Ultrasonic C-scan Images Research Articles

Ultrasonic C-scan techniques for the evaluation of impact damage in CFRP

Abstract In the present work, different ultrasonic C-scan approaches were used to evaluate Carbon Fiber Reinforced Polymers (CFRP) submitted to impacts of low energy, in order to evaluate their effectiveness for the detection and characterization of small defects. In particular, as to the question how useful could be the air-coupled C-scan approach, using low frequencies, for in-service application. For that goal, several samples with different stacking sequences and thicknesses were impacted with 1.5 and 3 J. Then, ultrasonic C-scan images were produced by immersion pulse-echo (in amplitude and time-of-flight (TOF)) and immersion through-transmission, and also by air-coupling through-transmission. The immersion C-scan images were produced using 5, 10 and 20 MHz probes and the air-coupled C-scan was made using two 400 kHz probes. The obtained images for the considered samples show that all used methods are able to detect the defects and give acceptable information about their size and shape. However, if the way of delamination evolving over thickness is of interest, the images by TOF should be used. As expected, good image resolution with sharp contour defects require high frequencies. Nevertheless, the air-coupled C-scan demonstrated similar capabilities to detect defects, with the advantage that the coupling medium is air, thus widening the range of applications, such as real-time damage monitoring of composite structures. As a disadvantage, the air C-scan system requires high power emission signals, and also great amplification of the received signals, to face the considerable attenuation in the air.

Mechanical and ultrasonic characterisation of copper-nickel diffusion bonded joints

ABSTRACT The diffusion bonding of commercially pure Cu/Ni was made to diffuse at different bonding conditions allowing the bonding to proceed across the planar interfaces. The mechanism, microstructure and mechanical properties which are the effects of diffused bonding were studied. The Nondestructive Ultrasonic C-scan testing method was used to characterise the bond interface. The area of fraction bonded, and its reflection coefficients were computed using ultrasonic C-scan images to determine the quality of bonding. The inferences arrived based on nondestructive testing were checked by conventional destructive test methods which were well correlated. The SEM micrograph was used to compute the interface line width of the diffused bond. The EDS was used to study the mapping of elemental distribution and the fractured surface area of the diffused bonded joints.

Self-Healing of Wind Turbine Blades by Pressurized Delivery of Healing Agent

Abstract Self-healing is a promising way to solve the difficulty in wind turbine blades repair, yet the embedded healing agent may have a disadvantage because of being exposed to outdoor for a long time. Pressurized delivery of the healing agent in real-time when the blade is damaged may be the solution to avoid the disadvantage healing agent. In this paper, the healing agent was pumped to the damaged area by a peristaltic pump, and the healing effect was evaluated by the recovery rate of the residual flexural strength after impact and the image of ultrasonic C-scan. To evaluate the healing effect of different damage degrees, 10 J, 15 J, 20 J, and 25 J impact energies were applied. The fluid tracer test showed that the healing agent could penetrate in the damaged areas after the impact of 15 J, 20 J and 25 J, while the three-point bending test revealed that the healing efficiency was the highest with 20 J (85.2%). The ultrasonic C-scan and optical photos of the sample showed that the images of the healing area were almost consistent with those of the un-impacted area, indicating that the damaged area is healed well. Based on the success of plate samples, the self-healing of the wind turbine blade-scale prototype was then carried out. Twenty-joule impact was exerted on the blade prototype, and the healing agent was pumped to the damaged area using the peristaltic pump similar to the same procedure as that of the plate specimen. Ultrasonic C-scan and optical images of the damaged area showed that the prototype was healed well in comparison with those of the plate specimens, indicating that the application of pressurized delivery of the healing agent system in the self-healing of wind turbine blade prototype was successful.

Damage assessment of NCF, 2D and 3D woven composites under compression after multiple-impact using acoustic emission

This study is devoted to the damage characterization of Non-Crimp Fabric (NCF), 2D plain-woven (2D-PW) and 3D orthogonal plain-woven (ORT-PW) carbon/epoxy laminates, subjected to compression after multiple-impact loading, using Acoustic Emission (AE). The ultrasonic C-scan images showed that the interlaminar damage area induced by the single and 3-impact in ORT-PW architecture is 3 and 2 times smaller than NCF and 2D-PW architectures respectively. The impacted specimens were then subjected to the in-plane compression load. Two indices, one based on the mechanical response and another one based on the AE behavior of the laminates, were proposed to compare the performance of different architectures. These indices showed that the ORT-PW had the best performance among all the architectures. Finally, AE was used to distinguish the different damage mechanisms including: matrix cracking, intra and inter-yarn debonding, defected-fiber breakage, intact-fiber breakage and z-binder fiber breakage in the CAI tests of the architectures.

