Types of Non-Destructive Testing
April 14th, 2010
The tensile-strength test is basically fruitless; during the process of fostering research, the sample is destroyed. Although this is not a problem when a safe supply of the sample material is at hand, nondestructive methods are better for materials that are dear or hard to create or that have been made into finished or semifinished items.
Liquids
One commonly used nondestructive process, used to detect surface marks and weaknesses in samples, uses a penetrating fluid, which is either visibly dyed or fluorescent. After being left on the surface of the material and set to soak into any tiny markings, the liquid is wiped off, leaving brightly perceptible markings and imperfections. An analogous test, applicable to nonmetals, uses an electrically charged fluid painted on the material surface. After superfluous liquid is removed, a dry powder of opposite charge is sprayed onto the sample and attracted to the cracks. Neither of these techniques, however, can find internal weaknesses.
Radiation
Internal, as well as external weaknesses, can be detected by X-ray or gamma-ray techniques in which the radiation passes through the sample and implicates on a suitable photographic film. Under some circumstances, it may be possible to focus the X rays to a particular section in the metal, permitting a 3-dimensional image of the flaw shape along with its site.
Sound
Ultrasonic inspection of areas involves transmission of sound waves above human hearing range within the test sample. By the reflection process, a sound wave is targeted from one side of the test material, reflected from the far side, and returned back to a receiver that is located at the original side. Upon locating a mark or crack in the test sample, the signal is reflected and its movement changed. The actual delay becomes a signal of the flaw’s location; a map of the test material can then be created to illustrate the location and shape of the flaws. Using the through-transmission technique, the transmitter and receiver need to be placed at the opposite ends of the test piece; delays in the movement of sound waves are used to find and measure marks. Usually a water medium is employed in which transmitter, sample, and receiver should be immersed.
Magnetism
As the magnetic elements of a object are heavily formed by its overall shape, magnetic techniques can be utilized to reveal the location and indicative geometry of flaws and breaks. In magnetic testing, an object is used that holds a big coil of wire through which flows a steady alternating current (primary coil). Held within the first object is a smaller coil (the secondary coil), to which is linked an electrical measuring device. The steady current in the larger coil forces further current to charge through the secondary coil by the method of induction. When an iron rod is inserted into the secondary coil, obvious changes in the secondary current should signal defects in the rod. This process only detects differentiations in parts along the length of a rod and cannot isolate longer or continuous defects that often. A parallel technique, employing eddy currents induced with a primary coil, also may be used to isolate errors and breaks. A steady current is induced within the test object. Marks that lie in the path of the current alter resistance of the test piece; this adaptation will then be measured by better items.
Infrared
Infrared processes have sometimes been used to isolate material continuity in intricate structural materials. By testing the strength of adhesive joints in the sandwich core and facing sheets within a ordinary sandwich structure object like plywood, for example, heat is applied in the surface of the sandwich skin item. When bond lines are found to be continuous, the core parts provide a heat signature on the surface material, and the local temperatures of the face should fall spaciously along the bond lines. In the case that a bond line appears to be inadequate, missing, or faulty, however, temperature should not adapt. Infrared photography of the face does show the situation and shape of the marked adhesive. Another kind of method employs thermal coatings that will change hue on reaching a determined temperature.
In conclusion, nondestructive test procedures also are being shown to allow a whole understanding of the mechanical elements of a test piece. Ultrasonics and thermal procedures appear the most promising in this situation.
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