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Non-destructive testing (NDT) is a testing and analysis technique used by industry to evaluate
the properties of a material, component, structure or system for characteristic differences or
welding defects and discontinuities without causing damage to the original part. NDT also
known as non-destructive examination (NDE), non-destructive inspection (NDI) and non-
destructive evaluation (NDE).
MPI (MAGNETIC PARTICAL INSPECTION)
The magnetic particle test method of Non-Destructive Examination was developed in the USA,
in the 1930s, as a way to check steel components on production lines. The principle of the
method is that the specimen is magnetized to produce magnetic lines of force, or flux, in the
material. If these lines of force meet a discontinuity, such as a crack, secondary magnetic poles
are created at the faces of the crack. Where these secondary magnetic fields appear at the surface of the metal, they can be revealed by applying magnetic particles, as a powder, or in a liquid suspension, to the surface. The particles are attracted to the flux leakage and clump round the flaw, making it visible. The particles may be black, or coated with a fluorescent dye to increase their visibility.
The magnetic flux lines should be at right angles to a flaw to give the best indication, as this
creates maximum flux leakage. This governs the choice of a suitable magnetizing technique.
Often, more than one technique must be used to give a complete inspection. A flaw attracts more particles if it cuts more magnetic lines of force, so the ability to show a flaw depends on the depth of the flaw, the angle of the flaw to the lines of force, and the magnetic field strength induced during magnetization. The method is limited to ferromagnetic materials – iron, cobalt and nickel – as other paramagnetic and diamagnetic materials cannot hold a flux which is strong enough to attract particles.
DYE PENETRATION TEST
Dye Penetrant Inspection (DPI), also called Liquid Penetrant Inspection (LPI) or Penetrant
Testing (PT), is one of the oldest and simplest NDT methods where its earliest versions (using
kerosene and oil mixture) dates back to the 19th century.
Liquid penetrant inspection is used to detect any surface-connected discontinuities such as cracks from fatigue, quenching, and grinding, as well as fractures, porosity, incomplete fusion, and flaws in joints. DPI is based upon capillary action, where low surface tension fluid penetrates into clean and dry surface-breaking discontinuities. Penetrant may be applied to the test component by dipping, spraying, or brushing. After adequate penetration time has been allowed, the excess penetrant is removed, a developer is applied. The developer helps to draw penetrant out of the flaw where an invisible indication becomes visible to the inspector. Inspection is performed under ultraviolet or white light, depending upon the type of dye used – fluorescent or nonfluorescent (visible).
UT (ULTRASONIC TEST)
Ultrasonic inspection can be used to detect surface flaws, such as cracks, seams, and internal
flaws such as voids or inclusions of foreign material. It’s also used to measure wall thickness in
tubes and diameters of bars. An ultrasonic wave is a mechanical vibration or pressure wave
similar to audible sound, but with a much higher vibration frequency. For NDT purposes, the
range is usually from 1MHz to 30MHz. Depending on the test requirements, these waves can be
highly directional and focused on a small spot or thin line, or limited to a very short duration.
UPV (ULTRA PULSE VELOCITY TEST)
An ultrasonic pulse velocity (UPV) test is an in-situ, nondestructive test to check the quality of concrete and natural rocks. In this test, the strength and quality of concrete or rock is assessed by measuring the velocity of an ultrasonic pulse passing through a concrete structure or natural rock formation. This test is conducted by passing a pulse of ultrasonic through concrete to be tested and measuring the time taken by pulse to get through the structure. Higher velocities indicate good quality and continuity of the material, while slower velocities may indicate concrete with many cracks or voids.