Enhanced resistance-welding hybrid joints of titanium alloy/thermoplastic composites using a carbon-nanotube lamina

The joining of titanium (Ti) alloy and composite parts with weight reduction and cost saving is being developed to improve aircraft performance. The high-efficiency resistance welding technology was explored for joining Ti alloy plate and glass fabric reinforced polyetherimide (GF/PEI) laminate in this work and a carbon nanotube (CNT) reinforced laminae was embedded into the interface of the welding region to enhance the strength of the hybrid joints of Ti -GF/PEI. The CNTs were in-situ grown on the surface of the Ti alloy plate, and their structures and morphologies were characterized by Raman spectrum, scanning electron microscope (SEM) and transmission electron microscope (TEM). The single lap shear strength (LSS) of the hybrid joints was evaluated with tensile tests. The LSS values first increased and then decreased with welding time (tw) due to the combined effects of the improving molten state of PEI resin and its local thermal degradation induced by excessive heat accumulation. The optimum LSS of the hybrid joint with CNT reinforced lamina is increased by 146% to 17.28 MPa in comparison with that of the hybrid joint without CNTs. The failure modes shift from interlayer debonding of the Ti alloy plate and the implant to the cohesive fracture of the implant and the delamination of the GF/PEI laminate with increasing tw. The ultrasonic C-scan images also show an increase of the number of internal defects of GF/PEI laminates with tw.

Influence of Location and Thickness Variations on Guided Waves in Defective Carbon/Epoxy Plate

This paper addresses the effects of plate thickness and defect location on guided wave propagation in carbon/epoxy plates. A three-dimensional (3D) finite element model (FEM) of the plate was developed using MATLAB program codes, and simulated in Abaqus/Explicit. Referring to experimental ultrasonic C-scan images, the complex impact damage was modelled with irregular-shaped delamination and through-thickness matrix cracks. The simulated results show that a slower arrival time signal and amplitude drop of guided wave captured behind the defective region can be used as an indicator of the impact damage. A largOer scattering occurred when delamination was located closer to the plate surface. The extent of scattering gets larger, especially in the direction of 345o from the excitation point. It is also observed that the impact damage can still be detected through a line scan method across the impact damage, although the wave attenuation is greater in a thicker composite plate. By investigating these factors independently, the trends of the scattered guided ultrasonic waves can be classified and perhaps will revolutionize a smart non-destructive method for composite structure in the future.

Analysis of dynamic mechanical, low-velocity impact and compression after impact behaviour of benzoyl treated sugar palm/glass/epoxy composites

The objective of this work is to investigate the effect of glass fibre addition and benzoylation treatment on the dynamic mechanical, impact and post-impact properties of sugar palm composites. The hybrid composites were prepared using hand lay-up method with different volume fractions of sugar palm fibre and glass fibre. Dynamic mechanical analysis (DMA) results show that EP/30TSPF/70GF composites exhibited the highest loss and storage modulus compared to EP/UTSPF composites. For the low-velocity impact testing, the experimental results show that the impact energy displayed an excellent relationship with the impact response. The ultrasonic C-scan images show that the damaged area is increasing as the impact energy increasing. From the compression after impact (CAI) studies, it could be resolved that the impact damage exhibited outstanding connection with the impact response and compressive strength of the EP/30TSPF/70GF composite decreased as the impact energy increasing. Overall, it can be concluded that the benzoylation treatment and glass fibre addition on the sugar palm composites enhances the dynamic mechanical, impact and post-impact properties of sugar palm composites.

Ultrasonic C-scan detection research for effective connection area of arc-stud-weld joints

Ultrasonic echo signal analysis and C-scan detection are used to investigate arc-stud welds (ASWs). The different connection zones of ASW joints are identified based on the variation of the ultrasonic echo in the time domain and time–frequency domain. The amplitude of the first echo in the time domain is used to differentiate an ASW joint from the base metal. The analysis in the time–frequency domain shows that the fusion and weak connection zones of an ASW joint can be distinguished based on the amplitude at a frequency of 5.6 MHz, and the defects in an ASW joint can be precisely detected based on the amplitude at a frequency of 37.5 MHz. A compound ultrasonic C-scan image based on the amplitudes at these specific frequencies is adopted to intuitively display the actual connection status of an ASW joint. According to this image, the joint diameter and defect dimensions can be quantitatively analyzed. This method provides an effective quality assessment method for an ASW joint.

