Ultrasonic C-scan Research Articles

Modelling the effects of patch-plug configuration on the impact performance of patch-repaired composite laminates

The patch-plug configuration has been widely used to repair composite structures and restore the structural integrity of damaged composites. In the present research, single-sided CFRP patch-repaired panels, with different patch-plug configurations, are prepared. This is where a circular-shaped damaged area has been removed and a CFRP patch has been adhesively-bonded onto the panel. In some cases, a CFRP plug is inserted into the hole, caused by removal of the damaged area, before the patch is applied. Such patch-repaired panels, and the pristine CFRP panel, are subjected to a low-velocity impact at an energy of 7.5 J. These impacted pristine and repaired panels are then examined using ultrasonic C-scan and optical microscopy to inspect the impact-associated permanent indentation, interlaminar and intralaminar damage. A finite element analysis (FEA) model, which significantly extends a previously validated elastic-plastic (E-P) numerical damage model, has been developed to predict the impact behaviour of the pristine CFRP panel and the various designs of patch-repaired CFRP panels. The comparison between the experimental and numerical results for all the studied cases shows the maximum deviations for the loading response and the damage area are 12% and 15%, respectively. The good agreement between the experimentally-measured impact properties and those predicted using the numerical model demonstrates that the model is a useful design tool.

Development of thermoplastic self-healing panels by 3D printing technology and study extrinsic healing system under low-velocity impact analysis

This study introducing self-healing concept into thermoplastic panels for engineering applications. Hence, the base self-healing square shaped panels were manufactured using a common thermoplastic, polylactic acid (PLA), and glass/in-situ vascular channels as healing carriers via 3D printing additive process. 3D printing is the best approach for inserting glass vascular channels and creating insitu channels inside the panels. Two types of healing resins were prepared with polymethyl methacrylate (PMMA) and Acrylonitrile 1–3 Butadiene Styrene (ABS) solutions to adjust the chemistry of PLA and healing channels for effective healing without any trouble. A drop weight impact test with the low-velocity mode is conducted on prepared PLA panels. To study the self-healing behaviour of panels 23.05 N-m impact energy was applied to create the initial damage of the PLA panels. The impacted specimens were examined through ultrasonic C-scan for observing the healing process at damaged portion. The experimental results show the effective healing behavior of PLA panels were (after self-healing) about 75% and 80% of PLA-PMMA and PLA-ABS, respectively. This study also deals on the damage model-based numerical study was established on PLA panels using the Ansys workbench to investigate the failure mechanisms of PLA self-healing panels.

Fatigue state characterisation of steel pipes using ultrasonic shear waves.

The phenomenon of the reduction in the propagation speed of an ultrasonic wave when it travels through a fatigue zone has been well studied in the literature. Additionally, it has been established that shear waves are more severely affected by the presence of such a zone, compared with longitudinal waves. Our study utilises these phenomena to develop a method able to characterise the fatigue state of steel pipes. Initially, the existing theory regarding the increased sensitivity of shear waves to the presence of fatigue is validated through measuring and comparing the change in propagation speed of both longitudinal and bulk shear waves on flat geometries, at different fatigue states. The comparison is achieved with the aid of ultrasonic speed C-scans of both longitudinal and shear waves, with the latter now being obtainable through our implementation of advances in Electromagnetic Acoustic Transducers (EMAT) technology. EMATs have not been traditionally used for producing C-scans, and their ability do to so with adequate repeatability is demonstrated here; we show that shear wave scanning with EMATs now provides a possibility for inspection of fatigue damage on the inner surface of pressure-containing components in the nuclear power industry. We find that the change in ultrasonic wave speed is amplified when shear waves are used, with the magnitude of this amplification agreeing well with the theory. Following the verification of the theory, the use of EMATs allowed us to tailor the shear wave scanning method to pipe geometries, where C-scans with conventional piezoelectric transducers would not have been possible, with the results successfully revealing the presence of fatigue zones.

