Types of Non-Destructive Testing

The tensile-strength test is within itself fruitless; during the process of collating research, the sample is wasted. Although this is not an issue when a safe store of the material exists, nondestructive techniques are safer for materials that are expensive or arduous to create or that have been constructed into completed or semicompleted items.

Liquids

One tried and true nondestructive process, used to locate surface cracks and imperfections in samples, takes a penetrating liquid, which is either brightly dyed or fluorescent. After being smeared on the surface of the sample and left to sink into any perceptible cracks, the dye is removed, leaving brightly uncovered markings and weaknesses. Similarly, another technique, used for nonmetals, employs an electrically charged fluid smeared on the sample surface. After the extra fluid is cleared off, a dry powder of opposite charge is sprayed on the surface of the nonmetal and draws to the cracks. Neither of these processes, however, can identify internal breaks.

Radiation

Internal, as well as external flaws, can be identified with X-ray or gamma-ray technologies in which the radiation scans the material and impresses on an ideal photographic film. In some cases, it can be possible to focus the X rays to a significant section within the metal, bringing up a 3-dimensional description of the flaw geometry as well as its site.

Sound

Ultrasonic inspection of parts requires transmission of sound waves above human hearing range within the test sample. By the reflection technique, a sound wave is transmitted from one end of the material, reflected with the opposite area, then signalled onto a receiver that is situated at the first point. Upon impinging on a mark or weak point in the piece, the signal is reflected and its traveling time adapted. The actual delay is a signal of the location of the flaw; a map of the test piece can be generated to illustrate the area and geometry of the weaknesses. By the through-transmission method, the transmitter and receiver are started on the opposite parts of the subject; delays in the signal of sound waves are used to isolate and measure weaknesses. Sometimes a water medium is utilized by which transmitter, sample, and receiver are immersed.

Magnetism

As the magnetic characteristics of a object are strongly influenced by its overall structure, magnetic methods are sometimes utilized to isolate the area and relative geometry of flaws and breaks. By magnetic testing, an object is used that consists of a large coil of wire through which flows a steady alternating current (primary coil). Located within this first wire is a smaller coil (the secondary coil), to which is linked an electrical measuring tool. The steady current in the larger coil forces electrical current to flow within the secondary coil by way of the technique of induction. If an iron rod is placed in the secondary coil, sudden changes in the secondary current can indicate flaws in the bar. This method only isolates differentiations between sections within the length of a sample and will not locate long or continuous defects very easily. A parallel process, utilizing eddy currents induced in a primary coil, also may be used to detect flaws and weaknesses. A steady current is induced in part of the test object. Cracks that are found in the transmission of the current determine resistance of the test object; this change should be measured by suitable tools.

Infrared

Infrared techniques have sometimes been utilized to isolate material continuity in involved constructual objects. While testing the strength of adhesive bonds in the sandwich core and facing sheets with a standard sandwich structure sample like plywood, for example, heat is the face of the sandwich skin item. In the case that bond lines are found to be continuous, the core parts show a heat signature on the surface piece, and the general temperatures of the surface then fall steadily on the bond lines. When that bond line may be too small, missing, or in error, however, temperature does not adapt. Infrared photography of the face can then demonstrate the location and area of the erroneous adhesive. A similar method uses thermal coatings that change hue at reaching a determined heat.

Finally, nondestructive test methods also are sometimes sought to permit a total determination of the mechanical properties of a test sample. Ultrasonics and thermal procedures seem most promising in this situation.

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