METHODS

Radiographic   |   Ultrasonic   |   Liquid Penetrant   |   Magnetic Particle   |   Visual Inspection


Radiographic Inspection

Radiography is the most commonly used non-destructive method for the detection of subsurface volumetric discontinuities in metal. As with all testing methods, radiography has certain limitations. It is a method that simultaneously measures differences in the thickness and density of the material under examination. There must be enough difference to show up on a film image or other digital recording media. X-Rays and gamma rays both have extremely short wavelengths. This short wavelength radiation contains enough energy to ionize atoms should it strike them directly. It is this characteristic whick enables them to penetrate solid objects which would block ordinary light.

The two types of radiation affect photographic film in the same manner. Ionizing radiation is dangerous and exposure must be carefully controlled. Whenever radiography is performed a safe zone is established and warning signs posted. These barriers must never be crossed unless the technician in charge directs you to do so.


Ultrasonic Inspection

Ultrasonic examination utilizes sound frequencies between 20 Khz to approximately 10 Mhz (10,000,000 cycles per second). When examining welds the frequency range is quite high, generally in the range of 2.5 to 5 Mhz.

Ultrasonic sound waves are generated by applying electric pulses to piezoelectric crystals such as quartz or barium titanate. These crystals vibrate, and electrical energy is transferred to mechanical energy. The mechanical energy is transmitted through the particles of the material undergoing test. The velocity at which the ultrasonic beam moves through the material is constant for that specific material and for the wave mode. When a change occurs in the material (such as a void caused by a defect), the velocity of the beam changes and what is known as acoustic mismatch occurs. When this happens, part of the sound beam is reflected back to the crystal, transformed back into electrical energy and projected on to a screen. The phenomena of reflection due to acoustic mismatch is the basis of all ultrasonic testing.


Liquid Penetrant Inspection

Liquid Penetrant Inspection is one of the oldest of non-destructive testing, and is based on a very simple process. A liquid penetrant is applied to the surface of an object, and is permitted to “dwell” (remain on the surface) for a specific period of time which it penetrates into any discontinuity open to the surface. When the dwell time is completed, the excess penetrant remaining on the surface is carefully removed. Penetrant must be removed from the surface without cleaning it from potential discontinuities it has been drawn in to. (If liquids are used to remove excess penetrant material, the operator must ensure that the surface is dry before the next step in the procedure takes place.)

An absorbant light-coloured powder (developer) is now applied to the surface. This developer acts like a blotter and draws out the penetrant which has seeped into cavities in the surface. As the penetrant is drawn out, it spreads into the thin layer of developer forming visible indications. The indications are actually much wider than the cavities with which they are associated, and there is a direct correlation between the amount of “bleed out” of the penetrant material, and the volume of the cavity. The acceptance criteria of most standards is based upon the size of the penetrant indication not on assessing the depth or volume of the cavity.


Magnetic Particle Inspection

The magnetic particle method of testing is for locating surface and subsurface discontinuities in ferromagnetic material. It depends for its operation, on the fact that when the material or part under test is magnetized, discontinuities which lie in a direction generally transverse to the direction of the magnetic field will cause a leakage field to be formed at and above the surface of the part. The presence of this leakage field, and therefore the presence of the discontinuity, is detected by the use of finely divided ferromagnetic particles applied over the surface, some of these particles being gathered and held by the leakage field. The magnetically held collection of particles form an outline of the discontinuity and indicates its location, size, shape and extent.


Visual Inspection

Visual inspection is of great importance because it constitutes the principle basis of acceptance for many types of materials. It is one of the most extensively used methods of inspection because it is easy to apply, is fast, is relatively inexpensive and, provided the inspection report format is properly organized, gives very important information about all aspects of fabrication.