Ultimate Guide to NDT: Methods, Tools, and Applications

Sep. 23, 2024

Ultimate Guide to NDT: Methods, Tools, and Applications

WHAT IS NDT (NON-DESTRUCTIVE TESTING)?

NDT refers to an array of inspection techniques that allow inspectors to collect data about a material without damaging it.

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NDT stands for Non-Destructive Testing. It refers to an array of inspection methods that allow inspectors to evaluate and collect data about a material, system, or component without permanently altering it.

In the field, NDT is often used as an umbrella term to refer to non-destructive inspection methods, inspection tools, or even the entire field of non-destructive inspections.

For commercial applications, the goal of NDT is to ensure that critical infrastructure is properly maintained in order to avoid catastrophic accidents.

While NDT methods are typically associated with industrial use cases, like inspecting weak points in a boiler at an oil refinery, uses in medicine are actually some of the most common.

For example, an expecting mother getting an ultrasound to check on the health of her baby would be considered an NDT use case, as would getting an X-ray or MRI to learn more about an injury.

But it&#;s important to note that NDT does not necessarily require the use of special tools, or any tools at all.

For instance, when inspectors in industrial settings review the outside of a pressure vessel with their naked eye, that would fall under the NDT designation, since they are collecting data on the status of the boiler without damaging it. On the other hand, using a sophisticated tool like an ultrasonic sensor to look for defects in a certain material or asset would also be called NDT.

Regardless of the specific use case, the underlying commonality among all these examples is the collection of data in a non-intrusive manner.

What Is NDT?

We&#;ve already covered what NDT stands for and how the phrase is used in the field. Now let&#;s dive in and look more closely at some of the details that govern the world of NDT.

The Importance of NDT

When it comes to ensuring that assets are properly maintained, the importance of non-destructive testing cannot be over emphasized.

Here are the top reasons NDT is used by so many companies throughout the world:

Savings. The most obvious answer to this question is that NDT is more appealing than destructive testing because it allows the material or object being examined to survive the examination unharmed, thus saving money and resources.
Safety. NDT is also appealing because almost all NDT techniques (except radiographic testing) are harmless to people.
Efficiency. NDT methods allow for the thorough and relatively quick evaluation of assets, which can be crucial for ensuring continued safety and performance on a job site.
Accuracy. NDT methods have been proven accurate and predictable, both qualities you want when it comes to maintenance procedures meant to ensure the safety of personnel and the longevity of equipment.

"NDT is the life blood of a well-run facility, and the high importance of NDT is known by any trained inspector. NDT techniques and repeatable results depend on highly trained technicians with experience and integrity. Not only does the technician need to be certified in a specific NDT method, but they also need to know how to operate the equipment being used to gather data. Understanding equipment capabilities and limitations is the difference between making an accept or reject determination."

- Jason Acerbi, General Manager at MFE Inspection Solutions, "Your One Stop Inspection Source"

Where is NDT Used?

Depending on how broadly you define NDT you could say that it&#;s used in almost every industry in the world, since visual inspections (whether formalized or casual) take place in almost every workplace in some form or other.

That being said, there are specific industries that require NDT and have formalized processes for its use, as codified by those organizations we listed above like API and ASME.

These industries include:

Oil & Gas
Power Generation
Chemicals
Mining
Aerospace
Automotive
Maritime
Mining

In all of these industries, there are three different methods that inspectors use to gain access to hard-to-reach locations or locations at height:

Scaffolding. The use of scaffolding requires inspectors to work at height in person in order to collect inspection data.
Rope access. The use of rope access also requires inspectors to work at height in order to collect inspection data. Learn more about rope access and how it's used by inspectors in this guide.
Drones. Inspectors can use drones to collect inspection data remotely, allowing them to remain safely outside of confined spaces or safely below locations at height. Read the next section to learn more about how drones can be used for NDT.

NDT Codes and Standards

NDT techniques can be used for all kinds of inspections. But some of the most important types of NDT inspections are of assets like boilers and pressure vessels, which could be incredibly dangerous if not properly maintained.

Because proper maintenance of these assets is so important for the safety of those working nearby (or even at a distance, when it comes to nuclear power plants), most countries have laws requiring companies to adhere to specific inspection codes and standards when conducting inspections.

These standards and codes typically require inspections to be conducted periodically following specific guidelines. For the most assets that present the greatest risk, these inspections must be both conducted by a certified inspector and approved by a certified witness working for a formal inspection body.

