Types of Non-Destructive Testing

April 14th, 2010

The tensile-strength test is innately destructive; during the process of collating information, the sample is wasted. Although this is excusable when a plentiful sample of the sample material is available, nondestructive techniques are desirable for materials that are expensive or complex to create or that have been made into completed or semicompleted items.

Liquids

One tried and true nondestructive procedure, utilized to find surface cracks and flaws in metal samples, takes a penetrating liquid, either luminescently coloured or fluorescent. After being smeared on the surface of the metal and left to sink into any surface imperfections, the liquid is wiped off, leaving easily uncovered markings and weaknesses. A similar test, used for nonmetals, takes an electrically charged liquid painted on the material surface. After the extra liquid is cleaned off, a dry powder of opposite charge is sprayed on the sample and draws to the cracks. Neither of these techniques, however, can identify internal imperfections.

Radiation

Internal, like external weaknesses, can be detected through the use of X-ray or gamma-ray machines in which the radiation scans the metal and impresses on a subject photographic film. On some occasions, it can be possible to target the X rays onto a single area in the metal, permitting a 3-dimensional image of the flaw shape along with its position.

Sound

Ultrasonic inspection of sections requires transmission of sound waves higher than human hearing range through the test sample. In the reflection method, a sound wave is transmitted from one area of the piece, reflected with the other end, and returned back to a receiver that is located at the first end. Upon locating a break or crack in the sample, the signal is reflected and its traveling time disrupted. The actual delay then becomes a mark of the flaw’s location; a map of the subject can be made to isolate the area and geometry of the flaws. In the through-transmission technique, the transmitter and receiver need to be located at the opposite parts of the subject; delays in the movement of sound waves are found to target and measure marks. Often a water medium is used in which transmitter, sample, and receiver should be immersed.

Magnetism

As the magnetic characteristics of a test piece are very much shown by its overall structure, magnetic methods can be utilized to measure the placement and general dimensions of weaknesses and breaks. With magnetic testing, an item is used that contains a large length of wire through which flows a steady alternating current (primary coil). Held inside this primary wire is a smaller coil (the secondary coil), to which is connected an electrical measuring device. The steady current in the first coil makes the current to move through the secondary coil through the technique of induction. When an iron piece is put in the secondary coil, sharp changes in the further current will signal defects in the rod. This method only finds differentiations within zones along the length of a rod and does not isolate long or continuous marks that easily. A similar method, employing eddy currents induced with a primary coil, also can be employed to locate imperfections and marks. A steady current is induced in part of the test sample. Weaknesses that are located in the transmission of the current change resistance of the test object; this determination can be measured under better equipment.

Infrared

Infrared techniques also have been utilized to find material continuity in complex constructual situations. In testing the quality of adhesive joints between the sandwich core and facing sheets within a ordinary sandwich structure material like plywood, for example, heat is applied in the surface of the sandwich skin sample. In the case where bond lines are found to be continuous, the core areas provide a heat depression on the surface object, and the localised temperatures of the surface should drop steadily along those bond lines. Where that bond line may be insignificant, gone, or mistaken, however, local temperature does not change. Infrared photography of the surface will then demonstrate the placement and geometry of the broken adhesive. A variation of this technique utilizes thermal coatings that will change appearance when reaching a determined temperature.

Conclusively, nondestructive test techniques also are sometimes sought to reveal a entire knowledge of the mechanical aspects of a test object. Ultrasonics and thermal processes seem most trustworthy in this circumstance.

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