Inspection Of Composite Materials Research Articles

Lock-In Thermography with Cooling for the Inspection of Composite Materials.

This paper presents the development of the lock-in thermography system with an additional cooling system. System feasibility is tested by investigating a square-shaped glass fiber-reinforced polymer (GFRP) with artificially made outer flaws. The influence of heating mode and sinusoidal excitation period on the defect detectability is considered. Thus, the experiment is split into two modes: the sample is solely heated in the first mode or simultaneously heated and cooled in the second. In each mode, the temperature measurement is performed first with a shorter excitation signal period and second with a longer one. The signal-to-noise ratio (SNR) is used to assess defect detection quantitatively. The comparative analysis shows that employing a mixed heating-cooling mode improves the SNR compared to the conventional heating mode. The further enhancement of the SNR is obtained by extending the excitation period. The combination of simultaneous heating and cooling with longer periods of the excitation signal allows for the best SNR values for the most detected defects.

Automated woven background removal for enhanced infrared thermographic inspection of fabric composites

Infrared thermography is a promising non-destructive testing methodology for the inspection of composite materials. While there are many accounts in literature that report an excellent performance on unidirectional composite laminates, this is not the case for woven fabric composites. Their inspection is typically characterised by the presence of an imprint of the weave pattern in the thermographic recordings, which is most-often found to be disturbing for reliable anomaly detection. In this contribution, it is proposed to evaluate the thermographic data in ????-space in order to separate the weave pattern from the defect information. To this end, an algorithm is introduced to automatically perform this decomposition. The efficiency of the proposed methodology is demonstrated for several weave patterns with different defect cases.

Multi-label classification algorithms for composite materials under infrared thermography testing

ABSTRACT The key idea in this paper is to propose multi-labels classification algorithms to handle benchmark thermal datasets that are practically associated with different data characteristics and have only one health condition (damaged composite materials). A suggested alternative approach for extracting the statistical contents from the thermal images, is also employed. This approach offers comparable advantages for classifying multi-labelled datasets over more complex methods. Overall scored accuracy of different methods utilised in this approach showed that Random Forest algorithm has a clear higher performance over the others. This investigation is very unique as there has been no similar work published so far. Finally, the results demonstrated in this work provide a new perspective on the inspection of composite materials using Infrared Pulsed Thermography.

Suppression of front and back surface reflections in ultrasonic analytic-signal responses from composites

Pulse-echo ultrasound testing is the most prevalent method for inspection of composite materials in industry although evolving designs combined with the anisotropic nature of composites demands the constant development of more advanced signal-processing techniques and testing equipment. One problem that is frequently encountered in ultrasonic inspection, in pulse-echo mode, is the masking effect that occurs due to the strong surface reflections. This can prove critical for the detection of near-surface defects and accurate ply-tracking of the first and last two plies. The purpose of this study is to suppress the front- and back-surface reflections by first removing them from the measured ultrasonic response using the analytic signal and its instantaneous parameters. The first in the series of key steps the method includes is determining the shape of the input pulse using the measured front-surface reflection. After obtaining the input pulse, the front- and back-surface reflections are constructed artificially. The front-surface reflected pulse is the product of a complex reflection coefficient and the input pulse, while the back-surface reflected pulse is the product of a complex reflection coefficient, an attenuation term, and the incident pulse at the back surface. The next step involves subtracting the constructed surface pulses from the original response and substituting a reflection from a resin layer embedded in composite at the front and back surfaces. Those reflections are added back to the signal in order to make the ply extraction work consistently in the internal layers. The method has been tested using both simulated and real data. Subtraction of the front-surface was highly successful in a range of material configurations, but subtraction of the back-surface required algorithm refinements to cope automatically with all the scenarios tested. The ability of the method to improve detectability of defects and tracking of near-surface plies is demonstrated using data from real samples with near-surface delaminations, tape gaps and overlaps, and internal wrinkling.