Here are the most commonly followed organizations in the world for creating NDT standards and codes:

API (American Petroleum Institute)
ASME (American Society for Mechanical Engineers)
ASTM (American Society for Testing and Materials)
ASNT (American Society For Nondestructive Testing)
COFREND (French Committee for Non-destructive Testing Studies)
CSA Group (Canadian Standards Association)
CGSB (Canadian General Standards Board)

What Is the Difference Between Destructive Testing and Non-Destructive Testing?

Non-Destructive Testing (NDT) is used to collect information about a material in ways that do not alter it (i.e., without destroying it). Destructive Testing (DT) is used to collect information about a material in ways that do alter it (i.e., destroy it).

Essentially, the NDT and DT difference is that NDT doesn't require inspectors to damage the material they test, while DT does.

In Destructive Testing, for example, a piece of the material might be scraped away for analysis or altered in some other way onsite.

Here are some examples of destructive testing:

Macro sectioning. Macro sectioning tests a small section of a welded material by polishing and etching it for examination.
Tensile testing. Also called tension testing, this is a destructive testing technique that uses controlled tension applied to a sample material to see how it reacts. Tension could be applied to test certain loads or conditions, or to test a material&#;s failure point.
3 point bend testing. 3 point bend testing examines the soundness and flexibility (or ductility) of a material by taking a sample of it, called a coupon, and bending it in three points to a specified angle.

Read our guide on destructive testing to learn more.

NDT and NDE

While we're defining terms, it's important to note that there are a few common phrases that refer to testing materials without destroying them.

NDT is the most common phrase, but there are a few others as well:

NDE (non-destructive examination or non-destructive evaluation)
NDI (non-destructive inspection)

The NDE full form is non-destructive examination or non-destructive evaluation, and the NDI full form is non-destructive inspection.

The 8 Most Common NDT Methods

There are several techniques used in NDT and NDE for the collection of various types of data, each requiring its own kind of tools, training, and preparation.

Some of these techniques might allow for a complete volumetric inspection of an object, while others only allow for a surface inspection. In a similar way, some NDT methods will have varying degrees of success depending on the type of material they&#;re used on, and some techniques&#;such as Magnetic Particle NDT, for example&#;will only work on specific materials (i.e., those that can be magnetized).

Here are the eight most commonly used NDT techniques:

1. Visual Testing (VT)

Definition: Visual Non-Destructive Testing is the act of collecting visual data on the status of a material. Visual Testing is the most basic way to examine a material or object without altering it in any way.

How to Conduct Visual Testing

Visual Testing can be done with the naked eye, by inspectors visually reviewing a material or asset. For indoor Visual Testing, inspectors use flashlights to add depth to the object being examined. Visual Testing can also be done with an RVI (Remote Visual Inspection) tool, like an inspection camera. To get the camera in place, NDT inspectors may use a robot or drone, or may simply hang it from a rope.

Read our guide on visual testing.

2. Ultrasonic Testing (UT)

Definition: Ultrasonic Non-Destructive Testing is the process of transmitting high-frequency sound waves into a material in order to identify changes in the material&#;s properties.

How to Conduct Ultrasonic Testing

In general, Ultrasonic Testing uses sound waves to detect defects or imperfections on the surface of a material created.

One of the most common Ultrasonic Testing methods is the pulse echo. With this technique, inspectors introduce sounds into a material and measure the echos (or sound reflections) produced by imperfections on the surface of the material as they are returned to a receiver.

Here are some other types of Ultrasonic Testing:

Phased Array Ultrasonic Testing (PAUT)
Automated Ultrasonic Testing (AUT)
Time-Of-Flight Diffraction (TOFD)

Read our guide on ultrasonic testing.

3. Radiography Testing (RT)

Definition: Radiography Non-Destructive Testing is the act of using gamma- or X-radiation on materials to identify imperfections.

How to Conduct Radiography NDT Testing

Radiography Testing directs radiation from a radioactive isotope or an X-ray generator through the material being tested and onto a film or some other kind of detector. The readings from the detector create a shadowgraph, which reveals the underlying aspects of the inspected material.

Radiography Testing can uncover aspects of a material that can be hard to detect with the naked eye, such as alterations to its density.

Read our guide on industrial radiography.

