Non-Destructive Tests on Metals
Non-destructive tests include radiography, ultrasonic testing, magnetic particle testing, eddy current testing and dye penetrant testing. These tests enable the engineer to decide whether a part is likely to be reliable in the service.
Radiography
Radiography is usually confined to the testing of welds in pressure vessels such as starting air reservoirs. Large reservoirs for high pressure use may require only spot tests to be taken. X-rays or gamma rays are used to expose the emulsion of the radiographic film. Welding defects give a greater exposure of the film and show as darker areas. The use of radiographic equipment and interpretation of he negative requires considerable training and skill.
Ultrasonic Testing
Ultrasonic testing is is one of the non-destructive tests performed by equipment that transmits high frequency vibrations through the materials to be tested. The vibrations are reflected back from the opposite surface or from any discontinuity in the material. Ultrasonic methods can be used to measure the thickness of materials or to detect internal or surface defects in welds, castings or forgings, either during manufacture or when in service. Defects are shown as extra pulses to the transmitted and reflected pulses, on a cathode ray oscilloscope.
Magnetic Particle Testing
Magnetic particle testing is one of the non-destructive tests method can be used for detecting defects and near surface defects in materials that can be magnetized. When a magnetic field is induced in the part to be tested, defects allow a flux leakage to occur. This causes the magnetic particles used in the test to congregate at the leakage, indicating the location of the defect.
Magnetic particle testing is used mainly for checking the condition of the engine parts and shafting, which are liable to fatigue failure. The use of this type of test equipment requires skill and experiences.
Eddy Current Testing
Eddy current testing methods are used mainly in production line work during the manufacture of small ferrous material parts for use in either small or large engines. A coil is used in the tester, any defects present causing a change in the impedance of the coil. The change of impedance is utilized in various ways to call attention to the presence of a defect.
Dye-Penetrant Test
Dye penetrant tests are used to detect surface defects such a fatigue cracks in crankshaft and screw shafts. The part to be tested is thoroughly cleaned, and a dry penetrant is sprayed on the cleaned area. If any surface defect is present, capillary attraction draws the dye into the cracks. The dye is cleaned off (but remains in the crack) and absorbent or developer material is spray coated on the test area. The absorbent draws out the penetrant dye from the crack, showing a coloured line which indicates the presence and location of the defect.
References
“Lamb’s Questions and Answers on the Marine Diesel Engine” by Stanley G Christensen