NDT test is a method to find the possibility of defects and the characteristics of a material that might cause aircraft to experience interference, reactor failure, train derailments, burst pipes, and various other events.
NDT is used to ensure the quality of materials and joining procedures during the development and erection phases, and in-service NDT checks are used to ensure that the goods in use continue to have the integrity necessary to ensure their effectiveness and public safety.
There are different types of NDT test carried out for various kinds of applications such as:
- Visual and optical testing
- Magnetic particle testing
- Ultrasonic testing
- Penetrant testing
- Electromagnetic testing
- Leak testing
- Acoustic Emission Testing
Visual and optical testing:
The most basic NDT method is visual inspection. Visual testing is done by only looking at the object that is tested using the eye to see whether the surface imperfections are visible. Aids can also be used in the form of cameras and computer equipment to help vision and reach places where money is difficult to see directly.
RT uses gamma-ray or X-ray penetration of the test object to look for defects or examine internal or hidden features. An X-ray generator or radioactive isotope is used as a source of radiation. Radiation is directed through the test object and on film or other detectors.
The resulting shadowgraph shows the internal features and the physical condition of the test object. The thickness of the test specimen and the change in density are seen as lighter or darker colours on the film or detector.
Magnetic Particle Testing (MT):
The NDT method is carried out by inducing a magnetic field in the test specimen that is ferromagnetic and then sprinkles iron particles on the test surface (particles dry or dissolved in liquid). Defects on the surface and near-surface (subsurface) will disrupt the flow of magnetic fields in the test object and cause some magnetic lines to leak out on the surface (Flux leakage).
Iron particles will be attracted and concentrated in the location of the magnetic flux leakage. This results in an indication of defects on the surface of the material. The image below shows the components before and after the inspection using dry magnetic particles.
Ultrasonic Testing (UT):
In ultrasonic testing, high-frequency sound waves are transmitted to the test object to detect imperfections or to search for properties changes in the test material. The most commonly used testing technique is pulse-echo, where the sound is fired into the test object and the reflection (echo) of internal imperfections or geometric surfaces returns to the receiver.
Penetrant Testing (PT):
With this test method, the test object is moistened with a liquid containing visible or fluorescent dyes. The liquid will seep into the gap or discontinuity which is indicated as a defect. After the liquid has seeped, the excess fluid on the surface of the test specimen is cleaned. The developer is then sprayed to pull out the penetrant from the defect. The contrasting color difference between the developer and the emerging penetrant fluid represents an indication of a defect.
Electromagnetic Testing (ET):
There are several electromagnetic testing methods but the focus here will be on eddy current testing. In eddy current testing, an electric current (eddy current) is generated in the test material due to changes in the magnetic field. The strength of this eddy current can be measured.
Defects in the test object cause interruptions in eddy current flow, this interruption is observed by the Inspector as an indication of a defect or other changes in the test object. Eddy current is also influenced by electrical conductivity and the permeability of the test specimen.
Several techniques are used to detect and find leaks in pressure vessels, pipes, and other structures. Leaks can be detected using electronic leak test devices, pressure gauge measurements, penetrant and gas techniques, or simple soap bubble tests.
Acoustic Emission Testing :
When a solid material experiences stress, imperfections in the material emit short acoustic energy called “emissions.” As in ultrasonic testing, acoustic emissions can be detected with a special receiver. The source of emissions can be evaluated through studies of the intensity of the emissions captured and the time of arrival to gather information (such as location) about the sources of these emissions.