Visualization of subsurface damage in woven carbon fiber-reinforced composites using polarization-sensitive terahertz imaging

Polarization-sensitive terahertz imaging is applied to characterize subsurface damage in woven carbon fiber-reinforced composite laminates in this study. Terahertz subsurface spectral imaging based on terahertz deconvolution is tailored and applied to detect, in a nondestructive fashion, the subsurface damage within the first ply of the laminate caused by a four-point bending test. Subsurface damage types, including matrix cracking, fiber distortion/fracture, as well as intra-ply delamination, are successfully characterized. Our results show that, although the conductivity of carbon fibers rapidly attenuates terahertz propagation with depth, the imaging capability of terahertz radiation on woven carbon fiber-reinforced composites can nonetheless be significantly enhanced by taking advantage of the terahertz polarization and terahertz deconvolution. The method demonstrated in this study is capable of extracting and visualizing a number of fine details of the subsurface damage in woven carbon fiber-reinforced composites, and the results achieved are confirmed by comparative studies with X-ray tomography.The authors gratefully acknowledge the financial support of the Conseil Régional du Grand Est of the Fonds Européen de Développement Régional (FEDER), and of the Institut Carnot ARTS

Application of Ultrasonic Coda Wave Interferometry for Micro-cracks Monitoring in Woven Fabric Composites

The consequences of a four-point bending test, up to 12 mm, are examined by emitting 1 MHz ultrasonic guided waves in woven carbon fiber reinforced polymer specimens, using coda wave interferometry (CWI), revealing a potential use for nondestructive evaluation. It is known that CWI is more sensitive to realistic damage than the conventional method based on the first arriving time of flight in geophysical, or in civil engineering applications such as concrete structures. However, in composite materials CWI is not well established because of the involved structural complexity. In this paper, CWI is investigated for monitoring the occurrence of realistic defects such as micro-cracks in a woven carbon fiber composite plate. The micro-cracks are generated by a four-point bending test. The damage state is stepwise enhanced by gradually increasing the load level, until failure initiation. The damage is monitored, after each loading, using ultrasound. It is demonstrated that CWI is a powerful tool to detect damage, even low levels, in the sample. Two damage indicators based on CWI, i.e. signals correlation coefficient and relative velocity change, are investigated and appear to be complimentary. Under significant loading levels, the normalized cross-correlation coefficient between the waveforms recorded in the damaged and in the healthy sample (reference at 0 mm), decreases sharply; this first indicator is therefore useful for severe damage detection. It is also demonstrated, by means of a second indicator, that the relative velocity change between a baseline signal taken at zero loading, and the signals taken at various loadings, is linear as a function of the loading, until a critical level is reached; therefore this second indicator, is useful for low damage level detection. The obtained evolution of the relative velocity measurement is supported by relative comparison to the evolution of the bending modulus in function of displacement. The relative velocity change exhibits the same evolution as the bending modulus with loading. It could be used to indicate when the material stiffness has decreased significantly. The research is done in the framework of composite manufacturing quality control and appears to be a promising inspection technique.This work is supported by the Région Grand Est

Mechanical Defects Detection on Solar Panel with Ultrasonic Guided Waves

In order to ensure a solar power system runs at its optimum efficiency throughout the lifecycle, proper structuralhealth monitoring on photovoltaic module (PVM) should be performed as part predictive maintenance activities.Mechanical defects such as cracks may induce power loss, raise the risks of electrical failure, and compromise thePVM’s structural integrity. Therefore, early detection of their existence is crucial. This research emphasizes theutilization of ultrasonic guided waves (UGW) to develop an efficient inspection technique. Since the waves cantravel over large distances, they allow long-range defect detection, hence faster inspection process as compared to the conventional bulk wave scan. In this work, UGW experiments were done over areas with cracks of differentseverity. Dispersion characteristics of the propagating modes were evaluated through experiments and numerical simulations, where the obtained results were in agreement to each other. Time frequency analysis shows that certain modes have lower phase velocity in the area with cracks. In addition, the results demonstrate the capability of UGW to detect a crack that is invisible under visual inspection

Mechanical Defects Detection on Solar Panel with Ultrasonic Guided Waves

In order to ensure a solar power system runs at its optimum efficiency throughout the lifecycle, proper structuralhealth monitoring on photovoltaic module (PVM) should be performed as part predictive maintenance activities.Mechanical defects such as cracks may induce power loss, raise the risks of electrical failure, and compromise thePVM’s structural integrity. Therefore, early detection of their existence is crucial. This research emphasizes theutilization of ultrasonic guided waves (UGW) to develop an efficient inspection technique. Since the waves cantravel over large distances, they allow long-range defect detection, hence faster inspection process as compared to the conventional bulk wave scan. In this work, UGW experiments were done over areas with cracks of differentseverity. Dispersion characteristics of the propagating modes were evaluated through experiments and numerical simulations, where the obtained results were in agreement to each other. Time frequency analysis shows that certain modes have lower phase velocity in the area with cracks. In addition, the results demonstrate the capability of UGW to detect a crack that is invisible under visual inspection

