Published on 19-Jun-2023

What Is Nondestructive Testing?

What Is Nondestructive Testing?

Nondestructive testing (NDT) is a form of inspection that is popular in a wide variety of industries worldwide. Companies that work in aerospace, oil and gas, construction and many other sectors can rely on NDT to maintain the quality of their products without destroying any usable materials or halting the production process.

The nondestructive testing market as a whole is expected to grow by more than 6% by the year 2026. The industry is popular all over the world, particularly in Europe which has a share of more than 30% of the market, and NDT is used in major companies such as Applus+, Dekra and Nikon Metrology. NDT is quickly expanding and the need for qualified inspectors is expected to increase along with the industry.

Nondestructive testing, explained

Nondestructive testing is an umbrella term for 14 different inspection methods. While each method comes with its own requirements and on-the-job expectations, they are all able to examine products, materials and processes without destroying anything that could be used. 

In this way, nondestructive testing is less wasteful and more environmentally friendly than other forms of inspection. Companies are able to test their materials for imperfections, making sure their customers will be satisfied with the products that they receive, while also lowering the total cost of production and taking a closer look at the manufacturing processes.

NDT is used both during and after the manufacturing process. When inspectors work during the assembly process, they can make sure that all of the individual materials are measuring up to quality standards and that the process is functioning as expected. After assembly, inspectors can make sure that the final products meet safety standards and are usable for the customers. 

How it helps companies and organizations

The primary benefit of using NDT in the manufacturing process is that companies and organizations can check the quality of their products while reducing waste. Because of this, NDT is both an environmentally- and economically-friendly inspection option. Industries that utilize NDT in their manufacturing can end up saving millions of dollars each year.

Of course, companies that rely on NDT can save money just by maintaining the quality and reliability of their products. If an inspection finds that a piece of machinery isn’t functioning in the way that it should, company employees can fix the problem before it creates even bigger issues. For example, companies in the oil and gas industry can pay a small amount to examine and fix faulty equipment before a costly oil spill occurs.

Reducing waste is often a goal for companies that are trying to be more environmentally conscious. Once a product has been destroyed for an inspection, it can no longer be used and must be thrown out. NDT allows all materials to be used even after inspection and, as in the case with an oil spill, can help companies avoid any major problems that could harm the environment.

In addition to keeping the environment safe, NDT can also keep workers safe. Malfunctioning equipment can lead to serious injury; inspecting small imperfections in the equipment can pinpoint any potential safety hazards before workers are hurt (which, in turn, can also save the company money by avoiding any workers’ compensation fees).

Compared to more destructive forms of inspection, NDT can provide a more comprehensive look at the quality of a large number of products. Because the materials can continue to be used after inspection, nearly all products can be examined. Destructive inspections usually take a sample set of products to look for imperfections. If the damaged products are not included in the sample set, inspectors may miss an important detail.

Stopping a production process to conduct inspections can come with its own problems. The company may lose money by halting manufacturing, employees lose available working time and deadlines may not be met. NDT can keep production in motion because products can be inspected at any point in the process, even while everything is still running.

NDT testing methods

There are 14 total inspection methods that fall under the nondestructive testing umbrella, although six methods are used more frequently than others. These six methods include magnetic particle testing, liquid penetrant testing, radiographic testing, ultrasonic testing, electromagnetic testing, and visual testing.

Each method is classified by the equipment or penetrating medium that is used during the inspection; they are also generally divided into two groups, those involving advanced techniques and those using conventional techniques.

Advanced NDT techniques are methods that are typically newer and continue to evolve with advancing technology. They tend to be more complicated and confusing than conventional methods. Electromagnetic testing, laser testing methods, radiographic testing, and ultrasonic testing are all considered to be advanced techniques.

On the other hand, conventional techniques (which include all of the remaining NDT methods) are usually simpler than advanced techniques and don’t necessarily require the same level of specialized training that NDT technicians need for some of the advanced techniques. They are usually simpler than advanced techniques and don’t necessarily require the same specialized training that NDT technicians need for some of the advanced techniques.


Magnetic particle testing (MT) uses magnetic fields to find flaws on or near the surface of materials. When the magnetic fields are applied to the surface of an object, they will create a magnetic flux leakage when they make contact with a discontinuity. MT can be used on metals that are susceptible to magnetization, such as nickel, iron, or certain alloys. 

MT is divided into two different types of inspection: wet magnetic particle testing (WMPT) or dry magnetic particle testing (DMPT). Both processes start by sending a magnetic field through the material being inspected. After that, metal particles will be spread over the material; the magnetic flux leakages that are created during the first step draw the metal particles in, making the surface flaws easily visible.


