Nondestructive Evaluation Of Composite Materials Research Articles

Multi-sensor spatial augmented reality for visualizing the invisible thermal information of 3D objects

In recent years, infrared imaging technology has been widely used to capture valuable thermal information of target objects for various applications such as construction engineering, advanced manufacturing, and medical diagnosis. However, it still remains a challenging task to visualize the invisible thermal information of three-dimensional objects for further evaluation and decision making. In this paper, we build a multi-sensor system consisting of a long-wave infrared camera, a RGBD camera, and a digital projector for multimodal data acquisition, real-time 3D thermographic reconstruction, and projector-based spatial augmented reality. We propose a hybrid method that combines the benefits of model-based ray casting and frame-based image warping to generate high-quality projected images with complete view coverage and abundant thermal details for visualizing the invisible thermal information on 3D objects. The effectiveness of the proposed method has been validated for two typical thermal imaging applications including human body temperature monitoring and non-destructive evaluation of composite materials.

Characterization of Global and Local Damages in Composite Images Using Geometrical and Fourier-Hu Moment–Based Shape Descriptors

Abstract In this work, damages in composite materials are characterized using geometrical and Fourier-Hu shape descriptors. Digital images are acquired from the front and rear sides of composite materials after 5-, 6-, and 7-mm indentations. An anisotropic diffusion filter is used to remove the noise and to smooth the digital images. The global and local damages in the composite materials are segmented using K-Means (KM) clustering. Geometrical features and Fourier-Hu–based moments are calculated from the extracted regions. The global and local damages due to different indentations are classified using random forest (RF). Results show that the KM is able to segment the global and local damages with cluster centers three and four, respectively. The Fourier-Hu moment suggested that the damage dimension is high in the rear side compared with the front side. The Hu moment performed better compared with using geometrical features to characterize the damages. RF with both geometrical and Fourier-Hu moments performed better than the individual features. The image-based analysis performed in this study concludes effective characterization of damages; this framework can be applied in the nondestructive evaluation of composite materials.

Application of Subspace-Based Distorted-Born Iteration Method in Imaging Biaxial Anisotropic Scatterer

Various natural and artificial materials are anisotropic. The inverse scattering problem of anisotropic scatterers is widely involved in oil detection, nondestructive evaluation of composite materials and microscopic imaging of biological tissue. In this contribution, the two-dimensional inverse scattering problem of biaxial anisotropic scatterers illuminated by the TE-polarized incident wave is investigated. Since the biaxial anisotropic scatterer has different permittivity components along different transverse directions, the problem faced with is more complex than in the scalar TM-polarized case. The subspace-based distorted-Born iteration method (S-DBIM) is employed. Only one regularization term is involved in the inversion, which is proven to be quite robust against noise and flexible to be chosen. Both synthetic and experimental results are given to prove the validity of the proposed method. The results illustrate that as the interaction between the incident electric field and the scatterer induces a directional scattered field, the images constructed appear clear into the strongly scattered directions, but blurred into weakly ones. Overall, S-DBIM is shown to yield super-resolved images for the biaxial anisotropic scatterers, while being quite robust with respect to noise.

Distributed point source modeling of the scattering of elastic waves by a circular cavity in an anisotropic half-space

