Last revised: February 14, 2025

Nondestructive Testing (NDT) equipment is used to detect, inspect, and measure flaws, bond integrity, and other material conditions without permanently altering or destroying the examined part or product. NDT equipment includes a wide variety of instruments and systems. Examples include thickness gauges, flaw detectors, material condition testers, and eddy current instruments, as well as devices that measure conductivity, resistivity and corrosion. Some NDT equipment can be mounted on a rack, in a cabinet, or on a printed circuit board (PCB). Other devices are designed to be held by hand or operated from a benchtop. Common technologies for NDT equipment include radiography or X-ray analysis, laser holography or gaging, penetrant testing, and magnetic particle testing.

Acoustic Emission Instruments and Noise Detectors

Acoustic emission instruments and noise detectors are NDT equipment for monitoring conditions and detecting changes in mechanical, electrical and process systems. Acoustic emission instruments are used to detect shorting or arcing in electrical power distribution systems. In mechanical systems, flaws also provide specific acoustic or vibrational responses. For example, if a break, deformation or other failure occurs, acoustic emission sensors can detect the burst of high frequency caused by the event. Noise detectors are used to detect leaks or changes in process components such as steam traps, pipes, valves, and pressure vessels. Ultrasonic (UT) noise detectors can identify changes in bearings, gearboxes, and rotating machinery due to changes in wear or load.  Other ultrasonic inspection methods such as the pulse-echo technique detect flaws and estimate sizes by comparing the amplitude of a reflected echo from an interface with that of a reference interface of known size.

Eddy Current Instruments

Eddy current instruments are NDT equipment used to induce detectable eddy currents in conductive materials. They detect flaws, determine thickness, inspect welds, measure conductivity, and sort alloys. Eddy current instruments include a straight or angled magnetic probe and an analog or digital meter with a zero reference point. Moving the magnetic probe over the surface of a conductive material such as a metal tube induces circulating currents (eddies) of electrons that oppose the externally applied magnetic field from the probe. Surface irregularities such as cracks and corrosion interrupt the surface flow of eddy currents and are detected.

Uses

Infrared (IR) and radiographic NDT equipment is used to determine the thickness or basis weight of webs, sheet materials, or coatings. With nonmetallic materials such as plastic films or webs, radiation is reflected back or transmitted in order to determine absorption levels. Increases in mass, density, or thickness result in increased absorption. Gauges are often calibrated with samples of a known thickness, density, or mass. Equipment that uses penetrating X-rays or gamma rays is commonly available. Densitometers are used to quantity the density variants in X-ray images. Penetrameters or other X-ray opaque gage references are positioned with the part during imaging for sizing internal cracks, pores, defects and other features.

Penetrant testing systems are NDT equipment that detects discontinuities in the surface of components. Parts are sprayed with aerosols, immersed in liquids, or dusted with powders. Capillary action pulls the penetrant into surface flaws. Next, a cleaner is used to remove the residual surface penetrant from the part that the only penetrant that remains is in the flaws or cracks. Many NDT penetrant systems are available. Red dye penetrants are visible under normal light. Fluorescent penetrants may require ultraviolet (UV) light or backlight illumination.

NDT Equipment FAQs

What are the key differences between the various types of nondestructive testing methods?

NDT methods are essential for evaluating the properties of a material, component, or system without causing damage. Here are some key differences between various types of NDT methods based on the information provided:

Ultrasonic Testing (UT)

• Uses high-frequency acoustic energy beams to detect flaws or measure thickness.
• Suitable for inspecting non-conductive materials like composites, plastics, wood, and ceramics.
• Can indicate material thickness, speed of sound in material, elastic modulus, leak detection, and fault detection.

Eddy Current Testing (ET)

• Induces eddy currents in conductive materials to detect flaws, measure thickness, and verify heat treatment.
• Useful for inspecting metals and alloys, especially on rough surfaces or surfaces with coatings.
• Does not require contact with the work surface

Radiographic Testing (RT)

• Uses X-rays or gamma rays to capture images of the internal structure of a part.
• The density and composition of internal features alter the intensity in the X-ray image.
• Suitable for detecting surface and subsurface flaws.

Magnetic Particle Testing (MT)

• Involves magnetizing a part and using magnetic particles to detect flaws at magnetic poles.
• Effective for detecting surface and near-surface discontinuities in ferromagnetic materials.

Liquid Penetrant Testing (LP)

• Involves applying a liquid to the surface of a material and then using a developer to draw out the liquid from any flaws.
• Suitable for detecting surface-breaking defects in non-porous materials.

Acoustic Emission Testing

• Monitors acoustic or vibration responses to detect flaws or structural inadequacies.
• Useful for assessing the structural adequacy of tanks and pressure vessels.

Optical-Based Testing

• Includes methods like laser shearography and holographic interferometry to detect flaws or measure residual stress.
• Utilizes optical techniques to provide detailed surface and subsurface information.

How does Radiographic Testing (RT) work?

Radiographic Testing (RT) is a nondestructive testing method that uses penetrating X-rays or gamma rays to capture images of the internal structure of a part or finished product. Here's how it works:

Radiation Source: RT employs X-rays or gamma rays as the radiation source. These rays are capable of penetrating materials to varying degrees depending on the material's density and thickness.

