Non-destructive testing (NDT) is a technique used in the aerospace and defense industry to evaluate the properties of a material, component, or structure without causing any damage or altering its physical and chemical characteristics. NDT techniques are based on the physical principles of different phenomena, such as ultrasonic waves, X-rays, magnetic fields, and thermal conductivity.
NDT methods are critical in ensuring the safety, reliability, and integrity of various parts and components used in aerospace and defense applications such as aircraft, missiles, satellites, and other defense systems.
According to BIS Research, the global non-destructive testing services in the aerospace and defense market are estimated to reach $1.81 billion 2033 from $1.29 billion in 2022, at a CAGR of 3.34% during the forecast period 2023–2033.
This article explores the top five non-destructive testing techniques that are commonly employed in the aerospace and defense industry.
1. Ultrasonic Testing
Ultrasonic testing (UT) is a popular NDT technique used in aerospace and defense industries to detect internal defects in materials and components. This method uses high-frequency sound waves that penetrate the fabric and bounce back when they encounter a fault, such as a crack or void. Moreover, UT is commonly used to inspect aircraft components, such as fuselage, wings, missiles, and rocket components.
There are several types of ultrasonic testing used in aerospace and defense, such as:
- Pulse-Echo Testing- This is one of the most common ultrasonic testing techniques used in the industry. It uses a single transducer to transmit and receive sound waves, making it suitable for testing relatively thin parts. Pulse-echo testing is particularly effective for detecting internal defects in materials and components, such as cracks and voids.
- Pitch-Catch Testing- This ultrasonic testing technique uses two transducers to transmit and receive sound waves. This method is commonly used for testing thicker parts, such as castings and forgings, where the sound waves must travel through a greater material thickness. Pitch-catch testing is particularly effective for detecting surface and near-surface defects.
- Time-of-Flight Diffraction (TOFD)- This ultrasonic testing technique uses two transducers and is suitable for detecting defects at the surface and near subsurface areas of materials. TOFD is particularly effective for detecting small cracks and defects that other testing techniques may miss.
- Phased Array Ultrasonic Testing (PAUT)- This newer ultrasonic testing technique uses a phased array transducer to scan and inspect large areas quickly and efficiently. PAUT is particularly effective for detecting defects in complex structures, such as aircraft engines and missile guidance systems.
2. Radiographic Testing
Radiographic testing (RT) is a widely used non-destructive testing technique that employs X-rays or gamma rays to inspect the internal structure of materials and components. This method is beneficial in detecting internal defects such as cracks, corrosion, and other discontinuities that may not be visible to the naked eye.
In the aerospace and defense industry, radiographic testing is commonly used to inspect vital components, such as aircraft engines or missile guidance systems, subject to extreme operating conditions. Therefore, detecting and correcting any internal flaws in these components is crucial before they can lead to catastrophic failures or accidents.
There are various types of radiographic testing methods available to cater to different applications and component types, such as:
- X-Ray Radiography- This radiographic testing method uses X-rays to create an image of the internal structure of a component, such as for testing metal components such as castings and welds. This method works by directing a beam of X-rays at the component, penetrating the material, and creating an image on a film or digital detector on the other side of the component.
The density of the component determines how much radiation is absorbed, and the resulting image shows the areas of different radiation absorption, revealing any flaws in the material. - Gamma Radiography-This is another radiographic testing method that uses gamma rays to create an image of the internal structure of a component. Gamma rays have a higher energy than X-rays, which can penetrate thicker materials or components such as tanks and pipelines. This method uses a radioactive source to produce gamma rays that penetrate the component and create an image on a film or digital detector.
- Computed Radiography (CR)- This radiographic testing method uses a unique imaging plate to capture the X-ray or gamma-ray image. The imaging plate is scanned with a laser, which releases the stored energy from the radiation exposure, creating a digital image. The resulting image is then digitized and processed using computer software to produce a detailed radiographic image. This method offers high-quality images and is more efficient than traditional film-based radiography.
- Digital Radiography (DR)- This radiographic testing method uses a digital detector to capture the radiographic image. The sensor produces a digital image processed using computer software to produce a detailed radiographic image. Like CR, this method is faster, more efficient, and offers high-quality images compared to traditional film-based radiography.
- Computed Tomography (CT)- This advanced radiographic testing method generates 3D images of a component using X-rays to visualize the internal features of high-density materials used in the aerospace industry.
CT scans use a rotating X-ray beam to capture images of the component from multiple angles, which are then processed using computer software to create a 3D image of the internal structure of the component.
3. Eddy Current Testing
Eddy current testing uses the principles of electromagnetic induction to generate and detect eddy currents in the material being tested. The aerospace and defense industries commonly use eddy current testing to inspect aircraft and missile components, such as landing gear, engine parts, and fuselage.
