NDT – Non Destructive Testing

Non-destructive testing is a wide group of methods used to detect discontinuities that cannot be identified by the naked eye or those in the surface of the material without damaging the material. Most common methods are Visual Testing, Magnetic Particle Testing, Penetrant Testing, Ultrasonic Testing, Radiographic Testing, Acoustic Emission and Eddy Current. In these tests, defects such as corrosion, cracks, decrease in wall thickness or gaps in internal structures are identified in ferritic and austenitic steels, aluminum alloys, nickel, copper and titanium alloys during production or usage. Non-destructive testing methods can change depending on the procedure, size, thickness and structure of the material.

These tests should be performed by a qualified staff. We, at SZUTEST, provide services of non-destructive testing for industrial and welded products and applications according to ISO 9712 (EN 473) and ASNT TC 1A standards with our Level II and Level III qualified expert staff.

History does not provide a certain date for Non-destructive testing to began as a known technology. Humans have performed Non-destructive tests since the beginning. Many years before Non-destructive testing was first used, people were looking at objects to determine size, shape and even visual surface imperfections.

NON-DESTRUCTIVE TESTING METHODS

Visual testing is the fastest and cheapest method of Non-destructive testing. It’s the first step of every inspection before any other Non-destructive test starts. When performing visual test with naked eye, equipments such as magnifying glass, light source, boroscope and mirror can also be used.

The condition of the surface is important in order to detect discontinuities such as cracks, porosities and undercuts. Required cleanings must be finished before visual testing starts. It can be applied on any metallic or non-metallic materials.

Visual testing may seem as an easy method, but it has it’s own inspection terms and the experience of the staff is important. Test must be performed under enough light, minimum 500 lux, with an angle not lower than 30° and the distance between eye and the surface shouldn’t be less than 300 mm.

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Advantages

  • Fast and immediate results.
  • Requires minimum preparation.
  • Inexpensive.
  • Cheap equipments.

Disadvantages

  • Only surface indications can be detected.

history

Magnetic particle testing was being executed earlier than radiographic testing. Englishman S.M. Saxby in 1868 and American William Hoke in 1917 tried to detect cracks in gun barrels by magnetic indications. The industrial applications were made by Victor de Forest and Foster Doane later than 1929. In 1934, they formed a company with the name of Magnaflux.

what is it?

Magnetic Particle Testing is a test method to detect defects on surface or open to surface. The main principle of this method is; it includes application of magnetic field externally or applying electric current through the material which in turn produces magnetic flux in the material. At the same time visible ferrous particles are sprayed on the test material. Surface or open-surface defects in the material creates distortion in the magnetic flux which causes leakage of the magnetic fields around the defect. The magnetic particles are pulled by the surface field in the area of the defect and indicates the location of the defect.

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This method can be applied to ferromagnetic materials only. Any other materials should be tested with different methods.

Magnetic particle test techniques are given below. Most commonly used one is

“Portable electromagnet”.

Current flow techniques

  • Axial current flow
  • Prods; Current flow
  • Induced current flow

Magnetic flow techniques

  • Threading conductor
  • Adjacent conductor(s)
  • Fixed installation
  • Portable electromagnet (Yoke)
  • Rigid coil
  • Flexible coil
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Portable electromagnet (Yoke) 

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Flexible cable technique

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Coil technique  

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Threading bar or cable technique

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Prods; Current flow  

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Current flow technique

Advantages

  • Fast and immediate results.
  • Doesn’t require a detailed cleaning and can be applied on coated materials. (Up to 50µm)
  • Inexpensive.
  • Can detect surface and open-surface defects.

Disadvantages

  • It can be performed only on ferromagnetic materials.
  • Needs equipments working with AC or DC current. Can be expensive in certain situations.
  • Parts may require demagnetization and cleaning when the inspection ends.
  • Requires a constant power source to work.

HISTORY

Penetrant Testing has started in the second half of 19th century. To detect surface defects on the materials, the parts were covered in oil, then cleaned and powder was applied. If there was a defect on the materials, the oil would get into it. The powder would stick to the oil in the defect, making it more visible to the eye. This is called the “oil and whiting” method.

Before and during the World War II, fast growing aircraft-industry used more and more non-magnetic light metals, which wasn’t possible to test with MT. Magnaflux together with Switzer in USA, Brent Chemicals in GB started production of fluorescent and dye penetrants.

WHAT IS IT?

Penetrant testing is another method to detect surface-connected defects. Any other ones below the surface cannot be detected. It is important to have a clean and smooth surface. After mechanical, chemical precleaning the surface must be dry and any dirt such as rust, oil, or paint should be cleaned from the surface as it will affect the process.

The biggest advantage of this method is, it has no restrictions about the material. Means can be used on steel, ceramic, glass and many others.

