Types of Non-Destructive Testing

The tensile-strength test is within itself damaging; in the process of collecting data, the sample is destroyed. Although this is not an issue when a plentiful store of the sample exists, nondestructive techniques are better for materials that are dear or difficult to create or that have been constructed into finished or semifinished items.

Liquids

One tried and true nondestructive procedure, used to locate surface breaks and imperfections in metal samples, uses a penetrating fluid, either luminescently dyed or fluorescent. After being left on the surface of the sample material and allowed to soak into any surface flaws, the fluid is removed, leaving easily uncovered markings and imperfections. An analogous process, applicable to nonmetals, takes an electrically charged liquid rubbed on the material surface. After excess fluid is removed, a dry powder of opposite charge is sprayed on the surface of the sample and sinks into the breaks. Neither of these tests, however, can find internal weak points.

Radiation

Internal, like external weaknesses, can be detected with X-ray or gamma-ray tests in which the radiation scans the object and impinges on a suitable photographic film. On some occasions, it is possible to nominate the X rays onto a single section within the metal, bringing up a three-dimensional view of the flaw shape along with its position.

Sound

Ultrasonic inspection of areas requires transmission of sound waves above human hearing range through the test material. By the reflection technique, a sound wave is transmitted over one part of the piece, reflected off the far part, then returned onto a receiver situated at the original end. Upon locating a weakness or imperfection in the sample, the sound wave is reflected and its signal changed. The actual delay becomes a measure of the flaw’s location; a map of the material can then be made to reveal the location and dimensions of the flaws. By the through-transmission technique, the transmitter and receiver need to be located on opposite parts of the sample; delays in the signal of sound waves are used to find and measure flaws. More often than not a water medium is used through the use of which transmitter, sample, and receiver are immersed.

Magnetism

As the magnetic characteristics of a object are heavily reflected by its overall structure, magnetic techniques are employed to measure the placement and relative dimensions of weaknesses and marks. By magnetic testing, a tool is used that holds a big length of wire through which flows a steady alternating current (primary coil). Placed inside this initial piece is a shorter coil (the secondary coil), to which is linked an electrical measuring device. The steady current in the initial coil generates current to charge through the secondary coil by way of the method of induction. When an iron bar is slotted into the secondary coil, acute changes in the second current will isolate defects in the rod. This method only isolates changes within areas on the length of a rod and does not locate longer or continued defects that readily. A parallel method, employing eddy currents induced with a primary coil, also can be employed to locate flaws and breaks. A steady current is induced within the test sample. Cracks that exist within the signal of the current alter resistance of the test object; this determination should be measured by the correct methods.

Infrared

Infrared processes have sometimes been utilized to find material continuity in complicated structural situations. In testing the durability of adhesive conjoinments with the sandwich core and facing sheets within a standard sandwich construction object such as plywood, for example, heat is the surface of the sandwich skin piece. In the case that bond lines are continuous, the core areas provide a heat sink in the surface material, and the local temperatures of the surface then fall lightly on the bond lines. When the bond line may be not enough, missing, or mistaken, however, temperature will not fall. Infrared photography of the surface can then demonstrate the situation and dimensions of the failing adhesive. Another kind of technique uses thermal coatings that will change appearance on reaching a set temperature.

Conclusively, nondestructive test methods also are now being shown to permit a complete study of the mechanical properties of a test material. Ultrasonics and thermal processes appear the most reliable in this regard.

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