High-Speed Time- and Frequency-Domain Terahertz Tomography of Glass-Fiber-Reinforced Polymer Laminates with Internal Defects

Composite materials are increasingly being utilized in many products, such as aircrafts, wind blades, etc. Accordingly, the need for nondestructive inspection of composite materials is increasing and technologies that allow nondestructive inspection are being studied. Existing ultrasound methods are limited in their ability to detect defects due to high attenuation in composite materials, and radiographic examination methods could pose a danger to human health. Terahertz (THz) wave technology is an emerging approach that is useful for imaging of concealed objects or internal structures due to high transmittance in non-conductive materials, straightness, and safety to human health. Using high-speed THz tomography systems that we developed, we have obtained THz tomographic images of glass-fiber-reinforced polymer (GFRP) laminates with artificial internal defects such as delamination and inclusion. The defects have various thicknesses and sizes, and lie at different depths. We present THz tomographic images of GFRP samples to demonstrate the extent to which the defects can be detected with the THz tomography systems.

A differential temperature contour map to characterize the detection limit of pulsed thermography of materials

Pulsed thermography has been widely used in the inspection of composite materials. The detection limit of pulsed thermography of the materials is often a question being concerned by end users. However, the detection limit cannot be described only by a simple parameter, and is affected by many factors associated with the defect, material, excitation, infrared camera and data processing algorithm. The current concept of detection limit defined with the minimum detectable size, the maximum detectable depth or the minimum aspect ratio depends on the specific experimental conditions, and it lacks universality when the extrinsic experimental conditions change. The aim of this paper is to establish a unified standard model of the detection limit which can be applied to various experimental conditions. The solution was to decouple the material’s intrinsic thermal property and extrinsic experimental condition dependencies and then allow them to be described independently. A new expression of the detection limit of the PT was firstly proposed by creating a differential temperature contour map. Taking delaminations in a C/SiC composite as detecting defects, an experiment was conducted to assess the feasibility and the detection limit, and to evaluate the noise level in practice. The influences of defect size and defect depth on the defect informative parameters were analyzed by using the finite element method based on 2D modeling. Eventually, the differential temperature contour map was calculated for the pulsed thermography of delaminations of C/SiC composites. The results show that the differential temperature contour map proposed can strictly describe the detection limit due to its differential temperature invariance property, and it provides a comprehensive tool to consider many experimental parameters including thermal excitation energy intensity, noise level, and SNR threshold by using a criterion equation in practice.

Evaluation of Heat Treatment Effect on Impact Strength of Glass Fiber–Reinforced Composite Material Using NDT Technique

Abstract Of late, the frequent use of composite materials merits effective inspection techniques to ensure structural integrity after manufacturing as well as during service. The inspection of composite materials is undertaken using various nondestructive testing (NDT) techniques. Ultrasonic testing (UT) is one of the popular NDT techniques used for testing of composites. The aim of the research is to establish a quantitative relationship between UT signal’s features and the mechanical property, i.e., impact toughness of the composite material heat treated at different temperatures. Polyester glass fiber–reinforced composite undergone post-cure heat treatment is the composite used in the research work. The glass fiber–reinforced plastic (GFRP) specimens have been heat treated at different temperatures to mimic different levels of thermal aging. The impact toughness of the composite material is determined using the Charpy impact test. Then, variation in attenuation of UT signals is related to the change in impact toughness of GFRP composite material heat treated at different temperatures. A possible classification of UT signals acquired from GFRP specimens in terms of different level of thermal aging has also been investigated through application of Hilbert–Huang transform (HHT) on the acquired UT signals. Hilbert energy frequency distribution-based feature extraction has been successfully used to analyze the effect of post-cure heat treatment on GFRP composite. An empirical relationship is finally obtained between Charpy impact energy and HHT energy. The proposed technique will be useful in prediction of strength and remaining useful life of GFRP composites through analyses of acquired UT-based NDT signals.

