Defects In Composite Materials Research Articles

Metal inclusion influence on mechanical behaviour of plates made of araldite

Due to the need for creating new, more efficient, materials for use in structures manufacturing, a high interest issue for researchers and engineers is to analyze the plastic or composite material defects and their influence over the mechanical behavior of the structures. Presented in this paper are the results of experimental analysis and its validation, made by finite element numerical analysis. The first part of the analysis process consisted in obtaining the reference values from plastic plates containing no metallic inclusions, which were analyzes both experimentally and numerically, by subjecting them to a flexural load. A numerical validation of the experimental model was obtained. Next, same kind of numerical and experimental analyses were performed, but this time plastic plates containing metallic inclusions were subjected to the same loads as before. In doing so, the study revealed the local and global influences of inclusions over the mechanical behavior of plastic plates.
 

Universal installation for studying structural defects in electrical and optical fiber materials

A universal device for experimentally studying the migration of microscopic structure defects and the features of dielectric relaxation is proposed. it allows using the thermo stimulated depolarization method, in combination with the measurement of the tangent of the angle of dielectric losses and the thermo stimulated polarization current, to perform dielectric spectroscopy of hydrogen-bonded crystals and perform analysis of the properties and parameters of structure defects. A smaller (in comparison with the existing installation) additional compact device for measuring small values of electrical capacitance and the tangent of the angle of dielectric losses, including an electrometer B7-30, was designed, measurement was carried out using a q-factor meter VM 560. When measuringtgδ > 0.1, the VM-507 device was used. An experimental methodology is proposed that allows, in combination with the method of minimizing the comparison function (MFC - method), with a high degree of accuracy, to calculate the molecular characteristics of structural defects in composite materials based on semiconductors and dielectrics used in the electrical and optical fiber industry, electric power and insulation technology.

Terahertz Spectroscopy and Imaging Detection of Defects in Civil Aircraft Composites

Composite materials have increasingly become a high proportion of the structural weight of aircraft due to their excellent performances. Different types of damages may occur in the aircraft service period, which will bring potential safety risks to aircrafts. To investigate the defect damage detection and its spectral characteristics and imaging of carbon-fiber-reinforced polymer composite laminates, defects from the low-velocity impact damage in composites were measured by the THz time-domain reflection imaging system. Results show that there exists obvious THz spectral differences between the impact damaged defects and nondefect. The effective detection frequency band for the low-speed impact damaged defect is 0.12–2.0 THz. In the time domain, there are attenuations and delays in the spectra of defects relative to those of nondefects. In the frequency domain, with the increase of frequency, the power spectral density of the defect first increases and then decreases, and the absorption coefficient increases slowly. In general, the imaging results in time-domain imaging are better than those from the frequency-domain imaging, which not only is suitable for the qualitative detection of defects but also has great potential and application prospects in quantitative detection. This work shows an important guide for the application of THz technology to detect the composite material defects in civil aircraft.

Spatial-Neighborhood Manifold Learning for Nondestructive Testing of Defects in Polymer Composites

The subspace learning (dimensionality reduction) algorithms have played an important role in the analysis of thermographic data: a key step in infrared thermography-based nondestructive testing of subsurface defects in composite materials. However, one of its branches, manifold learning, with excellent ability to preserve local data structure, is rarely applied. In this article, a spatial-neighborhood manifold learning (SNML) framework is proposed for thermographic data analysis. Different from traditional manifold learning methods, SNML uses the spatial-neighborhood information instead of the traditional k-nearest neighbors, or ε-neighborhood, to construct the adjacency graph. This overcomes the difficulty of parameter selection and extracts local features in images in a more reasonable way. Additionally, the data preprocessing step and the means of thermographic data normalization in the proposed framework are discussed. For performance comparison, three traditional manifold learning methods are also implemented. The experiments on carbon fiber-reinforced polymer specimens demonstrate the validity and feasibility of SNML.

Improvement of defect size and morphological estimation in shearography inspection by wavelet transform

Speckle shearography is often used in the aerospace industry to detect defects in composite materials because it allows noncontact, full field, and fast measurements. However, despite this advantage, this nondestructive technique is still not widely established, due to its limitations in performing defect quantitative estimations. The main issue lies in the extrapolation of phase profile boundaries, which represent defect edges. In this paper, this was achieved by studying the multimodal distribution of a characteristic parameter, called structural intensity, which is the probability of finding a denser population of local maxima of wavelet coefficients at a given position and adding an adaptive threshold selection in the structural intensity distribution. Once the damage boundaries were detected, the defect dimensions were computed by subtracting the local shearing amount, which was defined by a circle detection code, from each coordinate in both Cartesian directions. The paper shows an improvement to the algorithms able to detect defect area and perform morphological estimation previously developed by the authors and an in-depth study of the parameters involved in these algorithms. This resulted in both a high accuracy (0.04%) revealed by the calibration process on a PVC test panel and a close morphological agreement with the ultrasound reconstruction of unknown defects in composite materials. Thanks to this improvement, shearography becomes a quantitative inspection technique that takes few minutes compared to ultrasound inspection of the same area, which may require up to an entire day.

