NDE Methods Research Articles

Artificial Neural Network based Defect Detection of Welds in TOFD Technique

Time of Flight Diffraction Technique is one of the NDE methods, used in weld inspection to identify the weld defects. The classification of defects using the TOFD technique depends on the knowledge and experience of the operator. The classification reliability of defects detected by this technique can be improved by applying the Artificial Neural Network. In this work, four austenitic stainless steel weldments with defects viz, Lack of Fusion, Lack of Penetration, Slag, Porosity and one with out any Defect were fabricated. TOFD experiment is conducted on these weldments. Discrete wavelet transform based denoising methods were applied to denoise the resultant A scan signals. Time scale features are extracted from the denoised signals. A multi layer feed forward network with Resilient Back Propagation algorithm has been applied for classification of the signals. The number of hidden layers in the network are increased from 0 to 6. Various performance functions are also employed to achieve a better classification efficiency. The results are promising to proceed the automatic defect classification by TOFD technique.

Prediction of Initiation Site of Destruction of Flat Braided Carbon Fiber Composites Using HTS-SQUID Gradiometer

Abstract Carbon fiber reinforced polymers (CFRPs) are composite materials with lightweight and high specific strength. As the braided CFRPs have continuous carbon-fiber bundles in their longitudinal direction, they are stronger than conventional CFRPs. In this study, we applied a current-injection-based NDE method using a HTS-SQUID gradiometer to the flat braided CFRPs with and without carbon nanotubes (CNTs), and estimated conditions of the carbon fibers while applying a step-by-step tensile load to the CFRPs. From the results, a possibility to predict an initiation site of the destruction in the braided CFRPs was demonstrated.

OS0715 空気超音波法に対する3次元数値シミュレーション

This paper presents a 3D numerical simulation for air-coupled ultrasonic testing. Air-coupled ultrasonic testing has some advantages because this method is one of non-contact ultrasonic NDE methods. However, the positioning of air-coupled transducers is very sensitive, because the difference of acoustic impedance between air and solid is very large. In order to use this method for evaluation of various materials, it needs to simulate wave propagation in a solid medium. A BEM (boundary element method) is an effective method for wave propagation problems. Therefore, the BEM formulation using the CQM (convolution quadrature method) is developed for air-coupled ultrasonic testing and the numerical results for 3D wave simulation are shown.

Technological Study on Manufacturing of Multifinger Module of He-Cooled DEMO Divertor and Investigation of NDE Method

Technological Study on Manufacturing of Multifinger Module of He-Cooled DEMO Divertor and Investigation of NDE Method

Application of Noise Cancelling and Damage Detection Algorithms in NDE of Concrete Bridge Decks Using Impact Signals

Delamination is a commonly observed distress in concrete bridge decks. Among all the delamination detection methods, acoustic methods have the advantages of being fast and inexpensive. In traditional acoustic inspection methods, the inspector drags a chain alone or hammers on the bridge deck and detects delamination from the “hollowness” of the sound. The signals are often contaminated by ambient traffic noise and the detection of delamination is highly subjective. This paper describes the performance of an impact-based acoustic NDE method where the traffic noise was filtered by employing a noise cancelling algorithm and where subjectivity was eliminated by introducing feature extraction and pattern recognition algorithms. Different algorithms were compared and the best one was selected in each category. The comparison showed that the modified independent component analysis (ICA) algorithm was most effective in cancelling the traffic noise and features consisting of mel-frequency cepstral coefficients (MFCCs) had the best performance in terms of repeatability and separability. The condition of the bridge deck was then detected by a radial basis function (RBF) neural network. The performance of the system was evaluated using both experimental and field data. The results show that the selected algorithms increase the noise robustness of acoustic methods and perform satisfactorily if the training data is representative.