4. Eddy Current (Electromagnetic) Testing (ET)

Definition: Eddy Current Non-Destructive Testing is a type of electromagnetic testing that uses measurements of the strength of electrical currents (also called eddy currents) in a magnetic field surrounding a material in order to make determinations about the material, which may include the locations of defects.

How to Conduct Eddy Current Testing

To conduct Eddy Current Testing, inspectors examine the flow of eddy currents in the magnetic field surrounding a conductive material to identify interruptions caused by defects or imperfections in the material.

Read our guide on eddy current testing.

5. Magnetic Particle Inspection (MPI)

Definition: Magnetic Particle Non-Destructive Testing is the act of identifying imperfections in a material by examining disruptions in the flow of the magnetic field within the material.

How to Conduct Magnetic Particle Inspection

To use Magnetic Particle Inspection, inspectors first induce a magnetic field in a material that is highly susceptible to magnetization. After inducing the magnetic field, the surface of the material is then covered with iron particles, which reveal disruptions in the flow of the magnetic field. These disruptions create visual indicators for the locations of imperfections within the material.

Read our guide on magnetic particle inspection.

6. Acoustic Emission Testing (AE)

Definition: Acoustic Emission Non-Destructive Testing is the act of using acoustic emissions to identify possible defects and imperfections in a material.

How to Conduct Acoustic Emission Testing

Inspectors conducting Acoustic Emission Tests are examining materials for bursts of acoustic energy, also called acoustic emissions, which are caused by defects in the material. Intensity, location, and arrival time can be examined to reveal information about possible defects within the material.

Read our guide on acoustic emission testing.

7. Dye Penetrant testing (PT)

Definition: Dye Penetrant Penetrant Non-Destructive Testing (also called Liquid Penetrant Testing) refers to the process of using a liquid to coat a material and then looking for breaks in the liquid to identify imperfections in the material.

How to Conduct Penetrant Testing

Inspectors conducting a Penetrant Test will first coat the material being tested with a solution that contains a visible or fluorescent dye. Inspectors then remove any extra solution from the material&#;s surface while leaving the solution in defects that &#;break&#; the material&#;s surface.

After this, inspectors use a developer to draw the solution out of the defects, then use ultraviolet light to reveal imperfections (for fluorescent dyes). For regular dyes, the color shows in the contrast between the penetrant and the developer.

Read our guide on dye penetrant testing.

8. Leak Testing (LT)

Definition: Leak Non-Destructive Testing refers to the process of studying leaks in a vessel or structure in order to identify defects in it.

How to Conduct Leak Testing

Inspectors can detect leaks within a vessel using measurements taken with a pressure gauge, soap-bubble tests, or electronic listening devices, among others.

Read our guide on leak testing.

Welding NDT

Welding NDT is the use of non-destructive testing to inspect a weld.

Welds are one of the most common parts of industrial assets that inspectors test. Using non-destructive weld testing equipment, inspectors can determine whether a weld is strong or has potential defects that could compromise its integrity.

The most common flaws found in weld NDT are:

Poor weld quality due to the presence of slag
Fatigue caused by human error
Flaws caused by incorrect technique or setup in how the weld was created
Environmental damage to the weld (i.e., temperature extremes, the presence of moisture, or the use of incompatible metals)

The most common welding NDT methods include:

Ultrasonic testing NDT
Magnetic particle inspection NDT
Acoustic emission NDT
Dye penetrant NDT
Radiography NDT
Eddy Current NDT

A visual inspection can be used for weld NDT, to help inspectors make basic determinations about the strength of a weld, though its findings may be more limited than the above welding testing methods.

Welding NDT test of a crack performed via magnetic particle inspection

The goal of using NDT for welding is to identify defects on the surface or within the weld that could cause the weld to deteriorate or fail.

If a weld fails, the consequences can be very serious, since welds are often crucial for the integrity of large industrial assets, like boilers or pressure vessels.

What is the best NDT welding inspection method?

Although all the methods listed above can be used for welding NDT, the best method is ultrasonic testing using the phased array approach.

This welding test method can be done fairly quickly without a lot of setup work required or extra NDT equipment, providing high quality data in a short period of time.

Destructive and Non-Destructive Testing in Welding

We've already covered the primary welding NDT methods, which are inspection methods inspectors use to test weld without causing any damage to them.

But inspectors also use destructive testing to inspect welds.