Abnormalities in Ultrasonic (C-Scan) Images of Composite Structures: Impact Damaged Versus Hole Damaged

This paper is about applying image processing to detect different types and levels of damage in composite structures using in C-scan images produced by ultrasonic testing. This method plays an important role as it can detect any discontinuities or flaws in the specimen. To analyze such critical image in an, automated inspection environment an image processing method that correlates Threshold values to Gradient Field values is used to extract important information from the c-scan images in order to detect abnormalities that may exist. The paper presents an improved approach to composite damage characterization using Threshold Level Variation as an input variable to Gradient Fields. Furthermore, the detailed results showed an important dependency between what can be detected and the used threshold level. At each level certain abnormalities appeared. Setting threshold value is proved to be a function of image type and quality, purpose and application.

3D reconstruction for ultrasonic C-scan images of tissue-mimicking phantom based on an improved K-nearest neighbor filtering

Although the ultrasonic C-scan technique has been extensively applied in nondestructive testing (NDT) in recent years, 3D reconstruction from ultrasonic C-scan images has not been well addressed. This paper develops a novel and efficient 3D reconstruction technique based on an improved K-nearest neighbor filtering for ultrasonic C-scan data of the tissue-mimicking phantoms. An edge-points-predicting approach based on K-nearest neighbor filtering is first proposed to predict the undetected edge points and to reduce the noise points for 2D ultrasonic images. Then, the 3D model is reconstructed from the clean edges by utilizing the surface rendering algorithm. The proposed approach is validated using the ultrasonic C-scan data of a liver model embedded in a tissue-mimicking phantom. The comparisons with other methods are presented in the experiments. The results demonstrate the effectiveness and the significantly improved reconstruction results of the proposed approach.

Pulsed Phase Thermography Approach for the Characterization of Delaminations in CFRP and Comparison to Phased Array Ultrasonic Testing

Pulsed phase thermography (PPT) is a well-established algorithm used for processing thermographic data in frequency domain with the aim to extract information about the defect size and depth. However, few works demonstrated the capability of PPT technique in defects evaluation in real components. The aim of this work is the assessment of capability of PPT technique in determining delaminations in CFRP components used in aeronautics. The component chosen for implementing the technique has a non-uniform geometry and the defects inside it are not simulated, but they are real and generated during the production process. The specimen has been investigated through the application of both the ultrasonic technique and the thermographic one. Thermographic phase images elaborated with a suitable computational processing have been compared with Ultrasonic C-scan images and, the agreement between the location and depth of defects has been verified. Besides, the ultrasonic technique has been used to validate the PPT results.

3D Surface Profile Construction and Flaw Detection in a Composite Structure

The nondestructive method of evaluating material properties and structural integrity is built upon ultrasonic scanning, raw data acquisition and processing, coming from an ultrasonic transducer. Commercial software LabVIEW designed by National Instruments Inc. is aimed at processing ultrasonic images. An ultrasonic C-scan image system was developed. This system is capable of constructing a 3D surface profile and detecting flaws inside the composite structure. The technique for nondestructive inspection and geometry estimation is validated experimentally on an aluminum plate with small holes and composite structure with a delamination defect.

Barely visible impact damage assessment in laminated composites using acoustic emission

Despite the key advantages of Fiber Reinforced Polymer (FRP) composites, they are susceptible to Barely Visible Impact Damage (BVID) under transverse loadings. This study investigates BVID in two quasi-isotropic carbon/epoxy laminates under quasi-static indentation and Low-Velocity Impact (LVI) loadings using Acoustic Emission (AE). First, the evolution of interlaminar and intralaminar damages is studied by analyzing the AE signals of the indentation test using b-value and sentry function methods. Then, the specimens are subjected to the LVI loading and the induced damages are compared with the indentation test and the percentage of each damage mechanism is calculated using Wavelet Packet Transform (WPT). In consistent with the mechanical data, ultrasonic C-scan and digital camera images of the specimens, the AE results show a considerable similarity between the induced BVID under quasi-static indentation and LVI tests. Finally, the obtained results show that AE is a powerful tool to study BVID in laminated composites under quasi-static and dynamic transverse loadings.

