1. Introduction
Non-destructive testing (NDT) techniques are used to ensure that materials or products are devoid of defects or flaws, and are also used for salvaging a component, evaluating the component post-repair, and making recommendations for the early identification of damage before it escalates into a serious issue.
2. Aerospace and Defence (A&D)
The aerospace and defence industry concentrates on the development of aircraft and weapon systems such as commercial airliners, military jets, space shuttles, missiles, drones, etc.
3. Challenges in the A&D Industry
- Compliance with stringent quality standards for various components and structures, and ensuring the precision, accuracy and reliability of various equipment.
- Regular assessment of damage and deterioration, particularly for structures with irregular shapes that pose inspection challenges, requires highly qualified operators and precision NDT.
- The use of advanced materials presents inspection challenges that call for specialised techniques.
Several inspection issues include the detection of hidden corrosion, cracks in multi-layered or thick structures, and damage due to fatigue. Debond, delamination, and barely visible impact damage (BVID) are different issues in composites.
4. NDT to Meet the Challenges
Various challenges are precisely addressed by NDT techniques, which are extensively employed at different stages, starting from raw material inspection to manufacturing, during maintenance and inspection. Airworthiness of aircraft, along with the safety of crew and passengers, is guaranteed through the use of NDT [1]. Various NDT techniques used in the A&D industry are described below. The capability of NDT is further enhanced through automation, embedded sensors, artificial intelligence (AI), and machine learning (ML).
5. NDT Techniques
5.1. Visual
Visual inspection is done to confirm that the surface of aircraft and military structures is free from corrosion, wear, or damage. The inside of fuel tanks in an aircraft is inspected by remote visual tools since any flaw or corrosion heightens the risk of fuel leaks. Engines also undergo remote visual inspection.
5.2. Dye Penetrant (DP)
Landing gear and structural components like wings, fuselage, and tail section of aircraft are inspected by dye penetrant [2]. Surface flaws are detected that jeopardise the structural integrity of these parts. The integrity and reliability of naval vessels, armored vehicles, and weapon systems are confirmed by DP.
5.3. Magnetic Particle (MP)
Various structural parts of an aircraft, such as fuselage, wings, and tail, along with bolts, fasteners, and rivets, as well as engine parts like turbine blades, compressor disks, and engine casings, are subjected to MP testing. MP is applied to examine landing gear parts, including landing gear struts, axles, and associated components. In defence, this method is instrumental at identifying defects in missile casings, gun barrels, weaponry components, military vehicles, engine components, etc.
5.4 Eddy Current (EC)
Eddy current is used for testing components made of conductive materials like steel, titanium, and aluminium used in the A&D sectors. Aircraft structural, engine, and landing gear components are inspected by EC and used to find voids and delamination in composite materials, corrosion as well as cracks hidden beneath rivet heads in lap joints. Flexible EC sensors are found useful for inspecting components with intricate shapes and curved surfaces, and located in inaccessible regions. The undersides of aircraft wings, consisting of several layers, are inspected precisely by low-frequency EC in conjunction with either a giant magneto-resistive (GMR) sensor or a superconducting quantum interference device (SQUID). The healthiness of heat exchanger tubes of space shuttle engines is ascertained through EC testing.
For military components, EC is proficient at identifying corrosion and erosion. Material properties evaluation, flaw detection, and verification of structural soundness in vital defence systems, including aircraft, artillery, armored vehicles, and various other equipment, are accomplished through EC testing. EC is also utilised to inspect artillery tubes and other weapon parts for signs of cracks, wear, and other imperfections.
Several advanced eddy current techniques used are as follows:
Pulsed and Multifrequency Eddy Current (PMEC)
Pulsed and multifrequency EC is used to detect corrosion and cracks in multilayered structures, such as riveted joints, fuselage, wings, and turbine blades. For identifying subsurface cracks and fatigue damage, PMEC, GMR sensors, electromagnetic acoustic transducers (EMATs), and SQUIDs are effective.
Eddy Current Array (ECA)
Eddy current arrays remove the need for raster scanning and are particularly effective for examining surfaces with complex shapes in aircraft and for precisely identifying very small defects across extensive areas. ECAs are utilised to verify the structural soundness of armored vehicles and other essential components.
5.5. Ultrasonic Testing (UT)
The fuselage, wings, tail, and landing gear of aircraft, missiles, rockets, and submarine components of defence are suitable for the application of UT. While pulse-echo testing is used for detecting delamination in composites, the Pitch-Catch technique becomes useful for examining larger parts, such as castings and forgings, and for identifying surface and near-surface defects. Structures used in the military for shielding personnel, equipment, and infrastructure from threats like explosions, ballistic impacts, etc, are assessed by UT. Discontinuities in steel fibre-reinforced concrete are located through UT [3]. Solid rocket motors in defence are tested using the acousto-ultrasonic method.
Various advanced ultrasonic techniques, as stated below, are found useful in the A&D industry.
Phased Array UT (PAUT)
Phased array UT is used for accurately characterising materials with anisotropies such as CFRP, and for detecting delamination and porosity. Curved or multilayer structures, such as wing skins, aircraft engines, and missile guidance systems, are inspected faster by PAUT than with traditional UT. Wingboxes in an aircraft are inspected using PAUT.
Guided Wave Ultrasonic
Longitudinal wing stringers of an aircraft need to be disassembled for inspection. Guided Wave UT brings a solution to such cases. It is also used for deicing aircraft to improve flight safety [4].
Non-contact UT
Utilising lasers in UT removes the necessity for couplants and allows for quick scanning of composite materials.
