What is Non-Destructive Testing?
Non-destructive testing is an effective technique that allows inspectors to collect data on a product without damaging it. It is used to examine defects and deterioration of products inside the object without disassembling or breaking them.
Non-destructive testing (NDT) which is synonymous with non-destructive inspection (NDI) is the testing of the object without causing damages. In other words, NDT is used for nondestructive testing, while NDI is used for pass/fail inspections.
Japanese Industrial Standards (JIS) Z 2300 defines Non-destructive testing as "Testing to determine the presence or absence of flaws, their location, size, shape, distribution state, etc., without destroying the material or product". And Non-destructive testing is defined as "a judgment based on the non-destructive testing result according to standards or criteria.
In some cases, however, non-destructive inspection is similarly used to NDT. As the methods of the non-destructive inspection in this section also includes NDT's, we recommend separating the use depending on your applications and purposes.
The most two NDT purposes are:
- Quality Assessment :
- Checks for problems with manufactured products and parts. For example, it is used to check shrinkage cavities in casting, welding defects, etc.
- Lifetime evaluation :
- Confirms safe usage in the product operation. It is available to check for abnormalities in structures and infrastructures that will be used for a long period.
Advantages of Non-destructive Testing
Non-destructive testing offers safe and effective ways of inspecting objects as follows.
- High accuracy, easy to find defects that cannot be seen from the surface.
- No damage to objects, available for all inspection.
- Increasing product reliability
- Identify timely repair or replacement
The non-destructive testing is particularly accurate effective because it can identify internal defects of the objects without destroying them. The method is the same as an X-ray examination that shows broken parts that are difficult to discern externally.
The NDT is available for the inspection of products before shipment since the method does not contaminate or damage them. This makes it helpful in ensuring better-inspected products by all inspection, increasing product reliability. In some cases of the NDT, however, you might require many preparation processes that are relatively expensive.
Methods of Common NDT Methods
There are several techniques used in non-destructive testing, and they have varying degrees depending on the defects or materials to be examined.
|Internal defect||RT : Radiographic Testing|
|UT : Ultrasonic Testing|
|Surface defect ET : Electromagnetic (Eddy Current) Testing||ET : Electromagnetic (Eddy Current) Testing|
|MT : Magnetic Particle Testing|
|Surface opening defect||PT : Penetrant Testing|
Radiographic Testing (RT)
Radiographic testing (RT) uses X-rays and gamma rays on objects. The RT detects defects utilizing thickness differences in images at various angles. Computed Tomography (CT) is one of the industrial NDT imaging methods that can provide cross-sectional and 3D images of the object during inspection. The feature allows detailed analysis of the internal defects or thickness. It is suitable for thickness measurement of steel plates and interior survey of buildings.
While NDT is beneficial, some considerations must be made before handling the system: Extreme caution to using radiation is required.
RT is used for the internal analysis of lithium-ion batteries and mounting electronic circuit boards. It is also available for detecting defects of pipes and welds placed in power plants, plants, and other buildings.
Ultrasonic Testing (UT)
Ultrasonic testing (UT) uses ultrasonic waves on objects. Using the ultrasonic wave property that reflects off the boundary surfaces of materials, UT inspects internal conditions of objects by measuring sound reflections. UT is commonly used in many industries as one of many Non-Destructive Testing without having to damage materials. It detects internal product defects and flaws inside homogeneous materials, such as rolled products and materials.
The UT system is safe and easy to use, but it has limitations where it is difficult to inspect irregularly shaped materials. It is used to detect internal defects in products and to inspect flaws inside homogeneous materials such as rolled products and materials.
Eddy Current (Electromagnetic) Testing (ET)
In Eddy Current (Electromagnetic) Testing (ET), a coil carrying an AC current is placed closed to the object surface. The current in the coil generates a circulating eddy current in the object close to the surface following the electromagnetic induction. Then, defects such as cracks on the surface are measured. ET is one of the most common NDT methods easily used in automated inspections because it requires no pre-processing or post-processing. It is suitable for thickness measurements and building surveys, and is often used in manufacturing facilities.
However, the ET is available in probes into a conductive material.
Magnetic Particle Testing (MT)
Magnetic Particle Testing (MT) is used to detect defects just beneath the surface of materials in an inspection solution containing magnetic powder. An electric current is applied to the object to inspect it by changing the magnetic powder pattern on the object's surface. When the current encounters defects there, it will create a flux leakage field where the defect is located.
It is used to detect shallow/fine cracks in a surface, and it is available for aircraft, automobile, and railroad parts.
Penetrant Testing (PT)
Penetrant Testing (PT) refers to the process using capillary phenomenon where a penetrant is used to coat an object and fill the inside of the defect.
After the process, the surface penetrant is removed.
The penetrant that has entered the defect cannot be washed out and remains, so by supplying a developer, the defect is absorbed and becomes visible. The PT is suitable only for surface defect inspection. It needs longer processes and greater time to complete, unsuitable for internal inspection.
It is used to inspect turbine blades of jet engines and automobile parts.
Hammering testing system has been conventionally handled by operators performing hammering and checking the internal condition by the sound. It utilizes the same principle that an intact teacup produces a clear sound when struck, but a cracked teacup produces a muddy sound. The testing method is also used for checking bolt looseness, inspection of railroad axles and exterior building walls.
Visual checking is one of the easiest and simplest methods of Non-destructive testing. the visual appearance of objects is checked by staff.
The non-destructive testing helps improve the safety and security of industrial plants by offering advantages in quality control of castings, forgings, rolled products, pipes, welding processes, etc. It also provides better living reliability by maintaining transportation infrastructure such as bridges, tunnels, railroad wheels and axles, aircraft, ships, vehicles, etc., and inspecting daily life infrastructure such as turbines, pipelines, and tanks in power plants.
Moreover, NDT techniques will become more significant for advances in non-industrial situations such as surveys of cultural assets, art objects, fruit sorting, and thermographic testing.
Qualifications for Non-Destructive Testing
In Japan, The Japanese Society for Non-Destructive Inspection (JSNDI), is an academic research organization promoting Non-Destructive testing. In cooperation with the Ministry of Economy, Trade and Industry (METI), Japanese Standards Association (JSA) related organizations, JSNDI conducts personnel qualification and certification activities in order to maintain a high level of NDT technical expertise. And JSNDI Standardization has been established and reviewed to standardize inspection technology and promote their broad application.
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