NDT (2)

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NON DESTRUCTIVE TESTING NDT - general name for all methods which permit testing / inspection of materials / parts without impairing its future usefulness


TYPES OF DEFECTS inherent defects - present during initial production of raw materials processing defects - resulted from the manufacturing process service defects - occurred during the operation

Examples of causes of defects:

Examples of causes of defects Blowholes - gas trapped during solidification process Segregation - during solidification of alloy Scale - oxide formation on the surface of a metal heated to high temperature Stress - residual stress after cold working or rapid cooling


Cracks quenching / hardening cracks - rapid volume change tempering cracks - rapid heating shrinkage cracks - rapid cooling grinding cracks - friction heating also caused by residual stress, collapsed blowholes, improper rolling, sharp edge of dies etc. etc.


TEST METHODS visual pressure and leak test penetrant thermal radiography acoustic magnetic electrical / electrostatic electromagnetic induction - eddy current optical holography electron probe etc.etc

Visual Testing:

Visual Testing most widely used experienced inspector knows where are likely cracks , orientation of cracks relative to various zones in the weld, surface porosity, weld penetration, potential weakness such as sharp notches or misalignment

Pressure and Leak Test:

Pressure and Leak Test common form is hydrostatic test hydrostatic test often required for pressure vessels, pipes, valves. normally pressurised to 1.5 or 2 times the working pressure for sensitive leak test, radioactive material, halogen or helium gases are used


Penetrant extension of visual inspection for detection of surface flaws two kinds ; dye penetrant and fluorescent penetrants

Basic steps of dye penetrant testing:

Basic steps of dye penetrant testing clean the surface apply penetrant remove excess penetrant apply developer inspect / interpretation penetrant seep into flaw developer draws penetrant onto surface

Magnetic particle testing:

Magnetic particle testing for locating surface & subsurface discontinuities in ferromagnetic materials leakage current occurs at the discontinuities / surface flaws when magnetized fine particles collect at the leakage sites

Ultrasonic testing:

Ultrasonic testing use beams of high frequency sound waves to probe the material internal flaws and locations can be determined by analyzing reflected sound waves

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