If you’ve spent any amount of time in the construction and manufacturing industries you’ve probably heard of non-destructive testing (NDT) or nondestructive examination (NDE) as these are terms used in fabrication and in-service inspections.
What you may not be familiar with is that NDT more easily finds differences and discontinuities in material characteristics. But, did you know that there are several different types of NDT?
Around the world, NDT and NDE are critical within the construction and manufacturing industries. We’ll discuss five NDT types in this article which differ in the tools and evaluation method used.
#1. Visual Testing (VT)
Visual testing (VT) can be done with the naked eye or with tools like mirrors, flashlights, magnifying glasses, drones, borescopes, and other computer equipment for remote viewing. This is the most common method of testing performed because it is low-cost and easy to perform. External cracks are just one problem that can be found with visual testing.
This type of inspection is a simple yet powerful procedure used to check for corrosion, fractured surfaces, abnormal phenomena, and imperfections in welds, storage tanks, piping, boilers, and pressure vessels. Visual testing is typically the first method used in an NDT examination.
#2. Radiographic or Digital Testing (RT/DR)
To view the inside of a component, radiographic testing (RT) is an imaging test often used. Depending on size or component, radioactive material or an X-ray tube is used to irradiate a film. Imperfections such as pore or slag can be found easier with this type of test. There is no need for intermediate scanning or developing chemicals.
Digital radiography (DR) has become increasingly popular and is carried out separately after radiographic testing (RT) finds imperfections. These images can be evaluated and stored digitally and are often carried out by service providers. Flat-panel detectors provide high-quality digital images with a better signal-to-noise ratio, improving the dynamic range and providing high sensitivity for radiographic applications.
#3. Liquid Penetrant Testing (PT)
Liquid penetrant examination has been called many names over the years:
– Penetrant testing (PT)
– Dye penetrant testing (DP)
-Liquid penetrant testing (LP)
The American Society for Nondestructive Testing (ASNT) uses liquid penetrant testing (PT). The National Board Inspection Code (NBIC) uses the name liquid penetrant examination. Whatever name it goes by, liquid penetrant testing is a process that uses the capillary forces of pores and cracks. Fluorescent or color penetrant (contrast) is applied to a component that has been cleaned, usually with a spray can followed by a developer. This penetrant can creep into small cracks and reveal imperfections.
Welds are the most commonly inspected item, but bars, pipes, castings, plate, and forgings are commonly inspected with a penetrant examination. When dye penetrant testing is done, the colors white and red are most often used.
However, this method does have its limitations, as this can lead to so-called false readings because of color intensity on brittle or rough surfaces. The depth of the crack is not always appropriately measured when using dye.
#4. Ultrasonic Testing (UT)
Ultrasonic testing (UT) allows the inside of the component to be viewed for flaw detection and thickness measurements. This is done with a probe that is moved over the component’s surface, where ultrasonic waves are emitted, and their reflections are viewed on a screen. More straightforward interpretation is done by phased-array imaging.
Voluminous and flat imperfections can be inspected in this type of testing. Ultrasonic testing provides information on the uniformity and strength of many steels, concrete, rock, ceramics, refractory materials, to name a few. For surface imperfections, it is often a more desirable method than radiographic testing (RT) and is used to measure wall thickness with simple geometries with angled and vertical probes. Ultrasonic testing can be used to localize delaminations, cracks, honeycombs, and defects.
There are multiple types of UT inspection techniques, all of which are widely utilized and highly popular in the industry. One of the most popular is also one of the more straightforward and affordable, this is known as Ultrasonic Thickness Testing. This type of testing allows a material to be examined to determine the thickness remaining and its corrosion rate. This provides quantifiable data to be gathered and computed to aid in rendering a more empirical determination for the mechanical integrity of a structure or mechanism both now and in the future.
#5. Magnetic Particle Testing (MT)
A quick way of testing is to use magnetic particle testing (MT). This method for testing materials that are magnetizable uses a minimum amount of time where a powder or liquid is used to apply magnetic particles to the test subject. A change in the magnetic field reveals magnetic inconsistencies by causing the particle’s alignment to be different from the good part of the object.
Inclusions or cracks of non-magnetic materials can be found quickly with this method of testing. A small crack with a width of only 0.0001mm and a depth of 0.01mm can be detected. This is amazing when you consider the thickness of a human hair is 0.04mm or more.
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