Evaluation of debonding in CFRP-epoxy adhesive single-lap joints using eddy current pulse-compression thermography

Contaminations arising during the manufacturing or usage of Carbon Fiber Reinforced Plastic (CFRP)-epoxy adhesive single-lap joints can drastically affect the mechanical performance of the structure and eventually lead to its catastrophic failure. Eddy current stimulated thermography has been recently proposed for detecting and characterizing defects in composite materials. The characterization of debonding in adhesive single-lap joints in composites by eddy current stimulated thermography can be challenging due to the anisotropic thermal diffusion caused by the anisotropic conductivity and multi-layer nature of the composites. This work tackles the challenge of detecting the interface contamination by combining eddy current pulse-compression thermography and image post-processing algorithms. The impulse responses from samples containing either brass, release film or release agent contaminations obtained through pulse-compression combined with Eddy Current Pulsed Thermography (ECPT). Non-uniform heating patterns are removed by partial least square technique. Then, the time instants containing meaningful information about the contaminated interface layer are inferred by comparing each norm of kernel principal component over suitable time windows. The evaluation of contamination depths and properties are conducted in selected time windows by principal component analysis and time-phase analysis. Results proved that release agent contamination plays the minor role in changing the electrical and thermal properties of the single-lap joints compared with brass and release film.

Independent component thermography for non-destructive testing of defects in polymer composites

Thermographic data processing and analysis is critical for effective infrared thermography non-destructive testing of defects in composite materials. In this research work, the concept of blind source separation is introduced into this field for facilitating subsurface defect detection in polymer composites. In detail, a thermal image generated by infrared thermography is viewed as a linear mixture of several independent and non-Gaussian sources, which can be decomposed by adopting the proposed independent component thermography (ICT) technique. After the decomposition, the non-Gaussian sources are extracted and plotted as a number of images with the same size as the original thermograms. Eventually, the defect signals can be separated from the non-uniform backgrounds which are usually caused by uneven heating. As a result, the location and shape information of the subsurface defective regions is highlighted. The feasibility of the ICT method is illustrated with its application to a carbon fiber reinforced polymer specimen.

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.

Identification of structural defects in composite materials by the thermal imaging non-destructive testing method

The article discusses the practical aspects of applying the thermal imaging non-destructive testing method of layered polymer composites. The specific purpose of the work is to demonstrate the possibilities of detecting in material and geometric identification of initial defects such as debonding.

An investigation of the stepped thermography technique for defects evaluation in GFRP materials

The ability of thermography to detect defects in composite materials has been demonstrated and showed in various works and in many applications. In this regard, various NDT techniques are currently used for defect detection in composites such as Lock-in Thermography (LT), Pulsed (PT) Stepped Thermography (ST/SH), all of which have their own peculiarities and capabilities. A critical aspect concerns the overall lengthy time required for testing and analysis of thermographic data above all, for large structure where it is necessary a scanning approach. In this work, two algorithms based on the stepped thermography approach were investigated in quantitative way with the aim to optimizing the testing parameters and data analysis in terms of testing time and signal to noise ratio. In particular, several tests were carried out on a sample specimen with simulated defects and the well-established lock-in thermography technique has been used as comparison.

Influence of Defects on Bending Properties of 2D-T700/E44 Composites Prepared by Improved Compression Molding Process.

2D-T700/E44 composite materials were prepared by improved compression molding process (ICM) then microstructure and properties of the composites were analyzed and summarized by scanning electron microscope (SEM) and electronic universal testing machine. It is found that defects will occur when the process parameters are not controlled properly and the main defects of composite materials include inadequate resin impregnation, weak interlaminar binding force, fiber displacement warping, hole and brittle fracture. Moreover, there are significant differences in the infiltration microstructure, bending properties, and fracture morphology of the composite materials with different defects. When the defects of weak interlaminar binding force and brittle fracture occur, bending properties of composite materials are relatively low, and they are 220 MPa and 245 MPa, respectively, which reach 34.9% and 38.9% of the bending strength of composite material whose defects are effectively controlled. When the process parameters are reasonable and the defects of the composite materials are effectively eliminated, the bending strength can reach 630 MPa. This will lay a foundation for the preparation of 2D-T700/E44 composite materials with ideal microstructures and properties by ICM.

