Nondestructive Inspection Techniques Research Articles

Quantification of pitting corrosion from thermography using deep neural networks.

Pipelines serve as the major infrastructure for transmission and distribution in oil and gas and water industries. The health condition of a metallic pipe is determined mainly by the extent to which it is corroded. Thus, the quantification of pitting corrosion in terms of metal loss is required for the understanding of pipe condition. There are different ways to quantify corrosion pit geometry. Direct methods measure the pit depth of pipe samples, which are sand/grit blasted to remove corrosion products, and are often adopted in the laboratory. Indirect methods employ non-destructive inspection techniques to detect and quantify the corrosion without sandblasting, which is preferred for a field test. In this study, pulsed thermography is considered for the quantification of pitting corrosion in a metallic pipe. Thermography testing can generate a sequence of infrared images, which reflect the diffusion process of heat through the pipe wall. This paper proposes a new deep neural network-based approach to quantify the pitting corrosion damages from the acquired thermography images. Through extensive experimental tests, the quantified results demonstrate a reasonably good linear relationship with metal loss of pipe, as the quantification accuracy was better than 98%.

Training AI-Based Feature Extraction Algorithms, for Micro CT Images, Using Synthesized Data

X-ray computed tomography (CT) is a powerful technique for non-destructive volumetric inspection of objects and is widely used for studying internal structures of a large variety of sample types. The raw data obtained through an X-ray CT practice is a gray-scale 3D array of voxels. This data must undergo a geometric feature extraction process before it can be used for interpretation purposes. Such feature extraction process is conventionally done manually, but with the ever-increasing trend of image data sizes and the interest in identifying more miniature features, automated feature extraction methods are sought. Given the fact that conventional computer-vision-based methods, which attempt to segment images into partitions using techniques such as thresholding, are often only useful for aiding the manual feature extraction process, machine-learning based algorithms are becoming popular to develop fully automated feature extraction processes. Nevertheless, the machine-learning algorithms require a huge pool of labeled data for proper training, which is often unavailable. We propose to address this shortage, through a data synthesis procedure. We will do so by fabricating miniature features, with known geometry, position and orientation on thin silicon wafer layers using a femtosecond laser machining system, followed by stacking these layers to construct a 3D object with internal features, and finally obtaining the X-ray CT image of the resulting 3D object. Given that the exact geometry, position and orientation of the fabricated features are known, the X-ray CT image is inherently labeled and is ready to be used for training the machine learning algorithms for automated feature extraction. Through several examples, we will showcase: (1) the capability of synthesizing features of arbitrary geometries and their corresponding labeled images; and (2) use of the synthesized data for training machine-learning based shape classifiers and features parameter extractors.

Combining eddy current, thermography and laser scanning to characterize low-velocity impact damage in aerospace composite sandwich panels

Aerospace sandwich panels subject to low-velocity impacts can develop damage in both the face sheet and the core that may compromise structural integrity. The standard repair manual for each aircraft specifies allowable damage limits that consider surface dents and the presence of disbonds or delaminations. A strategy for identifying the types of impact damage according to the standard repair manual using a series of non-destructive inspection techniques is presented for honeycomb sandwich panels with composite facesheets and metallic core. Eddy current is first used to identify impact sites, followed by thermography to detect disbonds or delaminations and laser scanning to measure the size of surface dents. Images of damage sites in honeycomb sandwich panels from low-velocity impacts created using a drop tower show that when used together, these inspection methods are able to detect and distinguish between three different damage configurations; core crush, surface dents, and adhesive disbonds or face sheet ply delaminations. While these three methods are commonly used non-destructive evaluation techniques, this paper focuses on their combined use in identifying different types of impact damage in aerospace sandwich panels in-situ according to the requirements of the standard repair manual. The combined methods are shown to address requirements of dent depth, dent area, and the identification of disbonds or delaminations in a more rapid, repeatable and robust manner than is currently employed.

