How Cracks are Detected in Metals in Air Craft Industries
Normal visual inspection cannot be relied upon to disclose very fine surface cracks nor can it reveal internal faults such as may develop in highly stressed components. Therefore specified components must be subjected to a crack detection test.
The general crack detection methods are described below :
(a) A magnifying glass will be useful for examining for signs of cracks.
(b) Hot Fluid Chalk Test. This is a satisfactory method, but can be applied only to smaller components which can be removed from the aircraft. The process consists of heating a mixture of Kerosene (3 parts) and lubricating oil (1 Part) in a bath to 90° C. The component to be tested is then immersed in the fluid to soak until it attains the bath temperature. The component is then removed, cleaned and dried as quickly as possible and while the component is still hot, it is rolled in French chalk. Surplus chalk is removed by tapping and when the component is cooled off, the contraction of the component on cooling will force the oil out of any crack and stain the french chalk with a yellowish color.
(c) Cold Fluid Chalk Test. This method should be applied to components which cannot be readily dismantled. The method is same as above, except that the fluid is applied on the parts to be tested, cleaned and then the parts are painted with a mixture of methylated spirit and french chalk. When the spirit evaporates, the component can be examined for cracks by the stains on the thin coating of french chalk.
(d) Electro-magnet Crack Detection. This test can only be used on parts capable of being magnetized (i.e. most ferrous metals). This is done by magnetizing the part to be tested either by placing it directly across the poles after the application of a direct current to an electro-magnet or by passing an alternating current through a conductor utilizing the magnetic field thus produced to magnetize the component. Application of detecting ink over the components under test will indicate the presence of defects by a thin black line, as the fine solid particles in the ink adhere to the poles.
(e) Dye Penetrant Crack Detection. (Ardrox 996). This method of crack detection gives a more definite indication of cracks than the chalk methods, and the suspected parts may be tested without dismantling. A kit containing penetrant, penetrant remover and developer may be used. As the developer dries, the cracks will be shown as red lines; and a series of red spots is . the indication of slight cracks.
(f) X-rays, X-rays are now widely used in the detection of flaws in castings and forgings. The presence of flaws generally corresponds to a decrease in absorption of X-rays and hence a denser portion will appear on the radiograph.
(g) Super-sonic testing for detection of flaws is also used in certain cases.