Non-Destructive Testing is any of a wide group of analysis techniques used in science and technology to evaluate the properties of a material, component or system without causing damage. Here, we look at the different types of Non Destructive Testing and the advantages the different techniques offer.
Non-Destructive Testing (NDT), also known as Non-Destructive Inspection or Non-Destructive Examination, offers a number of distinct advantages. The most obvious of these is the fact that the items being inspected are left undamaged and usable. Because NDT does not permanently alter the article being inspected, it is a highly valuable technique that can save both money and time in product evaluation.
What are the types of NDT?
NDT falls into six main categories: visual, ultrasonic, magnetic particle, liquid penetrant, radiographic and eddy current inspections.
Visual and optical testing
The most basic NDT method is visual examination. Visual examiners follow procedures that range from simply looking at a part to see if surface imperfections are visible, to using computer controlled camera systems to automatically recognize and measure features of a component.
Remote visual inspection uses a borescope with a rigid or flexible probe to look into components, pipes or tanks.
Ultrasonic inspection
In ultrasonic testing, an inspector will use a probe or transducer to send sound waves through the material. If there are no defects in the material, the sound waves will pass through it, but if the sound waves hit a defect, they will bounce off of it, indicating their presence.
Ultrasonic thickness testing measures the time for a sound wave to travel through to the back wall of a component or pipe and return to the transducer. This time is used based on the sound velocity of the material to calculate thickness.
Magnetic particle testing
This NDT method is accomplished by inducing a magnetic field in a ferromagnetic material and then dusting the surface with iron particles (either dry or suspended in liquid). Surface and near-surface flaws disrupt the flow of the magnetic field within the part and force some of the field to leak out at the surface. Iron particles are attracted and concentrated at sites of the magnetic flux leakages producing a visible indication of defect on the surface of the material.
Penetrant testing
With this testing method, the test object is coated with a visible or fluorescent dye with exceptionally low surface tension. This is allowed to dwell on the surface to allow penetration into fine defects or cracks. Excess solution is removed from the surface of the object but remains in surface breaking defects. A developer is then applied to draw the penetrant out of the defects. With fluorescent dyes, ultraviolet light is used to make the bleed-out fluoresce brighter, allowing imperfections to be readily seen. With visible dyes, a vivid colour contrast between the penetrant and developer makes the bleed-out easy to see.
Radiography
This involves using penetrating gamma or X-radiation on materials and products to look for defects or examine internal or hidden features. An X-ray generator or radioactive isotope is used as the source of radiation. Radiation is directed through a part and onto film or detector. The resulting shadowgraph shows the internal features and soundness of the part. Material thickness and density changes are indicated as lighter or darker areas on the film or detector in a similar manner to medical X-rays.
Eddy current testing
In eddy current testing, electrical currents (eddy currents) are generated in a conductive material by a changing magnetic field. The strength of these eddy currents can be measured. Material defects cause interruptions in the flow of the eddy currents which alert the inspector to the presence of a defect or other change in the material. Eddy currents are also affected by the electrical conductivity and magnetic permeability of a material, which makes it possible to sort some materials based on these properties.
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Featured image: technicians testing the welds of pipe with ultrasonic test method. Image: Funtay/Sutterstock.com