Dispersion relation of Lamb waves in cross-ply composite laminates using multi-layered models

Lamb waves can be used in carbon fiber reinforced plastics (CFRP) for establishing both structural health monitoring systems and nondestructive inspection techniques. In this study, we investigated the dispersion relation of Lamb waves propagating in a cross-ply CFRP laminate. Using a formalism of the multi-layer Lamb wave model, we compared a homogeneous single-layer model and multi-layer models. The results suggest that the velocity of the S0 mode was the same for all the models in the low-frequency region. Thus, the use of homogeneous parameters is valid for the S0 mode. However, it was found that in high-frequency regions, there was a discrepancy between the velocities of Lamb wave modes for the homogeneous model and multi-layer models. This is because Lamb waves can be localized in a particular layer, and the velocity can be affected by the stiffness constants of the layer. This work elucidates the characteristics of Lamb wave propagation by showing that the stacking sequence of the laminate can have an effect on the wave velocities and wave structures, enabling researchers to improve Lamb wave-based defect detection.

Mean local frequency-wavenumber estimation through synthetic time-reversal of diffuse Lamb waves

Inspection of the elastic properties of plate-like structures is an important and challenging problem in many engineering disciplines. There exist multiple non-destructive inspection techniques based on active ultrasonic transducers to tackle this problem. In recent years, the usefulness of passively induced waves has also been explored. It has been shown that the diffuse waves, often referred to as coda waves, carry profound information about the elastic properties of the supporting medium. They can effectively probe the volume of a medium due to multiple scattering and long propagation paths. Moreover, the measured diffuse field can be synthetically focused using the correlation of wavefield’s time histories. The local elastic properties of the medium can be inferred from the focal spot size.In this work, we demonstrate the algorithm of mean local frequency-wavenumber estimation through synthetic focusing of diffuse Lamb waves. We provide theoretical foundations for expanding the concept of mean frequency estimation in purely temporal signals to mean wavenumber estimation in a spatiotemporal Lamb wave field. The applicability of this approach for the damage detection in plate-like structures is demonstrated on aluminum and carbon fiber reinforced polymer (CFRP) plates with localized defects. Test samples are excited with sequences of white noise signals, to create optimal diffuse field conditions, and wavefield is captured using a Scanning Laser Doppler Vibrometer (SLDV). We show that the spatial maps of mean wavenumber allow for precise localization and sizing of damage.

3D Imaging of CRP and Ultrasonic Tomography to Detect Decay in a Living Adult Holm Oak (Quercus ilex L.) in Sardinia (Italy)

A field-integrated methodology using 3D ultrasonic tomography supported by close range photogrammetry (CRP) has been developed and evaluated as a tool to detect the presence and patterns of decay forms in a living adult holm oak (Quercus ilex L.) in an urban green area of the city of Cagliari, Sardinia, Italy. Close range photogrammetry was used to compute a high resolution 3D model of the studied tree, texturized with natural colors. Moreover, following the implemented workflow process it was possible to evaluate the deformation pattern of the studied tree over time. In a second step of our integrated approach, and in order to diagnose the state of health of the inner part of the studied tree in a non-invasive way, laboratory and in situ non-invasive ultrasonic techniques were applied. The results of the close range photogrammetry analysis supported the optimal design of the 3D ultrasonic tomography of the living adult holm oak. Ultrasonic tomography is one of the most powerful non-destructive testing techniques for the full-volume inspection of a structure. It produced physical information on the inner structure of the stem of the investigated tree. The results of the study show that the integrated application of close range photogrammetry and 3D ultrasonic tomography is a powerful tool for a highly accurate and objective evaluation of the external and internal decay of trees and for monitoring their conservation states. With the fully integrated approach, the diagnostic process aimed to prevent instability and the failure of trees can be greatly improved.