Assortment of ultrasonic nondestructive evaluation signals using the centroid searching method with the LMS algorithm and SAFT

NDE using ultrasonic signals is a very useful technique for the assessment of solid materials, construction, food, and biomedicine. Among many NDE methods, the ultrasonic inspections may involve the extraction of an appropriate set of features or a neural network for the classification of the signals in the feature space. This paper presents an approach that uses a geometric method and the LMS (Least Mean Square) algorithm to determine the coordinates of the ultrasonic probe followed by the SAFT (Synthetic Aperture Focusing Technique) and centroid searching technique to estimate the location of the ultrasonic reflector. The proposed method is employed to classify ultrasound NDE signals from cracks and deposits within steam generator tubes in a nuclear power plant.

Comparison of NDE techniques for assessing mechanical properties of unjointed and finger-jointed lumber

Abstract Non-destructive evaluation (NDE) methods are common for grading structural lumber with static bending as the traditional NDE method for strength. More recently, longitudinal and transverse vibration techniques have also been proposed for grading lumber. In this study, unjointed and finger-jointed sawn lumber has been evaluated by these traditional and relatively new NDE methods. In total, 188 pieces of 38 mm×89 mm black spruce lumber were tested. Of these, 40 were unjointed, 47 had 2–3 finger joints, and 101 had 5–7 finger joints. The main objective was to evaluate the reliability of the various NDE techniques in predicting the bending stiffness and tensile strength of finger-jointed lumber with different number of finger joints. Results show that all NDE methods provide stiffness values of unjointed and finger-jointed lumber that correlate well with laboratory measured static bending stiffness with R2 values ranging from 0.76 to 0.97. Moreover, lumber with finger joints has lower bending stiffness than unjointed lumber. Based on the correlation coefficients, there is no evidence that finger joints affect the precision of the strength prediction by NDE methods.

Development of a scanning system to detect corrosion in wire bundles and pipes using magnetoresistive sensors

Ageing and deterioration of the main functional components of civil structures are some of the main concerns that affect their safety. In the case of relatively old suspension bridges, problems emerge due to corrosion and wire breaks in the main cables. Decisive information, reliable monitoring and evaluation are factors of great relevance to prevent catastrophic damage to the structure and, more importantly, to people. The main challenge for NDE methods of inspection arises in dealing with the signal barrier created by steel wrapping around the suspension cable. The wrapping is used to shield, shape and hold the bundles. This paper presents a scanner system based on magnetoresistive sensors that has been designed to monitor and evaluate suspension bridge cables. Modelling, signal acquisition, signal processing, experiments, on-site tests and visualisation results are presented and discussed in detail.

Non-destructive evaluation of concrete quality using PZT transducers

This paper presents a new concept of using PZT (lead zircornate titanate) transducers as a non-destructive testing (NDT) tool for evaluating quality of concrete. Detection of defects in concrete is very important in order to check the integrity of concrete structures. The electro-mechanical impedance (EMI) response of PZT transducers bonded onto a concrete specimen can be used for evaluating local condition of the specimen. Measurements are carried out by electrically exciting the bonded PZT transducers at high frequency range and taking response measurements of the transducers. In this study, the compression test results showed that concrete specimens without sufficient compaction are likely to fall below the desired strength. In addition, the strength of concrete was greatly reduced as the voids in concrete were increased. It was found that the root mean square deviation (RMSD) values yielded between the EMI signatures for concrete specimens in dry and saturated states showed good agreement with the specimens' compressive strength and permeable voids. A quality metric was introduced for predicting the quality of concrete based on the dry-saturated state of concrete specimens. The simplicity of the method and the current development towards low cost and portable impedance measuring system, offer an advantage over other NDE methods for evaluating concrete quality.

MAGNETIC MEMORY TESTING: Studies on stress-magnetism coupling effect for 35 steel components

The stress-magnetism effect can be used as a novel and convenient potential NDE method, called the magnetic memory method. However, whether and how this can be used as a quantitative measurement are still subjects that are as yet rarely studied by researchers. In this paper, circle tensile stress within the elastic region was applied to a ferromagnetic sample under geomagnetic field. The relationships between the surface leakage magnetic field and the tensile stress are presented. The stress-magnetic field equivalent model is derived and discussed and reconciled with the experimental results. A new model of stress-permeability that represents the relationship of the material permeability versus stress is proposed, which reconciles with the experimental results. It confirms that quantitative measurement and estimation of the integrity of ferromagnetic items by magnetic memory testing is feasible.

