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Non-destructive testing and evaluation of composite materials/structures: A state-of-the-art review
Bing Wang, Shuncong Zhong, Tung-Lik Lee, Kevin S Fancey, Jiawei Mi
TL;DR
Composite materials are increasingly used in load-bearing structures, making reliable NDT important for safety and maintenance costs. This paper reviews established NDT techniques, compares their capabilities and limitations, and concludes that inspection will increasingly require intelligent, automated analysis.
Problem
Increasing use of composites in load-bearing aerospace, wind-turbine, transportation, and medical structures makes NDT essential for reducing safety concerns and maintenance costs.
Method
The paper reviews established NDT techniques for defect and damage evaluation, comparing their benefits, limitations, capabilities, and applications.
Results
NDT techniques are invaluable for testing and evaluation; ultrasound, IRT, and DIC are versatile, but appropriate technique selection must account for method limitations.
Takeaways & Limitations
Future composite NDT will increasingly depend on intelligent, automated systems that provide fast and accurate analysis through artificial neural methods.
Takeaways & Limitations
Laboratory X-ray imaging systems have limited penetration capability, while DIC details vary with supplier-specific equipment flexibility.
Abstract
from arXiv · showhide
Composite materials/structures are advancing in product efficiency, cost-effectiveness and the development of superior specific properties. There are increasing demands in their applications to load-carrying structures in aerospace, wind turbines, transportation, and medical equipment, etc. Thus robust and reliable non-destructive testing (NDT) of composites is essential to reduce safety concerns and maintenance costs. There have been various NDT methods built upon different principles for quality assurance during the whole lifecycle of a composite product. This paper reviews the most established NDT techniques for detection and evaluation of defects/damage evolution in composites. These include acoustic emission, ultrasonic testing, infrared thermography, terahertz testing, shearography, digital image correlation, as well as X-ray and neutron imaging. For each NDT technique, we cover a brief historical background, principles, standard practices, equipment and facilities used for composite research. We also compare and discuss their benefits and limitations, and further summarise their capabilities and applications to composite structures. Each NDT technique has its own potential and rarely achieves a full-scale diagnosis of structural integrity. Future development of NDT techniques for composites will be directed towards intelligent and automated inspection systems with high accuracy and efficient data processing capabilities.
1 Introduction
Composite applications are expanding, increasing the need for reliable NDT because complex, multiscale damage threatens structural integrity. The paper provides a practical state-of-the-art review of established and emerging methods, including principles, standards, equipment, benefits, limitations, and applications.
- Composite use is increasing in load-bearing aerospace, wind-turbine, transportation, and medical-equipment structures.
- Non-homogeneous, anisotropic composites develop defects across locations and scales, making damage detection and structural-integrity evaluation challenging.
- The review covers visual inspection, acoustic emission, ultrasonic testing, infrared thermography, terahertz testing, shearography, digital image correlation, and X-ray/neutron imaging.
- It compares method benefits, limitations, capabilities, and applications while incorporating ASTM standard practices and practical guidance for researchers and engineers.
- The paper concludes that future NDT development will emphasize intelligent, automated inspection with high accuracy and efficient data processing.
2 Defects and damage evolution in composites
Composite defects arise during manufacturing and service, accumulate through interacting failure mechanisms, and span poorly bounded scales. These flaws can concentrate stress and reduce effective structural performance and service time.
- Manufacturing can produce voids, porosity, inclusions, fibre or ply misalignment, waviness, warping, buckling, and residual-stress-related defects.
- Flaws and defects act as stress-concentration points that promote crack propagation and delamination, reducing effective strength, stiffness, and service time.
- Defect and damage scales have no clear boundaries and depend on composite constituents, so published literature provides general rather than fixed guidance.
- In-service failure accumulates through matrix cracking, interface debonding, fibre fracture and pull-out, micro-buckling, delamination, and wrinkling.
- Damage initiates at nano- or micro-scale levels and can progress through crack propagation, debonding, fibre fracture, delamination, and out-of-plane wrinkling.
3 Non-destructive testing & evaluation techniques
NDT methods use different physical principles and are grouped into visual, acoustic, optical, imaging, and electromagnetic categories. The review focuses on eight techniques, whose research activity and practical suitability vary by material and application.
- The paper reviews eight techniques spanning visual inspection, acoustic waves, optical methods, and X-ray/neutron imaging, while excluding electromagnetic-induction methods.
