03-09-2012, 12:39 PM
A Brief Description of NDT Techniques
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Introduction
Non-destructive Testing is one part of the function of Quality Control and is
complementary to other long established methods.
By definition non-destructive testing is the testing of materials, for surface or internal
flaws or metallurgical condition, without interfering in any way with the integrity of the
material or its suitability for service.
The technique can be applied on a sampling basis for individual investigation or may
be used for 100% checking of material in a production quality control system.
Whilst being a high technology concept, evolution of the equipment has made it
robust enough for application in any industrial environment at any stage of
manufacture - from steel making to site inspection of components already in service.
A certain degree of skill is required to apply the techniques properly in order to obtain
the maximum amount of information concerning the product, with consequent feed
back to the production facility.
Non-destructive Testing is not just a method for rejecting substandard material; it is
also an assurance that the supposedly good is good. The technique uses a variety of
principles; there is no single method around which a black box may be built to satisfy
all requirements in all circumstances.
Radiography - X And Gamma
This technique is suitable for the detection of internal defects in ferrous and nonferrous
metals and other materials.
X-rays, generated electrically, and Gamma rays emitted from radio-active isotopes,
are penetrating radiation which is differentially absorbed by the material through
which it passes; the greater the thickness, the greater the absorbtion. Furthermore,
the denser the material the greater the absorbtion.
X and Gamma rays also have the property, like light, of partially converting silver
halide crystals in a photographic film to metallic silver, in proportion to the intensity of
the radiation reaching the film, and therefore forming a latent image. This can be
developed and fixed in a similar way to normal photographic film.
Material with internal voids is tested by placing the subject between the source of
radiation and the film. The voids show as darkened areas, where more radiation has
reached the film, on a clear background. The principles are the same for both X and
Gamma radiography.
In X-radiography the penetrating power is determined by the number of volts applied
to the X-Ray tube - in steel approximately 1000 volts per inch thickness is necessary.
In Gamma radiography the isotope governs the penetrating power and is unalterable
in each isotope. Thus Iridium 192 is used for 1/2" to 1" steel and Caesium 134 is
used for 3/4" to 21/2" steel.
Ultrasonic Flaw Detection
This technique is used for the detection of internal and surface (particularly distant
surface) defects in sound conducting materials.
The principle is in some respects similar to echo sounding. A short pulse of
ultrasound is generated by means of an electric charge applied to a piezo electric
crystal, which vibrates for a very short period at a frequency related to the thickness
of the crystal. In flaw detection this frequency is usually in the range of one million to
six million times per second (1 MHz to 6 MHz). Vibrations or sound waves at this
frequency have the ability to travel a considerable distance in homogeneous elastic
material, such as many metals with little attenuation. The velocity at which these
waves propagate is related to the Young’s Modulus for the material and is
characteristic of that material. For example the velocity in steel is 5900 metres per
second, and in water 1400 metres per second.
Ultrasonic energy is considerably attenuated in air, and a beam propagated through a
solid will, on reaching an interface (e.g. a defect, or intended hole, or the backwall)
between that material and air reflect a considerable amount of energy in the direction
equal to the angle of incidence.