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Non-Destructive Tests on Metals
Destructive and non destructive tests are carried out upon materials to determine their suitability for use in engineering. Apart from tests which are used to determine the dimensions and physical or mechanical characteristics of the materials, the main non-destructive tests are those used to locate defects.
This can be used for the examination of welds, forgings and castings. X-rays or gamma rays which can penetrate up to 180 mm of steel, pass through the metal and impinge upon a photographic plate or paper to give a negative. Due to the variation in intensity of the metal, the absorption of the rays is non uniform, hence giving a shadow picture of the material. It is a compact and simple system but requires skilled personnel for safe and accurate testing.
Here there is no limitation of thickness of the metal as we have seen with radiography. Sound waves with high frequency are directed to the metal to be tested. If there is no imperfections the sound waves travel along the metal and reflect back from the bottom most part. In case there is any cracks or defects in between, part of the the sound wave reflects back before reaching bottom and these results are displayed in a cathode ray oscilloscope for evaluation. The test is used for both finding defects and material thickness. Also it can be applied for parts under manufacturing process or those components which are already in use or service.
Magnetic Particle Testing
Magnetic particle testing method is used for finding surface and sub surface defects for magnetizing metals. When a magnetic field is applied to the material to be tested, any surface or sub surface defects allow a leakage of magnetic flux. The magnetic particles sprayed at the top of the metal try to accumulate near the defect, if present. Hence the location and magnitude of the defect can be ascertained. This test is normally used for engine parts which are subjected to fatigue stresses such as crank shafts, etc. Skilled personnel is required for the operation of the test equipment. One disadvantage is that the test cannot be used for non-magnetizing materials.
Eddy Current Testing
This is the testing method that can be used for small ferrous metals. The test is conducted while manufacturing of the components parts of large and small machinery. Here impedance of the coil in the tester varies according to the nature of the metal defect. The variation in impedance is analyzed by testing personnel for ascertaining the metal imperfections to alter manufacturing processes.
Dye penetrant test is used to detect surface defects. It is a simple test and usually carried out on board on auxiliary engine connecting rods, bearing shells, crank shaft, screw shafts, etc. First the surface to be tested is cleaned with a cleaner solution. After cleaning a dye is applied all over the surface. Keep it undisturbed for 10 minutes. Dye enters into the surface cracks, if present. Now wipe out the dye and apply a developer over the surface. This developer draws the dye out of the cracks and shows location of the surface cracks. Since the dye is normally pink and developer is white in colour, it is easy to identify the cracks.