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The Types of Non-Destructive Testing and Their Advantages

Magnetic Particle Testing for Non Destructive Testing as part of structural steel fabrication

Non-destructive testing (NDT) or non-destructive evaluation (NDE) is used in manufacturing, fabrication and in-service inspections to ensure product integrity and reliability, to control manufacturing processes, lower production costs and maintain quality.

NDT and NDE are forms of testing or evaluating fabricated components that do not destruct, destroy or affect the serviceability of the component. Discontinuities and differences in material characteristics are more effectively found by NDT.

NDT and NDE play a critical function within the Construction and Manufacturing Industries in Canada and around the world.

In this blog, we’ll discuss four main types of NDT/NDE – Liquid Penetrant Inspection, Magnetic Particle Testing, Radiographic Testing and Ultrasonic Testing.

Introduction

There are many different forms of NDT/NDE. Some are considered volumetric, and others are categorized as surface only. Visual Testing, Magnetic Particle and Liquid Penetrant Inspections are surface only, whereas Ultrasonic and Radiographic are considered volumetric, meaning that they are able to “look” inside the component to find defects that would not be visible without cutting into the component.

Depending on which code or standard a fabrication is built and welded to, there are different acceptance and rejection criteria for the different NDE disciplines.

At Saskarc, we regularly perform Magnetic Particle, Liquid Penetrant, Ultrasonic and Radiographic Inspections. Some of these disciplines can be performed in-house, pro-actively, but all disciplines require a third party to come in and perform the testing and provide the reports.

Ultimately, NDT/NDE is a way of confirming the quality, integrity and reliability of a weldment without doing any harm to the end product. The NDT/NDE reports give our customer’s the satisfaction that their product is up the code or standard it was designed to.

Here a deeper look at some of the most common types of NDT/NDE:

A close-up image of a metal surface undergoing Liquid Penetrant Inspection. A red liquid, the penetrant, has seeped into a tiny crack, highlighting it against the silver metal.
Liquid Penetrant Inspection, pictured above, is performed with a liquid with low viscosity – called a penetrant, which is applied to the surface of a part. It then will penetrate into fissures and open to the surface. Once the excess penetrant is removed, the penetrant trapped in those voids will creep back out through capillary action, creating an indication.

Liquid Penetrant Inspection

The basic principle of Liquid Penetrant Inspection (LPI) is that when a liquid with low viscosity – called a penetrant, is applied to the surface of a part, it will penetrate into fissures and open to the surface.

Once the excess penetrant is removed, the penetrant trapped in those voids will creep back out through capillary action, creating an indication.

Penetrant testing can be performed on magnetic and non-magnetic materials, but it doesn’t work well on porous materials. Penetrants may be “visible”, meaning they can be seen in ambient light, or fluorescent, requiring the use of a “black” light.

After applying the penetrant, it sits on the surface for a specified period of time, coined as the “penetrant dwell time”, then the part is carefully cleaned to remove excess penetrant from the surface. A light coating of developer is then applied to the surface and given time to allow the penetrant from any voids or fissures to seep up into the developer, creating a visible indication.

Following the developer dwell time, the part is inspected visually, with the aid of a black light when using fluorescent penetrants. Most developers are fine-grained, white talcum-like powders that provide a color contrast to the penetrant being used.

Magnetic Particle Testing

Magnetic Particle Testing uses magnetic fields to locate surface and near-surface discontinuities in ferromagnetic materials. Any defects found are measured against standards such as CSA W59 and ASME B31.3, to name a few.

The magnetic field can be applied with a permanent magnet, electromagnet or by running a current through the component. The magnetic field can be orientated circularly or longitudinally. Circular magnetic fields are generated by passing a current through a conductor surrounded by the component. A longitudinal magnetic field is produced when using a coil, permanent magnet or electromagnet.

An orientation of 45 to 90 degrees between the magnetic field and the defect is necessary to form an indication. When this happens, the flux lines produce a magnetic leakage.  Because magnetic flux lines don’t travel well in air, when very fine colored magnetic particles are applied to the surface of the part the particles will be drawn into the discontinuity. This reduces the air gap, and produces a visible indication on the surface of the part.

The magnetic particles can be a dry powder, or suspended in a liquid solution, and they may be colored with a visible dye or a fluorescent dye that fluoresces under an ultraviolet light.

Radiographic Testing

Industrial radiography works by exposing a test object to radiation, which passes through the object to a recording medium, which is placed against the opposite side.

For thinner or less dense materials like aluminum, electrically generated X-rays are commonly used. For thicker or denser materials, gamma radiation is usually used.

Gamma radiation is given off by decaying radioactive materials. The two most commonly used sources of gamma radiation are Iridium-192 and Cobalt-60. Iridium-192 or IR-192 is generally used for steel up to 2-1/2 – 3 inches, depending on the Curie strength of the source. Co-60 is usually used for thicker materials due to its greater penetrating ability.

The recording media can be an industrial x-ray film, or a radiation detector.  With both, the radiation passes through the test object.

If there is a void or defect in the part, more radiation passes through, showing a darker image on the film or detector.

This allows a fabricator to look for defects and test the quality of work before it is shipped out.

Ultrasonic Testing

Ultrasonic testing uses the same principle as Sonar. An ultra-high frequency sound is introduced into the part being inspected. If the sound hits a material with a different acoustic velocity, some of the sound will reflect back, and is presented on a visual display.

By knowing the speed of the sound through the part, which is called the acoustic velocity, and the time required for the sound to return to the sending unit, the distance to the reflector can be determined.

The two most common types of sound waves used in industrial inspections are the compression wave and the shear wave.

Compression waves cause the atoms in a part to vibrate back and forth parallel to the sound direction and shear waves cause the atoms to vibrate perpendicularly to the direction of the sound.  Shear waves travel at approximately half the speed of longitudinal waves.

Sound is introduced into the part using an ultrasonic transducer that converts electrical impulses from the UT machine into sound waves.  Then it converts returning sound back into electric impulses that can be displayed on an LCD or CRT screen.

When the machine is properly calibrated, the operator can determine the distance from the transducer to the reflector, and the operator can determine the type of discontinuity (like slag, porosity or cracks in a weld) that caused the reflector.

Because ultrasound will not travel through air, a liquid or gel is used between the face of the transducer and the surface of the part. This allows the sound to be transmitted into the part.

Saskarc has over 25 years of experience providing superior steel fabrication solutions achieved by combining project analysis, stakeholder communications and design innovation for modularization. See what our customers have to say about working with us and contact us today to learn more about how we can make your fabrication project a success by utilizing modularization.