On the assessment of non-metallic inclusions by part 13 of API 579 -1/ASME FFS-1 2016

Improvement of nondestructive inspection techniques has made more frequent the detection of zones containing closely spaced zones of non-metallic inclusions in pressure vessels made of low carbon steel. In the present study, closely spaced inclusions in an in-service cylindrical horizontal pressure vessel were detected by Scan-C ultrasonic inspection and considered as laminations to be assessed by Part 13 of the API 579-1/ASME FFS-1 2016 standard. The outcoming results were of rejection for Level 1 assessment, and a repair or replacement of the component was required, even though it retained a significant remaining strength. Thus, an alternative procedure to assess the mechanical integrity of pressure vessels containing zones of non-metallic inclusions is proposed with by adequation of some criteria of the API 579-1/ASME FFS-1 Part 13 standard procedure, taking into consideration the dimensioning and grouping characteristics of the inclusion zones, providing more reasonable results.

Edge Effect Analysis and Edge Defect Detection of Titanium Alloy Based on Eddy Current Testing

Titanium alloy is widely used in the area of aerospace and aviation due to its excellent properties. Eddy current testing (ECT) is among the most extensively used non-destructive techniques for titanium alloy material inspection. However, most previous research has focused on inspecting defects far from the edge of the material. It is a challenging task for edge crack detection because of edge effect. This study aims to investigate the influences of sensor parameters on edge effect and defect detection capability, and in the meantime, optimize sensor parameters to improve the capability of edge defect detection. The simulation method for edge effect evaluation is proposed including the 2k factorial design used for factor screening, and the regression model is fitted and validated for sensor design and optimization for edge defect detection. A simulation scheme is designed to investigate the defect detection capability. An approach comprehensively analyzing the influence of coil parameters on edge effect and defect detection capability is applied to determine the optimal coil parameters for edge defect detection.

Comparative Study for Inspection Planning of Aircraft Structural Components

In the aircraft operation, the metallic airframes are usually exposed to the extreme loading conditions during the flights due to which the risk of failure should be assessed and held to an allowed level to prevent the associated loss. If the components are inspectable, the components should be inspected routinely so that the fatigue damage can be detected before failure, which usually requires non-destructive inspection (NDI) techniques to evaluate the damage growth during the maintenance action. There have been several methods to determine the inspection plan in the aircraft industry for this purpose. In this study, four methods: Aircraft Structural Integrity Program (ASIP), Reliability Centered Maintenance Analysis (RCMA), Modified RCMA and Stochastic Life APproach (SLAP), are selected to evaluate the performance in comparative manner for a couple of crack growth problems with the aim to help the maintenance planning engineers select their own inspection plan properly based on the performance of each method. The methods are briefly reviewed, implemented, and examined in comparison for the fatigue test data of the fastener hole specimen under an aircraft load spectrum.

Portable digital holographic camera featuring enhanced field of view and reduced exposure time

Digital holography offers a full-field and high-resolution technique for non-destructive testing and inspection of electro-opto-mechanical components and products. But as holography is an interference-based technique so environmental and system vibrations put requirement for development of holographic systems in laboratory on optical vibration isolation platforms. Another drawback associated with digital holography is its small field of view due to limited spatial resolution of the available digital sensors. In this work, we report design, realization and validation of a compact and portable digital holographic camera (DHC) with enhanced field of view. We used an efficient opto-mechanical design to simultaneously increase the field of view and reduce exposure time and hence the effect of vibrations. The field of view of proposed DHC is 192% more compared to that of conventional digital holographic system. The temporal stability of the developed DHC is measured to be 48 nm over an operation time of 10 min. This development may enable use of digital holography for wide range of non-destructive testing applications.