Liquid penetrant testing (PT) is actually conducted much in the same way as MT, but instead of using magnetic particles, a low viscosity liquid (known as a penetrant) is spread over the surface of a material and will attract to shallow flaws. This means that PT can be performed on both magnetic and non-magnetic materials, but will not work with porous surfaces.

Once the penetrant (which can be seen either by the naked eye or, if it is fluorescent, with a black light) is applied to the surface of a material, it is left to sit on the surface for a period of time, which is called the “penetrant dwell time.” The material will then be cleaned off, removing the penetrant from the surface but leaving some in the cracks and fissures on the object. 

A coating of a developer will be applied after the penetrant is cleaned off of the surface, then let sit for a “developer dwell time.” The penetrant will move upward and combine with the developer, creating a visible indication of where the surface flaws are on the object.


The simplest way to understand how radiographic testing (RT) works is to understand how an x-ray works. In fact, x-rays are often used on thinner or less dense materials, while gamma rays are preferred for materials that are thicker or denser. 

When a material is exposed to radiation, the radiation rays will pass through the object and to a recording medium that is placed on the other side. When more radiation passes through the object, it will create dark areas on the recording medium, and when less radiation passes through, it will create light areas. A different amount of radiation from the surrounding material will pass through any defects or discontinuities.


Ultrasonic testing (UT) works much in the same way as a fetal ultrasound during pregnancy: a high-frequency sound is sent through the material being inspected. When the sound waves hit part of the material with a different density and acoustic velocity, they will reflect back to the sending unit. The reflected sound waves can then be displayed as an image of the material’s density.

The sound frequencies that are typically used in UT are between 1.0 and 10.0 MHz. These frequencies are too high to be audible and they will not travel through the air, so generally a liquid or gel (called a “couplant”) is used between the face of the sound machine and the material being inspected. Lower frequencies can penetrate deeply, but aren’t as sensitive to flaws as higher frequencies.


There are three main techniques used in electromagnetic testing (ET); eddy current testing, which uses an alternating current coil to create a magnetic field (the field is disrupted when it comes into contact with a flaw in the material being inspected), alternating current field measurement, which uses a specialized probe to create the magnetic field, and remote field testing, which is most often used to inspect ferromagnetic tubing.


Visual testing (VT) is by far the most commonly used inspection method, as well as the simplest. VT requires NDT technicians to examine the surface of a material with their eyes to find any imperfections or problems. Sometimes technicians will use magnifying glasses, mirrors, borescopes or other forms of technology to get a better look at the material being inspected.


In acoustic emission testing (AE), an external force such as a change in temperature or pressure will be applied to the material being inspected. When the material is put under stress, high-frequency elastic waves will be produced on the material’s surface.

AE uses sensors to detect the elastic waves on the surface of the material being inspected; multiple sensors will record data that, when analyzed, will provide information about any discontinuities that can be found in the material.


Guided wave testing (GW) is predominantly used to inspect the structure and stability of piping. GW sends ultrasonic waves along the length of the pipe using a transducer ring or exciter coil. GW inspectors can use a computer to both send the ultrasonic waves across the pipe and to record the results. 

The equipment used to create ultrasonic waves can be designed specifically for the pipe that is being inspected, and it has the ability to perform inspections without having to remove any coatings or insulation from the pipe. GW is able to find both inside-diameter and outside-diameter flaws, but is unable to distinguish which is which.


Laser testing methods (LM) is a broader term for three different types of inspection: holography, profilometry and shearography. All three methods use lasers in some way to detect imperfections on the surface of the piece of equipment being inspected.

In holographic testing, the laser will detect any changes on the surface of the material while some form of stress (like heat or pressure) is applied. The laser will then produce an image of the results. In profilometry, the laser will be applied to the surface of the material, then an image of the results will be sent to a computer. The process will happen again when applying stress to the material, then the two images will be superimposed, revealing any defects.

Shearography will scan the surface of a pipe or tube and pass any reflected light to a lens, focusing the light onto a photo-detector. When the distance between the laser and the surface of the tube changes, the reflected light will begin to create a three-dimensional image of the object’s surface.


Like LM, leak testing (LT) consists of different inspection methods: bubble leak testing, which looks for gas leaking from a pressurized system, pressure change testing, which looks for leaks by either pressurizing or pulling a vacuum on an object, halogen diode testing, which pressurizes a system with a combination of air and a halogen-based tracer gas, and mass spectrometer testing, which pressurizes the object being inspected with either helium or a helium-air mixture, then examines the surface using a sniffer.