Detection and Evaluation of Pre-Preg Gaps and Overlaps in Glare Laminates

Gaps and overlaps between pre-preg plies represent common flaws in composite materials that can be introduced easily in an automated fibre placement manufacturing process and are potentially detrimental for the mechanical performances of the final laminates. Whereas gaps and overlaps have been addressed for full composite material, the topic has not been extended to a hybrid composite material such as Glare, a member of the family of Fibre Metal Laminates (FMLs). In this paper/research, the manufacturing, the detection, and the optical evaluation of intraply gaps and overlaps in Glare laminates are investigated. As part of an initial assessment study on the effect of gaps and overlaps on Glare, only the most critical lay-up has been considered. The experimental investigation started with the manufacturing of specimens having gaps and overlaps with different widths, followed by a non-destructive ultrasonic-inspection. An optical evaluation of the gaps and overlaps was performed by means of microscope image analysis of the cross sections of the specimens. The results from the non-destructive evaluations show the effectiveness of the ultrasonic detection of gaps and overlaps both in position, shape, width, and severity. The optical inspections confirm the accuracy of the non-destructive evaluation also adding useful insights about the geometrical features due to the presence of gaps and overlaps in the final Glare laminates. All the results justify the need for a further investigation on the effect of gaps and overlaps on the mechanical properties.

The interaction problem of two Zener–Stroh cracks with a nearby inhomogeneity

In this paper, the interaction among two Zener–Stroh cracks (with plastic zone correction) and a nearby circular inclusion are investigated. To evaluate the plastic zone sizes at crack tips in the current physical problem is a great challenge. As the first attempt to explore the multiple defects’ interaction effect on the yielding behavior of a crack, we focused on the analysis of one target crack, while the other crack and the circular inhomogeneity are treated as influence factors. With the help of coordinate transformation and superposition procedure, the formulated singular integral equations can be solved numerically. The influence of material properties, crack–crack positions and other parameters, such as crack length and Burgers vector of the Zener–Stroh crack, on the target crack tip stress intensity factor, plastic zone size and crack tip opening displacement are examined. It is found that the effects of the aforesaid parameters on the cracks are all inter-related and dependent on each other. This observation reveals the complexity of fracture analysis and the necessity to have a deep research on interacting defects in composite materials.

Modeling and classification of defects in CFRP laminates by thermal non-destructive testing

Abstract Pulsed thermography has been used for many years to investigate the presence of subsurface defects in composite materials for aeronautics. Several methods have been proposed but only few of them include a complete automated approach for the effective defect characterization. This paper presents a novel method which approximates the thermal decays on the laminate surface, induced by a short heat pulse, by means of an exponential model in three unknowns (model parameters), estimated in the least squares sense. These parameters are discriminant and noise-insensitive features used to feed several classifiers, which are trained to label possible defects according to their depths. Experimental tests have been performed on a carbon-fiber reinforced polymer (CFRP) laminate having four inclusions of known properties. The comparative analysis of the proposed classifiers has demonstrated that the best results are achieved by a decision forest made of 30 trees. In this case the mean values of standard and balanced accuracies reach 99.47% and 86.9%, whereas precision and recall are 89.87% and 73.67%, respectively.

Far field microwave NDE of composite structures using time reversal mirror

Microwave NDE techniques rely on the ability to propagate through Fiber Reinforced Polymer (FRP) composites and provide information about the structural integrity of composite materials. Time reversal (TR) property of the wave equation offers an effective method for interpreting the microwave NDE data. In TR, the fields back-propagate to focus at the source. TR techniques using microwave measurements have proved to be promising in radar imaging and nondestructive evaluation (NDE). This paper investigates the feasibility of using microwave TR for NDE of composite materials. Simulation studies demonstrate the feasibility and robustness of this approach to detect disbonds in metal-composite joints. An experimental system is built using a pulsed time domain setup coupled with the TR method. Experimental results demonstrate the capability of this technique for detecting single as well as multiple defects in composite materials.

