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One of the many Non Destructive Testing (NDT) techniques used for the inspection of welding is surface crack detection. We shall briefly examine the two most common methods of surface crack detection, Liquid Penetrant Testing (PT), sometimes referred to as Dye Penetrant Inspection, and Magnetic Particle Testing (MT). We shall consider how they are used, and what types of welding discontinuities they can be expected to find. We shall examine their advantages over other inspection methods and their limitations.
Liquid Penetrant Testing (PT) – This is probably the most common method of surface crack detection used. We shall consider the two most common methods of this testing method, Color Contrast and Fluorescent Dye. Both these methods use the same fundamental procedure. First, the application of a penetrating liquid to the surface of the weld to be tested, followed by a predetermined soak period to allow for adequate penetration of the liquid penetrant into any surface breaking discontinuities. Second, the careful removal of any excess penetrant, usually with a solvent or sometimes with a water wash, dependent on the method used. Third, the application of a developer to withdraw penetrant left behind within any discontinuity. These three steps are followed by the interpretation and evaluation of the test results. This will involve the detection of penetrant bleed-out from within any surface discontinuity. The method of detection is different for the color contrast and the fluorescent dye. The color contrast method is dependant on the bright contrast between the red penetrant dye and the white developer background covering the surface of the weld being tested, and the evaluation of the test is conducted in ordinary light. The fluorescent dye method is assisted by the use of an ultraviolet light (black light) that is used to illuminate the fluorescent dye and assist in the interpretation of the test.
This type of testing is limited to the detection of surface breaking discontinuities, that is, discontinuities which are open to the surface to which the penetrant has been applied. It cannot detect discontinuities that are sealed within the body of the weld such as internal porosity, or fusion defects. It is not usually suitable for testing rough or porous materials because interpretation of the test results can be hindered by false indications.
When compared to unassisted visual inspection, this type of inspection can provide a more sensitive inspection method that is more likely to detect smaller and finer surface breaking discontinuities, such as hair line cracks and micro surface porosity. This type of inspection may be suitable for both ferrous and nonferrous materials.
Magnetic Particle Testing (MT) – This is an NDT method used for detecting cracks, porosity, seams, inclusions, lack of fusion, and other discontinuities in ferromagnetic materials. Surface discontinuities and shallow subsurface discontinuities can be detected by using this method. This testing method consists of establishing a magnetic field in the part to be tested, applying magnetic particles to the surface of the part, and examining the surface for accumulations of particles that indicate discontinuities. A magnet will attract magnetic particles to its ends or poles, as they are called. Magnetic lines of force or flux flow between the poles of a magnet. Magnets will attract magnetic materials only where the lines of force enter and leave the magnet at the poles. If a magnet is bent and the two poles are joined so as to form a closed loop, no external poles will exist and consequently it will have no attraction for magnetic material.. This is the basic principle of magnetic particle testing. As long as the part has no cracks or other discontinuities, magnetic particles will not be attracted. When a crack or other discontinuity is present in the part being tested, north and south magnetic poles are set up at the edge of the discontinuity.
Only ferromagnetic materials can be tested by this method. Ferromagnetic parts that have been magnetized during testing may retain a certain amount of residual magnetism. Certain parts may require demagnetization if they are to function properly in service.
When using surface crack detection, whether liquid penetrant testing or magnetic particle, you should always consult the relevant specification involved for levels of acceptability and qualifications for equipment and operators. These methods of inspection are specialized and should be carried out by suitably trained and qualified inspection personnel.