Types of Non-Destructive Testing

April 14th, 2010

The tensile-strength test is inherently destructive; at the time of the process of collating material, the sample is ruined. While this is excusable when a plentiful store of the material is available, nondestructive techniques are better for materials that are costly or hard to make up or that have been formed into completed or semicompleted products.

Liquids

One common nondestructive test, employed to detect surface markings and imperfections in metal samples, employs a penetrating liquid, which needs to be luminescently dyed or fluorescent. After being smeared on the surface of the sample material and allowed to sink into any surface flaws, the liquid is rubbed away, leaving brightly revealed cracks and weaknesses. Another such technique, used for nonmetals, employs an electrically charged fluid pasted on the sample surface. After superfluous fluid is cleared off, a dry powder of opposite charge is sprayed on the sample and draws to the flaws. Neither of these methods, however, can identify internal weak points.

Radiation

Internal, as well as external weaknesses, can be identified by X-ray or gamma-ray tests in which the radiation passes through the material and impresses on an appropriate photographic film. On some occasions, it may be possible to target the X rays toward a single plane in the piece, creating a 3rd dimensional description of the flaw identity along with its location.

Sound

Ultrasonic inspection of sections takes transmission of sound waves higher than human hearing range within the sample. In the reflection process, a sound wave is sent over one area of the subject, reflected from the far part, then signalled back to a receiver that is located at the first part. By impinging on a weakness or weak point in the piece, the signal is reflected and its traveling time changed. The actual delay is a measure of the location of the mark; a map of the test piece can then be created to locate the location and form of the weaknesses. By the through-transmission technique, the transmitter and receiver need to be located at the opposite areas of the material; interruptions in the signal of sound waves are utilized to find and measure cracks. Usually a water medium is used through the use of which transmitter, sample, and receiver should be immersed.

Magnetism

As the magnetic traits of a sample are heavily influenced by its overall shape, magnetic processes are employed to isolate the situation and indicative dimensions of flaws and marks. In magnetic testing, an object is employed that consists of a sizeable measure of wire through which flows a steady alternating current (primary coil). Held in this first coil is a shorter coil (the secondary coil), to which is attached an electrical measuring device. The steady current in the initial coil makes electrical current to move in the secondary coil by the method of induction. When an iron bar is inserted in the secondary coil, acute changes in the secondary current should indicate defects in the rod. This method only finds differentiations in sections on the length of a sample and will not isolate long or continued flaws that readily. An analogous technique, using eddy currents induced by a primary coil, also can be employed to detect errors and breaks. A steady current is induced in part of the test subject. Flaws that exist in the path of the current alter resistance of the test piece; this determination can be measured with the correct tools.

Infrared

Infrared methods also have been employed to find material continuity in complex structural items. While testing the quality of adhesive conjoinments with the sandwich core and facing sheets by a usual sandwich construction sample such as plywood, for example, heat is the surface of the sandwich skin item. When bond lines are continuous, those core materials show a heat sink within the surface sample, and the local temperatures of the face then appear evenly along those bond lines. Where a bond line may be too small, missing, or mistaken, however, this temperature can not drop. Infrared photography of the front can then isolate the situation and geometry of the failing adhesive. Another such method employs thermal coatings that will change hue at reaching a specific temperature.

Finally, nondestructive test processes also are sometimes sought to reveal a complete knowledge of the mechanical characteristics of a test sample. Ultrasonics and thermal methods seem to be most trustworthy in this circumstance.

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