Application steps are;

  1. SURFACE PREPARATION: The surface to be insptected should be free from dirt, oil or coating. Water, cleaner solvent can be used to clean contaminants. After these steps are done, the surface should be dried so that neither water nor solvents remains in the discontinuities.
  1. APPLICATION OF PENETRANT: Penetrant can be applied to the part to be tested by spraying, brushing, flooding, dipping. The penetration time depends on the properties of the penetrant, tempature, the material and defects to be found. Waiting time should meet the requirments from the standard which is between 5-60 minutes.
  1. EXCESS PENETRANT REMOVAL: When removing the excess penetrant, cleaning must be done carefully. Water or cleaner solvent can be used again. One thing to keep in mind that during the cleaning process, water or the cleaner solvent shouldn’t be applied to the surface directly, as it might get in the discontinuities and remove the penetrant.
  1. APPLICATION OF DEVELOPER: After the unwanted penetrant is removed, developer should be applied immediately. It is being used to drag out the penetrant that’s inside of the discontinuities and also to make a contrast. Waiting time should meet the requirements from the standard which is between 10-30 minutes.
  1. EVALUATION: The penetrant will start to come out, that’s when the first evaluation starts. Size of the discontinuities should be measured and noted. Bigger the leak is, deeper the discontinuity is.

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Types of penetrants, cleaners and developers are specified in EN 3452-1 standard.

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Advantages

  • Inexpensive.
  • Can be applied to pretty much every material.
  • Equipments are easy to carry.
  • Small objects with different shapes can easily be inspected.
  • Doesn’t require a power source to work.

Disadvantages

  • Can only detect discontinuities that is open to surface.
  • Much slower compared to MT.
  • Cannot be applied on paint.
  • Cleaning, surface conditions are important.

history

Latest method to come into industrial use. The methods of ultrasound were discovered in 1847 by James Precott Joule and in 1880 by Pierre Curie and his brother Paul Jacques. Not before than 1912 a first application was proposed after the Titanic incident. Englishman Richardson confirmed the identification of icebergs by ultrasound in his patent applications. In France, Chilowski and Langevin started their development to detect submarines by ultrasound during World War I.

In 1929, Sergei Y. Sokolo proposed to use ultrasound for testing castings, same year he created high-frequency vibrations in materials using a quartz crystal. The detection of laminations in plates and fine non-metallic inclusions in hot-rolled profiles became necessary during World War II. Already existing NDT methods such as X-Ray, MT, PT and ET were unable to solve these issues.

Industrial use of ultrasonic testing started in three countries: America, England and Germany. Main persons were Adolf Trost, Donald O. Sproule and Floyd Firestone. Sproule and Trost used transmission-technique with seperate transmitter and receiver probes. Trost invented the so-called “Trost-Tonge”. The 2 probes were contacted on opposite sides of a plate, held in same axis by a mechanical device – the tonge – and coupled to both surfaces by continuously flowing water. Sproule placed the 2 probes on the same side of the workpiece. So he invented double-crystal probes. He also used this combination with varying distances from each other. Firestone was the first to realize the reflection-technique.

what is it?

Ultrasonic testing is based on transmitting high frequency sound waves to materials and receiving them back in order to detect any discontinuities. Probes with pulser/receiver speciality are being used to send sound waves. A probe connected to an ultrasonic testing device sends sound waves into the material. The sound waves pass through the material and reflect back to the probe. With a discontinuity found, sound waves will return to the probe before the distance is complete, this will show a discontinuity was there.

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Ultrasonic testing is able to scan any welding seams, castings and forgings. Some of the most common industries that uses ultrasonic testing are petrochemical, automotive, aerospace and structural steel, among many others.

Advantages

  • Fast and highly accurate results.
  • It has no harm to environment or humans.
  • Practical and easy to carry equipments.
  • Requires minimum preparation.
  • Can also be used to measure thickness.
  • Internal discontinuities can be detected.

Disadvantages

  • Surface must be accessible.
  • Linear defects oriented parallel to the sound wave might not be found.
  • Equipments are expensive. .
  • Materials in different shape, small or thin are difficult to inspect.
  • Requires a coupling to transfer the sound wave to test material.

history

X-Ray Technique was the first NDT method to come industrial application.

Wilhelm Conrad Roentgen discovered X-rays in 1895 during his experiments with cathode rays. He won Nobel Prize in Physics in 1901. In his first publication he described all effects including possible flaw detection.

During that time industry didn’t need this invention yet, but medicine did. Medical equipments were first to develop. Only side effect Röntgen could not predict was that X-rays were harmful to human health. Before radiation protection came out, many people lost their life.

First technical X-ray applications were done by Richard Seifert around 1930 in Germany. He improved medical equipment, cooperated with welding-institutes. Radiation testing can also be done with radioactive isotopes. This was discovered by Marie Curie, she received Nobel Prize for physics in 1903.

what is it?

This method of NDT is a technique that uses either x-rays or gamma rays to see the internal structure of a material. High-energy electromagnetic waves penetrate the material. Radiation penetrating the material affects the radiation-sensitive film placed on the other side of the material. When developed, this film reveals the image of the inner part of the material through which the beam passes. The darker areas on the image are evaluated as the indicators of discontinuities.