Ultrasonic Welding of Glass Fiber Reinforced PP Thermoplastic Composites: An Investigation of the Outer Layer Orientation and the Fiber Volume Fraction

This paper presents an experimental study of the influence of the orientation of the outer layer of polypropeylene (PP) reinforced with E-glass fiber laminate (GF/PP) and the influence of the fiber volume fraction on the quality of the welded joint using an ultrasonic welding process. An orthogonal L 16 array (OA) design of experiment was conducted in this paper based on the Taguchi method to evaluate the effect of the orientation of the outer layer and the fiber volume fraction, on the welding process parameters; the welding energy, the amplitude of vibration, the welding pressure, the holding pressure and the holding time were considered in order to achieve a high weld quality. The experiments were carried out using a 15 kHz ultrasonic welding unit with a maximum supplied power of 4000-Watt. GF/PP laminates with fiber volume fraction of 36% and 46% were used in this paper, and the GF/PP laminates were either unidierctional or had a 90 degree outer layer orienation. A 0.127 mm thick polypropeylene film was used as a flat energy director (ED). The evaluation of the weld quality was measured by the apparent shear strength of the single lap welded joints, and by using laser shearography as a non-destructive inspection technique . The failure mechanism of the single lap joint was monitored, using a high speed digital imaging system. A combination of the highest selected level of welding energy, lowest level of amplitude, lowest level of welding pressure, and the lowest level of both hold time and hold pressure of a unidirectional GF/PP with the lowest fiber volume fraction, were found to achieve a higher apparent shear strength of the welded adherends, as compared with the apparent shear strength obtained with the presence of the flat energy director for the same level of factors. A confirmation experiment was conducted to measure the predicted apparent shear strength and compare it with the measured apparent shear strength from the test.

Laser ultrasonics for defect evaluation on coated railway axles

This scientific paper focuses on the application of an advanced non-destructive technique for an effective inspection of railway axles. The method pertains to ultrasonic techniques, which are widely used in the railway field. The experimental investigation was carried out on simulated defects tooled near the cross section reduction of the axle, in order to simulate fatigue cracks which, due to notch effect, can trigger crack propagation and axle failure.The aim of this research activity is to evaluate how efficiently the proposed technique detects defects and to verify its applicability to axles with a black coating for protection. In view of the experimental setup, comprising a pulsed laser for ultrasonic waves generation and a continuous beam laser plus an interferometer as the receiving unit to measure surface displacements, the presence of a black coating represents a major challenge in terms of signal detection. Nonetheless, defects were detected by collecting all the waves in a B-scan map and data were processed by cross-reading A-scans and B-Scans in correspondence of each defect analysed. Results of the analysis show how very promising and robust the application of the proposed laser ultrasonics technique is in defect detection on painted railway axles.

A railway tunnel structural monitoring methodology proposal for predictive maintenance

Structural health monitoring (SHM) on railway tunnels is increasingly demanding inspection methodologies based on full-field image-based techniques. The relevance of adequate inspection methodologies to signal potential problems that need increased attention is at the forefront of any monitoring system for problems that can disturb tunnel's integrity. Detecting defects is decisive for an efficient inspection, investigating tunnel status, and rescheduling maintenance operations, when and where required. This paper introduces a methodology design, based on acquisition of tunnel's 3D geometry and monitoring it through time to signal the possible emergence of structural defects and return the exact position where monitoring equipment will have to be deployed to. A demonstrator is built, and the applicability and accuracy of the system is assessed through implementation of a model prototype, carried out on a scaled tunnel model. At a secondary stage inspection, imposed geometrical changes and defects of the tunnel are monitored and characterised by an optical nondestructive inspection technique based on the digital image correlation (DIC). It enables the appraisal of structural integrity on the tunnel lining through displacement and strain fields acquisition. The chief advantage of the deployed inspection system, amongst others, lies on its robust functionality in both geometrical and structural integrity inspection aspects, contributing to the establishment of a robust SHM methodology for railway tunnels over time. In conclusion, experimental results are presented for evaluating scaled tunnel condition and monitoring the tunnel profile, and further DIC data are presented inferring displacement field progress of an introduced structural defect.