Modeling of ultrasonic wave propagation in composite laminates with realistic discontinuity representation

This paper presents a method for embedding realistic defect geometries of a fiber reinforced material in a finite element modeling environment in order to simulate active ultrasonic inspection. When ultrasonic inspection is used experimentally to investigate the presence of defects in composite materials, the microscopic defect geometry may cause signal characteristics that are difficult to interpret. Hence, modeling of this interaction is key to improve our understanding and way of interpreting the acquired ultrasonic signals. To model the true interaction of the ultrasonic wave field with such defect structures as pores, cracks or delamination, a realistic three dimensional geometry reconstruction is required. We present a 3D-image based reconstruction process which converts computed tomography data in adequate surface representations ready to be embedded for processing with finite element methods. Subsequent modeling using these geometries uses a multi-scale and multi-physics simulation approach which results in quantitative A-Scan ultrasonic signals which can be directly compared with experimental signals. Therefore, besides the properties of the composite material, a full transducer implementation, piezoelectric conversion and simultaneous modeling of the attached circuit is applied. Comparison between simulated and experimental signals provides very good agreement in electrical voltage amplitude and the signal arrival time and thus validates the proposed modeling approach. Simulating ultrasound wave propagation in a medium with a realistic shape of the geometry clearly shows a difference in how the disturbance of the waves takes place and finally allows more realistic modeling of A-scans.

Capacitive imaging of impact damage in composite material

Abstract This work presents capacitive non-destructive imaging of impact damage in woven fibre CFRP using a coplanar capacitive sensor. Novelty of the approach consists in that spectral characteristics of the capacitive sensor response to CFRP properties are firstly used to establish excitation frequencies for optimum imaging of delamination. For the tested samples optimum excitation frequencies were found to be above 200 MHz. Clear capacitive images of delaminations were obtained on each sample. Ultrasonic (UT) C-scans of delaminations due to impacts were acquired using phased array technique to confirm presence and extent of the delaminations. Capacitive imaging technique can be used as complementary to UT for imaging shallow defects in composite materials at depths where UT has insufficient time domain resolution.

Active Millimeter-Wave Radiometry for Nondestructive Testing/Evaluation of Composites—Glass Fiber Reinforced Polymer

Manufacture and in-service assessment of composite materials can be challenging since there is, yet, no standard method for testing them. Nevertheless, regular inspection is necessary to maintain the structural integrity. This paper describes a nondestructive, broadband noise mapping method that uses a millimeter-wave radiometer to detect defects in composite materials. The $W$ -band system is configured for transmission mode measurement in which an amplified photonic source illuminates a glass fiber reinforced polymer containing machined defects. A high sensitivity radiometer is used for imaging the sample. The radiometer consists of a Schottky diode-based frequency tripler and a fundamental mixer.

Lock-in thermography as a rapid and reproducible thermal characterization method for magnetic nanoparticles

Lock-in thermography (LIT) is a sensitive imaging technique generally used in engineering and materials science (e.g. detecting defects in composite materials). However, it has recently been expanded for investigating the heating power of nanomaterials, such as superparamagnetic iron oxide nanoparticles (SPIONs). Here we implement LIT as a rapid and reproducible method that can evaluate the heating potential of various sizes of SPIONs under an alternating magnetic field (AMF), as well as the limits of detection for each particle size. SPIONs were synthesized via thermal decomposition and stabilized in water via a ligand transfer process. Thermographic measurements of SPIONs were made by stimulating particles of varying sizes and increasing concentrations under an AMF. Furthermore, a commercially available SPION sample was included as an external reference. While the size dependent heating efficiency of SPIONs has been previously described, our objective was to probe the sensitivity limits of LIT. For certain size regimes it was possible to detect signals at concentrations as low as 0.1mg Fe/mL. Measuring at different concentrations enabled a linear regression analysis and extrapolation of the limit of detection for different size nanoparticles.

Noncontact Sonic NDE and Defect Imaging Via Local Defect Resonance

A selective acoustic activation of defects based on the concept of local defect resonance enables to enhance considerably the intensity of defect vibrations and makes it possible to reduce the input acoustic powers to the levels permissible for noncontact nondestructive inspection. Since for cm-size defects in composite materials, the LDR frequencies lie in the low kHz-range, the resonant noncontact activation shifts to an audible frequency range and can be provided by conventional sonic equipment. In this paper, the feasibility of the resonant noncontact inspection is validated for the most “problematic” methodologies of nonlinear, thermosonic and shearosonic NDE that usually require an elevated acoustic power and, therefore, a reliable contact between the specimen and the transducer. In contrast, the noncontact versions developed employ commercial loudspeakers which can simultaneously insonify large areas and be applied for a contactless sonic inspection of different materials and various scale components.