Nondestructive Ultrasonic Inspection of Composite Materials: A Comparative Advantage of Phased Array Ultrasonic

Carbon- and glass fiber-reinforced polymer (CFRP and GFRP) composite materials have been used in many industries such as aerospace and automobile because of their outstanding strength-to-weight ratio and corrosion resistance. The quality of these materials is important for safe operation. Nondestructive testing (NDT) techniques are an effective way to inspect these composites. While ultrasonic NDT has previously been used for inspection of composites, conventional ultrasonic NDT, using single element transducers, has limitations such as high attenuation and low signal-to-noise ratio (SNR). Using phased array ultrasonic testing (PAUT) techniques, signals can be generated at desired distances and angles. These capabilities provide promising results for composites where the anisotropic structure makes signal evaluation challenging. Defect detection in composites based on bulk and guided waves are studied. The capability of the PAUT and its sensitivity to flaws were evaluated by comparing the signal characteristics to the conventional method. The results show that flaw sizes as small as 0.8 mm with penetration depth up to 25 mm can be detected using PAUT, and the result signals have better characteristics than the conventional ultrasonic technique. In addition, it has been shown that guided wave generated by PAUT also has outstanding capability of flaw detection in composite materials.

Control Quality of Composite Materials by Using Subminiature Eddy Current Transducers

Based eddy current transducer (ECT), a probe has been designed to research composite materials. Defects inspection of composite materials is performed to determine the following standard defects: defect of the metallic and (or) polymer layer uniformity. The subminiature ECT of the original design is used as a sensor in this device, it is made according to a differential scheme of switching on of the coils of a transformer ECT and allowing to localize the control area up to 0.1-0.5 mm. The measurement procedure allowing one to detect defects in composite materials with a high accuracy is described. The sensor was tested on the composite material consisting of paper or low-density polyethylene and aluminum layers in which the model defect was placed. The dependences of the ECT signal on the defect in this structure are given. The determined dependence of electrical conductivity of composite materials on model defects make it possible to carry out defects inspection of composite materials.

Automated Quality Characterization for Composites Using Hybrid Ultrasonic Imaging Techniques

ABSTRACTAn enhanced technique using image processing has been developed for automated ultrasonic inspection of composite materials, such as glass/carbon-fibre-reinforced polymer (GFRP or CFRP), to ascertain their structural healthiness. The proposed technique is capable of identifying the abnormality features buried in the composite by image filtering and segmentation applied to ultrasonic C-Scan images. This work presents results performed on two composite samples with simulated delamination defects. A local gating scheme is applied to raw A-Scan data for improved contrast between defective and healthy regions in the produced C-Scan image. In this test campaign, different filtering and thresholding algorithms are evaluated and compared in terms of their effectiveness on defect identification. The accuracies of less than 3 mm and 1.11 mm were attained for the defect size and depth, respectively. The results demonstrates the applicability of the proposed technique for accurate defect localization and characterization of composite materials.

Extraction of Independent Structural Images for Principal Component Thermography

Thermography is a powerful tool for non-destructive testing of a wide range of materials. Thermography has a number of approaches differing in both experiment setup and the way the collected data are processed. Among such approaches is the Principal Component Thermography (PCT) method, which is based on the statistical processing of raw thermal images collected by thermal camera. The processed images (principal components or empirical orthogonal functions) form an orthonormal basis, and often look like a superposition of all possible structural features found in the object under inspection—i.e., surface heating non-uniformity, internal defects and material structure. At the same time, from practical point of view it is desirable to have images representing independent structural features. The work presented in this paper proposes an approach for separation of independent image patterns (archetypes) from a set of principal component images. The approach is demonstrated in the application of inspection of composite materials as well as the non-invasive analysis of works of art.

Porosity evaluation of in-service thermal barrier coated turbine blades using a microwave nondestructive technique

Porosity inspection of composite materials, ceramics and polymers is an important practical consideration in the nondestructive evaluation/testing (NDE/T) industry. The porosity of ceramics such as yttria stabilized zirconia (YSZ), the most widely used thermal barrier coating (TBC), for hot section metal components in a gas turbine is highly correlated with the thermal conductivity performance of the component. In-situ porosity inspection of TBCs after cyclic service is crucial to ensure the operational quality of TBCs and avoid any catastrophic system failure. The ability of microwave signals to propagate through a dielectric material and interact with the underlying surface and intrinsic properties, combined with the availability of high-resolution portable measurement equipment, make microwave imaging an attractive candidate for in-situ porosity inspection. In this paper, an extensive analysis of the simulation and measurement using a X-band open-ended rectangular waveguide (OERW) energy launch to evaluate the porosity of the TBCs of turbine blades is given. Existing mixing models are verified using simulation to determine the porosity and its relation to dielectric properties. The results show that the OERW microwave NDT/E technique can be used to characterize the porosity of TBCs, after cyclic services, using the phase of the reflection coefficient, S11. The results are validated using waveguide dielectric property measurements and image analysis produced using a scanning electron microscope (SEM).