Investigation of Damage in Composites Using Nondestructive Nonlinear Acoustic Spectroscopy

International audienceThe presented experimental work describes the nondestructive damage examination of polymer-matrix composites using acoustic methods under the consideration of nonlinear effects. The aim is to analyze these nonlinear effects in order to provide a quantification of the nonlinear acoustic transmission which is related to the damage state and its severity in the composite material. The first objective was to study the effectiveness of the distortion evaluation method and its related parameter: the BTotal Difference Frequency Distortion^ (TDFD) parameter. The TDFD was utilized as a new damage indicator to quantify the progressive damage state in composite materials. The TDFD method had initially been proposed to characterize the distortion of audio amplifiers. A custom-made setup was developed that imposes acoustic signals to the structure. The samples’ vibrations were afterwards analyzed by a laservibrometer and further spectrum evaluations. The developed method was applied to two composite materials, both reinforced with taffeta woven glass-fibers, but having different thermoset polymer matrix, i.e. vinylester and epoxy. The damage was introduced in the specimen by tensile tests with a stepwise increase of the tension loading. It was observed that damage influences the intensity of nonlinear intermodulation after having introduced two harmonic and constant signals of different and randomly chosen frequencies in the specimen. The nonlinear intermodulation was then quantified by computing the TDFD parameter. In the specific case of epoxy based composites, high frequency peaks were noted for the high tensile loading levels only. The TDFD parameter was then modified in order to take into account this effect. For both studied composites, the modified TDFD parameter increases with the damage accumulation caused by the applied stepwise tensile loading

Assessing the number of twists of stranded wires using ultrasound

Wiring, of different degrees of complexity, is a dominant part of mechanical support in constructions, electromagnetic and telecommunication signal transmission cables, among other applications. Single and manifold twisted wires are prominent examples of such utilities and are susceptible to mechanical irritations and deterioration. They require ultrasonic non-destructive testing and health monitoring. The objective is to develop an ultrasound-based technique to automatically measure the number of twists per meter in winded wire strands implementable in the industry, to be used during an ultrasonic scan and provide the number of twists per meter during cable production, for instance, to verify that calibration is still in place. Fourier transformation is applied as an expedited non-destructive testing method of twisted wires. Digital signal processing to obtain spatial and time spectral representation recognition due to amplitude variance, induced by the varying distance between the transducer and wire, is developed depending on the number of twists. Two different spatial spectral analyses satisfactorily quantify the number of twists by providing the distance between each twist. The method is robust and applicable when the distance between the transducer and strand is not constant, as the industry requires

An Investigation on the Lateral Propagation of Ultrasound Waves in Multi-layered Structures

Multi-layered structures have become a cornerstone in various sectors of industry. The use of composite materials continues to grow in aviation and automotive production. Solar photovoltaic modules, equally multi-layered structures, also become valuable elements in a carbon-free electricity generation system in the energy sector. Non-destructive testing using ultrasound C-scan techniques is frequently used for the inspection of these structures. However, lateral wave propagation through these structures may have more benefits. Thus, in this work, the authors investigate phenomena that occur when signals are emitted in a direction lateral to the plate's surface and make a comparison with recently obtained results for isotropic mono-layered structures, such as defect localization and edge detection. The plate under investigation is a multi-layered structure that consists of different materials bonded with an adhesive polymer. Experiments are designed to study the acoustic waves field resulting from parallel propagation of signals in layers with different stiffness properties. The investigation focuses on the propagating waves' interaction with discontinuities within the laminae, such as cracks or debonding. This work is an ongoing endeavour in which the first three authors, all PhD students, ordered in alphabetical order, contribute equally

Terahertz pulsed imaging of low velocity impact damage in woven fiber composite laminates

Recently, terahertz pulse imaging has been used extensively to characterize internal defects in various electrically insulating materials. In this work, terahertz pulse imaging is used to identify the damage induced by low-velocity impact on woven glass-fiber reinforced polyamide laminates. Several impact energies are considered to study the damage initiation and propagation. The permanent indentation, known to be a relevant damage indicator, is extracted from the terahertz results and validated through comparison with profilometry. Further, the criticality of the damage, in terms of the number of composite plies in which the cracks propagate, can be clearly determined from terahertz imaging. These observations are validated through X-ray tomographic observations and analysis. Finally, a strong connection is established between the evolution of the permanent indentation and the appearance, and criticality, of the low-velocity impactinduced damage. These observations suggest that terahertz imaging is a reliable technique for the nondestructive assessment of impact damage in glass fiber composites