Magnetic flux leakage (MFL) will apply a magnetic field to a ferromagnetic material, then will find any discontinuities in the standard flux patterns. MFL is typically used to inspect piping, tubing or tank floors, although there are other applications as well.


Neutron radiographic testing (NR) will penetrate a metallic material using low energy neutrons. The force of the neutrons is reduced when they come into contact with most organic materials, then allowing the materials to be seen within the object being inspected. 


Thermal/Infrared testing (IR), also called infrared thermography, will examine the surface temperatures of an object when heat flows from, to or through the object. IR relies on infrared radiation to measure the surface temperatures during the inspection process.


In vibration analysis (VA), the vibration signatures of a piece of rotating machinery will be analyzed using a type of sensor. The three most commonly used sensors in VA are displacement sensors, velocity sensors, and accelerometers. The results that are produced when the vibration signatures are analyzed will produce information about the equipment’s condition.

Getting an education in NDT

In order to officially become certified as a Level I, II or III NDT inspector, all prospective inspectors must obtain a certain amount of training and supervised work experience. Those hours will change depending on the level of certification and the specific inspection method they are becoming certified in.

When a potential inspector is working on their certification prerequisites, they have a number of avenues in which they can obtain their training and work experience. NDT training can be completed through a college or university, vocational or technical school, the U.S. Armed Forces, commercial training companies or through the training department at a specific company

Inspectors who have completed at least two years of post-high school education in either science or engineering often have a lower number of required training hours as they will receive some basic training through their educational institution. 

Formal education can provide many other benefits for NDT technicians apart from required training. Inspectors may be able to cross-train, taking courses or earning certifications in more than one inspection method. For example, the NDT program at Ridgewater College in Willmar, MN, covers all of the major inspection methods. This can make NDT technicians more competitive when looking for a job. 

Students who choose to pursue four years of NDT education can be presented with a variety of additional opportunities within the field. Technicians with a bachelor’s degree may begin a career in quality assurance, management, research or engineering design, or they may decide to become an NDT trainer or teacher. Those who wish to go on to a graduate or doctoral program can start their education by obtaining an NDT-related bachelor’s degree. 

When gaining an education in NDT, future inspectors are able to network with their professors and fellow students, gaining connections that can help them to further their careers. Networking can help students to find jobs within their field or, if they’re interested in conducting NDT research, to find opportunities to research either with other NDT students or with professors. 

Possibly one of the greatest benefits to come from gaining an education in NDT is an increase in salary; undergraduates can earn $70,000 – $90,000 in their first jobs after graduation. 

Technicians that gain more experience and in-depth knowledge through education may be able to advance to higher certification levels, which also come with higher average salaries. Level II-certified inspectors have an average salary range of $60,000 – $80,000, while Level III-certified inspectors have an average salary range of up to $120,000 – $150,000. 

NDT technicians who have already began their careers can also benefit from going back to school. Because NDT is so technology-centric, technicians are expected to stay up-to-date with changing technologies and NDT techniques. Even if a student does not want to pursue a full degree, they may be able to audit NDT classes at their local university or technical school. 

NDT training centers can also provide future technicians with the education that they’ll need in their careers. Some organizations, like the American Institute of Nondestructive Testing, will provide training programs that are at least partially virtual, meaning students will not have to travel for the entire duration of their training (in this case, AINDT only requires 18 days of training in-person at the facility in Baxter, MN). 

The in-person training is a key part of NDT education. While the students can learn from reading assignments, lectures, quizzes and essays through the online training portion, in-person training will allow them to obtain a more hands-on education, applying the knowledge they’ve already gained and practicing on real NDT equipment with the aid of their instructors. 

Although students will not obtain a degree from an NDT training center, they will gain the knowledge and training needed for their intended NDT inspection method. They may also be able to benefit from job placement services, such as the ones offered by AINDT, which holds a 90% placement rate for graduates. 

But formal schooling isn’t the only education option for prospective inspectors. Each branch of the United States military can use NDT technicians to inspect the ships, tanks or other equipment that they frequently use. 

Technicians will have to complete basic training for their preferred branch, which can range from 7.5 to 12 weeks. Basic training will give them the general knowledge needed to begin a career in that branch. After that training is completed, technicians will then go on to a specialized school where they can gain the specific education needed to become an NDT inspector

Surehand wants all NDT technicians and industrial inspectors—regardless of their education, certification level or inspection method—to advance in their careers and find the opportunities that are right for them. That is why our job placement process is so simple. Certified technicians only need to create an online profile—including their job preferences, certifications, work history, and training—then let their future employers come to them. 


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