The most common types of destructive weld testing are:

Guided bend weld test. Bending a sample section of the weld to predetermined radius to make determinations about its internal structure.
Macro etch weld testing. Removing a small sample from the weld, polishing the samples, then etching on the samples with an acid mixture in order to test the internal makeup of the weld.
Transverse tension test. Testing the tensile properties of the base metal, the weld metal, and the bond between them.

How Drones Can Help with NDT

In the last several years drones equipped with cameras have become another tool commonly used in NDT for collecting visual data.

Due to limitations in the technology, for some time drones could only provide supplementary visual data for inspectors, but could not take the place of inspectors physically collecting visual data themselves.

However, as drone technology has improved, inspectors have been able to use drones more and more as RVI tools, in some instances completely replacing the need for them to collect visual data manually.

Here are two of the primary ways drones are helping with NDT these days:

Safety

By removing the need for inspectors to enter dangerous confined spaces in order to collect visual data drones are helping improve safety in the workplace.

Are you interested in learning more about Non Destructive Testing Products? Contact us today to secure an expert consultation!

For outdoor inspections of assets like power lines or towers, using a drone to collect visual data reduces the amount of time a person needs to physically be in the air on the tower or line.

For indoor inspections of assets like pressure vessels or boilers, using a drone like the Elios 3 to collect visual data means the inspector does not have to enter a confined space to do so, again helping significantly reduce the exposure to risk.

Savings

Drones can help companies improve their ROI in both indoor and outdoor scenarios, but savings are especially significant for indoor inspections.

Using a professional indoor drone instead of sending an inspector in to collect visual data manually means that companies save on not having to build and take down scaffolding, and can reduce downtimes associated with those requirements, in some cases by as much as one to two days.

Because a drone inspection can be mounted quickly, inspections can be conducted more frequently, helping to identify issues earlier and increasing the longevity of the asset. These early discoveries can save companies hundreds of thousands of dollars with a single inspection.

What&#;s Next for Drones in NDT?

To date, the primary use case for drones in NDT has been for the collection of visual data.

But in the last few years, in addition to cameras, several types of sensors used in NTD have been attached to drones, including thermal, ultrasonic, magnetic, and radiographic sensors.

New sensors supporting NDT methods are constantly being developed for use with drones, allowing inspectors to collect an even wider variety of data without even touching an asset.

In addition, software developed to meet the needs of inspectors is growing in both market and capability. The software can work directly with a drone&#;s data collection, like Inspector 4.0, the most up-to-date version of Flyability&#;s inspection software.

Using Inspector 4.0, inspectors can quickly create a sparse 3D model of the asset they&#;re inspecting after the data collection process that shows exactly where defects are located in the asset, successfully addressing a long-time pain point for inspectors. Inspectors can see a defect with the imagery the drone collected and then know where it was using the model's locational information, saving hours of work.

Manually reviewing the mountains of data provided by inspection drones would overwhelm a human. As a result, software designed to help inspectors make sense of the information gathered has become a necessity rather than a luxury.

Aided by machine learning and AI, these advanced programs can process and review images of the asset and flag issues for human review. WinCan, for example, is a company that makes software just for sewer inspections. Using an AI-powered algorithm, its software can take raw visual data from a sewer inspection and identify all of the potential defects that might require further inspection or repair.

Advances in drone technology, payload diversity, and data processing are creating a new era of efficiency for NDT and safety for inspectors.

Read about the seven major benefits drones provide for NDT.

Non-Destructive Testing (NDT): Types and Techniques

What is Non-Destructive Testing (NDT)?

1.1.

The Importance of Non-Destructive Testing

1.2.

Destructive vs Non-Destructive Testing: Key Difference

When Is NDT Testing Required?

Key NDT Methods: Overview

3.1.

Visual Testing (VT)

3.2.

Magnetic Particle Testing (MT)

3.3.

Magnetic Flux Leakage Testing (MFL)

3.4.

Liquid Penetrant Testing (PT)

3.5.

Ultrasonic Testing (UT)

3.6.

Guided Wave (GW) Testing

3.7.

Radiographic Testing (RT)

3.8.

Eddy Current Testing

3.9.

Ground Penetrating Radar (GPR)

3.10.

Acoustic Emission Testing (AE)

3.11.

Thermal/Infrared Testing (IR)

3.12.

Microwave Testing

3.13.

Laser Testing

3.14.

Leak Testing

NDT Testing: Regulations and International Standards

Streamline NDT Processes with Voliro

Discover Voliro inspection drone and payloads.

