The tensile-strength test is basically destructive; at the time of the process of collecting information, the sample is wasted. Although this is acceptable when a large store of the sample material exists, nondestructive methods are desirable for materials that are costly or hard to make up or that have been shaped into finished or semifinished samples.

Liquids

One tried and true nondestructive process, utilized to target surface markings and flaws in metals, uses a penetrating liquid, which needs to be visibly coloured or fluorescent. After being pasted on the surface of the sample material and set to impress into any surface markings, the dye is rubbed away, leaving readily perceptible imperfections and weaknesses. An analogous process, used for nonmetals, uses an electrically charged fluid rubbed on the material surface. After excess fluid is removed, a dry powder of opposite charge is sprayed on the surface of the sample and attracted to the cracks. Neither of these processes, however, can identify internal flaws.

Radiation

Internal, like external flaws, can be detected by X-ray or gamma-ray tests in which the radiation passes through the material and impinges on a subject photographic film. In some cases, it may be possible to nominate the X rays on a significant plane within the material, permitting a 3-dimensional perspective of the flaw identity along with its site.

Sound

Ultrasonic inspection of sections involves transmission of sound waves above human hearing range through the sample. Under the reflection process, a sound wave is sent from one side of the sample, reflected by the far end, and signalled onto a receiver located at the original point. By finding a mark or failure in the test sample, the sound wave is reflected and its transmission changed. The actual delay becomes a measure of the location of the crack; a map of the piece can be generated to show the location and geometry of the flaws. By the through-transmission process, the transmitter and receiver are located at the opposite sides of the sample; interruptions in the signal of the sound waves are studied to locate and measure weaknesses. Often a water medium is used by which transmitter, sample, and receiver should be immersed.

Magnetism

As the magnetic characteristics of a sample are largely influenced by its overall form, magnetic methods are sometimes employed to demonstrate the situation and relative size of weaknesses and marks. In magnetic testing, an apparatus is used that consists of a big coil of wire through which flows a steady alternating current (primary coil). Held in the first wire is a smaller coil (the secondary coil), to which is connected an electrical measuring device. The steady current in the larger coil generates the current to move through the secondary coil by the process of induction. When an iron piece is put into the secondary coil, obvious changes in the second current can implicate defects in the bar. This process only isolates changes in sections on the length of a rod and will not locate elongated or continued flaws that readily. A parallel skill, utilizing eddy currents induced in a primary coil, also might be utilized to locate marks and cracks. A steady current is induced in part of the test material. Cracks that are found across the path of the current alter resistance of the test material; this alteration should be measured under suitable items.

Infrared

Infrared techniques have also been utilized to locate material continuity in complicated structural situations. By testing the durability of adhesive joins with the sandwich core and facing sheets with a ordinary sandwich construct material such as plywood, for example, heat is used against the surface of the sandwich skin material. In the case where bond lines are continuous, the core parts provide a heat signature in the surface object, and the localised temperatures of the skin will appear lightly along these bond lines. In the case where a bond line may be too small, gone, or erroneous, however, the local temperature does not drop. Infrared photography of the surface can then reveal the geography and dimensions of the marked adhesive. Another such method utilizes thermal coatings that can change appearance upon reaching a devised temperature.

Finally, nondestructive test methods also are now being seen to reveal a whole knowledge of the mechanical aspects of a test object. Ultrasonics and thermal techniques seem to be most reliable in this instance.

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