This method can be used to detect the internal and surface flaws in all metallic or non-metallic materials.

In the industry to produce gamma rays Iridium 192, Selenium 75 and Cobalt 60 are being used, for X–ray x-ray tubes are being used.

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Advantages

  • Internal and surface discontinuities can be detected.
  • Requires minimum surface preparation.
  • Test results can be saved for a long time.
  • Applicable to many materials.

Disadvantages

  • Radiation coming out during the process is harmful to human body.
  • Equipments are expensive.
  • Slow process.
  • Test material must be accessible from both sides.
  • Not possible to determine the depth of the discontinuities.

history

Michael Faraday has discovered of electromagnetic induction in 1831. Faraday was a chemist in England, in 1800’s he is credited with the discovery of electromagnetic induction, electromagnetic rotations, the magneto-optical effect, diamagnetism. A scientist named Hughes recorded changes in the properties of a coil when placed in contact with metals of different conductivity and permeability in 1879. However, it was not until the Second World War that these effects were put to practical use for testing materials. In 1950’s and 60’s many work had been done already, especially in the aircraft and nuclear industries.

what is it?

Eddy-Current testing is a method to identfy surface and near-surface defects. When an energy coil is brought close to the surface of a material, the variable magnetic field of the coil creates eddy currents on the material. These currents tend to magnetize against the original magnetic field. The impedance of the coil near the material is impacted by the presence of induction currents induced in the material.

When the eddy currents in the material change due to defects or material variations, the coil impedance changes. This change is measured, and displayed as indicating the defect or the material variation. It can be applied on all metallic and alloy materials with electrical conductivity. With eddy current testing, it is also possible to classify materials based on properties such as electrical conductivity or magnetic permeability. In addition, it is also possible to measure coating thickness or thickness of thin metal sheets.

Advantages

  • Can detect small defects.
  • Fast and highly accurate results.
  • Doesn’t require a coupling, able to test without a contact.
  • Requires minium surface preparation.

Disadvantages

  • Can only detect surface and near-surface defects and defects parallel to surface can not be detected.
  • Surface condition is important, otherwise it will affect the testing.
  • Effects such as vibrations and hits will make harder to find defects.
  • Works only on conductive materials.

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Acoustic emission is the phenomenon of radiation of acoustic waves in solids that occurs when a material undergoes irreversible changes in its internal structure, for example as a result of crack formation or plastic deformation due to aging, temperature gradients or external mechanical forces. In particular, AE is occurring during the processes of mechanical loading of materials and structures accompanied by structural changes that generate local sources of elastic waves. This results in small surface displacements of a material produced by elastic or stress waves generated when the accumulated elastic energy in a material or on its surface is released rapidly. The waves generated by sources of AE are of practical interest in structural health monitoring (SHM), quality control, system feedback, process monitoring and other fields. In SHM applications, AE is typically used to detect, locate and characterise damage.

Acoustic emission is the transient elastic waves within a material, caused by the rapid release of localized stress energy. An event source is the phenomenon which releases elastic energy into the material, which then propagates as an elastic wave. Acoustic emissions can be detected in frequency ranges under 1 kHz, and have been reported at frequencies up to 100 MHz, but most of the released energy is within the 1 kHz to 1 MHz range. Rapid stress-releasing events generate a spectrum of stress waves starting at 0 Hz, and typically falling off at several MHz.

The three major applications of AE techniques are: 1) source location – determine the locations where an event source occurred; 2) material mechanical performance – evaluate and characterize materials/structures; and 3) health monitoring – monitor the safe operation of a structure, for example, bridges, pressure containers, and pipe lines, etc. More recent research has focused on using AE to not only locate but also to characterise the source mechanisms such as crack growth, friction, delamination, matrix cracking, etc. This would give AE the ability to tell the end user what source mechanism is present and allow them to determine whether structural repairs are necessary.

The technique is used, for example, to study the formation of cracks during the welding process, as opposed to locating them after the weld has been formed with the more familiar ultrasonic testing technique. In a material under active stress, such as some components of an airplane during flight, transducers mounted in an area can detect the formation of a crack at the moment it begins propagating. A group of transducers can be used to record signals, then locate the precise area of their origin by measuring the time for the sound to reach different transducers. The technique is also valuable for detecting cracks forming in pressure vessels and pipelines transporting liquids under high pressures. Also, this technique is used for estimation of corrosion in reinforced concrete structures.

Advantages of AE

  • High sensitivity, early and fast detection of discontinuities.
  • Able to scan a structure in one phase, tests doesn’t take long.
  • Having access to sensors is enough, instead of the whole surface.
  • Doesn’t depend on the size of the defect. (MT, UT, RT does)
  • Real time monitoring, results can be received during the test.
  • Allows to locate discontinuities on big structures. (e.g. LPG tanks or storage tanks)

References

SZUTEST > Non-Destructive Testing