10-05-2014, 11:39 AM
Non Destructive Testing
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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 internalflaws 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 maybe 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.
Radiography
Introduction to Radiography
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
To produce an X or Gamma radiograph, the film package (comprising film and intensifying screens - the latter being required to reduce the exposure time – enclosed in a light tight cassette) is placed close to the surface of the subject.
Magnetic Particle Inspection
Introduction to Magnetic Particle Inspection
This method is suitable for the detection of surface and near surface discontinuities in magnetic material, mainly ferritic steel and iron.
The principle is to generate magnetic flux in the article to be examined, with the flux lines running along the surface at right angles to the suspected defect. Where the flux lines approach a discontinuity they will stray out in to the air at the mouth of the crack. The crack edge becomes magnetic attractive poles North and South. These have the power to attract finely divided particles of magnetic material such as iron fillings. Usually these particles are of an oxide of iron in the size range 20 to 30 microns, and are suspended in a liquid which provides mobility for the particles on the surface of the test piece, assisting their migration to the crack edges. However, in some instances they can be applied in a dry powder form.
Introduction to Eddy Current Testing
The main applications of the eddy current technique are for the detection of surface or subsurface flaws, conductivity measurement and coating thickness measurement. The technique is sensitive to the material conductivity, permeability and dimensions of a product.
Eddy currents can be produced in any electrically conducting material that is subjected to an alternating magnetic field (typically 10Hz to 10MHz). The alternating magnetic field is normally generated by passing an alternating current through a coil.
The coil can have many shapes and can between 10 and 500 turns of wire. The magnitude of the eddy currents generated in the product is dependent on conductivity, permeability and the set up geometry. Any change in the material or geometry can be detected by the excitation coil as a change in the coil impedance. The most simple coil comprises a ferrite rod with several turns of wire wound at one end and which is positioned close to the surface of the product to be tested. When a
crack, for example, occurs in the product surface the eddy currents must travel farther around the crack and this is detected by the impedance change.