Introduction in NDT

Nondestructive testing is the examination of an object or material with technology that does not affect the object's future usefulness. The use of NDT is to determine the integrity of a material, component or structue tested.

Ultrasonic

Ultrasonic inspection is a NDT method in which beams of high-frequency sound waves are introduced into materials for detection of subsurface flaws in the test objects.

Ultrasonic testing uses the transmission of high frequency sound waves, into a material to detect imperfections within the material or changes in material properties.

The most commonly used ultrasonic testing technique is pulse echo wherein sound is introduced into the test object and reflections (echoes) are returned to a receiver from internal imperfections or from geometrical surfaces of the part.

Eddy Current

In this method, current passed through wire coils generates a magnetic field in the test specimen.

The field in turn creates eddy currents, which produce electro-motive force in the exciting coil or an auxiliary coil.

The phase and amplitude of the electro-motive force reveal characteristics of the specimen, including presence or absence of defects.

Magnetic Particle

It is used for detecting discontinuities in ferromagnetic parts.

The part is magnetized by using an electrical current that induces a magnetic field in the part.  A discontinuity, which crosses the magnetic field, creates north and south poles on either side of the defect area.

When magnetic particles are applied to the part, the poles attract the particles and an indication of the discontinuity is formed.

Liquid Penetrant

It is a NDT method of revealing discontinuities that are open to the surfaces of solid and essentially nonporous materials. Liquid penetrant testing is probably the most widely used NDT method. The test object or material is coated with a visible or fluorescent dye solution.

The excess dye is removed from the surface, and then a developer is applied. the developer acts like a blotter and draws penetrant out of imperfections which open to the surface.

With visible dyes, the vivid color contrast between the penetrant and the developer makes the “bleedout’ easy to see. With fluorescent dyes, and ultraviolet lamp is used to make the “bleedout” fluoresce brightly, thus allowing the imperfection to be seen readily.

Radiography

This NDT method involves the use of penetrating X or gamma radiation to examine parts and products for imperfections.

An X-ray machine or radioactive isotope is used as a source of radiation.

Radiation is directed through a part and onto film.

When the film is developed, a shadowgraph is obtained that shows the internal soundness of a part.

Possible imperfections show up as density changes in the film.