Inspection (MPI), is a vital nondestructive testing (NDT) method designed to detect surface and near-surface discontinuities in ferromagnetic materials such as iron, nickel, cobalt, and their alloys. This technique is particularly effective for identifying defects like cracks or voids that could potentially compromise a material’s integrity.

Magnetization Techniques

MT involves creating a magnetic field within the material being tested. Magnetization can be accomplished through two primary methods:

Direct Magnetization

This method involves passing an electric current directly through the test object, which induces a magnetic field within the material.

Indirect Magnetization

Instead of passing a current through the object, a magnetic field is applied from an external source. In both direct and indirect magnetization, the magnetic lines of force are oriented perpendicular to the direction of the current flow, utilizing either alternating current (AC) or direct current (DC).

Detection of Flaws

The presence of a flaw disrupts the magnetic field flowing through the material, causing magnetic flux to leak out. Since air cannot support as much magnetic field density as metals, these leakage fields can be readily detected:

Application of Ferrous Particles

To visualize these leakages, small ferrous particles, either dry or suspended in a liquid, are applied to the component’s surface. These particles are attracted to areas of flux leakage, accumulating and forming visible indications of the underlying defects.

Evaluation of Indications

The patterns formed by the ferrous particles on the material’s surface help in evaluating the characteristics of the discontinuity, providing insights into the defect’s size, shape, and potential impact on the material’s performance.

Operator Training and Surface Preparation

Operator Expertise

While the basic training required for MT is straightforward, practical experience dramatically enhances the inspection’s effectiveness and accuracy.

Surface Preparation

The surface under inspection must be thoroughly clean and free from any contaminants that could obscure defects or cause particles to adhere improperly, leading to false readings. Techniques such as acid etching may be employed to remove surface smearing and reopen closed defects, ensuring the test results are reliable and accurate.

Magnetic Particle Testing is a crucial technique in industries where the structural integrity of ferromagnetic components is paramount. Proper execution of this testing method ensures the safety and functionality of critical machinery and infrastructure.