Достижения в области НК авиационных деталей: новые методы и решения на основе использования роботизированных систем

The application of nondestructive testing to the aerospace sector is constantly improving, as fast as technology allows, due to increasing presence of composites in structures, the use of new materials and the new geometries emerging. This makes necessary to innovate in the development of NDT methods, like ultrasonics or thermography, and in particular in ultrasounds, from using conventional piezoelectric transducers to phased-array technology, that allows the generation of the sound beam easily controlled and configurable based on the use of a multi-elements matrix of probes that can be electronically trigger. Moreover, the application of popular UT techniques (pulse-echo and through transmission) has been extended to other new innovative techniques, such as laser ultrasonic and air coupled ultrasounds. 
 Tecnatom participates in different innovation project in NDT and develops new equipment for inspection of composite materials. One of the studies is in the comparison of different NDT methods and techniques for the inspection of cured and non-cured materials in aeronautical components with the help of air-coupled ultrasound and thermography.
 In the article are described the use of robots in automated Tecnatom system, developed in cooperation with KUKA Robotics. The use of its own developed electronics, allow working efficiently with materials with high attenuation, and specialized software, allows effectively inspecting the complex geometries aviation parts, taking into account the scanning trajectory and robot movement and control all the inspection process.

Damage Localization in Composite Structures Using Nonlinear Vibration Response Properties

Subsurface damage in composite materials is difficult to detect using visual techniques, and other current inspection methods lack the ability to perform quick, wide-area inspections without the need for reference signatures or baseline measurements. This paper presents a method for detecting and locating subsurface damage in composite materials without historical reference measurements by considering the nonlinear behavior of the material in the vicinity of damage. Nonlinear behavior is identified by comparing frequency response functions measured at different input amplitudes. It will be shown that the nonlinear behavior of the material is most evident in the areas nearest to the damage. The proposed inspection method is demonstrated both analytically and experimentally. First, a finite element model of a sandwich beam is developed using Bernoulli–Euler beam elements to represent each layer of the beam and springs to represent the interface between the layers. A bilinear stiffness nonlinearity is simulated to represent disbond damage between the top and core layers of the beam. The simulated disbond damage is localized by identifying degrees of freedom which indicate significant nonlinear response through a comparison of frequency response functions measured at various input amplitudes. Next, the method is demonstrated experimentally by identifying disbond damage in a fiberglass sandwich panel. A three-dimensional scanning laser vibrometer is used to measure the forced frequency response of the panel in its damaged state as it is excited at two or more amplitudes of excitation by a piezoelectric actuator. Comparisons of the frequency response functions measured at different input amplitudes show that the subsurface damage introduces nonlinear behavior which resembles a bilinear stiffness nonlinearity, and the differences in the frequency response functions are largest in the vicinity of the damage location. In addition, it was found that improved localization of the damage is achieved by investigating the response at higher frequencies. This work has application as a nondestructive method for detecting and locating subsurface damage in composite materials and, by using a laser vibrometer for noncontact measurement, allows for quick, wide-area inspection of composite materials without the need for reference signatures or baseline measurements.

X-ray inspection of composite materials for aircraft structures using detectors of Medipix type

This work presents an overview of promising X-ray imaging techniques employed for non-destructive defectoscopy inspections of composite materials intended for the Aircraft industry. The major emphasis is placed on non-tomographic imaging techniques which do not require demanding spatial and time measurement conditions. Imaging methods for defects visualisation, delamination detection and porosity measurement of various composite materials such as carbon fibre reinforced polymers and honeycomb sendwiches are proposed. We make use of the new large area WidePix X-ray imaging camera assembled from up to 100 edgeless Medipix type detectors which is highly suitable for this type of measurements.