- Electromagnetic-induction methods are limited mainly to conductive materials, whose applicability to most composites is constrained by low, inhomogeneous conductivity.
- Acoustic emission has a long, established history and remains relatively steady, whereas terahertz testing became promising within the last decade.
- Recent equipment, computing, imaging, and acquisition advances increased applications of infrared thermography, ultrasonic testing, digital image correlation, and radiography.
- Shearography is widely used industrially, especially in aerospace, while neutron imaging remains relatively uncommon because neutron generation is more expensive than X-ray generation.
3.1 Visual inspection
Visual inspection provides basic, economical surface assessment, while acoustic-emission methods monitor damage-generated stress waves and can detect several composite failure modes. Their effectiveness is balanced by depth, interpretation, and analysis constraints.
- Visual inspection: Visual inspection is a basic NDT method using accessible tools such as miniature cameras or endoscopes to inspect damage and surface imperfections.
- Visual inspection: Visual inspection is quick, economically viable, and flexible, but its accuracy depends on training, procedures, and inspection apparatus.
- Visual inspection: Visual methods are particularly effective for macroscopic flaws, including poor joints, erroneous dimensions, poor surface finish, and poor fits.
- Acoustic emission: Acoustic emission detects stress waves emitted by damage during loading and monitors dynamic defect development, benefiting fatigue tests.
- Acoustic emission: Acoustic emission can detect fatigue cracks, fibre fractures, matrix micro-cracks, interface debonding, and delamination.
- Acoustic emission: Acoustic-emission analysis is time-consuming and requires skill because overlapping amplitude distributions can make damage mechanisms difficult to identify.
3.3 Ultrasonic testing
Ultrasonic testing uses elastic waves to inspect composites for flaws, damage, and structural features, with guided-wave analysis suited to anisotropic materials. The section also describes system configurations, applications, and relevant standards.
- Applications: UT is used across manufacturing and in-service detection, with guided-wave mode selection, modelling, simulation, signal processing, and interpretation documented in prior reviews.
- Principles: UT uses pulsed elastic waves in reflection, transmission, and back-scattering modes to inspect composite material systems.Guided Lamb waves propagate selectively because composites are anisotropic.
- Capabilities: Ultrasonic testing can detect flaw size, crack and delamination locations, fibre waviness, ply fibre orientation, and layup features.
- Equipment: Guided-wave UT supports multiple generation approaches, including probes, lasers, piezoelectric elements, interdigital transducers, and optical fibres.Systems may use angled transmitters or phased arrays.
- Standards: UT techniques for composites have standards covering time-of-flight examinations, flat panels and sandwich structures, and filament-wound pressure vessels.The cited standards are ASTM E2373, E2580, and E2981.
3.4 Infrared thermography
Infrared thermography detects and maps infrared energy to produce temperature-distribution images for composite inspection. It supports non-contact, real-time, high-resolution measurements over large volumes and can be used throughout a product’s lifecycle.
- Principles: IRT detects infrared emissions by measuring and mapping thermal distributions, which are converted into temperature-distribution images.A radiometer converts infrared energy into electrical signals for display.
- Capabilities: IRT provides non-contact, non-invasive, real-time measurement, high resolution, and coverage of large volumes.
- Lifecycle applications: The method supports manufacturing process control, finished-product NDT evaluation, in-service maintenance, and diagnostics.
- Composite applications: Composite IRT applications include detecting inclusions, debonding, delamination, and cracked networks, including aerospace structural health monitoring.Boeing and Airbus have used IRT for composite-product integrity monitoring.
- Implementation: IRT can be implemented through passive thermography or active thermography, in which heating or cooling reveals internal structures through thermal responses.Active approaches include optical, induction, and mechanical thermography.
- Development: Recent signal-processing and equipment developments have made active thermography more practical and effective than conventional approaches.
3.5 Terahertz testing
Terahertz testing uses reflected or transmitted THz waves to analyse phases, inclusions, defects, and damage in composite materials. Its penetration and resolution support multi-scale inspection, although standardised practice remains under development.
- Capabilities: THz-based NDT offers higher resolution and better penetration in most materials compared with other techniques.
- Principles: THz waves penetrate non-metallic, non-polar materials, including resin, foams, ceramics, glass, rubber, and composites.
- Principles: THz inspection determines internal structures by analysing waves reflected or transmitted after interacting with phases, inclusions, defects, or damage.
- Capabilities: Composite multi-phase and multi-layered structures are well-suited to THz NDT, which offers multi-scale, comprehensive information about internal structures and damage.
