Non destructive testing (NDT) refers to the process of performing inspections, evaluations and tests on materials, assemblies or parts with the objective of detecting discontinuities or variations in characteristics. The process is conducted without destroying the materials, assemblies or parts. Essentially, they can still be put to use upon completion of the test.
Unlike non destructive testing, other tests destroy materials, assemblies and parts. That is why the tests are only performed on a few samples instead of the materials, assemblies or parts that would actually be used. Destructive tests are usually used to establish impact resistance, fatigue strength, ductility, fracture toughness, yield strength, tensile strength and other physical characteristics of materials. However, NDT is more effective in establishing the discontinuities and variations in material properties.
Today, modern NDT inspections are carried out in manufacturing, in-service examinations and fabrication to guarantee reliability and integrity of products, maintain standardised quality level, control manufacturing processes and minimise production costs.
Non destructive tests are often performed during construction works to guarantee quality of joining processes as well as quality of materials at the erection stage and fabrication phase too. Additionally, in-service inspections are conducted to make sure that the commodities used continue having the integrity required to ensure that they are safe for use by the general public and are also useful.
It is important to note that even though the field of medicine makes use of similar processes, NDT is a term that is not used in the description of medical processes.
NDT test methods generally refer to equipment used for carrying out the tests or mediums that the equipment penetrates.
Current methods used for non destructing testing include: Acoustic Emission Testing, Visual Testing, Electromagnetic Testing, Vibration Analysis, Guided Wave Testing, Ultrasonic Testing, Ground Penetrating Radar and Infra-red Testing/ Thermal Testing.
Other NDT methods are: Laser Testing, Radiographic Testing, Leak Testing, Neutron Radiographic Testing, Magnetic Flux Leakage, Magnetic Particle Testing, Microwave Testing and Liquid Penetrate Testing as well.
Among the above-mentioned methods, there are six techniques that are commonly used not just in the United Kingdom, but also in North America, Asia and Africa. The six methods are described here-under:
This technique makes use of multiple magnetic fields to detect discontinuities and dissimilarities on the near- surfaces as well as surfaces of ferromagnetic materials. Electromagnets or permanent magnets are used to create magnetic fields. When using the former, the field only exists when electric current flows through. When the field comes across a discontinuity that crosses the specific magnetic field direction, magnetic flux seepage field is produced by the flux lines.
Given that magnetic flux lines cannot traverse well through air, coloured ferromagnetic particles will be attracted into the discontinuity upon their application to the surface. Consequently, the air gap will get reduced, thus producing a noticeable indication. The ferromagnetic particles can be suspended in a suitable liquid solution or may comprise of dry powder. The particles can be colored with a fluorescent dye to make them more visible.
There are five main Magnetic Particle Testing techniques. They involve use of:
ii. Central conductors
Visual Testing (VT) is the most common technique used in industries. Given that most testing methods force operators to examine the surfaces of inspected parts and systems, VT is intrinsic in nearly all the methods. Visual Testing, as the words suggest, is a technique that involves visual surveillance of surfaces of materials, assemblies or parts to determine the existence or non-existence of discontinuities or variations on surfaces.
Visual Testing may involve direct viewing or viewing using line of sight. It can also be improved using magnifying glasses, computer- assisted systems, mirrors, charge-coupled-devices and boroscopes, among other optical instruments.
Some of the discrepancies and discontinuities that can be discovered by VT include: corrosion, cracks, misalignment and physical damage, among others.
When a penetrate (liquid with very high fluidity/ extremely low viscosity) is applied on a surface, the fluid finds its way into voids and cracks that exist on the surface. Penetrate ensnared in the voids and cracks flows back out when the surplus is removed. Consequently, an indication is created. This is the fundamental principle of LT.
The test can be carried out not only on magnetic materials, but also on non-magnetic materials. However, the method is unsuitable for use on porous/ absorbent materials. The penetrate may either be fluorescent or visible. The former requires use of reliable source of black light, whilst the latter requires use of ambient light.
When conducting a PT inspection, the surface must be clean and devoid of liquids and other foreign substances that might obstruct entry of the liquid into voids and cracks.
This is a broad test category that comprises of Eddy Current Testing, Remote Field Testing and Alternating-Current Field-Measurement. Even though Magnetic Particle-Testing falls under ET, it is generally considered an independent testing method because of its widespread use.
The four techniques mentioned above make use of electric current induction or magnetic field induction. The resultant outcomes are then recorded and assessed.
In Eddy Current Testing, a weak current is generated around the MFF (magnetic- flux field) when an AC coil creates an electromagnetic field (EF) into a test piece.
Remote Filed Testing is a technique that is usually applied in the examination of ferromagnetic tubing because of the powerful skin-effect that is usually present in such tubes. The results produced by RFT are more accurate than those given by regular Eddy Current methods due to equal sensitivity at the tube’s OD surface and ID surface.
Alternating-Current Field-Measurement makes use of a dedicated probe that brings in AC into the test piece. This results in the formation of a magnetic field.
This technique involves the exposure of a test material to penetrating radiation to enable passage of the radiation rays through the object under examination, and a recorder placed on the opposite side.
For aluminium and other less dense objects, electrically generated X-rays are usually used. Gamma radiation is normally used for dense materials.
UT is another method that is commonly used to perform NDT inspections. To complete the process, a technician can make use of a Straight Beam, Time-Of Flight Diffraction, Angle Beam, Phased Array, Immersion Testing or Through Transmission.
Industries that deal with the production, transportation, storage and sale of oil and oil products, petrochemicals and gas heavily rely on non destructive testing. NDT methods also play pivotal roles in chemical, defence, aerospace, manufacturing and automotive industries. The main objective of carrying out NDT is to detect any faults in components, so as to enhance reliability and minimise failure rates.
In industries that deal with petrochemicals, the inspections are usually conducted all through the life cycle. The strategy plays a very important role in the management of asset integrity. Additionally, NDT inspections give historical data regarding process units of the facility, as well as information about frequency of inspection, replacement or repair of vital components.
The life cycle of equipment determines the type and frequency of inspections. It may seem costly to carry out multiple tests all through the life cycle of equipment, but the benefits far outweigh the costs. You can save millions of pounds if inspections are performed at specific intervals. This is because assessments may uncover threats, thus allowing you to conduct repairs before possible occurrence of catastrophic failures or complete shutdown of the facility.
Equipment that are usually subjected to NDT inspections include (but are not limited to the following):
a) Storage tanks
b) Pipes and piping systems
c) Heat exchange systems
d) Pressure vessels
There are four main factors that must be considered when planning for non destructive test inspections:
1) Type of damage method to be examined for.
2) Inadequacies and sensitivities of the technique.
3) Minimum measurable fault size, orientation and shape of flaw.
4) Location of defect.
If you take the above-mentioned factors into consideration, it will be very easy to achieve optimal production, improve environmental safety and enhance the safety of people who work in the facility as well.
So what is non-destructive testing? As mentioned at the beginning of this article, non destructive testing refers to the process of performing inspections, evaluations and tests on materials, assemblies or parts with the objective of detecting discontinuities or variations in characteristics.
Other terms that are commonly used in place of non-destructive testing are NDE (non destructive examination), NDI (non destructive inspection) and NDE (non destructive evaluation).
Apart from NDT, there are many other testing services that we offer. Please get in touch with us if you need Radiographic Inspection, Non-Destructive Hardness-Testing, Eddy Current Testing, Visual Inspection, Liquid Penetrate Inspection, Ultrasonic Testing or Magnetic Particle Inspection services.
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Format NDT provide NDT services and are based in Merseyside.
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