Wavelets and curvelets transform for image denoising to damage identification of thin plate

As a common structural form, thin plates are widely used in civil engineering. Since the thin plate needs to face harsh work conditions, the damage inevitably to be accumulated, thus affecting the stability and safety of the application components. Therefore, it is of great application significance to quantify and characterize the damage of thin plates. However, the raw images produced by current inspection techniques such as Ultrasonic immersion C-scan technology, Metal sheet Lamb wave inspection technology, etc applied to thin plates usually bring various noises and imperfections during the reception, encoding, and transmission. In this paper, wavelet transform and Curvelet transform are used to denoise the detected noise image. First, we outline the numerical implementation of two newly developed multi-scale representation systems. Curvelet transform is a new multi-scale transform based on wavelet transform after 1999. The purpose of this paper is to analyze the influence of wavelet and Curvelet transform on image denoising. These methods can also be applied to the problem of image restoration from noisy images, and the effects of denoising on images are compared. The results show that the Curvelet transform can accurately identify the damage location for the thin plate damage degree, damage range, strip damage, and multiple damage conditions, and its energy focusing is better than that of the wavelet transform in each type of thin plate damage.

A novel nondestructive testing method for honeycomb structure using acoustic band gap

Honeycomb structure is increasingly applied in various fields due to its high strength-to-weight ratios. However, due to the complex wave field in the structure, how to detect the defect accurately and efficiently is always an important problem. In this article, the honeycomb core, instead of being an obstacle to obtain the dispersion characteristic for traditional guided wave method, provides a way to calculate the wave propagation by being taken advantage of its space periodicity. By analyzing, simulating, and experimental testing, it is exhibited that there is a gap in the dispersion relation of the honeycomb structure which caused by the interaction between the local resonant vibration of the skins and the bending vibration of the structure. This gap disappears in the debonding area. Then a nondestructive testing method is proposed to detect the debonding defects based on this feature. The detection results of this proposed method are accord with that of ultrasonic C-scan. The results indicate that this method is suitable for detecting and locating the debonding defects in honeycomb structure with quite high sensitivity and efficiency, and it has a great potential to be widely used in filed test of honeycomb structure for its high feasibility.

Adhesive bond quality classification using machine learning algorithms based on ultrasonic pulse-echo immersion data

In this study, we explored the detection of weak bonds (WBs) due to contamination and faulty curing (FC) using linear ultrasound and machine learning. For this purpose, aluminium single-lap adhesive joints containing three variants of bonding quality were investigated: perfect bond, WB due to release agent (RA) contamination, and WB due to FC. The data, according to the deviation of the bonding protocol, were arranged in two groups, creating two datasets: distinct and complete. Each dataset included all bonding conditions (perfect, RA, and FC), although the distinct dataset contained only marginal cases, which were expected to be well separable, whereas the complete dataset included data with minor deviations from the bonding protocol. Pulse-echo C-scan images were acquired for all prepared samples in the immersion tank, and 45 features were initially extracted from the time traces representing each bonding group. The initial data were analysed via a t-test and pairwise correlation analysis to reveal statistically significant features. Then, we performed dimensionality reduction using tree-based, recursive, sequential, and linear discriminant analysis (LDA) feature selectors to explore feature importance and classification accuracy with different feature subsets. Finally, the important features identified with the different feature selectors were fed to support vector machine (SVM) classifiers, and the classification accuracies were compared amongst the different feature subsets. The classification accuracy using a distinct dataset in some cases demonstrated nearly 99% accuracy, indicating that significant bonding protocol deviations could be easily detected. It was demonstrated that classification accuracy increased with the number of features. However, even in the case of the 2D feature space obtained using linear discriminant analysis, the bonding quality classification accuracy remained higher than 84%. The feature subspace reduction with LDA demonstrated sufficient classification accuracy and an improvement of nearly 40% in training time compared with that for the initial feature set. Thus, the classical ultrasonic pulse-echo C-scan with an LDA feature transformation and SVM classifier could be used to identify the deviations in the bonding protocol in aluminium single-lap adhesive joints.