5.6. Radiographic Testing (RT)
Radiography proves to be a powerful tool for inspecting aircraft engines, landing gear, the undersides of wings, and missile guidance systems. Porosity and cracks in submarine parts are detected by RT. Cutting-edge radiographic methods like Digital Radiography (DR) and Computed Tomography (CT) are extremely useful for examining components in the A&D sectors.
5.7. Computed Tomography
CT is employed to examine aircraft engine components. Rockets, missiles, and torpedoes used in the military need to be inspected precisely to guarantee their reliability. Any defect, irregularity, or assembly fault could affect the performance of different systems, i.e. guidance systems, propulsion systems, and warheads. All of these issues are solved through CT scanning. Bombs and explosives are examined by CT, which accurately captures images of circuitry, detonators, and casing designs, thereby enhancing the efficiency of bomb disposal systems. The CT is also used for assessing damage in composite ballistic helmet shells.
5.8. X-ray Diffraction
Identification and characterisation of materials are done through X-ray Diffraction; this includes substances such as explosives and narcotics utilised in the military. Residual stress in aircraft landing gears is determined by this technique.
5.9. Shearography
In aerospace and defence, shearography, a laser-based NDT, is commonly used to find ply wrinkling, kissing bonds, and debonds in composites. This technique is utilised to assess armor and parts of weapon systems, in addition to aerospace components, including aircraft wings, fuselage, engine, and rotor blades.
5.10. Terahertz (THz) Imaging
Hybrid bonds, including those between CFRP and titanium in aircraft nacelles, are mapped by THz imaging. Ballistic protection armor in defence is inspected by this [5]. Metallic interfaces in aircraft are inspected by a Terahertz-ultrasonic system.
5.11. Infrared Thermography (IRT)
IRT is employed to examine both primary and secondary structures, as well as components of aero-engines and spacecrafts, for issues such as debonds, delamination, and water ingress. Surveillance, targeting, and navigation of aircraft, vessels, and vehicles in the military is done by thermography. The IR radiation emitted by objects guides soldiers to see in low-light conditions, through smoke, and other visual obstacles.
A few examples of advanced thermography applications are as follows:
Lock-in Thermography
The integrity of bonds such as bonds between CFRP and titanium fuselage brackets is assessed by IRT. But this becomes complicated because of the variation of emissivity at interfaces. This is circumvented by employing lock-in thermography that differentiates flaw signals, enhancing the precision of detection.
Pulsed Thermography
Parts made of composites, bonded joints, corrosion in metallic components, impact damage, etc., in aircraft and military systems are inspected by pulsed thermography.
5.12. Automation
Automation in NDT is essential for the inspection of distant sites and efficiently covering large areas in a limited timeframe. Devices such as scanners, crawlers, and robots facilitate automated inspection and imaging of hard-to-reach places, such as inside pipes and tubes. An automated NDT sensor is very useful for conducting a raster scan of an aircraft's surface.
5.13. Embedded sensors
Embedded in aircraft structures, sensors (piezoelectric, fibre optic, eddy current, or acoustic/vibration) monitor for cracks, corrosion, and any other damage. In defence, these are integrated into weapon systems, such as gun barrels and missile casings, to detect cracks or defects. Sensors embedded in military infrastructure, like bridges and buildings, monitor their structural integrity. These are also used for condition monitoring purposes.
5.14. Artificial Intelligence and Machine Learning
The application of an artificial neural network (ANN) boosts the effectiveness of NDT. For example, ANN enhances the signal-to-noise ratio of eddy current signals towards successful inspection of the aluminium skin of an aircraft. AI augments capabilities in areas of monitoring, logistics, and cyber warfare in defence. AI-assisted systems are progressively utilised for the operation of autonomous vehicles, drones, border surveillance, and lethal autonomous weaponry. AI models applied to Radiography data are useful for defect detection in rocket motors.
Implementation of ML enhances the precision, accuracy and reliability of NDT data, thus further improving the defect detection capability of NDT.
6. Summary
NDT techniques are indispensable for the evaluation of new and advanced materials and for the inspection of parts and components used in aerospace and defence. Advancements in methodology, automation, artificial intelligence and machine learning are a step forward to enhance the precision, speed, reliability, and accuracy of the evaluation and inspection processes for materials and structures utilised in these two vital industries.
7. References
- Çınar, Z.M., Nuhu, A.A., Zeeshan, Q., Korhan, O., Asmael, M. and Safaei, B. 2020. Machine learning in predictive maintenance towards sustainable smart manufacturing in Industry 4.0. Sustainability, 12(19), 8211.
- Özlem Ulus, Furkan Eren Davarci and Elif Eren Gültekin, “Non-destructive testing methods commonly used in aviation”, International Journal of Aeronautics and Astronautics, 2024, VOL:5, ISS:1, 10-22.
- Zezulová Eva, Hasilová Kamila, Komárková Tereza, Stoniš Patrik, Štoller Jirí and Anton Ondrej, “NDT Methods Suitable for Evaluation of the Condition of Military Fortification Construction in the Field”, Appl. Sci. 2020, 10, 8161.
- Yibin Wang, et. al. (2017). Progress on ultrasonic guided waves de-icing techniques in improving aviation energy efficiency. Renewable and Sustainable Energy Reviews, 638-645.
- Norbert Pałka, Kamil Kamiński, Marcin Maciejewski, Dawid Pacek and Waldemar Świderski, “Terahertz nondestructive testing of alumina-based ceramic ballistic protection armor”, Infrared Physics & Technology, Vol. 137, March 2024, 105163.
Author: Chandan Mukhopadhyay