Nondestructive Internal Defect Detection Using a CW–THz Imaging System in XLPE for Power Cable Insulation

The demand for internal nondestructive testing and inspection techniques is rapidly increasing. Using a continuous wave (CW) terahertz (THz) imaging system, we demonstrate that the internal defects in cross-linked polyethylene (XLPE) plates for power cable insulation can be detected. In a coherent detection scheme based on photomixers, which serve as the THz emitters and receivers, the change of phase occurring with the defects inside the XLPE plates is distinctly measured by the change in the amplitude of the transmitted THz waves. According to the two-dimensional images of THz waves transmitted through the XLPE plates, defects of up to 0.5 mm size located inside the XLPE plates can be detected by the internal nondestructive examination method based on CW–THz waves. This technique will be useful for internal nondestructive testing and inspection of insulation materials that require high resolution in various industries, including the automobiles, electronics, and electrical power industries.

Detection and Characterization of Hidden Corrosion in Aircraft Lap Joints by Optical NDT Method

Abstract Hidden corrosion in lap splice faying surfaces can directly affect the airworthiness of an aircraft. The corrosion thinning not only reduces the residual strength of aircraft components but also adds stresses from corrosion growth in confined spaces (e.g., corrosion pillowing). This article presents an optical nondestructive inspection technique for the detection and quantification of hidden corrosion in multilayer structures. Based on the bright field illumination technique with a laser light, a calibration procedure is developed to correlate the amplitude of the pillowing deformation of lap joints to the intensity changes of laser light. Using a finite element model of the lap joint, it is possible to relate pillowing deformation to the amount of material loss. Applying the proposed method to an aircraft lap joint shows very promising results.

Optimization of digital radiography for large metallic additively manufactured components

PurposeThe continued improvement of additive manufacturing (AM) processing has led to increased part complexity and scale. Processes such as electron beam directed energy deposition (DED) are able to produce metal AM parts several meters in scale. These structures pose a challenge for current inspection techniques because of their large size and thickness. Typically, X-ray computed tomography is used to inspect AM components, but low source energies and small inspection volumes restrict the size of components that can be inspected. This paper aims to develop digital radiography (DR) as a method for inspecting multi-meter-sized AM components and a tool that optimizes the DR inspection process.Design/methodology/approachThis tool, SMART DR, provides optimal orientations and the probability of detection for flaw sizes of interest. This information enables design changes to be made prior to manufacturing that improve the inspectabitity of the component and areas of interest.FindingsValidation of SMART DR was performed using a 40-mm-thick stainless-steel blade produced by laser-based DED. An optimal orientation was automatically determined to allow radiographic inspection of a thickness of 40 mm with a 70% probability of detecting 0.5 mm diameter flaws. Radiography of the blade using the optimal orientation defined by SMART DR resulted in 0.5-mm diameter pores being detected and indicated good agreement between SMART DR’s predictions and the physical results.Originality/valueThis paper addresses the need for non-destructive inspection techniques specifically developed for AM components.

Towards a digital twin for mitigating void formation during debulking of autoclave composite parts

High-performance polymeric composites are increasingly used in the design of aircraft structural components; however, susceptibility to manufacturing irregularities, including porosity/voids, remains a primary challenge delaying the implementation of advanced composites in modern aircraft. Voids may be precursors to structural damage significantly affecting structural integrity and remaining useful life of the aircraft. High-performance aerospace composite parts are commonly manufactured from resin-saturated pre-impregnated plies of uncured material laid-up over a rigid tool and consolidated and cured in an autoclave. In many applications, especially in thick and curved composite sections common in aerospace designs, autoclave consolidation alone is not sufficient to remove the air, or bulk, that might have been entrapped within the laminate during the layup process. Vacuum consolidation, or “debulking”, has been a standard practice extensively used by manufacturers to reduce the amount of bulk prior to autoclave curing. The debulking process typically takes a long time and requires intensive manual operations. The underlying physical principles governing the formation and evolution of voids during these early stages of the manufacturing process are not yet well understood, and controlling parameters remain largely determined empirically or based on prior experience. Driven by the need to improve such understanding, there has been a recent interest in using newly available high-fidelity non-destructive inspection (NDI) techniques, such as X-ray Computed Tomography, for in situ observations of composites internal structure during the early stages of manufacturing. In situ observations are allowing researchers to identify the driving mechanisms involved during defect formation and develop improved predictive models. The objective of this work is to show that high-fidelity NDI data and new developments in numerical modeling can be combined to create a Digital Twin for mitigation of void formation in composite parts. In particular, X-ray CT data is used to extract bulk content and distribution in uncured carbon-epoxy curved-beam specimens after manual layup, and such information is transferred into a finite element (FE) model for simulation of debulking. The FE model uses a fracture-based approach that relies on pore-pressure cohesive zone modeling recently proposed for the discrete representation of entrapped air pockets in uncured resin-saturated prepregs. Preliminary results support a promising ability of the Digital Twin concept for optimizing the debulking process of autoclave composites towards mitigating void formation.

