Non-Destructive Testing (NDT) refers to a collection of methods designed to evaluate the properties of materials, components, or systems without causing damage, ensuring the tested objects remain fully intact and functional.1
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Advancements in optical science have greatly refined NDT methods. Tools such as lasers, lenses, and optical fibers offer non-contact, highly sensitive solutions that are resistant to electromagnetic interference.
For example, optical fiber-based NDT uses sensors like Fiber Bragg Gratings (FBG) to detect and transmit light signals. These sensors measure strain, temperature, and vibration, making them suitable for real-time monitoring of critical structures, including bridges, pipelines, and aircraft.2-3
Laser-Based Methods
Laser Ultrasonics
Laser ultrasonics is a method for generating and detecting ultrasonic waves in materials without physical contact. The process relies on a pulsed laser that causes rapid localized heating through the thermoelastic effect, leading to thermal expansion and the generation of elastic waves. These waves propagate through the material, reflecting or scattering at boundaries, defects, or internal structures.
A second laser, typically coupled with an interferometer, measures the surface displacements caused by these waves, enabling precise mapping of subsurface features.
This method is effective for inspecting complex materials and structures where traditional contact-based techniques may not be practical.4 In aerospace, laser ultrasonics is used to identify internal cracks, delaminations, and other subsurface defects in aircraft fuselages, wings, and engine parts. It is also applied in manufacturing to verify the integrity of welds, composites, and metal structures during production.5
Laser ultrasonics is particularly useful for inspecting high-temperature or moving parts without interrupting functionality. While it provides high precision in detecting subsurface flaws, challenges include the high cost of equipment, the complexity of the setup, and the need for skilled operators to interpret data accurately.4,5
Laser Shearography
Laser shearography is an advanced NDT method that uses lasers and interferometric techniques to detect and analyze surface deformations and defects.
The process starts by illuminating a material’s surface with coherent laser light, creating a speckle pattern that reflects surface irregularities. A shearing device splits the image into two slightly displaced versions, which are combined into an interferometric speckle pattern.
When the material is stressed—through thermal, acoustic, or vacuum loading—surface deformations alter the speckle pattern. These changes, caused by the interference of coherent light, result in phase differences in the interferogram. These phase changes correspond to displacement gradients on the surface, enabling the detection of defects such as delaminations, cracks, and disbonds.4
Compared to traditional interferometric methods like Electronic Speckle Pattern Interferometry (ESPI), shearography is less sensitive to external vibrations and motion, making it better suited for industrial environments and fieldwork.
Applications include inspecting composite structures, turbine blades, and aircraft tires in aerospace, as well as detecting defects in electronic components and semiconductor wafers in manufacturing. It is also used in the rubber industry to identify tire delaminations and in art preservation to analyze wooden panel paintings.6
The advantages of laser shearography include its ability to provide fast, full-field measurements and its resistance to external vibrations, making it effective for use in environments where conditions may involve movement, high noise levels, or unstable surfaces.6
However, the technique does have limitations. The setup is costly and complex, requiring skilled operators and appropriate excitation methods to identify defects accurately. Additionally, shearography only measures surface strain, so it is less effective at detecting bulk internal defects.4,6
Laser Shearography NDT
Lens-Based Methods
Optical Coherence Tomography
Optical Coherence Tomography (OCT) is a non-invasive imaging technique that uses light waves to produce high-resolution, cross-sectional, and 3D images of an object’s internal structure.
OCT operates on the principle of low-coherence interferometry, where light from a broadband source is split into two paths: one directed at the sample and the other at a reference mirror. When reflected light from both paths combines, it creates an interference pattern that reveals structural details. With resolutions ranging from 2.6 to 10 micrometers, OCT is well-suited for detecting minute defects or anomalies.7,8
Compared to traditional NDT methods like ultrasound, OCT offers faster imaging, enabling real-time or near-real-time inspections that improve productivity. Its non-contact nature is particularly useful for inspecting delicate materials, such as medical devices and electronic components, as it ensures the integrity of the object is preserved during the inspection process.7
OCT has applications across multiple industries. In the medical device sector, it ensures the structural integrity of implants like stents and prosthetics and detects defects in biomaterials. In display and panel manufacturing, OCT evaluates LCD and OLED screens, identifying imperfections at the pixel level.8 It is also used in aviation and automotive manufacturing to inspect turbine blades, engine parts, and composites for structural abnormalities.8
While OCT offers high-resolution imaging and rapid inspection capabilities, it is limited by its shallow penetration depth compared to ultrasound and the high cost of equipment.7,8
Visual Inspection with High-Resolution Lenses
Visual inspection remains one of the simplest and most commonly used NDT methods. It is often enhanced with high-resolution lenses and optical tools. Devices such as magnifiers, microscopes, borescopes, and endoscopes improve the ability to detect surface defects.