Whether you&#;re in the automotive or petroleum industry, you&#;ve probably heard about non-destructive testing. It&#;s either part of your product development to enhance new parts&#; quality or a regulatory requirement.

This guide explains when non-destructive tests are used, which assets to inspect regularly, and the best testing methods for different materials.

What is Non-Destructive Testing (NDT)?

Non-destructive testing (NDT) refers to quality assurance and material examination techniques that preserve the integrity of inspected materials, components, and assets.

Industrial assets such as oil storage tanks, lightning prevention systems on wind turbines, piping, and supporting pylons require regular examination for maintenance and regulatory purposes. NDT inspections identify defects, flaws, irregularities, or wear and tear in structures to ensure safety and robustness

Non-destructive testing is the only way to get high-fidelity material characteristics of an asset without damaging it.

The most common types of non-destructive testing methods assume the usage of sound waves, acoustic impulses, or gamma rays to perform in-depth material assessments. By combining different methods, NDT technicians can test objects for the smallest cracks, voids, or weld discontinuities, invisible to the naked eye.

Types of materials you can test with NDT:

Metals
Alloys
Plastics
Ceramics
Composites
Concrete
Coatings
Wood
Glass
Fiberglass
Rubber
Electronic components
Weldments
Pipelines
Castings
Forgings

The Importance of Non-Destructive Testing

NDT serves two primary purposes: quality control and asset maintenance.

Aerospace and automotive companies have little margin for error when developing new product components. Aircraft engine turbine blades must operate under extreme temperatures and pressure while maintaining optimal speeds. Quality assurance engineers use NDT techniques like ultrasonic testing (UT) or radiographic testing (RT) to inspect the blades for material inconsistencies or cracks after crash tests to ensure durability. NDT allows inspection without interfering with a product&#;s final use, balancing quality control and cost-effectiveness.

Regular NDT assessments are a compliance requirement in many industries. Operators of oil rigs, power plants, and chemical facilities must comply with safety standards and ensure assets don&#;t leak, deform, or degrade to the point where they become hazardous.

Magnetic particle testing is used to detect surface-level cold cuts or shrinkage cracks in castings, forgings, and metal components. Eddy current testing is a good method for detecting early signs of corrosion or material loss in pipelines, boilers, and storage tanks.

Early issue detection prevents costly leaks, deformations, and sudden equipment failures. Fortune Global 500 manufacturing and industrial firms lose over $1.5 trillion yearly to unplanned downtime. Regular NDT testing helps avoid operational failures, save on repairs, and extend assets&#; lifespan.

Industries relying on NDT for asset evaluations and inspections:

Aviation & aerospace
Automotive
Oil & gas
Manufacturing
Power generation
Infrastructure
Telecom
Mining
Chemicals
Construction
Rail & transportation
Marine & shipbuilding

Destructive vs Non-Destructive Testing: Key Difference

Destructive and non-destructive testing gauge material endurance using different principles. Non-destructive tests aim to preserve material integrity, while destructive testing (DT) applies different pressures (e.g., high temperature, extreme load) until asset deformation or destruction.

Destructive tests help determine the point of failure for the material or component, while NDT tests provide data on asset condition, quality, and reliability.

Non-destructive testing examplesDestructive testing examples1. Directing ultrasonic waves against a metal sheet to detect cracks.

2. Using electromagnetic waves to detect changes in concrete thickness.1. Bending a metal sheet until it fractures.

2. Dropping a weight on a concrete platform to estimate deformation and cracks under pressure.Sample NDT MethodsSample NDT Methods1. Ultrasonic testing (UT)
2. Magnetic particle testing (MT)
3. Eddy current testing1. Tensile testing
2. Metallographic Analysis
3. Impact testing

When Is NDT Testing Required?

NDT inspection frequency varies depending on the assets in use, established by regulatory bodies and certification organizations like SAFed, IoP, or ASNT.

In the UK, lifting equipment (e.g., aerial loading platforms) and associated accessories used to lift people must be inspected every six months, and other lifting equipment (e.g., hoists) must be inspected every twelve months. The American Petroleum Institute Standard (API 653) states that internal and external storage tank inspections must be done at least every 5 years.

Even when NDT isn&#;t directly required, running regular tests is more cost-effective than replacing equipment. Although most wind turbines have a lightning strike prevention (LPS) system, lightning damages cost operators over $100 million annually and account for 60% of blade losses. This leads to extra downtime, maintenance expenses, and higher insurance costs. These losses can be minimized with regular LPS tests with drones &#; a cost-effective method, requiring only 20 minutes versus 6 hours with rope access.