Morphological study of damage evolution in woven-laminates subjected to high-velocity impact

The aim of this experimental work was to study the evolution of damage in woven E-glass fiber/polyester laminates of four different thicknesses. After being impacted, these plates were inspected by nondestructive inspection techniques: ultrasound by C-Scan and B-Scan and visual inspection in order to confirm the qualitative and quantitative results. The damage presented an opening cone shape towards the rear face of thin laminates. On the other hand, two different opening cones of delamination grew towards the front and rear faces after a certain thickness in thick laminates.

A Simulative and Experimental Approach Toward Eddy Current Nondestructive Evaluation of Manufacturing Flaws and Operational Damage in CFRP Composites

Abstract The manufacturing process of carbon fiber reinforced polymer (CFRP) composite structures can introduce many characteristic defects and flaws such as fiber misorientation, fiber waviness, and wrinkling. Therefore, it becomes increasingly important to detect the presence of these defects at the earliest stages of development. Eddy current testing (ECT) is a nondestructive inspection (NDI) technique that has been proven quite effective in detection of damage in metallic structures. However, NDI of composite structures has mainly relied on other methods such as ultrasonic testing (UT) and X-ray to name a few and not much on ECT. In this paper, the authors explore the possibility of using ECT in NDI of CFRP composites by conducting simulations and experiments thereafter. This research is based on the fact that the CFRP displays some low-level electrical conductivity due to the inherent conductivity of the carbon fibers. This low-level conductivity may permit eddy current pathways to cause the flow of eddy currents in the CFRP composites that can be exploited for nondestructive damage detection. An invention disclosure describing our high-frequency ECT method has also been processed. First, the multiphysics finite element method (FEM) simulation was used to simulate the detection of various types of manufacturing flaws and operational damage in CFRP composites such as fiber misorientation, waviness, wrinkling, and so on. Thereafter, ECT experiments were conducted on CFRP specimens with various manufacturing flaws using the Eddyfi Reddy eddy current array (ECA) system.

Porosity analysis of carbon fibre-reinforced polymer laminates manufactured using automated fibre placement

Automated fibre placement-based manufacturing technology is increasingly being used in several engineering applications. Manufacture of carbon fibre-reinforced plastic’s small/large structures have been made possible due to its remarkable capabilities like productivity and accuracy. Nevertheless, making high-quality composite laminate using automated fibre placement relies on the proper selection of critical processing variables to avoid internal flaws during the fibre placement process. Consequently, a reliable non-destructive inspection technique is required for quality assurance and structural integrity of fabricated laminates. Neutron radiography/tomography offers unique imaging capabilities over a wide range of applications including fibre-reinforced polymer composites. The application of this technique towards tomographic reconstruction of automated fibre placement-made thermoplastic composites is presented in this paper. It is shown that the porosity analysis using neutron imaging technique provides reliable information. Additionally, using such technique valuable data regarding the size and the location of the voids in the laminate can be acquired and informed. This will assist the composite structural analysts and designers to select the appropriate processing parameters towards a defect free automated fibre placement part manufacture.