Adhesive debonding inspection with a small EMAT in resonant mode

In this paper, a noncontact ultrasonic testing method with using a small electromagnetic acoustic transducer (EMAT) in resonant mode is proposed for metal-based adhesive debonding inspection. The small EMAT with a new magnet configuration and a small coil is developed firstly to improve the spatial resolution and detecting ability for minute defects. Different from conventional pulse echo method using short pulse excitation for debonding inspection, a new method based on power spectrum estimation of the resonant ultrasonic signal with long burst excitation is applied to increase the signal-to-noise ratio (SNR) and detecting ability of the small EMAT. Finally, ultrasonic C-scan images of specimens with various adhesive bonding are obtained to demonstrate the capability of this technique.

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.

Inspection of delamination defect in first wall with a flexible EMAT-scanning system

In-vessel inspection of delamination defects in first wall (FW) is one of the major maintenance challenges for ITER and future fusion reactors. In this paper, a noncontact and flexible ultrasonic scanning system with combining an electromagnetic acoustic transducer (EMAT) and a robotic inspection manipulator is proposed and investigated for the delamination defects inspection in the FW. Instead of using pulse echo method in conventional ultrasonic method for delamination inspection, a new method based on measuring the ultrasonic resonance signal at the delamination area is applied here. The use of resonant ultrasonic signals and signal processing with power spectrum analysis can significantly enhance signal amplitude of the EMAT and its ability to resist electromagnetic interference from the servo motors of the robotic manipulator. Finally, ultrasonic C-scan images of the delamination defects in an Aluminum-Copper welding specimen are obtained to demonstrate the capability of this technique.

Clustering of interlaminar and intralaminar damages in laminated composites under indentation loading using Acoustic Emission

This study focuses on the clustering of the indentation-induced interlaminar and intralaminar damages in carbon/epoxy laminated composites using Acoustic Emission (AE) technique. Two quasi-isotropic specimens with layups of [60/0/-60]4S (is named dispersed specimen) and [604/04/-604]S (is named blocked specimen) were fabricated and subjected to a quasi-static indentation loading. The mechanical data, digital camera and ultrasonic C-scan images of the damaged specimens showed different damage evolution behaviors for the blocked and dispersed specimens. Then, the AE signals of the specimens were clustered for tracking the evolution behavior of different damage mechanisms. In order to select a reliable clustering method, the performance of six different clustering methods consisting of k-Means, Genetic k-Means, Fuzzy C-Means, Self-Organizing Map (SOM), Gaussian Mixture Model (GMM), and hierarchical model were compared. The results illustrated that hierarchical model has the best performance in clustering of AE signals. Finally, the evolution behavior of each damage mechanism was investigated by the clustered AE signals with hierarchical model. The results of this study show that using AE technique with an appropriate clustering method such as hierarchical model could be an applicable tool for structural health monitoring of composite structures.

Nondestructive testing of additively manufactured material based on ultrasonic scattering measurement

To improve ultrasonic testing capability for additively manufactured materials, extreme value statistics is employed to calculate the experimental confidence bounds of structural noise, which can be treated as time-dependent thresholds for ultrasonic C-scan image segmentation. A 316L stainless steel sample manufactured by selective laser melting is used for ultrasonic scattering measurements with a focused transducer. Compared with the fixed threshold used in the traditional C-scan image segmentation, the time-dependent threshold can effectively distinguish the flaw echoes from the background of structural noise. The optical microscopy measurement results show that the present method can avoid both missed detections and false positives.

Effect of impact damage on the curved beam interlaminar strength of carbon/epoxy laminates

The bending interlaminar strength and bending interlaminar strength after impact of carbon/epoxy-laminated curved beams were studied experimentally using four-point bending test and low velocity impact. First, the post- impact damage of the laminated curved beams with different radii was analyzed based on ultrasonic C-scan images. Then, the effect of impact damage on both the interlaminar strength and the maximum interlaminar radial stresses of the laminated curved beams were investigated. Finally, the full-field displacement distributions of the laminated curved beams were obtained using digital speckle correlation method. Four-point bending experimental results play a significant role for interlaminar strength in evaluating the laminated curved beams with and without impact damage.

Ultrasonic C-Scan Image Processing Using Multilevel Thresholding for Damage Evaluation in Aircraft Vertical Stabilizer

Ultrasonic C-Scan Image Processing Using Multilevel Thresholding for Damage Evaluation in Aircraft Vertical Stabilizer