On damage behavior and stability of composite T-shaped stiffened panels under compression after impact considering impact locations

Impact locations play a critical role in determining the residual performance of the composite stiffened panel, since the local damage behavior at different locations is not identical. The objective of this paper is to systematically investigate the influence of impact locations on damage behavior, stability and residual compression strength of T-shaped stiffened CFRP panels. Low-velocity impact tests are carried out by impacting at the skin center or the flange tip. The damage patterns and post-buckling behavior are monitored by ultrasonic C-scan and digital image correlation. An effective computational framework with explicit formulations considering complicated damage mechanisms is proposed to investigate the details of damage behavior. The skin center impact causes severe fiber breakage and matrix squeezing, while the flange tip impact induces complex damage patterns including ply splitting, fiber breakage, matrix cracks, as well as extensive skin–rib interfacial delamination. Compression after impact results indicate that impact damage at the skin center has little effect on the residual performance of the structure, and the compression failure load is only reduced by 2.02%, from 276.8 kN to 271.2 kN. However, impact damage at the flange tip weakens the supporting effect of the stiffener, leading to early buckling of the local skin. The propagation of the interfacial delamination at the damaged flange tip directly triggers the premature compression failure, resulting in an 18.79% decrease in the failure load, 224.8 kN. Numerical predictions correlate well with experimental data. The reported results may be useful in structural design and maintenance.

A novel two-level approach to defect detection in braided CFRP using air-coupled ultrasonic testing

Air-coupled ultrasonic testing and C-scan technique has been increasingly applied to the braided CFRP structures owing to its non-destruction, non-contact and high visualization characteristics. Due to the noise, structural vibration, and airflow in the process of detection, the accuracy of defect identification is easily deteriorated. To address this issue and further determine the relationship between the ultrasonic acoustical pressure attenuation and structural parameters, a novel two-level identification method based on the modified two-dimensional variational mode decomposition (2D-VMD) has been proposed. In the first level, C-scan images have been sparsely decomposed into ensembles of modes by 2D-VMD method. Then, the modes have been screened by mutual information method to realize the reconstruction of new image in the second level. Experimental results have shown that the proposed method has the good ability to identify defects with a minimum detectable diameter of 1–2 mm. It has been noted that the ultrasonic acoustical pressure attenuation has become remarkably higher in the twill weave CFRP than the plain weave CFRP and the ratio of pressure attenuation between two weave types of CFRP has decreased with the defect depth increase. Meanwhile, shadows around defects in C-scan images have been suppressed to a great extent. It has been demonstrated that the capability of denoising has enabled the developed method with the accurate detection in terms of the shape, size, depth and weave type. With these advantages, the proposed method has provided valuable insights into the development of an effective method for defect detection of braided CFRP structures.

Manufacturing of Carbon Fibers/Polyphenylene Sulfide Composites via Induction-Heating Molding: Morphology, Mechanical Properties, and Flammability.

Conventional thermosetting composites exhibit advantageous mechanical properties owing to the use of an autoclave; however, their wide usage is limited by high production costs and long molding times. In contrast, the fabrication of thermoplastic composites involves out-of-autoclave processes that use press equipment. In particular, induction-heating molding facilitates a quicker thermal cycle, reduced processing time, and improved durability of the thermoplastic polymers; thus, the process cost and production time can be reduced. In this study, carbon fiber/polyphenylene sulfide thermoplastic composites were manufactured using induction-heating molding, and the relationships among the process, structure, and mechanical properties were investigated. The composites were characterized using optical and scanning electron microscopy, an ultrasonic C-scan, and X-ray computed tomography. In addition, the composites were subjected to flammability tests. This study provides novel insights into the optimization of thermoplastic composite manufacturing and thermoset composite curing processes.

Ultrasonic C-scan image restoration method using the Richardson-Lucy algorithm and a flaw measurement model

In ultrasonic imaging, blurred edges have a heavily negative impact on the accuracy of the flaw images. To improve ultrasonic C-scan image quality, this paper proposes a deconvolution-based ultrasonic C-scan image restoration method combining the Richardson-Lucy (RL) algorithm and a flaw measurement model. In order to implement an optimal restoration process, an ultrasonic C-scan imaging model is presented and employed to simulate the C-scan images. By restoring the simulated images, a relationship between the optimal iteration number and the ratio of the flaw size to the sound wavelength is established. Furthermore, the equivalent sizes of the flaws are extracted from a flaw measurement model-based non-destructive approach and are used to calculate the optimal iteration number of the RL deconvolution algorithm. Finally, the experimental ultrasonic C-scan images are restored by using the RL iterative algorithm with the optimal iteration numbers, 1, 2, 6, and 14, respectively. The experimental results show that the flaw sizes, obtained by the 6 dB drop method in the original C-scan images, have errors larger than 26 %. After restoration, the maximum error of the flaw size in the C-scan image is only 10 % and the minimum is 2 %, indicating that the proposed restoration method effectively improves the accuracy of the ultrasonic C-scan imaging.

