Types of Non-Destructive Testing

The tensile-strength test is basically futile; at the time of the process of fostering information, the sample is ruined. Although this is permissible when a safe supply of the material exists, nondestructive procedures are desirable for materials that are expensive or complex to create or that have been formed into completed or semifinished samples.

Liquids

One tried and true nondestructive method, utilized to locate surface marks and weaknesses in samples, employs a penetrating fluid, either luminescently dyed or fluorescent. After being left on the surface of the material and left to sink into any perceptible markings, the dye is wiped off, leaving totally visible cracks and imperfections. A similar process, applicable to nonmetals, employs an electrically charged fluid rubbed on the sample surface. After the extra liquid is removed, a dry powder of opposite charge is sprayed on the sample and draws to the breaks. Neither of these techniques, however, can locate internal breaks.

Radiation

Internal, as well as external weaknesses, can be found with X-ray or gamma-ray technologies in which the radiation passes through the object and impresses on a subject photographic film. In some cases, it can be possible to focus the X rays on a single area in the object, creating a three-dimensional view of the flaw shape along with its location.

Sound

Ultrasonic inspection of areas involves transmission of sound waves out of human hearing range through the test material. By the reflection method, a sound wave is targeted over one end of the piece, reflected off the other area, then signalled into a receiver situated at the starting point. When locating a flaw or imperfection in the sample, the sound wave is reflected and its traveling time changed. The actual delay is then a mark of the location of the crack; a map of the sample can be generated to show the location and form of the marks. In the through-transmission method, the transmitter and receiver need to be situated at opposite ends of the material; interruptions in the movement of the sound waves are used to isolate and measure flaws. Often a water medium is utilized in which transmitter, sample, and receiver will be immersed.

Magnetism

As the magnetic elements of a sample are strongly formed by its overall structure, magnetic processes can be used to characterize the placement and approximate geometry of voids and marks. For magnetic testing, an object is employed that consists of a sizeable stretch of wire through which flows a steady alternating current (primary coil). Placed in the initial piece is a shorter coil (the secondary coil), to which is linked an electrical measuring tool. The steady current in the larger coil forces further current to react in the secondary coil through the method of induction. If an iron sample is slotted in the secondary coil, sudden changes in the secondary current should indicate flaws in the piece. This process only detects differences between areas within the length of a bar and does not locate longer or continuous imperfections very often. Another such method, employing eddy currents induced with a primary coil, also should be employed to isolate errors and marks. A steady current is induced in the test item. Cracks that exist across the signal of the current alter resistance of the test piece; this change should be measured under better processes.

Infrared

Infrared techniques have sometimes been used to detect material continuity in complex structural situations. In testing the strength of adhesive joins with the sandwich core and facing sheets with a standard sandwich structure item such as plywood, for example, heat is applied in the surface of the sandwich skin material. Where bond lines appear to be continuous, those core areas allow a heat sink within the surface piece, and the general temperatures of the surface then spread spaciously on the bond lines. In the case where that bond line can be too small, missing, or erroneous, however, this temperature should not drop. Infrared photography of the front will then demonstrate the geography and area of the failing adhesive. Another such technique utilizes thermal coatings to change hue at reaching a specific heat.

In conclusion, nondestructive procedures also are seen to allow a complete knowledge of the mechanical aspects of a test object. Ultrasonics and thermal methods are most promising in this area.

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