Asset owners develop non-destructive inspection schedules based on their assets&#; risk levels and industry standards to prevent unscheduled downtime.

Key NDT Methods: Overview

NDT uses various analysis techniques to evaluate materials, parts, components, and large structures.

The main non-destructive testing methods are:

Visual Testing (VT)

Visual testing (VT) involves observing the test object&#;s surface for discontinuities or damages. Remote visual inspections effectively identify corrosion, physical damage, part misalignment, and cracks, especially in hard-to-reach areas.

NDT technicians use visual testing as a standalone method for evaluating visible damage like poor welds in oil piping or cracks in a storage tank. With the Voliro drone, technicians can inspect 8-10 storage tanks (inside or outside) in one day for signs of wear and tear, using payloads UT and EMAT payloads.

Source: Voliro. Voliro T outside tank inspection.

Visual testing is the first step in ultrasonic and radiographic inspection. It locates areas of interest and ensures the surface is free of contamination, coatings, or obstructions that may interfere with the tests.

The visual test is the second step in liquid penetrant and magnetic particle inspection. It confirms the correct application of penetrant or magnetic particles and captures defect indications.

Best NDT method for preliminary asset inspection and test site preparation.

Magnetic Particle Testing (MT)

Magnetic particle testing (MT) detects flaws or defects on metal surfaces or just beneath. Technicians create a magnetic field using a permanent magnet, electric coils, or handheld electrode prods.

When a magnetic field is applied to the metal, defects disrupt the field, causing magnetic force lines to leak out. These lines attract tiny metal particles, creating a visible mark that shows the defect&#;s location. Colored magnetic particles stick to the metal and are visible to the naked eye or under ultraviolet light.

Magnetic Particle Testing (MT). Source: Iowa State University.

Magnetic particle testing is the go-to method for testing welding on metal structures and cracks in power generation equipment, such as wind turbines, generators, boilers, and structural steel components.

Best for NDT testing method for easily accessible ferromagnetic assets.

Magnetic Flux Leakage Testing (MFL)

Magnetic flux leakage testing also leverages changes in magnetic fields to detect signs of corrosion or other damage in metal components. It&#;s like using a magnet to find hidden metal objects in your pocket. If there&#;s a problem in a metal pipe, like a hole or thinning, the magnetic field leaks out, and sensors can detect it.

Magnetic Flux Leakage Testing (MFL). Source: Semantic Scholar.

Unlike magnetic particle testing, MFL doesn&#;t use colored particles to detect surface damage. Instead, it uses data from various sensors (Hall effect, fluxgate, and coil sensors) to detect the smallest changes in magnetic fields indicative of a defect.

Most data is collected by sensors on specialized equipment, so MFL is used to test larger structures like storage tanks, tubes, and pipes. Inspection drones streamline the examination process by allowing close access to test pieces.

Best NDT testing method for large ferromagnetic assets and structures.

Liquid Penetrant Testing (PT)

Liquid penetrant testing (PT) is another method for finding surface-level defects in metal construction. Inspectors apply a highly fluid liquid penetrant to the structure. The substance seeps into cracks. Technicians apply a developer agent to the surface to color the trapped penetrant, exposing defects.

Liquid Penetrant Testing (PT). Source: ResearchGate.

Non-ferromagnetic materials like stainless steel, aluminum, or non-ferrous alloys can be tested with penetrant testing. This testing method also works better for structural components with irregular shapes (e.g., piping with multiple curves or bends).

Best NDT testing method for non-ferromagnetic assets with complex geometries.

Ultrasonic Testing (UT)

Ultrasonic testing (UT) leverages high-frequency sound waves for asset inspection. An irregularity in a wave causes some of the sound to bounce back, alerting the inspector about a possible defect. It&#;s like using a sonar fish finder to locate fish, but you&#;re fishing for flaw.

Different sound waves are used in UT. Some vibrate in the same direction as the sound (compression waves), while others vibrate perpendicular to the sound (shear waves).

Ultrasonic Testing (UT). Source: ResearchGate.