The Distributed Point Source Method (DPSM) is a modeling technique based on superposition of fundamental solutions corresponding to individual pair of source and target points. A collection of source points distributed over the boundaries and interfaces are responsible for transmission, reflection, and refraction of acoustic waves in the solution domain. The strength of the source points may not be known a priory. By imposing the prescribed conditions on the boundaries and interfaces, a system of equations with the source strengths as the unknowns is obtained. After finding the source strengths as the solution to this system of equation, the amount of the solution at any target point in the domain is obtained by superimposing the effect of all source points on that target point. DPSM is an efficient modeling technique for ultrasonic problems since it does not require discretization of the whole solution domain but only the boundaries and interfaces. The fundamental solution, or the Green’s function, between a pair of source and target points serves as the building block for DPSM. For an ideal fluid or a homogeneous isotropic solid the elastodynamic Green’s function is available as closed form algebraic expressions. But for an anisotropic solids, the set of governing equations are considerably more complex and the elastodynamic Green’s function needs to be evaluated numerically. In this study, an anisotropic half-space containing a flaw in the form of a circular hole is considered. The solid half-space is in contact with fluid and a transducer is located in fluid facing the solid half-space. Some efforts have been made to alleviate the computational intensity of the numerical evaluation of anisotropic Green’s function for this problem. Firstly, a technique called “windowing” is used to exploit the repetitive pattern of relative positions of the source and target points in order to considerably reduce the number of Green’s function evaluations. Secondly, the resolution of the integration for evaluation of the anisotropic Green’s function is changed based on the distance between the source and target points, and a calibration technique based on an equivalent isotropic stiffness tensor is suggested. This calibrated multi-resolution integration technique is combined with the windowing technique, and the developed DPSM model is applied to a numerical example containing a transversely isotropic half-space, to show the applicability and effectiveness of DPSM modeling for this class of problems. Important applications like non-destructive evaluation of composite materials may benefit from such modeling capability.

Restored pseudo heat flux (RPHF) algorithm for carbon fibre composite defect detection using thermography under uneven heating

Carbon fibre composite materials are widely used in high-value, high-profit applications – such as aerospace manufacturing and shipbuilding – due to their low density, high mechanical strength, and flexibility. However, existing techniques for non-destructive evaluation of composite materials are limited and not robust. Thermography has been demonstrated to be a promising technology; however, thermal diffusion and uncontrolled uneven heating considerably degrade its capacity to detect thin planar defects in carbon fibre composites. In this study, we have developed a method, restored pseudo heat flux (RPHF), for addressing this problem. Unlike many existing methods, RPHF does not require prior knowledge about defect depth. A numerical simulation was developed, and three sets of experiments were performed using a layered sample. Results suggest that in comparison with other existing methods, RPHF does not require an accurate value for its input, thermal diffusivity. It provides a good approximation to heat flux distribution and significantly improves the detection capability of thermography when compared to several existing methods.

Journal of Composites Science: A New Journal for Composite Materials, Structures and Experiments

The Journal of Composites Science is a new, online, open access journal, which aims to focus on advanced technology and the development of composites and composite structures.[...]

Air-Coupled Low Frequency Ultrasonic Transducers and Arrays with PMN-32%PT Piezoelectric Crystals.

Air-coupled ultrasonic techniques are being increasingly used for material characterization, non-destructive evaluation of composite materials using guided waves as well as for distance measurements. Application of those techniques is mainly limited by the big losses of ultrasonic signals due to attenuation and mismatch of the acoustic impedances of ultrasonic transducers and air. One of the ways to solve this problem is by application of novel more efficient piezoelectric materials like lead magnesium niobate-lead titanate (PMN-PT) type crystals. The objective of this research was the development and investigation of low frequency (<50 kHz) wide band air-coupled ultrasonic transducers and arrays with an improved performance using PMN-32%PT crystals. Results of finite element modelling and experimental investigations of the developed transducers and arrays are presented. For improvement of the performance strip-like matching elements made of low acoustic impedance, materials such as polystyrene foams were applied. It allowed to achieve transduction losses for one single element transducer −11.4 dB, what is better than of commercially available air-coupled ultrasonic transducers. Theoretical and experimental investigations of the acoustic fields radiated by the eight element ultrasonic array demonstrated not only a good performance of the array in a pulse mode, but also very good possibilities to electronically focus and steer the ultrasonic beam in space.

An Electromagnetic Sensor with a Metamaterial Lens for Nondestructive Evaluation of Composite Materials.