Depending on the application, inspections are performed either by direct visual observation or through specialized optical devices. High-resolution lenses allow for detailed examinations of small flaws and hard-to-reach areas, making this method both versatile and cost-effective.4, 9
Construction and chemical manufacturing industries rely heavily on visual inspection to ensure quality control and maintain safety standards. Borescopes are frequently used in construction to inspect masonry arch bridges for structural integrity.
Similarly, in the chemical industry, visual inspection helps evaluate combustion chambers, pressure vessels, and furnaces for signs of wear or damage. This method is also effective for assessing coatings, seals, and weld roots, providing a simple yet valuable approach to quality assurance.9
The advantages of visual inspection include its affordability, speed, and simplicity, making it ideal for quick assessments and initial screenings. However, it is limited by its inability to detect subsurface defects, reliance on surface cleanliness, and the subjective nature of analysis, which can impact accuracy.9,10
The Future of NDT
The potential for future innovations in optical NDT methods is vast. Advances in laser technology, imaging systems, and data processing are expected to further enhance the resolution, speed, and automation of these techniques.
The incorporation of artificial intelligence and machine learning could further refine defect detection and analysis, providing faster, more accurate, and consistent results.11 These developments will improve safety, operational efficiency, and quality assurance in various industries.
To learn more about these advancements and their applications, explore the following resources:
References and Further readings
1. Kumpati, R.; Skarka, W.; Ontipuli, SK. (2021). Current Trends in Integration of Nondestructive Testing Methods for Engineered Materials Testing. Sensors. https://www.mdpi.com/1424-8220/21/18/6175
2. Hassani, S.; Dackermann, U. (2023). A Systematic Review of Advanced Sensor Technologies for Non-Destructive Testing and Structural Health Monitoring. Sensors. https://pubmed.ncbi.nlm.nih.gov/36850802/
3. Liu, L.; Zhang, H.; Jiao, F.; Zhu, L.; Zhang, X. (2023). Review of Optical Detection Technologies for Inner-Wall Surface Defects. Optics & Laser Technology. https://www.sciencedirect.com/science/article/abs/pii/S0030399223002062
4. Kroworz, A.; Katunin, A. (2018). Non-Destructive Testing of Structures Using Optical and Other Methods: A Review. Structural Durability & Health Monitoring. https://www.techscience.com/sdhm/v12n1/28695
5. Jodhani, J.; Handa, A.; Gautam, A. (2023). Rana, R., Ultrasonic Non-Destructive Evaluation of Composites: A Review. Materials Today: Proceedings. https://www.sciencedirect.com/science/article/abs/pii/S2214785322074296
6. Gupta, M.; Khan, MA.; Butola, R.; Singari, RM. (2022). Advances in Applications of Non-Destructive Testing (Ndt): A Review. Advances in Materials and Processing Technologies. https://www.tandfonline.com/doi/full/10.1080/2374068X.2021.1909332
7. Aumann, S.; Donner, S.; Fischer, J.; Müller, F. (2019). Optical Coherence Tomography (Oct): Principle and Technical Realization. High resolution imaging in microscopy and ophthalmology: new frontiers in biomedical optics. https://pubmed.ncbi.nlm.nih.gov/32091846/
8. Heise, B.; Hannesschlaeger, G.; Leiss-Holzinger, E.; Peham, L.; Zorin, I. (2020). Optical Coherence Tomography in Nondestructive Testing. Optics and Photonics for Advanced Dimensional Metrology. https://doi.org/10.1117/12.2556832
9. Kumar, A.; Arnold, W. (2022). High Resolution in Non-Destructive Testing: A Review. Journal of Applied Physics. https://pubs.aip.org/aip/jap/article/132/10/100901/2837323/High-resolution-in-non-destructive-testing-A
10. Zhong, S.; Nsengiyumva, W. (2022). Visual Testing for Fiber-Reinforced Composite Materials. Nondestructive Testing and Evaluation of Fiber-Reinforced Composite Structures. https://link.springer.com/book/10.1007/978-981-19-0848-4
11. Sunil, P.; Chandu, P.; Vivek, D. (2024). A Review: Non-Destructive Testing (Ndt) Techniques, Applications and Futureprospects. Journal of Science & Technology (JST). https://jst.org.in/index.php/pub/article/view/35
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