14-05-2013, 01:21 PM
seminaron non destructive testing
destructive testing.pptx (Size: 723.26 KB / Downloads: 160)
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.
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.
TECHNIQUES OF NDT
The methods covered are:
• Radiography
• Magnetic Particle Crack Detection
• Dye Penetrant Testing
• Ultrasonic Flaw Detection
• Eddy Current and Electro-magnetic Testing
However, these are by no means the total of the principles available to the N.D.T. Engineer. ELECTRICAL POTENTIAL DROP, SONICS, INFRA-RED, ACOUSTIC EMISSION AND SPECTROGRAPHY, to name but a few, have been used to provide information that the above techniques have been unable to yield, and development across the board continues.
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 and dense, the greater the absorption.
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.
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.
DYE PENETRANT TESTING
This method is frequently used for the detection of surface breaking flaws in non - ferromagnetic materials.
The subject to be examined is first of all chemically cleaned.
Next the penetrant is applied and allowed to remain in contact with the surface for approximately fifteen minutes. Capillary action draws the penetrant into the crack during this period.
The surplus penetrant on the surface is then removed completely and thin coating of powdered chalk is applied.
After a further period (development time) the chalk draws the dye out of the crack, rather like blotting paper, to form a visual, magnified in width, indication in good contrast to the background.