Types of Non-Destructive Testing
The tensile-strength test is basically futile; during the process of gathering material, the sample is ruined. While this is acceptable when a safe store of the material exists, nondestructive procedures are safer for materials that are expensive or complex to make up or that have been shaped into finished or semicompleted products.
Liquids
One tried and true nondestructive procedure, used to find surface marks and imperfections in metal samples, requires a penetrating fluid, either luminescently coloured or fluorescent. After being smeared on the surface of the sample material and allowed to fill into any perceptible flaws, the liquid is wiped off, leaving readily perceptible breaks and flaws. Another such technique, better for nonmetals, employs an electrically charged fluid rubbed on the sample surface. After superfluous liquid is removed, a dry powder of opposite charge is sprayed on the material and attracted to the breaks. Neither of these tests, however, can detect internal flaws.
Radiation
Internal, like external imperfections, can be detected through the use of X-ray or gamma-ray technologies in which the radiation passes through the material and implicates on an appropriate photographic film. Under some circumstances, it may be possible to focus the X rays to a particular plane within the piece, creating a 3rd dimensional image of the flaw shape as well as its position.
Sound
Ultrasonic inspection of sections takes transmission of sound waves above human hearing range within the test sample. Under the reflection technique, a sound wave is targeted over one side of the piece, reflected off the opposite side, then returned onto a receiver located at the original point. When isolating a break or crack in the material, the sound wave is reflected and its transmission changed. The actual delay becomes a signal of the location of the crack; a map of the material can be made to locate the location and geometry of the flaws. Using the through-transmission technique, the transmitter and receiver need to be placed at opposite ends of the subject; delays in the signal of the sound waves are studied to isolate and measure cracks. Sometimes a water medium is used by which transmitter, sample, and receiver are immersed.
Magnetism
As the magnetic aspects of a test piece are largely influenced by its overall form, magnetic processes can be used to demonstrate the location and indicative geometry of flaws and breaks. For magnetic testing, a tool is used that contains a big measure of wire through which flows a steady alternating current (primary coil). Nested in the primary piece is a smaller coil (the secondary coil), to which is secured an electrical measuring device. The steady current in the larger coil forces further current to flow in the secondary coil through the technique of induction. If an iron bar is slotted in the secondary coil, obvious changes in the further current will indicate defects in the sample. This process only finds differentiations in parts on the length of a sample and will not isolate long or continued defects very easily. An analogous process, using eddy currents induced by a primary coil, also can be utilized to isolate errors and cracks. A steady current is induced in part of the test subject. Marks that lie across the track of the current determine resistance of the test item; this determination should be measured by appropriate processes.
Infrared
Infrared processes also have been used to isolate material continuity in complicated structural situations. While testing the value of adhesive joints in the sandwich core and facing sheets within a standard sandwich construction object like plywood, for example, heat is applied in the face of the sandwich skin item. Where bond lines appear to be continuous, those core materials show a heat marking within the surface material, and the localised temperatures of the skin then spread evenly on these bond lines. Where a bond line is not enough, disappears, or faulty, however, this temperature should not drop. Infrared photography of the area does reveal the situation and area of the flawed adhesive. A similar technique utilizes thermal coatings that will change hue on reaching a set temperature.
Finally, nondestructive procedures also are sometimes seen to permit a whole determination of the mechanical properties of a test item. Ultrasonics and thermal methods seem to be most valuable in this situation.
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