Investigation of Biaxial Properties of CFRP with the Novel-Designed Cruciform Specimens.

The biaxial loading properties of carbon-fiber-reinforced polymer (CFRP) are critical for evaluating the performance of composite structures under the complex stress state. There are currently no standardized specimens for the CFRP biaxial experiments. This work developed a new design criterion for the cruciform specimen coupled with the Hashin criterion. The finite element analysis was conducted to investigate the effect of geometric parameters on the stress distribution in the test area. The embedded continuous laying method (ECLM) was proposed to achieve the thinning of the center of the test region without introducing defects. The manufacturing quality of the cruciform specimens was verified by the ultrasonic C-scanning test. The biaxial test platform consisting of the biaxial loading system, digital image correlation (DIC) system, strain electrical measurement system, and acoustic emission detection system was constructed. The biaxial tensile tests under different biaxial loading ratios were conducted. The results showed that the biaxial failure efficiently occurred in the test area of the cruciform specimens designed and manufactured in this paper. The failure modes and morphology were characterized using macro/microscopic experimental techniques. The biaxial failure envelope was obtained. The results can be used to guide the design of composite structures under biaxial stress.

Damage evolution and failure mechanism of asymmetric composite laminates under low-velocity impact and compression after impact

The damage mechanisms of the balanced symmetrical laminates under low-velocity impact (LVI) and compression after impact (CAI) loading conditions are not fully applicable to composite stiffened panels. However, studies on LVI and CAI damage evolution and failure mechanism for asymmetric composite laminates are very rare, which poses challenges to the theoretical study, strength prediction, and application of composite structures, and increases the risk of sudden failure under load. In this study, the damage evolution and failure mechanisms of asymmetric composite laminates under LVI and CAI loading conditions were studied experimentally and numerically. Ultrasonic phased array C-scan technique was used to analyze damage state, damage size, and delamination damage induced by LVI under different impact energies in impact tests. Moreover, a three-dimensional digital image correlation (3D-DIC) technique was employed to monitor the evolution of full-field displacement and strain in the asymmetric composite laminates during the CAI tests. A three-dimensional damage model considering the interaction among the interlaminar delamination damage, intralaminar matrix damage, and intralaminar fiber damage was proposed, and an interface-based cohesive behavior embedded framework in ABAQUS/Explicit was used to define and capture the interlaminar damage. The damage initiation, evolution, and propagation behaviors of different damage modes were simulated to study the complex damage and failure mechanisms of the asymmetric laminates in the LVI and CAI processes. The relationships among the different impact energies and the impact damage modes, delamination morphologies, and compression damage propagation and failure modes were discussed.

Development of three-dimensional woven weft interlock carbon-fiber-reinforced plastic composites and experimental investigation on their out-of-plane flexural performance

Developing three-dimensional (3D) woven composites using various woven structures is important in the advanced composite research field. In this study, carbon-fiber 3D woven textile-reinforced epoxy–resin composites using four different woven structures (such as 2.5D, 3D-a, 3D-b, and 3D-c) were designed and fabricated based on a dobby-weaving loom, and vacuum-assisted resin transfer molding technology. In these 3D woven structures, part of weft insertions serves as binder yarns with a special weft interlock structural design, different weft-to-binder yarn ratios are obtained. Out-of-plane quasistatic three-point bending tests and dynamic low-velocity drop-weight impact tests were conducted on these woven composites to study the effect of woven structure on their flexural performance. Nondestructive ultrasonic C-scan and X-ray microcomputed tomography were applied to characterize the failure modes of the post impacted composites. We found that woven structures affect the flexural performance and failure mode of the developed composites, these structures with lower in-plane and binder yarn waviness exhibited higher quasistatic–flexural performance; 3D-c structure with serious in-plane yarn waviness combined with through-thickness binder yarn path exhibited superior impact resistance than other structures and survived more from composite through-thickness cracks and binder yarn breakage failures. Proper structural design is a key to improving flexural performance in specific engineering applications.