Image Capture: As the radiation passes through the material, it is absorbed at different rates by different features within the material. This variation in absorption creates a contrast on the radiographic film or digital detector, forming an image that reveals the internal structure.

Analysis: The resulting image is analyzed to identify any internal flaws or defects, such as cracks, voids, or inclusions. The density and composition of the internal features will alter the intensity or density of these features in the X-ray image, making it possible to detect both surface and subsurface flaws.

What are the applications of Eddy Current Testing (ET)?

Eddy Current Testing (ET) is a versatile nondestructive testing method with a wide range of applications, particularly in the inspection of conductive materials. Here are some of the key applications:

Flaw Detection

ET is effective in detecting surface and subsurface cracks in conductive materials. It is particularly useful for identifying small subsurface cracks and surface irregularities such as corrosion and cracks in metal sheets and tubing.

Thickness Measurement

ET can be used to measure the thickness of metal sheets and detect variations in thickness, which is crucial in applications like corrosion detection under aircraft skin.

Weld Inspection

ET is used to inspect welds, particularly for detecting surface cracks on weld caps and in heat-affected zones (HAZ). It is often used in conjunction with ultrasonic testing for comprehensive weld inspection.

Conductivity Testing

ET measures the electrical conductivity of materials, which can be used to sort alloys and verify heat treatment processes. This is useful in identifying and sorting ferrous and nonferrous alloys.

Surface Inspection

ET is employed to inspect machined parts and metal stock for surface cracks, including the area around fasteners in aircraft and other critical applications.

Corrosion Detection

ET can detect and quantify corrosion on the inside of thin metal structures, such as aluminum aircraft skin. It is particularly effective with low-frequency probes for locating corrosion on multiple layers of metal.

Bolt Hole Inspection

ET is used to detect cracking inside bolt holes, often utilizing automated rotary scanners for efficient inspection.

Tubing Inspection

ET is applied in both in-line inspection during manufacturing and field inspection of tubing, such as heat exchangers, to detect cracking and thickness variations.

What are the limitations of Eddy Current Testing?

Eddy Current Testing (ET) is a widely used nondestructive testing method, but it does have certain limitations. Here are some of the key limitations based on the information provided:

Material Limitations

ET is limited to conductive materials, meaning it cannot be used on non-conductive materials such as plastics or ceramics.

Depth of Penetration

The depth of penetration for eddy currents is limited. Eddy current density is highest near the surface, which means the technique is most effective for detecting surface and near-surface defects. The standard depth of penetration is defined as the depth at which the eddy current density is 37% of its surface value.

Sensitivity to Material Properties

Variations in material properties such as conductivity and magnetic permeability can affect the test results. These properties can generate noise that limits flaw resolution due to greater background variations.

Frequency and Coil Limitations

The test frequency and coil size can affect the sensitivity, resolution, and penetration of the test. Higher frequencies increase near-surface resolution but limit depth penetration, while lower frequencies increase penetration but reduce resolution.

Complexity and Cost

Eddy Current Testing can be relatively expensive and requires skilled technicians to interpret the results accurately. The technique involves complex equipment and interpretation of reference standards to ensure accurate and replicable measurements.

Surface Condition

While ET does not require contact with the surface, the surface condition can still affect the results. Rough surfaces or surfaces with coatings can influence the eddy current flow and the resulting signal.

How does Eddy Current Testing compare to other NDT methods like Ultrasonic Testing?

Eddy Current Testing (ET) and Ultrasonic Testing (UT) are both NDT methods used to evaluate the integrity of materials and structures without causing damage.

Material Suitability

Eddy Current Testing (ET): Primarily used for conductive materials, such as metals and alloys. It is not suitable for non-conductive materials like plastics or ceramics.

Ultrasonic Testing (UT): Can be used on both conductive and non-conductive materials, including composites, plastics, wood, and ceramics.

Depth of Penetration

ET: Has limited depth penetration, with eddy current density being highest near the surface. It is most effective for detecting surface and near-surface defects.

UT: Capable of deeper penetration, making it suitable for detecting subsurface flaws and measuring thickness.

Sensitivity and Resolution

ET: Sensitive to surface defects and small subsurface cracks. The sensitivity and resolution are affected by factors such as test frequency, coil size, and material properties.

UT: Provides high resolution for both surface and subsurface defects. It uses high-frequency acoustic energy beams, which can be adjusted for different inspection needs.

Applications

ET: Commonly used for flaw detection, thickness measurement, weld inspection, conductivity testing, and corrosion detection in conductive materials.

UT: Used for flaw detection, thickness measurement, and material characterization in a wide range of materials, including those that are non-conductive.

Surface Condition

ET: Does not require contact with the surface, making it useful for rough surfaces or surfaces with coatings.

UT: Typically requires a coupling medium to transmit sound waves into the material, which may be a consideration for certain surface conditions NDT Services.

Cost and Complexity:

ET: Can be relatively expensive and requires skilled technicians for accurate interpretation of results.

UT: Also requires skilled operators, but the equipment and techniques are well-established and widely used across various industries.

NDT Equipment Media Gallery

References

GlobalSpec—Nondestructive Examination Tools: An Introduction

GlobalSpec—What are Ultrasonic Flaw Detectors?