Types of Non-Destructive Testing

April 14th, 2010

The tensile-strength test is basically damaging; at the time of the process of collating information, the sample is ruined. Though this is permissible when a decent sample of the material is available, nondestructive tests are desirable for materials that are expensive or arduous to create or that have been formed into finished or semicompleted items.

Liquids

One commonly used nondestructive method, used to detect surface cracks and weaknesses in samples, uses a penetrating liquid, which is either visibly coloured or fluorescent. After being painted on the surface of the metal sample and left to sink into any tiny flaws, the dye is wiped off, leaving brightly revealed cracks and flaws. A similar process, applicable to nonmetals, uses an electrically charged liquid painted on the sample surface. After the extra liquid is rubbed off, a dry powder of opposite charge is sprayed on the surface of the nonmetal and attracted to the breaks. Neither of these techniques, however, can find internal imperfections.

Radiation

Internal, like external weaknesses, can be detected with X-ray or gamma-ray tests in which the radiation passes through the material and impinges on an appropriate photographic film. On some occasions, it is possible to target the X rays toward a particular section in the object, permitting a three-dimensional description of the flaw shape along with its position.

Sound

Ultrasonic inspection of parts requires transmission of sound waves higher than human hearing range within the test material. Under the reflection process, a sound wave is transmitted from one area of the sample, reflected with the opposite part, and returned back to a receiver located at the beginning side. Upon isolating a weakness or crack in the piece, the sound wave is reflected and its transmission changed. The actual delay is then a sign of the location of the crack; a map of the piece can then be formed to show the location and geometry of the flaws. With the through-transmission method, the transmitter and receiver need to be placed on opposite parts of the sample; interruptions in the signal of the sound waves are studied to target and measure cracks. More often than not a water medium is utilized through the use of which transmitter, sample, and receiver should be immersed.

Magnetism

As the magnetic elements of a material are very much formed by its overall shape, magnetic methods can be utilized to demonstrate the placement and relative size of failures and cracks. By magnetic testing, an object is employed that holds a sizeable measure of wire through which flows a steady alternating current (primary coil). Located in the initial coil is a shorter coil (the secondary coil), to which is connected an electrical measuring device. The steady current in the larger coil makes further current to react through the secondary coil by the method of induction. When an iron piece is slotted in the secondary coil, acute changes in the secondary current will implicate marks in the bar. This technique only isolates differentiations in parts in the length of a piece and does not locate elongated or continued imperfections very easily. An analogous process, using eddy currents induced by a primary coil, also should be employed to isolate errors and breaks. A steady current is induced in the test sample. Weaknesses that are located across the path of the current determine resistance of the test material; this change should be measured with the correct equipment.

Infrared

Infrared techniques also have been employed to detect material continuity in complicated construction materials. In testing the quality of adhesive conjoinments between the sandwich core and facing sheets by a ordinary sandwich structure material such as plywood, for example, heat is the surface of the sandwich skin object. In the case where bond lines are found to be continuous, those core parts reveal a heat sink for the surface material, and the local temperatures of the face then fall lightly on the bond lines. In the case that that bond line is too small, disappears, or faulty, however, this temperature can not change. Infrared photography of the surface can then reveal the location and area of the broken adhesive. Another such process employs thermal coatings that change appearance on reaching a set temperature.

Finally, nondestructive processes also are being seen to show a whole study of the mechanical elements of a test item. Ultrasonics and thermal methods seem most trustworthy in this regard.

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