A skin-core debonding quantitative algorithm based on hexagonal units reconstruction for air-coupled ultrasonic C-scan images of honeycomb sandwich structure

Honeycomb Sandwich Structure (HSS) is widely used in the aerospace industry. The skin-core debonding in HSS can grow to reach a critical size, leading to the sudden failure of structures. Due to the blurry edges on the C-scan image caused by honeycomb units with periodic honeycomb walls, accurate quantification of debonding is still a challenge. In this paper, accurate debonding quantification of HSS based on air-coupled ultrasonic C-scan and hexagonal units reconstruction is proposed. The debonding boundary identification is accurate to the interior of the honeycomb units by partial debonding units division based on calculating the total amplitude of all scanning points within the unit. The detailed process was presented with key steps as ultrasonic air-coupled C-scan for HSS, hexagonal units reconstruction based on peak points on C-scan images, units classification by K-means clustering algorithm, and units division according to the total amplitude. Four types of debonding, including film sticking, adhesive removal, slight subsidence, and severe subsidence, are used to evaluate the effectiveness of the method. The results show that the proposed method can describe and quantify-four kinds of debonding defects reliably and it is ideally suited for air-coupled ultrasonic C-scan image processing of periodic structures.

Experimental and numerical damage analysis of low-velocity drop weight impact of polyethylene self-reinforced composite laminates for transportation applications

This study covers a low-velocity impact property analysis of Polyethylene self-reinforced composites (PET-HDPE/HDPE SRC)through experimental and numerical methods emphasizing the influence of interfacial properties. Self-reinforcing a polymer material has gained a lot of attention due to maximum exploration of their light density characteristics with better chemical compatibility and interfacial adhesion of matrix and reinforcement, resulting in higher fuel efficiency. Reusability of self-reinforced thermoplastic composites even without separating their constituent materials has been observed to be aiding the circular economy strategies. The study aimed to analyze the peak load characteristics, energy-absorbing characteristics, and damage response of these materials processed through the hot compaction technique and subjected to a drop mass impact test. Fractographic visual inspection, optical macrographs, Scanning electron microscopy analysis, and through transmission ultrasonic C-scan were employed to study the damage evolution and failure mechanisms under the low-velocity impact. Three levels of impact energy were included in the test matrix, and initiation of the damage, nature of the matrix, interface debonding, matrix and fibre failures could be identified through the inspection of the damage cross-section at the impact eye. A finite element model developed through commercial FEA packages (Abaqus) was utilized for the complex mechanisms of damage and failure. Weak interfacial adhesion due to the self-lubricating nature of the polyethylene fibre surface contributed to the fibre pull out during the drop weight impact. A qualitative analysis and correlation of the nature of surface damage and internal damage were also studied, which are the factors that are necessary for the development of cargo containers and marine/aerospace ultralight luggage applications.

Analysis of multiple impact tests’ damage to three-dimensional four-directional braided composites

Abstract This article was designed with a plurality of impact tests of three-dimensional four-directional braided composites, and the impact response of specimen impacted by a circular punch was studied. Ultrasonic C-scanning was used to detect the internal damage area to study the damage propagation process under multiple impact loads. The finite-element software ABAQUS was used to model the meso-structure of three-dimensional four-directional braided composites. Based on material characteristics, the three-dimensional Hashin damage criterion was used for the fiber bundle, and the maximum stress criterion was used for the matrix to judge the material damage. Combined with test and simulation results, the failure mode and damage evolution process of the specimen under multiple impact loads were studied. The results showed that the impact resistance of the three-dimensional woven composite material is affected by the braided angle. The larger the braided angle of the specimen, the better the impact resistance. The damaged area of the large braided angle material expanded to the periphery, and the damaged area of the small braided angle material was primarily developed in the longitudinal direction. The failure modes of the specimen during the impact process were primarily a longitudinal tensile failure of fiber bundles, transverse tensile failure and transverse compression failure of fiber bundles and matrix.

