Technician examining an industrial radiographic film in front of a light box, focusing on the weld inspection of a tank.

Radiographic Testing: A Foundational Method for NDT

Discover the advantages of the radiographic testing (RT) method, understand the basic principles of RT, and explore the variety of techniques for applying this highly adaptable method in nondestructive testing across industries.

Table of Contents
OverviewAdvantages and LimitationsBasic PrinciplesTechniquesIndustry Applications

What Is Radiographic Testing and How Is It Used in NDT?

Radiographic testing (commonly referred to as industrial radiography) is a method of viewing the internal conditions and structure of a component using ionizing electromagnetic radiation of X-rays and/or gamma rays to produce visible images. Much like a doctor uses X-ray to see a broken bone in a patient, NDT inspectors use radiographic testing to see defects inside components used for critical industrial applications.

This method is highly beneficial to numerous industries due to its ability to find subsurface flaws that may lead to failure of a component in service. However, it does come with inherent safety risks, as radiation can be dangerous to humans without the proper precautions.

A skilled NDT specialist applies different RT techniques depending on the nature of the inspection while adhering to proper regulations that protect both themselves and the public.

The primary objectives of RT in industrial applications include:

  • Locating internal defects: RT can easily locate internal structural discontinuities using visual comparison with known geometric features of the object being inspected, and it creates a visual rendition of internal voids and fabrication errors that is easy to interpret.

  • Volumetric examination: RT is considered by many to be the most universal approach to volumetric examination, which examines the internal integrity of the object being inspected.

  • Creating a record: RT inspections produce a permanent record of the inspection—including evidence of the sensitivity of the test when image quality indicators (IQIs) are used—and can provide a digital record of the object being inspected for subsequent display.

Advantages and Limitations of Radiographic Testing in NDT

The ability of RT to evaluate the internal structure of an object without damaging it makes it invaluable in many industries.RT is a versatile method applicable in many industries, including manufacturing, aerospace, oil and gas, and more.

Advantages of Radiographic Testing

  • Discontinuity Evaluation: RT can easily locate internal structural discontinuities using visual comparison with known geometric features of the test object.

  • Versatility: RT can be applied to most types of materials.

  • Volumetric Inspection: RT is considered by many to be the most universal approach to volumetric examination.

  • Clarity: RT inspections yield a visual rendition of internal voids and fabrication errors that is readily interpretable.

  • Records: RT inspections create a lasting record of the inspection. When image quality indicators (IQIs) are used, they show how sensitive the test was and produce a digital record of the test object for later viewing.

  • Sensitivity: RT can detect small changes in thickness and density, down to about 1%, along the path of the X-ray beam.

Limitations of Radiographic Testing

  • Expense: RT is a relatively expensive method of nondestructive testing.

  • Complex Geometries: It is impractical to use RT on specimens of complex geometry.

  • Defect Size: Very small defects can be hard or impossible to see. Small, isolated defects that are less than 2% of the total thickness are usually not detected.

  • Access: There must be access to both sides of the object being inspected.

  • Discontinuity Orientation: The way a defect is positioned in relation to the direction of the radiation beam is very important. Some defects, like thin layers, can be missed by radiography if they are perpendicular to the radiation path.

  • Training and Experience: RT requires highly trained and skilled personnel to perform inspections correctly and safely. This makes RT training and certification essential.

  • Safety: The radiation hazards in RT mean that operating licenses issued by state and federal agencies are required for inspectors.

Modern advances make RT a method of great importance for continued development of automation and artificial intelligence. With the continued growth of additive manufacturing, radiographic inspection will find a home as one of the only ways to gather information regarding complex internal components that are otherwise inaccessible.”

Bryan Shumway
RT Level III

How Radiographic Testing Works: Basic Principles

To fully understand radiographic testing, it is important to be generally familiar with radiation and how it interacts with and affects materials at the atomic level.

Atomic Interaction with Materials

Understanding the fundamental building blocks of matter is crucial for comprehending the principles behind RT. Atoms, the basic units of matter, play a key role in how materials interact with radiation.

