What is Non Destructive Testing?

What is Non Destructive Testing?

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.

Non Destructive Test methods

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:

1. Magnetic Particle Testing

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:

i. Yokes
ii. Central conductors
iii. Prods
iv. Heads
Vi. Coils

2. Visual Testing

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.

3. Liquid Penetrate Testing

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.

4. Electromagnetic Testing

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.

5. Radiographic Testing

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.

6. Ultrasonic Testing

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.

Industry applications

What is non destructive testing, and what are its industrial applications?

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.

We have been in business for a very long time, and in the course of our journey, we have learned the benefits of providing high quality and customised services. We know what you and other prospective clients need, so there will be no disappointments whatsoever.

Format NDT provide NDT services and are based in Merseyside.

Get in touch with us today if you require Non Desctructive Testing services.

A Guide To Liquid Penetrant Testing

A Guide To Liquid Penetrant Testing

Liquid Penetrant Inspection or dye penetrant inspection is a method of NDT that was developed in the 1940s. Although times have changed today and newer and more innovative methods have been introduced to make it more efficient and accurate, the basic principles remain the same.

Because Liquid Penetrant Inspection is non-destructive, meaning it does not harm the inspected samples, it has proven highly effective in detecting flaws such as porosity, cracks, seams, fractures and laps. These flaws can be caused by fatigue, impact, overload, shrinkage, machining, quenching, grinding, forging or bursting. This makes Liquid Penetrant Inspection have many applications, such as being used for machined parts, weldments, castings, forgings, among other manufactured products that are put into service.

What Is NDT?

NDT is the acronym for Non Destructive Testing, which is a branch of engineering that is associated with methods of testing for, detecting and evaluating material flaws. NDT is important because these flaws are the primary reason why the material or the structure fails or becomes less serviceable. They can be cracks, inclusions or structural property variations. Through NDT, safety, quality control and longer plant life can be guaranteed.

It is important to note that Non-Destructive Testing is used for in-service inspection, component measurement and physical property measurements. NDT works well because it has no impactful effects on the structure or material being tested. It can be carried out in various ways, including visual inspection, radiography, ultrasonic tests, positron annihilation and magnetic particle crack detection. Furthermore, it can be automated or used in localised problematic areas.NDT is simple and cost-effective, but it can only detect surface breaking flaws like cracks, laps and porosity. To be able to detect it, the flaw must be relatively close to the surface or reach the surface itself.

What is Liquid Penetrant Testing?

Liquid Penetrant Testing is a method of NDT that improves upon visual inspection. It is faster, covers a larger area, and is relatively cheap.

When carrying out Liquid Penetrant Testing, the process occurs in 3 stages:

1. Surface cleaning

This is the stage where you clean the surface that needs to be tested in preparation for the test. You want to remove all the oil, dirt, grease, liquid, paint, scale or any other contaminant that may stand in the way preventing the penetrant from seeping into the flaws. The surface is de-greased, washed, wiped clean, or chemically cleaned among other cleaning methods. If the surface to be tested has been machined or sanded, it may need to be further etched to remove any contaminants. Surface cleaning is an essential step in the process of liquid penetrant testing. It can be done manually, semi-automatically or completely automatically.

2. Application of Penetrant Liquid

The penetrant liquid is then applied onto the surface. This can be done by dipping, spraying or brushing. You can also choose to dip the surface or component in a penetrant bath. The choice of application technique depends on:

– Material surface finish

– Compatibility of the penetrant and the material

– The degree of desired sensitivity

– The size of the surface to be tested

– The shape and accessibility of the surface to be tested

– The use or function of the component being inspected

– The equipment available for testing

Penetrant liquids can be categorized into three groups. The first group is the water-soluble penetrants, which, as the name suggests, are soluble in water. The next group contains all the penetrants that are post emulsifiable with water rinsing. Finally, the third group are the penetrants that are solvent removable. As you may have noticed, the primary difference between these three groups is the method used to remove excess penetrant.

3. Removal of excess penetrant liquid

After a specified period of time, the excess penetrant liquid is removed. The dwell time of the penetrant is usually a matter of preference, as long as it is not too long or too short. The type of penetrant used can also dictate how much time is needed. The surface finish, specifications of the test, and the type of material will also influence dwell time requirements. When removing the excess penetrant liquid, depending on the type of penetrant used, this is usually done using water or an acceptable solvent.