Damage prediction of thick composite laminates subjected to low-velocity impact loads

Experimental and numerical studies have been carried out to predict the behavior of composite laminates under low-velocity impact loads. Numerical models were built based on shell elements in order to simulate it efficiently. A strain-based failure theory was employed to predict the damaged area of laminates using a user-defined subroutine. Drop-weight impact tests were performed for verifying the analysis results of the low-velocity impact. The damaged areas were inspected using ultrasonic C-scans, and three types of thick composite laminates were examined. The impactor velocity was measured at time before and after contact with the target. Finally, the proposed numerical model with the failure theory was verified in that it well predicted the impact damage of the three composite laminates chosen for this study.

Damage and failure analysis of hybrid laminates with different ply-stacking sequences under low-velocity impact and post-impact compression

This study investigates the influence of carbon/glass hybrid laminates’ stacking sequences on the impact resistance and damage tolerances of the fiber-reinforced composites. Through a series of low-velocity impact tests, ultrasonic C-scan tests, and post-impact compression tests, the conventional non-hybrid laminates (carbon-fiber-reinforced plastic and glass-fiber-reinforced plastic) were compared with the carbon/glass hybrid laminates of different configurations. And an integrated computational damage model was proposed to predict the impact damage and residual compressive strength of the hybrid laminates of different configurations. The results show that the carbon/glass hybrid laminates are highly competitive in impact resistance and residual compressive strengths due to the hybrid synergistic effects. The optimized design of carbon/glass composite laminates with different configurations is carried out through the validated integrated finite element model, which provides guidance for subsequent engineering applications.

Combining digital image correlation and phased-array ultrasonics for non-destructive testing of translaminar fracture in composite laminates

A simple and efficient non-destructive testing method is presented to quantify translaminar fracture properties in quasi-isotropic [90/45/0/-45]3s T800SC/3900-2B carbon fibre-reinforced polymer laminates subjected to Over-height Compact Tension (OCT) and Compact Compression (CC) fracture tests. The combination of Digital Image Correlation (DIC) with phased-array ultrasonics enables the consistent measurement of the fibre-dominant translaminar crack length, damage height and hence damage resistance curves as well as the continuous monitoring of the Fracture Process Zone (FPZ). The length of the FPZ is determined by overlaying regions of saturated (fully developed) fibre damage obtained from DIC and ultrasonic C-scans. The analysis of the progressive fracture tests reveals average laminate fracture energies of 161 kJ/m2 and 68 kJ/m2 and FPZ lengths of up to 8–12 mm and 10 mm in tension and compression, respectively. It is shown that the proposed method works best when fibre-dominated damage is confined to a narrow band normal to the loading direction as observed in CC tests. In contrast, OCT test results show that delamination and the corresponding isolated ply behaviour can alter the DIC analyses and hence complicate the measurement of translaminar fracture properties.

Experimental investigation of high velocity impact response of CFRP laminates subjected to flyer plate impact

Carbon fiber reinforced plastic (CFRP) composites are widely used in aircraft and automotive industries due to their extremely high specific strength and stiffness. High velocity impact is inevitable throughout the service life of composite structure and it is important to understand its impact response. In this study, high velocity impact response of CFRP laminates subjected to flyer plate impact were investigated in the velocity range of 32∼100 m/s by a gas gun coupled with 3D Digital Image Correlation (DIC) analysis. For comparison purposes, similar tests were carried out on 2A12 aluminum alloy plates of equivalent thickness. The PVDF pressure sensor was used to measure the impact pressure of the target plate, and the influence of impact velocity and target type on the impact pressure was analyzed. The damage and failure mechanism of CFRP laminates during impact were analyzed by digital microscope, ultrasonic C-scan and sectional image. The results show that the variation trend of the theoretical impact pressure is consistent with that of the experimental impact pressure, and the experimental impact pressure is smaller than the theoretical impact pressure. The damage and failure mechanism of two target plates are different under high velocity impact. For 2A12 plate, a truncated circular cone-shape bulge is generated on the 2A12 plate and this damage patterns appears regardless of the impact velocity. For CFRP laminates, impact indentation with elongated splits extending from the indentation edges is observed on the impacted surface of the target plate. The delamination zone of the adjacent layers is distributed in a fan-shaped area and the delamination direction is consistent with the direction of the fibers in the front ply. The data from the experiments can be used to evaluate and refine composite material models and computational methodologies that are used to predict the impact response of composite.