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The tensile-strength test is within itself destructive; in the process of collecting data, the sample is wasted. Though this is acceptable when a good sample of the sample is at hand, nondestructive procedures are preferred for materials that are dear or hard to create or that have been shaped into finished or semicompleted samples.

Liquids

One tried and true nondestructive process, employed to locate surface cracks and weaknesses in samples, requires a penetrating liquid, either visibly coloured or fluorescent. After being left on the surface of the metal and set to sink into any surface imperfections, the dye is removed, leaving readily visible imperfections and weaknesses. An analogous method, better for nonmetals, employs an electrically charged liquid smeared on the nonmetal surface. After the extra liquid is rubbed off, a dry powder of opposite charge is sprayed on the surface of the sample and attracted to the breaks. Neither of these tests, however, can locate internal imperfections.

Radiation

Internal, like external imperfections, can be located through the use of X-ray or gamma-ray technologies in which the radiation scans the material and impinges on an ideal photographic film. In some cases, it can be possible to focus the X rays onto a single part within the material, permitting a 3rd dimensional image of the flaw shape as well as its location.

Sound

Ultrasonic inspection of areas requires transmission of sound waves above human hearing range through the test material. Under the reflection process, a sound wave is transmitted from one part of the piece, reflected off the other end, then signalled into a receiver located at the first area. By locating a break or crack in the test sample, the signal is reflected and its transmission altered. The actual delay is a mark of the location of the flaw; a map of the test piece can then be created to show the point and shape of the flaws. By the through-transmission technique, the transmitter and receiver need to be started at the opposite parts of the sample; interruptions in the transmission of sound waves are used to find and measure cracks. Sometimes a water medium is utilized through the use of which transmitter, sample, and receiver will be immersed.

Magnetism

As the magnetic elements of a sample are strongly shown by its overall form, magnetic processes can be employed to reveal the placement and indicative geometry of voids and imperfections. By magnetic testing, a tool is utilized that consists of a big stretch of wire through which flows a steady alternating current (primary coil). Held inside the larger wire is a smaller coil (the secondary coil), to which is attached an electrical measuring device. The steady current in the primary coil makes further current to charge within the secondary coil by the process of induction. If an iron rod is put into the secondary coil, obvious changes in the secondary current should isolate flaws in the sample. This process only finds differences within parts on the length of a piece and does not detect longer or continued marks very much. A similar method, utilizing eddy currents induced in a primary coil, also should be utilized to locate imperfections and breaks. A steady current is induced within the test material. Flaws that are found in the transmission of the current alter resistance of the test sample; this determination may be measured with appropriate processes.

Infrared

Infrared methods have also been used to find material continuity in complicated construction situations. In testing the durability of adhesive joins with the sandwich core and facing sheets with a typical sandwich structure material like plywood, for example, heat is the surface of the sandwich skin object. In the case that bond lines are found to be continuous, the core materials reveal a heat marking within the surface sample, and the local temperatures of the surface will appear evenly along the bond lines. When the bond line is too small, disappears, or in error, however, temperature does not drop. Infrared photography of the area can then indicate the situation and dimensions of the marked adhesive. Another kind of method employs thermal coatings that can change appearance at reaching a determined temperature.

Finally, nondestructive procedures also are shown to reveal a total determination of the mechanical aspects of a test piece. Ultrasonics and thermal processes are most valuable in this situation.

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