The tensile-strength test is basically damaging; at the time of the process of collating data, the sample is obliterated. Although this is not an issue when a good supply of the sample material is at hand, nondestructive tests are safer for materials that are dear or complex to make up or that have been constructed into completed or semifinished items.

Liquids

One commonly used nondestructive procedure, utilized to target surface marks and flaws in metal samples, takes a penetrating liquid, which is either luminescently coloured or fluorescent. After being rubbed on the surface of the metal and allowed to soak into any surface markings, the dye is removed, leaving brightly uncovered cracks and imperfections. Another such process, better for nonmetals, requires an electrically charged fluid painted on the sample surface. After excess liquid is cleaned off, a dry powder of opposite charge is sprayed onto the nonmetal and draws to the flaws. Neither of these techniques, however, can locate internal weaknesses.

Radiation

Internal, like external imperfections, can be located under X-ray or gamma-ray techniques in which the radiation scans the object and impresses on a subject photographic film. On some occasions, it can be possible to nominate the X rays on a single section within the piece, permitting a 3-dimensional description of the flaw geometry as well as its location.

Sound

Ultrasonic inspection of areas takes transmission of sound waves out of human hearing range through the test sample. By the reflection method, a sound wave is sent from one side of the subject, reflected from the opposite area, then signalled to a receiver located at the beginning point. By impinging on a flaw or weak point in the test sample, the signal is reflected and its signal altered. The actual delay is a sign of the location of the mark; a map of the test piece can be formed to show the point and shape of the cracks. In the through-transmission technique, the transmitter and receiver are situated on the opposite sides of the material; delays in the passage of sound waves are studied to find and measure cracks. More often than not a water medium is employed by which transmitter, sample, and receiver should be immersed.

Magnetism

As the magnetic aspects of a sample are largely reflected by its overall shape, magnetic techniques can be employed to characterize the location and relative shape of flaws and imperfections. By magnetic testing, an apparatus is utilized that contains a sizeable measure of wire through which flows a steady alternating current (primary coil). Placed in this initial piece is a shorter coil (the secondary coil), to which is attached an electrical measuring device. The steady current in the larger coil causes current to move through the secondary coil by the process of induction. If an iron bar is slotted into the secondary coil, sudden changes in the second current will isolate flaws in the piece. This method only detects differentiations in parts in the length of a bar and cannot find long or continued defects very much. A similar skill, making use of eddy currents induced with a primary coil, also might be used to isolate marks and cracks. A steady current is induced within the test object. Weaknesses that are found within the track of the current make for resistance of the test sample; this alteration will then be measured by appropriate equipment.

Infrared

Infrared methods have also been utilized to locate material continuity in involved structural objects. By testing the durability of adhesive joints in the sandwich core and facing sheets of a usual sandwich structure material such as plywood, for example, heat is used against the face of the sandwich skin item. Where bond lines are found to be continuous, those core samples reveal a heat depression in the surface sample, and the general temperatures of the face should spread spaciously on these bond lines. In the case where a bond line is too small, gone, or faulty, however, the local temperature will not change. Infrared photography of the face can then reveal the placement and area of the flawed adhesive. Another such technique uses thermal coatings that change colour at reaching a specific temperature.

Lastly, nondestructive test procedures also are being found to permit a total study of the mechanical elements of a test object. Ultrasonics and thermal processes are the most reliable in this instance.

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