This paper proposes the study and implementation of a sensor with a metamaterial (MM) lens in electromagnetic nondestructive evaluation (eNDE). Thus, the use of a new type of MM, named Conical Swiss Rolls (CSR) has been proposed. These structures can serve as electromagnetic flux concentrators in the radiofrequency range. As a direct application, plates of composite materials with carbon fibers woven as reinforcement and polyphenylene sulphide as matrix with delaminations due to low energy impacts were examined. The evaluation method is based on the appearance of evanescent modes in the space between carbon fibers when the sample is excited with a transversal magnetic along z axis (TMz) polarized electromagnetic field. The MM lens allows the transmission and intensification of evanescent waves. The characteristics of carbon fibers woven structure became visible and delaminations are clearly emphasized. The flaws can be localized with spatial resolution better than λ/2000.

Exploiting carbon nanotube networks for damage assessment of fiber reinforced composites

An approach for damage inspection of composite structures utilizing carbon nanotubes (CNT) networks is investigated. CNT are dispersed in an epoxy using a processing technique compatible with commonly employed composite manufacturing techniques and subsequently used as matrix for a structural glass fiber reinforced composite. The developed electrical conductivity of the composite system is verified experimentally. The electrically conductive CNT network within the GFRP is exploited through distributed electrical voltage measurements to sense and, ultimately, locate damage in the plane of the composite plate. Damage in the form of cracks or delamination interrupts the continuity of the CNT network separating and isolating regions of the conductive network. Employing electric potential fields these changes can become measurable and can provide information for inversely locating the damage. Electrical Resistance Tomography (ERT) is formulated and experimentally applied to measure changes in the potential fields and deliver electrical conductivity change maps which are used to identify and locate changes in the CNT networks. These changes are correlated to capture the damage in the composite. Different damage modes are studied to assess the capabilities of the technique. The technique shows sensitivity to very small damages; less than 0.1% of the inspected area. The solution of the inverse ERT problem delivers a conductivity change maps which offers an effective localization with nearly 10% error and an inspection area suppression of around 75%. The proposed methodology to create CNT networks enables the application of ERT for Non-Destructive Evaluation of composite materials, previously not possible due to lack of conductivity, thus offering damage sensing and location capabilities even in-situ.

Damage detection of unidirectional carbon fiber–reinforced laminates with embedded magnetostrictive particulates: A preliminary study

Defects in composite laminates are difficult to detect because of the conductive and paramagnetic properties of composite materials. Timely detection of defects in composite laminates can improve reliability. This research illustrates the preliminary analysis and detection of delaminations in carbon fiber laminate beams using a single layer of magnetostrictive particles and noncontacting concentric magnetic excitation and sensing coils. The baseline analytical models also begin to address the intrusive nature of the magnetostrictive particles as well as relate the applied excitation field with the stress and magnetic flux densities induced in the magnetostrictive layer. Numerical methods are used to begin to characterize the presence of magnetostrictive particles in the laminate and the behavior of the magnetostrictive particles in relationship to the magnetic field used during sensing. Unidirectional laminates with embedded delaminations are used for simulations and experimentations. A novel, yet simplified fabrication method is discussed to ensure consistent scanning and sensing capabilities. The nondestructive evaluation scanning experiments were conducted with various shapes and sizes of damages introduced into carbon fiber–reinforced polymeric composite structures. The results demonstrate high potential for magnetostrictive particles as a low-cost, noncontacting, and reliable sensor for nondestructive evaluation of composite materials.

NDE inspection of terahertz waves in wind turbine composites

An investigation of terahertz waves was made for the nondestructive evaluation of composite materials and structures in the wind turbine. The modalities of the terahertz radiation used were time domain spectroscopy (TDS) and continuous wave (CW). The composite materials and structures investigated include both non-conducting polymeric composites and conducting carbon fiber composites. Terahertz signals in the TDS mode resembles that of ultrasound; however, unlike ultrasound, a terahertz pulse was able to detect a smaller crack hidden behind a larger crack. This was demonstrated in thick GFRP (glass fiber reinforced plastics) laminates of the wind turbine using saw slots. In carbon composites the penetration of terahertz waves is quite limited and the detection of flaws is strongly affected by the angle between the electric field direction of the terahertz waves and the intervening fiber directions. The structures used in this study included both solid laminates and the parts in the wind turbine. The defects and anomalies investigated by terahertz radiation were foreign material inclusions, simulated disband, delamination and mechanical impact damage. The effectiveness and limitations of terahertz radiation are defined and noted for the NDE of composites.

