Nondestructive testing (NDT) is a method for detecting and evaluating material defects. In the aerospace industry, NDT plays a critical role in the design, manufacturing, and maintenance of aircraft.
The simplest and most accurate method for testing materials and components is typically destructive testing. In the aerospace industry, destructive testing is employed to determine the physical properties of materials, subassemblies, and assemblies. It can reveal valuable information about material characteristics, including ductility, tensile strength, and fracture toughness. However, when testing components and materials intended for use in aircraft or already installed on aircraft, destructive testing is not always feasible or desirable—primarily because of their high value. Instead, engineers and technicians rely on nondestructive testing techniques, equipment, and analytical methods to inspect materials and components before they are put into service and during operation, without causing any damage. Nondestructive testing can also detect defects and anomalies within materials that would otherwise be difficult or impossible to identify using destructive methods.
When is nondestructive testing used in the aerospace industry?
Safety is the paramount concern in the aviation industry. Nondestructive testing (NDT) is integral throughout a product’s entire lifecycle—ranging from the qualification of new materials and the design of new aircraft to in-service inspections of airframe structures and engines. During the development phase, researchers may employ NDT to facilitate the creation of lighter, more flexible aircraft materials and structures. Engineers utilize NDT tools and procedures during aircraft testing and certification to evaluate performance and durability and to investigate any issues that arise. Aircraft manufacturers rely on NDT to ensure the quality of materials and joining processes both during and after manufacturing. Finally, maintenance engineers and technicians use NDT to conduct routine inspections of in-service aircraft for damage and cracks, thereby safeguarding airworthiness and safety.
How is NDT used in the aerospace industry?
An important aspect of NDT is that the tested part or material remains intact and serviceable after inspection.
Nondestructive testing is generally divided into two fields: Surface-based and subsurface-based techniques. Within these two domains, a range of well-established and reliable tools are available, including impact testing with a hammer, as well as more recent and advanced technologies such as 3D computed tomography.
Visual inspection is the oldest and simplest nondestructive testing method. It is often said that the human eye is the most powerful NDT tool. Visual inspection of aircraft structures and components for damage such as cracks, corrosion, and misalignment typically provides the first indication of potential problems.
A variety of devices are used for visual inspection, ranging from magnifying glasses and mirrors to video borescopes, charge-coupled devices, and remote viewing systems designed to inspect hard-to-reach areas.
Visual inspection using a borescope
Penetrant testing is another simple and rapid method widely used in the aerospace industry for detecting surface defects and structural damage in non-porous materials.
The test specimen is coated with a high-viscosity dye. Once the dye penetrates any cracks or defects, the surface is cleaned, leaving only the dye that has seeped into the cracks. Any residual dye will then flow back, revealing evidence of the cracks and defects.
Penetrant testing is one of the most commonly used nondestructive testing techniques.
Fluorescent penetrants are typically used on sensitive materials and components. Liquid penetrant testing is a versatile technique that can be performed in the field, in the shop, or in the hangar. It is also frequently incorporated into cleaning and repair procedures, where the surface of an object is inspected after cleaning.
Acoustic emission testing involves applying a sudden force or “impact test” to the test object and analyzing the resulting signals. This analysis can be as simple as listening to the sounds, or it can involve multiple sensors to record the resulting stress waves and minor deformations.
Leak testing employs four primary techniques: bubble detection, pressure decay, halogen diode leak detection, and mass spectrometry, which involve pressurizing the test object and immersing it in a liquid to locate and record leaks. Aircraft and engines utilize large quantities of liquids and gases; therefore, leak testing is an essential component of both manufacturing and maintenance.
Radiography in aerospace can employ X-rays for thin materials and gamma rays for thicker materials. Traditionally, film was used to capture images, but this is now being replaced by digital methods.
Cross-section of aerospace components
Another recent advancement is 3D computed tomography (CT) scanning, which captures multiple X-ray images of the test object to construct cross-sectional views of the object on a computer.
Ultrasonic testing is the most common subsurface inspection technique, employing high-frequency sound waves to locate defects within components or materials. It is typically used to detect flaws in welds, fittings, joints, bolts, and adhesive bonds. This technique is analogous to sonar, as it records and analyzes the reflections of sound waves. 。
Ultrasonic testing of composite materials (Olympus equipment)
Aerospace engineers and technicians employ several ultrasonic testing techniques, including straight-beam testing, immersion testing, and phased-array testing. All of these methods utilize ultrasonic transducers—also known as probes—to transmit and receive sound waves and display the results graphically on a screen.
Magnetic particle inspection involves inducing a magnetic field in the test object, applying magnetic particles—either in dry form or as a liquid suspension (which may be colored or fluorescent)—to the surface, and then examining the object under appropriate illumination.
This technique is used to detect discontinuities in ferromagnetic materials. Depending on the inspection environment, several different types of equipment can be employed for magnetic particle testing, including yokes, probes, coils, and magnetic heads.
Eddy current testing is a subsurface technique that induces an electromagnetic field in conductive test objects and measures the secondary magnetic field generated around the induced eddy currents to locate defects. Eddy current testing is widely used in aircraft maintenance to detect cracks caused by fatigue or corrosion.
Eddy Current Tester
Vibration analysis monitors the vibration characteristics generated by rotating mechanical components and interprets these signals to detect anomalies. Displacement sensors, eddy-current sensors, velocity sensors, or accelerometers mounted on the machinery can be used for such monitoring.
Thermal/infrared inspection involves mapping the surface temperature of objects and can be used to detect defects such as corrosion, delamination, voids, and debonding. It works by detecting anomalies in heat flow within materials or components.
Laser-based testing encompasses techniques such as shearography, holography, and profilometry. All of these methods employ lasers in various ways to detect surface deformations on objects and use computational processing to compare loaded and unloaded conditions. This approach is particularly effective for detecting minute defects on the order of a few micrometers.
Nondestructive testing standards and training—while the task of detecting cracks and defects in aircraft and materials is daunting—can often mean the difference between life and death. The need to ensure high quality and safety throughout manufacturing, assembly, and service has driven the development of nondestructive testing standards and specifications for both civil and military aviation.
Several national and international organizations have established standards for NDT methods, equipment, and training. These include the American Society for Nondestructive Testing (ASNT)’s British Institute of Nondestructive Testing, the International Committee for Nondestructive Testing, and the European Union’s framework for nondestructive testing. International standards are also overseen by ISO and ASTM International (the American Society for Testing and Materials).
The standard applies to both civilian and military testing and training. Common certifications for personnel performing nondestructive testing include the ASNT Central Certification Program (ACCP), while SNT-TC-1A Qualification and Certification of Nondestructive Testing Personnel provides guidance and a framework for internal NDT certification programs. SNT-TC-1A establishes three distinct qualification levels, with each successive level entailing greater duties and responsibilities than the preceding one.
Conclusion
Thanks to advances in technology, connectivity, and data processing and analysis, the use of nondestructive testing in the aerospace industry is on the rise. At the same time, the complexity of these applications is increasing; nevertheless, conventional techniques—such as impact-echo testing and liquid penetrant testing—often remain effective methods for inspecting aerospace components. This segment of aerospace testing is critical to the safety of passengers and all other stakeholders in the aviation sector. To conduct nondestructive testing, it is essential to adhere to established standards and undergo comprehensive training, thereby ensuring that high levels of expertise are maintained going forward. 。