- Equipment: THz systems are implemented through time-domain spectroscopy or continuous-wave systems with different approaches to phase-information measurement.THz-TDS measures the time-dependent electric field of a pulse, whereas THz-CW records average electromagnetic-field intensity.
- Limitation: As an emerging technique, standardised practice for THz inspection is still developing.
3.6 Shearography
Shearography is a laser-based, non-contact, full-field technique that records speckle-derived fringe patterns during controlled stressing. It has been applied to diverse composite products and supports rapid inspection of large structures.
- Capabilities: Shearography is a non-contact, full-field surface-strain method that is resilient to environmental disturbance and suitable for large composite structures.
- Capabilities: The technique has been used to detect flaws, leakage, delamination, and damage, and to measure displacement, strain, curvature, residual stress, and vibration.
- Principles: Shearography illuminates a sample, shears the reflected speckle pattern into coherent images, and records deformation-related fringes with a CCD camera.
- Principles: A controlled stressing process can use thermal, vacuum, vibration, microwave, or mechanical loading to reveal structural information.
- Applications: Applications include pipes, sandwich structures, wind-turbine blades, aerospace structures, and racing tyres.
- Standards: ASTM E2581 represents standard practice for shearography of polymer composites and sandwich core materials in aerospace.
3.7 Digital image correlation
Digital image correlation (DIC) is an optical NDT technique that tracks surface patterns to calculate full-field deformation and strain during composite testing. It supports crack and deformation monitoring but remains difficult to standardize across situations.
- Surface changes detected by DIC reveal strain, deformation, and crack propagation, making it suitable for composite damage studies.
- DIC provides more accurate strain monitoring than conventional extensometers or strain gauges, which can suffer from attachment and gauge-length limitations.
- Typical systems use calibrated CCD cameras, illumination, a stochastic speckle pattern, and software that compares images to calculate strain or deformation.
- The speckle pattern strongly affects DIC accuracy and precision, while system flexibility makes universal standardization difficult or impossible.
3.8 Imaging techniques
Imaging-based NDT methods use X-rays, neutrons, and scattering approaches to characterize composite structures across scales. Laboratory X-ray systems are accessible but limited in penetration, whereas synchrotron and neutron techniques provide distinct advantages for demanding measurements.
- Synchrotron and neutron imaging: Synchrotron X-ray and neutron methods probe microstructure, residual stress, strain and stress fields, crystallographic texture, and other properties at atomic or crystalline levels.
- Synchrotron and neutron imaging: Diffraction characterizes crystalline structure and residual stress; small-angle scattering examines nanoscale structures; reflectometry studies layered surfaces; spectroscopy measures excitations and diffusion.
- Neutron imaging: Neutron imaging provides greater penetration depth than X-rays and better sensitivity to light elements, especially hydrogen, while non-destructively measuring composite strain and stress.
- X-ray imaging: Laboratory X-ray imaging is cheaper and easier to access, and is suitable for higher-phase-contrast materials such as glass-fibre-reinforced composites.
- X-ray imaging: Laboratory X-ray imaging lacks deep penetration into engineering materials, with performance depending on X-ray energy and wavelength.
- X-ray imaging: Synchrotron X-ray imaging offers higher signal-to-noise ratio, phase contrast, flux, and brightness, enabling fast high-resolution imaging of low-contrast composites.The TOMCAT beamline achieved 1 tomogram per second at 1.1 μm spatial resolution.
4 Conclusions and outlook
Established NDT techniques offer distinct capabilities and limitations for composite inspection, but no single technique rarely provides a full-scale diagnosis. Future progress emphasizes multi-technique, portable, intelligent, and automated systems with efficient data processing.
- Each reviewed NDT technique has its own potential but rarely achieves full-scale diagnosis of composite structural integrity.
- As composite parts become more voluminous and structurally complex, multi-NDT approaches are becoming increasingly popular for maintaining structural integrity.
- Ultrasound, infrared thermography, and digital image correlation are versatile, cost-effective solutions used extensively across industrial fields.
- Terahertz waves can penetrate opaque materials to detect internal defects and damage, while compact portable devices could facilitate in-service or in-situ inspections.
- Future NDT development will increasingly depend on intelligent and automated inspection systems with high accuracy and efficient data processing.
- X-ray and neutron imaging provide high-resolution defect characterization but require ionizing-radiation facilities, protection, and limited-access infrastructure.