Compression waves help detect:

Parallel surface cracks or fractures
Voids or porosity in the material

Shear waves help detect:

Laminations and delaminations (separations of layers within a material)
Inclusions or foreign material embedded in the tested asset

Ultrasonic testing can detect weld issues in structures, pipelines, pressure vessels, and tanks, as well as problems with piping systems (e.g., corrosion, erosion, wall thickness variations). It&#;s a common non-destructive examination method for pressure vessels and storage tanks to detect early signs of wall thinning, pitting, or stress corrosion cracking.

Technicians traditionally use various ultrasonic transducers and probes to inspect assets. These can be hand-held or attached to a drone to reach narrow cavities or high areas. UT inspection drones are a safer alternative to sending personnel up elevated structures, reducing risks.

Voliro has developed two ultrasonic transducer payloads for live scans at temperatures up to 260°C (500 °F), saving substantial time and effort on data collection. Boasting high dexterity, Voliro drones can perform thorough inspections in complex environments like chimneys, storage tanks, or stacks. UT inspections can be done in a day without shutdowns. Learn more about Voliro inspection drones.

Voliro T high temp UT inspection. Source: Voliro.

Guided Wave (GW) Testing

Guided wave testing is another non-destructive evaluation technique that uses ultrasonic waves to identify defects. Since ground wave testing doesn&#;t require direct contact, it&#;s often used to detect surface anomalies along large objects like pipes.

Inspectors place a transducer ring or exciter coil outside the pipe and direct ultrasonic waves along its sides. The waves bounce off any irregularities (like corrosion) on the surfaces inside and outside.

Guided Wave (GW) Testing. Source: ResearchGate.

The advantage of guided wave testing is that you don&#;t need to remove any coatings or insulation to examine the piping. The downside is that GW testing results have lower resolution, making it harder to detect smaller defects or identify their precise size, shape, or location.

Best NDT testing method for large pipelines and tubing.

Radiographic Testing (RT)

Radiographic testing (RT) uses radiation rays (aka X-rays) to examine dense materials. Gamma radiation passes through the test object, exposing a film or digital detector on the other side. Darker areas indicate where more radiation passes through, signifying gaps or cracks.

Radiographic Testing (RT). Source: ScienceDirect.

Industrial radiography uses two types of radioactive isotopes:

Iridium-192 (Ir-192) is used to examine objects with thicknesses up to 7 cm (3 inches).
Cobalt-60 (Co-60) is used to examine thicker objects.

Because we&#;re talking gamma radiation, inspectors must follow special safety protocols and always use appropriate personal protective equipment (PPE) to minimize exposure. Soindustrial radiography is rather laborious and expensive.

Eddy Current Testing

Eddy current testing is an electromagnetic method for inspecting conductive materials, like certain metals, alloys, and conductive paints or platings on metal surfaces. Unlike MPT and LT, direct contact between the surface and the testing equipment isn&#;t required with eddy testing.

During testing, inspectors apply an alternating current to materials to create a magnetic field, inducing eddy currents within them. Flaws or defects in the material alter the current&#;s pattern. Detecting and analyzing this change helps inspectors identify flaws with high precision.

Eddy Current Testing. Source: MDPI.

Eddy current testing is a cost-effective and reliable technique used for quality assurance and safety inspections of power cables, heat exchanger coils, condenser tubes, non-pyrogenic alloys, and carbon fiber composites.

Best NDT testing method for no-contact inspection of conductive materials.

Ground Penetrating Radar (GPR)

Ground penetrating radar (GPR) works by sending electromagnetic pulses into the ground and listening for their echoes as they bounce back. This signal can create a picture of what&#;s underground, similar to how ultrasound creates images of what&#;s inside the body.

Ground Penetrating Radar (GPR). Source: ResearchGate.

When the signal hits something underground, like a pipe or rock, it bounces back differently, and GPR can detect these differences. With GPR, asset owners can detect buried pipes, cables, and changes in the ground, like whether the soil is wet or dry. GPR can&#;t scan through metal (i.e., peak inside a metal pipe) &#; it can only indicate its location.

GPR is useful for finding buried utilities or hidden structures in concrete. Hence, this testing method is often used in civil engineering and construction.

Best NDT method for detecting underground objects or obstructions.

Acoustic Emission Testing (AE)

Acoustic emission testing (AE) captures mechanical vibrations of stressed materials or structures. Technicians attach sensors (e.g., piezoelectric sensors) or transducers (e.g., strain gauges) to the test object&#;s surface to convert stress waves into electrical signals. Then, they apply a sudden force, change in temperature, or pressure to the structure and analyze the generated vibration.

