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The tensile-strength test is innately destructive; during the process of collecting information, the sample is ruined. Although this is permissible when a plentiful supply of the sample material is at hand, nondestructive techniques are safer for materials that are dear or difficult to create or that have been shaped into finished or semifinished products.

Liquids

One tried and true nondestructive procedure, used to identify surface breaks and flaws in samples, takes a penetrating fluid, either brightly coloured or fluorescent. After being pasted on the surface of the material and left to sink into any perceptible flaws, the fluid is cleared, leaving readily visible breaks and imperfections. An analogous method, used for nonmetals, requires an electrically charged fluid painted on the nonmetal surface. After excess liquid is cleaned off, a dry powder of opposite charge is sprayed on the sample and attracted to the breaks. Neither of these methods, however, can find internal weaknesses.

Radiation

Internal, like external flaws, can be identified with X-ray or gamma-ray techniques in which the radiation scans the material and implicates on an ideal photographic film. Occasionally, it is possible to focus the X rays toward a particular part within the piece, permitting a 3-dimensional description of the flaw geometry along with its site.

Sound

Ultrasonic inspection of areas takes transmission of sound waves higher than human hearing range through the test sample. In the reflection technique, a sound wave is targeted over one area of the sample, reflected with the other side, then returned back to a receiver that is located at the first end. By locating a break or imperfection in the sample, the signal is reflected and its transmission altered. The actual delay becomes a signal of the location of the mark; a map of the sample can then be generated to isolate the point and shape of the marks. By the through-transmission process, the transmitter and receiver are located on the opposite areas of the subject; delays in the signal of the sound waves are utilized to isolate and measure weaknesses. Often a water medium is employed by which transmitter, sample, and receiver are immersed.

Magnetism

As the magnetic characteristics of a material are largely shown by its overall form, magnetic methods can be used to measure the location and indicative size of voids and cracks. In magnetic testing, a tool is used that consists of a sizeable length of wire through which flows a steady alternating current (primary coil). Held inside the larger piece is a smaller coil (the secondary coil), to which is secured an electrical measuring device. The steady current in the primary coil causes further current to flow through the secondary coil by the method of induction. If an iron bar is inserted within the secondary coil, sharp changes in the further current will signal marks in the piece. This method only isolates differences within areas in the length of a rod and cannot locate long or continued imperfections that readily. Another such technique, making use of eddy currents induced by a primary coil, also might be utilized to find imperfections and breaks. A steady current is induced in part of the test subject. Cracks that are found within the transmission of the current alter resistance of the test sample; this adaptation can be measured under suitable equipment.

Infrared

Infrared processes also have been used to find material continuity in complicated structural materials. While testing the strength of adhesive joints in the sandwich core and facing sheets in a ordinary sandwich structure object like plywood, for example, heat is used against the surface of the sandwich skin object. In the case where bond lines appear to be continuous, those core samples provide a heat depression on the surface sample, and the localised temperatures of the surface then appear spaciously along the bond lines. Where a bond line appears to be inadequate, disappears, or faulty, however, this temperature should not change. Infrared photography of the surface shall then reveal the location and dimensions of the broken adhesive. A variation of this process utilizes thermal coatings to change hue upon reaching a set temperature.

Conclusively, nondestructive techniques also are being shown to permit a complete study of the mechanical properties of a test object. Ultrasonics and thermal processes appear to be most valuable in this area.

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