Aeronautics composite material inspection with a terahertz time-domain spectroscopy system

The usability of pulsed broadband terahertz radiation for the inspection of composite materials from the aeronautics industry is investigated, with the goal of developing a mobile time-domain spectroscopy system that operates in reflection geometry. A wide range of samples based on glass and carbon fiber reinforced plastics with various types of defects is examined using an imaging system; the results are evaluated both in time and frequency domain. The conductivity of carbon fibers prevents penetration of the respective samples but also allows analysis of coatings from the reflected THz pulses. Glass fiber composites are, in principle, transparent for THz radiation, but commonly with significant absorption for wavelengths >1 THz . Depending on depth, matrix material, and size, defects like foreign material inserts, delaminations, or moisture contamination can be visualized. If a defect is not too deep in the sample, its location can be correctly identified from the delay between partial reflections at the surface and the defect itself.

Ultrasonic NDE for Internal Defect Detection in Multi-layered Composite Materials by Multi-resolution Signal Decomposition

Ultrasonic NDE has been a well known approach to investigate material’s microstructures, mechanical properties and structure integrity in industry. The internal structure of a material and position of anomalies can be recognised by the reactions of different materials to ultrasound. However, the interpretation of ultrasound signals is difficult in composite material inspection task due to the fact that the ultrasonic pulse is reflected not only by the defect occurred within the material but the microstructures and multiple lay ups of the material. This phenomenon causes the backscattering noise to hinder the real defect’s signal during the inspection. Backscattering noise exists in multiple frequencies. The objective of this study was to develop a new noise reduction method to enhance the defect detectability in coarse-grained structure material such as composites materials. This method increases Signal-to-noise Ratio (SNR) by means of decomposing the original signal into multiresolution representations. To prevent the loss of information, the signal is processed in both temporal and frequency domain. The proposed method has been tested on simulated signal and Glass Fiber Reinforced Plastics (GFRP) laminates. Both simulation and experimental results showed that this method can significantly reduce grain noise while preserving the resolution of the original signal of the defect.

Orientation-selective X-ray dark field imaging of ordered systems

X-ray scatter dark field imaging with a grating interferometer is becoming a standard tool for the characterization of microscopic texture of samples. Recently, it was shown that directional information could also be recovered when the sample displays an anisotropic ordering such as, for instance, a bundle of microscopic fibers. Here, we demonstrate that previously suggested approaches are ambiguous when multiple anisotropic orientations coexist in the sample. Therefore, we developed a new orientation-selective approach which allows for separating the contributions of individual orientations provided that these orientations are known a-priori. The method, demonstrated experimentally using a well-defined wood sample, is envisioned to be of high interest for the non-destructive inspection of composite materials.

In Introductory Review to THz Non-Destructive Testing of Composite Mater

As the volume of composite parts continues to rise, quality and performance specifications are becoming more and more demanding. In this context, robust and reliable nondestructive testing (NDT) techniques are necessary. Thanks to numerous recent technological breakthroughs in the THz field, the THz technology has proven to be a highly promising technology for composite NDT inspection. In this introductory review, the most relevant aspects of the THz technology as a tool for NDT inspection of composite materials are synthesized and analyzed based on a comprehensive review of the current literature. To this end, available THz experimental setups are described. Furthermore, THz imaging techniques suitable for composite inspection are analyzed and the THz spectroscopic approach is briefly presented. Finally, a review of the state of the art of THz based composite inspection systems is presented describing and analyzing recent most relevant milestones achieved in the field.

ThermoPoD: A reliability study on active infrared thermography for the inspection of composite materials

In this study, a Probability of Detection (PoD) experimental study was carried out in the framework of a Belgian-Quebec/Canada collaborative research project called ThermoPoD. Experiments were implemented on a Carbon Fiber Reinforced Plastic (CFRP) specimen with embedded material simulating delamination. For active infrared thermography, different heating sources (optical or ultrasound), heating forms (pulsed or lock-in), and data processing methods, such as Fourier Transform, Thermal Signal Reconstruction, Wavelet Transform, Differential Absolute Contrast, and Principal Component Thermography are of interest. In the present study, the effects of various data processing methods on PoD curves are compared.