Non-destructive evaluation of composite materials using a capacitive imaging technique

This paper describes the application of the capacitive imaging technique to the inspection of composite materials. The fundamental theory of the capacitive imaging technique was briefly described. Experiments using prototype capacitive imaging probes were also carried out. The proof-of-concept results indicated that the capacitive imaging technique could be used to detect cracks and delaminations in the glass fibre composite, defects in the aluminium core through the glass fibre face as well as surface features on the carbon fibre specimens.

Anisotropy of Plate Waves in Composites: Effects and Implications

Elastic wave is deemed to be an intrinsic material property which is fully determined by the matrix of elastic moduli. One of the results of this study is the finding that this is not true for ultrasonic waves in plates whose is shown to be a function of frequency. The of anisotropy occurs for plate waves in all anisotropic materials including fibre-reinforced composites. The physical reason for the dispersion of velocity is concerned with frequency dependent wave structure which activates different elasticity for various propagation directions. To probe of a certain stiffness component (e.g. Young's modulus), the plate wave fields should be kept similar by variation in operation frequency. The phase velocity and its frequency dependence provide frequency variation of the beam steering angle for plate waves (dispersion of beam steering). This effect is important to be accounted for in various applications since the plate waves are widely used for non-destructive evaluation of composite materials. A strong frequency-dependent focusing of ultrasonic energy (dispersion of phonon focusing) occurs for a 0 -waves in unidirectional carbon fibre reinforced composite. The phonon focusing prevents omni-directional wave propagation that might pose a problem in the plate wave applications for structural health monitoring. The results presented indicate that the effect of plate wave and its implications can be enhanced or diminished by matching the operation frequency.

Interpreting acoustic emission signals by artificial neural networks to predict the residual strength of pre-fatigued GFRP laminates

Artificial neural networks (ANNs) were used to predict the residual strength of glass fibre-reinforced plastic beams pre-fatigued in flexure up to different portions of their fatigue life. To this aim, the acoustic emission signals recorded during the tests for the measurement of the residual strength, and the associated applied stress, were provided as input. An optimisation of the network configuration was carried out, using the root-mean-square error calculated in the training stage as the optimisation parameter. The predictive accuracy of the optimised ANN, consisting of two nodes in the input layer, four nodes in the hidden layer, and a single node in the output layer, was tested by the “leave-k-out” method. From the results obtained, ANN provided quite reliable predictions when the applied load was sufficiently far from the failure load, performing better than a previous theoretical model, relying on fracture mechanics concepts. Therefore, ANN was shown to be a valid tool in the non-destructive evaluation of composite materials employed in fatigue-sensitive applications.

Non-destructive evaluation of composite materials with ultrasonic waves generated and detected by lasers

Measurement of the stiffness properties of composite materials with laser generated and detected ultrasound requires proper understanding of waves emanating from a line or point source in anisotropic and viscoelastic media. The paper briefly presents calculation results of waves radiated by such a source through or at the surface of a composite plate. Dispersion is represented as well as the multiple wave arrivals connected with the folded shape of the quasi-shear ray surface. Moreover, internal diffraction at the cusp edges is properly depicted. An identification method with specific signal processing have been used to measure the stiffness coefficients of composite materials. From group velocity data, the stiffness tensor of materials showing an orthorhombic symmetry can be identified. The stiffness tensor changes induced by elevated temperatures in a composite material were then measured. An alternative approach was developed which allows to measure the phase velocities of waves generated with laser line sources. The material characterisation reliability is then improved. Moreover, the method can be used in practical cases where the front side of the structure only is accessible with the experimental devices. Despite reflection at the rear interface of transient divergent waves which ray surfaces may contain caustics, this inverse problem can be solved in a simple and efficient manner.

