Types of Non-Destructive Testing

April 14th, 2010

The tensile-strength test is basically fruitless; in the process of collecting material, the sample is ruined. Although this is acceptable when a good supply of the sample is at hand, nondestructive tests are safer for materials that are expensive or hard to make up or that have been shaped into finished or semicompleted samples.

Liquids

One common nondestructive procedure, employed to locate surface cracks and weaknesses in metals, requires a penetrating liquid, which needs to be visibly coloured or fluorescent. After being left on the surface of the metal and set to soak into any surface imperfections, the dye is cleared, leaving readily visible breaks and weaknesses. A similar technique, used for nonmetals, takes an electrically charged liquid painted on the material surface. After superfluous liquid is removed, a dry powder of opposite charge is sprayed on the surface of the sample and draws to the cracks. Neither of these tests, however, can find internal weak points.

Radiation

Internal, like external imperfections, can be identified under X-ray or gamma-ray technologies in which the radiation passes through the sample and implicates on a subject photographic film. Occasionally, it can be possible to focus the X rays on a single section in the object, permitting a 3-dimensional description of the flaw identity as well as its location.

Sound

Ultrasonic inspection of sections takes transmission of sound waves out of human hearing range within the sample. Under the reflection process, a sound wave is sent over one end of the sample, reflected with the far end, and signalled into a receiver situated at the beginning end. Upon locating a break or failure in the test sample, the sound wave is reflected and its transmission altered. The actual delay is a sign of the location of the mark; a map of the material can be generated to reveal the area and shape of the flaws. With the through-transmission process, the transmitter and receiver are located at the opposite sides of the material; interruptions in the signal of the sound waves are utilized to find and measure cracks. More often than not a water medium is used in which transmitter, sample, and receiver will be immersed.

Magnetism

As the magnetic aspects of a object are very much formed by its overall structure, magnetic techniques are employed to reveal the area and relative geometry of weaknesses and breaks. For magnetic testing, an item is employed that contains a large length of wire through which flows a steady alternating current (primary coil). Held in the first piece is a smaller coil (the secondary coil), to which is secured an electrical measuring device. The steady current in the first coil forces electrical current to move within the secondary coil through the technique of induction. When an iron bar is put into the secondary coil, sudden changes in the further current can isolate flaws in the rod. This technique only isolates differences within areas along the length of a piece and does not isolate longer or continuous imperfections very often. An analogous method, using eddy currents induced by a primary coil, also may be used to locate flaws and weaknesses. A steady current is induced in the test object. Cracks that are found in the signal of the current make for resistance of the test material; this alteration can be measured with the correct tools.

Infrared

Infrared methods also have been used to detect material continuity in intricate constructual materials. While testing the strength of adhesive conjoinments with the sandwich core and facing sheets within a standard sandwich structure item such as plywood, for example, heat is used against the surface of the sandwich skin piece. In the case that bond lines are found to be continuous, those core areas reveal a heat signature in the surface piece, and the general temperatures of the face should appear evenly on those bond lines. In the case that a bond line can be inadequate, disappears, or in error, however, localised temperature will not adapt. Infrared photography of the surface can then isolate the situation and geometry of the defective adhesive. A variation of this method employs thermal coatings that will change colour when reaching a set temperature.

Finally, nondestructive test techniques also are sometimes seen to permit a whole knowledge of the mechanical characteristics of a test piece. Ultrasonics and thermal procedures appear the most valuable in this circumstance.

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