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The tensile-strength test is inherently futile; in the process of collecting research, the sample is destroyed. Although this is acceptable when a decent supply of the sample material is available, nondestructive procedures are safer for materials that are expensive or hard to make up or that have been constructed into completed or semicompleted items.

Liquids

One commonly used nondestructive procedure, used to locate surface breaks and weaknesses in metal samples, requires a penetrating liquid, which needs to be brightly coloured or fluorescent. After being pasted on the surface of the material and left to soak into any tiny breaks, the fluid is removed, leaving easily visible markings and weaknesses. Similarly, another method, applicable to nonmetals, requires an electrically charged fluid smeared on the nonmetal surface. After excess liquid is rubbed off, a dry powder of opposite charge is sprayed onto the nonmetal and sinks into the breaks. Neither of these methods, however, can detect internal weaknesses.

Radiation

Internal, like external imperfections, can be detected with X-ray or gamma-ray tests in which the radiation scans the sample and impinges on an ideal photographic film. In some cases, it is possible to nominate the X rays on a significant part in the piece, creating a 3rd dimensional view of the flaw identity as well as its position.

Sound

Ultrasonic inspection of sections requires transmission of sound waves above human hearing range within the sample. Under the reflection technique, a sound wave is sent from one side of the piece, reflected off the other part, then returned back to a receiver located at the original end. Upon isolating a weakness or failure in the piece, the sound wave is reflected and its transmission disrupted. The actual delay is a signal of the location of the imperfection; a map of the subject can be generated to illustrate the point and dimensions of the flaws. In the through-transmission process, the transmitter and receiver need to be located at opposite parts of the subject; delays in the transmission of the sound waves are utilized to locate and measure marks. Often a water medium is employed by which transmitter, sample, and receiver should be immersed.

Magnetism

As the magnetic characteristics of a object are largely reflected by its overall structure, magnetic processes can be utilized to measure the area and general size of weaknesses and imperfections. For magnetic testing, a tool is utilized that contains a large coil of wire through which flows a steady alternating current (primary coil). Nested in the first wire is a smaller coil (the secondary coil), to which is secured an electrical measuring tool. The steady current in the larger coil causes the current to charge within the secondary coil by the method of induction. When an iron rod is slotted within the secondary coil, acute changes in the secondary current can implicate defects in the piece. This method only detects differences within areas along the length of a rod and does not find long or continuous defects that often. A parallel technique, using eddy currents induced with a primary coil, also should be employed to isolate marks and marks. A steady current is induced in part of the test material. Marks that lie across the track of the current determine resistance of the test piece; this change will then be measured with better methods.

Infrared

Infrared processes have also been used to detect material continuity in intricate structural objects. In testing the value of adhesive joints between the sandwich core and facing sheets in a ordinary sandwich construction material such as plywood, for example, heat is applied in the surface of the sandwich skin item. Where bond lines are continuous, the core materials provide a heat signature within the surface piece, and the localised temperatures of the surface should spread steadily along those bond lines. Where that bond line appears to be insignificant, disappears, or in error, however, temperature will not drop. Infrared photography of the face shall then show the geography and geometry of the failing adhesive. A similar technique uses thermal coatings to change hue on reaching a specific temperature.

Finally, nondestructive techniques also are now being seen to allow a total study of the mechanical properties of a test piece. Ultrasonics and thermal techniques appear to be most reliable in this regard.

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