4. Developer application

The developer is then applied, which helps highlight problematic areas. This is the stage where the flaws will be revealed. Developers are typically dry powders, but there are some that are liquid solutions. Dry powders may be dusted onto the surface while wet developers can be lightly sprayed. Again, the type and method of developer application depend on:

– Material surface finish- Compatibility of the developer, the penetrant and the material- The degree of desired sensitivity- The size of the surface to be tested- The shape and accessibility of the surface to be tested- The use or function of the component being inspected

5. Surface Inspection

Once the developer has been applied, you will have to wait for a short amount of time before the surface needs to be inspected. This can be done visually or using cameras and other visual assistance equipment. The recording will then be replayed and problematic areas identified and marked out.

During the inspection, cracks, laps or bursts appear as continuous lines or broken dotted line marks. On the other hand, porosity in the material, shrinkage, improper bonding and leakages will appear as random dots or large localised areas of colour.

It is important to note that some instances require the inspection to be done by trained and certified professionals. They will carry out their visual inspections under a black light, UV lamp or white light, depending on the penetrant and the developer used. After the inspection is complete, the sample can either be rejected or accepted depending on the requirements. If the sample is rejected, the inspector needs to determine the cause of the flaw wherever possible.

Always verify that the inspectors you are using are qualified and certified to do the job. The laboratory you use also needs to have all the required certifications, and you need to establish whether there are any needed customer or end user approvals.

6. Post Inspection

After the inspection has been completed, cleaning of the surface needs to be done. Anti-corrosion solutions may be required at this stage.

The penetrant solution used usually contains a dye that makes the indication visible under white light. It may also contain a fluorescent material that lights up under ultraviolet light, typically UV-A. Fluorescent penetrants are much more costly, though, and should only be used when the maximum flaw sensitivity is required.

Similarly, if you require extremely high sensitivity, you can use radioactive tracers. However, these are highly dangerous and require the use of specialized testing teams and very stringent application and testing procedures. This, the use of radioactive tracers in Liquid Penetrant Testing is extremely rare.

How Does Liquid Penetrant Testing Work?

Liquid Penetrant Testing works under a very simple principle. The liquid penetrant is first drawn into the breaches or cracks on the surface of the material through capillarity. The excess penetrant is then removed, leaving the bit trapped in the problem areas. When a developer is added, it draws out the penetrant in the cracks and produces a surface indication of the problem. This is usually in the form of glowing lines that show up under special lights such as UV lights. The cracks on these surfaces can sometimes be very tiny, sometimes merely 150 nanometers wide. However, the indicators show up as much wider lines, which makes the flaws more visible and easier to identify.

Where To Use Liquid Penetrant Testing

Liquid Penetrant Testing can be applied to almost any non-porous surface. It does not work if the surface is porous or absorbs the penetrant. Similarly, the surface has to be clean. It can either be metallic or non-metallic, such as glass, plastics and ceramics, as long as it is not dirty or very rough. Very rough surfaces tend to trap the penetrant in the little fissures on their surfaces, giving false readings.

Liquid Penetrant Testing can also be automated. There are production lines where the units are cleaned, dipped in penetrant, washed, dried, and developed automatically. A recent development has also been the introduction of robotic inspectors, where robots are programmed to scan the surface of the material using cameras and through pattern recognition, identify flaws on the surface. Similarly, image enhancement techniques for the convenience of human inspectors have also been introduced.

Final Thoughts:

Here are a few pro tips:

1. Always check the required specifications for your industry before you begin inspection

2. If you can, buy all your penetrant products in kits, This will massively ease portability and convenience.

In summary, here is your concise guide to liquid penetrant testing:

1. Pre-cleaning

2. Application of penetrant and penetrant dwell time

3. Penetrant removal

4. Application of developer and developer dwell time

5. Inspection

6. Post-cleaning

People Also Ask:

1. Do Fluorescent Liquid Penetrants Have Different Sensitivities?
Yes. Fluorescent liquid penetrants are classified into four categories (1-4) depending on sensitivity. Category 1 fluorescent liquid penetrants are the least sensitive of the group, while category 4 fluorescent liquid penetrants are the most sensitive, being able to detect even the smallest flaws.

Share on facebook
Share on twitter
Share on linkedin

Comments are disabled from these blogs, if you have found an error or if we got something wrong, please contact us by clicking here.