Types of Non-Destructive Testing

The tensile-strength test is within itself damaging; in the process of gathering information, the sample is destroyed. Though this is not a problem when a decent supply of the material exists, nondestructive techniques are safer for materials that are costly or hard to create or that have been formed into finished or semifinished samples.

Liquids

One commonly used nondestructive test, used to detect surface cracks and imperfections in metal samples, employs a penetrating liquid, either visibly dyed or fluorescent. After being left on the surface of the metal and left to fill into any perceptible cracks, the dye is rubbed away, leaving totally revealed breaks and imperfections. Another such test, used for nonmetals, requires an electrically charged fluid pasted on the nonmetal surface. After the extra liquid is rubbed off, a dry powder of opposite charge is sprayed onto the sample and attracted to the breaks. Neither of these methods, however, can detect internal flaws.

Radiation

Internal, as well as external weaknesses, can be detected through the use of X-ray or gamma-ray machines in which the radiation passes through the metal and impinges on a suitable photographic film. Occasionally, it can be possible to target the X rays toward a particular plane within the metal, allowing a 3-dimensional description of the flaw identity along with its site.

Sound

Ultrasonic inspection of areas involves transmission of sound waves above human hearing range through the sample. Under the reflection process, a sound wave is transmitted over one area of the subject, reflected off the far side, and returned back to a receiver situated at the original side. When finding a weakness or crack in the test sample, the signal is reflected and its signal disrupted. The actual delay becomes a signal of the flaw’s location; a map of the test material can be generated to isolate the location and form of the cracks. In the through-transmission process, the transmitter and receiver need to be started on the opposite parts of the sample; interruptions in the signal of the sound waves are studied to isolate and measure cracks. Often a water medium is used by which transmitter, sample, and receiver are immersed.

Magnetism

As the magnetic characteristics of a object are largely shown by its overall structure, magnetic processes are sometimes utilized to reveal the location and indicative geometry of voids and marks. For magnetic testing, an item is utilized that contains a sizeable length of wire through which flows a steady alternating current (primary coil). Held in the initial object is a smaller coil (the secondary coil), to which is attached an electrical measuring device. The steady current in the first coil forces the current to charge within the secondary coil by the method of induction. If an iron bar is slotted into the secondary coil, sharp changes in the second current will isolate marks in the rod. This process only finds differentiations in zones on the length of a piece and cannot locate longer or continuous flaws very readily. Another such skill, making use of eddy currents induced by a primary coil, also might be utilized to detect imperfections and breaks. A steady current is induced in the test item. Marks that lie across the signal of the current change resistance of the test item; this determination may be measured with better processes.

Infrared

Infrared techniques have also been utilized to locate material continuity in complex construction materials. While testing the durability of adhesive joins in the sandwich core and facing sheets within a typical sandwich structure object like plywood, for example, heat is the face of the sandwich skin item. Where bond lines are found to be continuous, the core materials provide a heat marking in the surface object, and the local temperatures of the surface will drop spaciously along these bond lines. In the case where that bond line appears to be insignificant, gone, or mistaken, however, the local temperature can not fall. Infrared photography of the face can then show the placement and dimensions of the marked adhesive. Another such method utilizes thermal coatings that change appearance upon reaching a set temperature.

Finally, nondestructive methods also are seen to show a whole determination of the mechanical elements of a test piece. Ultrasonics and thermal techniques seem to be the most reliable in this area.

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