By exploring the structure of atoms, elements, isotopes, and radioisotopes, we can better understand how RT detects and analyzes defects within materials.

Atoms

Atoms are the basic units of matter, consisting of a nucleus with protons and neutrons, and electrons orbiting it. Atoms have an equal number of protons and electrons, and the number of protons determines the type of atom.

Element

Matter that is composed of only one type of atom. Understanding the elemental composition helps in predicting how materials will interact with radiation.

Isotope

A variant of an element with a different number of neutrons in its nucleus. The number of neutrons is important to balance the stability of the nucleus.

Radioisotopes

Isotopes that are radioactive due to unstable nuclei. When nuclei are too heavy, they emit radiation to become stable in a process called radioactive decay.

Radiation Types in RT

When atoms become radioactive, they can emit different types of radiation that each have different ways of interacting with materials. Gamma and X-ray radiation are used in radiographic testing due to their high energy and ability to penetrate through many materials.

X-ray Radiation: X-rays are generated from a tube head powered by an electrical current. High-speed electrons are abruptly halted by a tungsten target, converting their kinetic energy into heat and X-ray photons. Unlike gamma radiation, X-rays have controls that allow for adjustment in both quantity and energy intensity.

Gamma Ray Radiation: Energy photons (packets of electromagnetic radiation) are emitted from the nucleus of unstable isotopes. Gamma waves penetrate most matter but can be stopped by a dense (high atomic number) material such as lead, depleted uranium, and very thick steel. It is most applicable in remote areas where electrical power to produce X-rays is not practical.

Illustration showing the penetration power of different types of radiation: neutron (n) particle radiation, gamma rays (γ) electromagnetic radiation, X-rays (X) electromagnetic radiation, beta (β) particle radiation, and alpha (α) particle radiation. It depicts how each type of radiation is attenuated by different materials such as water, lead, wood, and paper.

Exposure Devices and Radiation Sources

Radiography requires a medium to capture and evaluate the image of a test subject. Traditionally, film was used. However, with advances in computer and medical radiographic technology, digital radiography (DR) and computed radiography (CR) techniques have become popular for industrial applications.

When NDT professionals are setting up for RT, regardless of the imaging method used, they must:

  1. Aim X-ray or isotope radiation at the test object.

  2. Place a film or digital sensor behind the object.

  3. Allow radiation to pass through the object to the sensor for a set time.

  4. Process the film or sensor.

  5. Examine the resulting image.

The production and process is slightly different depending on the type of radiation source.

X-ray Sources

  • Require electricity to produce radiation.

  • The exposure time and energy level are adjusted based on the material being tested.

Gamma Ray Sources

  • Radiation is generated by small pellets of radioactive material isotopes.

  • Isotopes are selected based on the energy and intensity needed for the material being tested.

Technician performing an X-ray inspection on an aircraft to ensure structural integrity and safety compliance.

Aligning and measuring the distance of an X-ray tube head to the inspection area of interest on an aircraft door frame.

A specialist reviews computed radiography images for quality assurance and defect detection in industrial components.

Radiographic Testing Techniques Used in NDT

Radiographic Testing (RT) employs a variety of techniques to detect and measure discontinuities in materials. These techniques can be categorized based on how the ultrasonic waves are introduced into the material and the specific modes and advanced methods used to enhance flaw detection and measurement.

There are four primary methods used.

Film Radiography

  • The oldest and “traditional” technique of image acquisition.

  • Uses radiographic film to capture images.

  • An Image Quality Indicator (IQI) is used to show that the appropriate radiograph quality level has been achieved.

  • The radiographic film requires processing (development) in a darkroom to view the image.

Computed Radiography (CR)

  • Uses a reusable phosphor plate or cassette instead of film.

  • Imaging plates contain photostimulable phosphors that store radiation as a latent image. The image is scanned by a laser to produce an electrical signal.

  • This signal is captured and converted to a digital image for viewing on a high-resolution monitor.