The mentioned components are typically made of conductive materials such as aluminum, titanium, and steel, ideal for eddy current testing.
The eddy current testing method involves placing a coil close to the material's surface and passing a high frequency alternating current through it, which induces eddy currents that generate magnetic fields that interact with the coil. The resulting changes in the coil's impedance can be measured and analyzed to detect anomalies or variations in the material's electrical conductivity.
There are several types of eddy current testing used in aerospace and defense, such as:
- Surface Testing- This is useful in detecting surface-breaking cracks, corrosion, and other surface irregularities. This method involves placing a single probe in contact with the material's surface and scanning it for defects, such as cracks or corrosion. It is commonly used to inspect aircraft skins, turbine blades, and other components with a smooth surface.
- Immersion Testing- This examines the internal surface of tubes, pipes, and other complex geometrical components. This method involves immersing the component in a liquid, such as water or oil, and then passing the probe through the liquid to scan the component for defects. Moreover, the liquid used in immersion testing can be formulated with various additives to enhance its ability to detect defects.
- Array Probe Testing- This is an advanced eddy current testing method that uses multiple coils to scan large areas of components. This method helps detect inaccessible defects in inaccessible regions and parts with complex geometries.
4. Magnetic Particle Testing
Magnetic particle testing (MPT) is a non-destructive testing (NDT) method to detect flaws or defects in ferromagnetic materials such as iron, nickel, and cobalt. This method is commonly employed in the aerospace and defense industries to inspect aircraft and missile components, such as landing gear, engine parts, and fuselage.
The method works by applying a magnetic field to the surface of the material being tested and then using a fine powder of magnetic particles, usually in a liquid suspension, to the surface.
When a magnetic field is applied to the surface of a ferromagnetic material, the field penetrates the material and causes the magnetic domains to align with the area. This creates magnetic poles on the material's surface, which attract magnetic particles.
Suppose the material has cracks, inclusions, or other discontinuities. In that case, magnetic particles will accumulate at the site of the defect due to the magnetic field, which creates a visible indication of the flaw, making detection easier for the inspector.
Following are the types of magnetic particle testing used in the aerospace and defense industry:
- Wet Magnetic Particle Testing- This is the most common type of MPT used in the aerospace and defense industry. It involves applying a liquid suspension of magnetic particles to the surface of the tested material. The particles are usually suspended in oil or water-based solutions to create a slurry that can be easily applied to the test surface.
The magnetic particles adhere to the magnetic fields generated by the surface defects, creating a visible indication that the inspector can detect. Wet magnetic particle testing is particularly effective for inspecting large or complex parts, such as aircraft engine components, requiring a uniform and consistent magnetic field. - Dry Magnetic Particle Testing- This tests smaller or more superficial parts, such as fasteners, bolts, and nuts. Dry magnetic particle testing uses a dry powder of magnetic particles dusted onto the surface of the tested material. The powder adheres to the surface defects, indicating the inspector can detect them. It is often used when liquid-based inspections are not feasible and could pose a safety hazard.
5. Penetrant Testing
Penetrant testing or liquid penetrant inspection (LPI) method detects surface-breaking defects in various materials, including metals, plastics, ceramics, and composites, by applying a liquid penetrant to the surface, which is drawn into any defects. The excess penetrant is removed, and a developer is applied, causing the penetrant in the defects to become visible to the inspector.
There are two types of penetrant testing used in aerospace and defense non-destructive testing-:
- Visible Dye Penetrant Inspection (VPI)- This method uses a visible dye applied to the surface of the tested material. The dye penetrates any surface-breaking defects, and the excess dye is removed. Next, a developer is applied that draws the penetrant out of the defects, making them visible to the inspector. VPI is often used when inspections are performed in well-lit areas, and the inspector can easily see the penetrant indications.
- Fluorescent Dye Penetrant Inspection (FPI)- This method uses a fluorescent dye applied to the surface of the tested material. The dye penetrates any surface-breaking defects, and the excess dye is removed. The part is then inspected under ultraviolet (UV) light. Any penetrant that has entered the defects will fluoresce, making them visible to the inspector. FPI is often used in low-light environments or when the defects are too complex to see using VPI.
The Future of NDT Techniques
Non-destructive testing (NDT) techniques are becoming increasingly efficient and cost-effective, brightening the future potential of NDT in the aerospace and defense industries.
The increasing demand for aircraft, the need for regular maintenance and inspections of aircraft components, and the increase in space activities, such as the deployment of satellite constellations, are expected to boost the NDT services market.
Furthermore, technological advancements, such as the development of new sensors, imaging technologies, and software, will further improve NDT services' accuracy, speed, and reliability, leading to a further increase in demand for these services.
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