Signal-processing methods for analysing the structure of carbon-epoxy-resin composite materials

The non-destructive evaluation of composite materials is needed in order to check their mechanical viability. Several classical ultrasonic signal-processing methods may be used to perform this task, such as resonance spectroscopy and extraction of features from time echograms. Neural-network processing methods may allow the evaluation of structural parameters (ply thickness, interface quality and so on) from ultrasonic echograms. The aim of this work is to implement such techniques and to apply them to ultrasonic echograms obtained from theoretical models and industrial samples, so that meaningful comparisons may be performed in order to assess the validity of the neural-network method. The neural-network based method appears very flexible and less computationally intensive in the recall phase than are the other two methods. It gives a precision sufficient for most purposes and so constitutes a good candidate for evaluation of composite-material (and more generally multi-layer) structures.

Coherence-based imaging through turbid media by use of degenerate four-wave mixing in thin liquid-crystal films and photorefractives

We describe a coherence-based imaging technique based on degenerate four-wave mixing in two materials. First, a 45 degrees -cut BaTiO(3) crystal was used as a self-pumped phase conjugator to obtain depth-resolved images through a 4 mean-free-path (mfp) scattering media. In addition, an HITC dye-doped K15 liquid-crystal layer was used to provide single-shot image acquisition through a 2 mfp scattering media. This technique can be used to provide instantaneous (single-pulse), depth-resolved, two-dimensional images of the internal structure of scattering materials. Potential applications of this technique include subsurface imaging of biomedical tissue and nondestructive evaluation of composite materials.

Nondestructive Evaluation of Composite Materials by ESPI

Electronic speckle pattern interferometry (ESPI) was used to perform nondestructive evaluation of carbon-fiber reinforced plastic (CFRP) laminate plates containing various sizes and shapes of defects located at different depths. A specially designed vacuum box was used to provide the deformation of the test specimen. Not the same as the traditional ESPI, the decorrelation between two speckle patterns was used to determine the size, and shape of a defect. By using the speckle decorrelation, the location, size and shape of a defect can be easily determined. A series of computer programs was developed on the ESPI system to acquire and analyze the interferometric patterns. Although the detected shape and size do not match well with the originally embedded one, the ESPI procedures developed in this paper should still be a potentially quantitative nondestructive method for detecting the location and size of the defect in composite materials.

Nondestructive Evaluation of Composite Materials by ESPI

Abstract Electronic speckle pattern interferometry (ESPI) was used to perform nondestructive evaluation of carbon-fiber reinforced plastic (CFRP) laminate plates containing various sizes and shapes of defects located at different depths. A specially designed vacuum box was used to provide the deformation of the test specimen. Not the same as the traditional ESPI, the decorrelation between two speckle patterns was used to determine the size, and shape of a defect. By using the speckle decorrelation, the location, size and shape of a defect can be easily determined. A series of computer programs was developed on the ESPI system to acquire and analyze the interferometric patterns. Although the detected shape and size do not match well with the originally embedded one, the ESPI procedures developed in this paper should still be a potentially quantitative nondestructive method for detecting the location and size of the defect in composite materials.

Nondestructive Evaluation of Composite Materials by ESPI

Electronic speckle pattern interferometry (ESPI) was used to perform nondestructive evaluation of carbon-fiber reinforced plastic (CFRP) laminate plates containing various sizes and shapes of defects located at different depths. A specially designed vacuum box was used to provide the deformation of the test specimen. Not the same as the traditional ESPI, the decorrelation between two speckle patterns was used to determine the size, and shape of a defect. By using the speckle decorrelation, the location, size and shape of a defect can be easily determined. A series of computer programs was developed on the ESPI system to acquire and analyze the interferometric patterns. Although the detected shape and size do not match well with the originally embedded one, the ESPI procedures developed in this paper should still be a potentially quantitative nondestructive method for detecting the location and size of the defect in composite materials.