  • Acts as a bridge between film radiography and digital radiography.

Digital Radiography (DR)

  • Uses fixed flat panel radiation detector (digital detector array) to capture images directly by converting radiation into an electric charge and then into a digital image.

  • The electrical charge is sent to a processor that assembles an image as a digital file.

  • The speed of image display of DR is much faster than even that of CR (near real-time).

Computed Tomography

  • Uses a similar detector as digital radiography.

  • Multiple radiographic exposures are taken at different angles to create cross-sectional images.

  • The multiple images taken create a 3D map of the conditions of the object being tested.

Add Radiographic Testing Certification to Your Qualifications

ASNT certifications enable you to become a qualified Level II or Level III in RT or obtain your industrial radiography radiation safety personnel (IRRSP) certification.

What Certification Is Right for Me?

Application of Radiographic Testing in NDT Across Industries

Radiographic Testing (RT) can be used on a wide range of materials, including metals, composites, and concrete. Its ability to offer detailed internal imaging, high penetration capability, and permanent records makes it invaluable for ensuring the integrity and safety of critical components and structures.

These characteristics make it an important nondestructive testing (NDT) method that’s widely used across a range of industries.

Energy

In the oil and gas industry, NDT professionals use RT responsible for inspecting pipelines, storage tanks, and offshore structures for defects and corrosion to ensure compliance with safety standards and prevent environmental hazards. RT is crucial in ensuring the safety and integrity of nuclear power plants.

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"A composite image showcasing various energy sources: solar panels in the foreground, oil pump jacks in the middle ground, and wind turbines and a power plant in the background. The scene illustrates the diversity of energy production methods at sunset.

Aerospace

RT is used in the aviation industry to ensure the structural integrity of aircraft components, engines, and airframes. Professionals in aviation NDT conduct inspections using techniques such as X-ray inspection and computed tomography to check critical components and ensure they meet stringent safety standards.

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A technician performing maintenance or inspection work on the landing gear of a large commercial airplane inside an aircraft hangar. The scene is illuminated with a blue tint, highlighting the aircraft's engines and the structural details of the hangar.

Transportation

RT is vital for inspecting critical components for boats and submarines, ensuring castings made with copper-nickel alloys are free of conditions such as shrinkage or porosity.

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A modern high-speed train moving swiftly through a train station at sunset. The motion blur effect emphasizes the train's speed, with vibrant colors in the sky and station lights creating a dynamic and futuristic atmosphere.

Manufacturing

RT is used to ensure the quality and reliability of automotive components such as engine parts, chassis, and welds, to detect defects and deviations from specifications. With new technologies like 3D printing and additive manufacturing, NDT is crucial for quality control and ensuring dimensional accuracy and material integrity.

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A modern manufacturing facility with robotic arms working on an automated assembly line. The scene is well-lit with blue overhead lighting, showcasing advanced machinery and precision engineering in a clean, industrial environment.

Infrastructure

In infrastructure and construction, NDT practitioners use RT is used to inspect structural steel, welds, and other critical components during construction and maintenance to detect flaws such as cracks, voids, and inclusions.

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A large infrastructure project featuring a highway under construction. Several cranes are positioned along the unfinished sections of the elevated roadway and bridge. The scene is set on a clear, sunny day with blue skies and some scattered clouds.

Example: RT in the Real World

RT techniques are used in the preservation and conservation of art and cultural heritage artifacts. Techniques such as X-ray fluorescence spectroscopy and digital imaging enable conservators to noninvasively analyze materials, detect hidden features, and assess the condition and authenticity of artworks, sculptures, historical artifacts, and archaeological specimens.

X-ray image of an ancient ritual figure, showing its internal structure and the joints or materials used in its construction, with the figure's form partially blurred, emphasizing its arm and leg positioning.

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Radiographic Testing (RT) Courses

Advance your skills and knowledge with courses and webinars on radiographic testing.

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Programmed Instruction Series: Introduction to NDT

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Nondestructive Testing Handbook, Vol. 10: Overview, 3rd ed.

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