Acoustic Emission Testing (AE). Source: ResearchGate.

AE tests detect surface changes caused by stress waves (cracks, deformation, etc.), indicating hidden problems or weaknesses. It&#;s great for quality assurance of critical assets like supporting structures, towers, bridges, and individual system components (pumps, compressors, bearings).

Best NDT testing method for structural bearing components, pressure vessels, and storage tanks.

Thermal/Infrared Testing (IR)

Thermal testing (also known as infrared thermography) measures the apparent surface temperature of tested objects to examine their thermal conductivity. Overheating metal parts in a motor can deform, causing performance issues. Poorly isolated wiring in electrical panels can cause short circuits. Thermal tests help detect those problems.

Thermal/Infrared Testing (IR). Source: MDPI.

Infrared thermography is primarily used to identify energy loss in buildings. Although manufacturing companies also use thermal scanning to locate cracks or delaminations in polymers, plastics, ceramics, and semiconductors. Infrared testing helps localize areas under high thermal stress, fatigue, or degradation, which can change material integrity.

Best NDT testing method for detecting energy loss and thermal-induced deformations.

Microwave Testing

Microwave testing (MW) uses electromagnetic waves in the microwave frequency range to scan for irregularities. By analyzing reflected wave signals, teams can identify abnormalities such as cracks, voids, or inconsistencies.

Microwave Testing. Source: ResearchGate.

Microwave (MW) inspection is gaining popularity for examining plastic and composite materials due to its effectiveness, especially with complex composite materials like glass fiber-reinforced polymers (GFRP).

The non-contact nature of this NDT technique allows for accurate, reliable, and repeatable MNDT readings on composites in high or low temperatures and complex electrostatic environments (e.g., DC biasing fields, ionizing radiation, etc.). On the downside, microwave testing doesn&#;t work great for metallic or other conductive materials. Delivering accurate results requires extensive setup, leading to higher testing costs.

Best NDT method for examining plastic and composite materials.

Laser Testing

Laser testing uses helium-neon, diode, Nd: YAG, and excimer lasers to detect surface-level issues. Inspectors first apply stress to the material (e.g., bend it). Then use laser light to create images of surface changes.

Laser Testing. Source: ResearchGate.

The two main testing techniques are holography and shearography. Here&#;s how they work:

Holography uses laser light to create a detailed image of a material&#;s surface and subsurface. When the material is stressed, defects cause tiny changes in the surface that can be seen in the holography image. However, ambient vibration in the test object can degrade these results

Shearography uses laser light to detect surface changes caused by stress. When the material is stressed, defects create small surface deformations visible in the shearography image. Due to its low sensitivity to ambient vibration, it&#;s better for examining larger objects like rail cars or aircraft parts.

Both approaches are non-contact fast, and they can detect a variety of defects without closely following a material&#;s shape. However, the test setup process is rather laborious.

Best NDT testing method for aerospace components, semiconductor chips, and electronic assemblies.

Leak Testing

Leak testing uses non-destructive testing methods to locate breaches in the integrity of sealed or pressurized systems like gas tanks, refrigeration systems, or chemical basins. These tests ensure asset integrity, safety, and regulatory compliance.

Leak Testing. Source: ResearchGate.

The four most common leak testing methods are:

Bubble leak tests monitor for visual signs of a gas (usually air) leak from a pressurized system.

Pressure change tests monitor for loss of pressure or vacuum in the test object under stress conditions.

Halogen diode tests infuse the system with halogen-based tracer gas and detect leaks in sealed systems using diode sensors.

Mass spectrometer tests infuse the system with helium or a helium/air mixture and then use mass spectrometer equipment to detect gas leaks.

In some cases, inspectors may suggest using alternative techniques like dye penetrant or acoustic emission testing for more precise results.

Best NDT method for container assets.

NDT Testing: Regulations and International Standards

Non-destructive inspections must be conducted at regular intervals, according to specific standards from regulatory bodies.

The main regulatory standards for NDT testing include:

The above standards provide guidelines for the qualification, certification, and performance of NDT personnel, as well as the proper use of different NDT techniques and interpretation of the results.

Additionally, each industry must meet compliance requirements for regular asset management prescribed by respective regulatory bodies. Here&#;s a quick overview of the main regulatory bodies and standards related to asset maintenance.

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Ultimate Guide to NDT: Methods, Tools, and Applications