Joining of thick section steels using hybrid laser welding

A recently completed project called Economical and Safe Laser Hybrid Welding of Structural Steel (HYBLAS) has developed the use of hybrid laser welding for thicker section steels up to 690 MPa yield strength. The full project involved several European organisations, was part funded by the European Research Fund for Coal and Steel (ERFCS) and was led by Corus RD&T. This paper presents an outline of those parts of the project which relate to those developments which resulted in being able to laser hybrid weld, in a single pass, up to 25 m plate thickness using 20 kW of laser power at speeds of ∼1 m min-1. Multipass and dual sided welding techniques have also been developed up to 30 mm plate thickness and fillet welds up to 20 mm steel thickness. The project examined the weldability of steels from 180-690 MPa and operational windows for defect free welding were defined. In addition various NDE methods were studied for their efficiency in regard to the defect types which can occur in laser hybrid welds. The fracture and mechanical properties of the joints were shown to be perfectly acceptable for all structural uses and an extensive fatigue testing programme demonstrated that the fatigue behaviour of the welded joints exceeded conventional welding expectations. Finally full scale industrial components were manufactured, inspected and tested to demonstrate that the anticipated fatigue benefits were obtained.

An investigation to determine the location of fatigue crack initiation sites from ultrasonic images of corrosion

Future corrosion control management on aircraft is planning to move from a Find-and-Fix approach to an Identify-and-Manage approach. Central to this change is the development of suitable life-prediction models that can be used to predict the effects of corrosion on structural integrity. This new approach requires NDE methods that can detect, characterise and monitor the corrosion damage. If the corrosion pit initiating the fatigue crack which leads to failure can be reliably identified in advance, existing life-prediction models can be employed for life prediction by using the dimensions of the corrosion pit as the initial flaw. This study has analysed 50 MHz ultrasonic amplitude C-scan images of pitting and crevice corrosion specimens to determine a parameter whose location in the image correlates with the point of failure of the specimen. Texture analysis was applied to ultrasonic amplitude images of corrosion using three texture filters, namely range, standard deviation and entropy. In addition, Laplacian filtering was applied to the images. The parameters of the filtered images chosen to correlate with the location of the point of failure were maximum range, maximum standard deviation, maximum entropy and maximum and minimum Laplacian values. It was found that the location of these parameters did not consistently coincide with the location of the corrosion pit that initiated failure.

Fretting damage assessment in Ti-6Al-4V by magnetic sensing

As a result of recent technological advances in high-sensitivity magnetic sensors, state of the art magnetometers, make it possible to obtain new assessment measurements of the near-surface layers (5–60 μm) in fretting damage zone. In particular, the experimental method used in this investigation is based on magnetic sensing of local thermoelectric currents produced by material imperfections such as texture, inclusions, residual stress, cold work, heat treatment etc., when a temperature gradient is established in the material. These thermoelectric currents generate flux magnetic densities that are detected by a highly sensitive magnetometer. According to our preliminary results, the magnetic flux density measurements was found to be rather sensitive to changes in near-surface layers produced by the fretting damage in Ti-6Al-4V alloy specimens with the exception of the intrinsic material background magnetic signal that affected deeply the detectability of subtle material variations in noncontacting thermoelectric measurements. This study shows a non-destructive test method that can detect the level of fretting damage in metals, that is difficult to characterize by other well known NDE methods.

Application of Magnetostrictive Transducer for the Long-Range Guided Wave Inspection

The leakage of the pipes is a major issue for the safety of industrial structures. However, in many cases, because of their geometrical complexity and inaccessibility, it is difficult to inspect them by the conventional NDE method. A long-range guided wave inspection, thus, is an option to inspect them. A torsional vibration mode, T(0,1) shows many advantages in a long-range guided wave examination of a pipe, such as no dispersion characteristic, no radial displacement and low attenuation. However, it is not easy to fabricate a transducer with an array of piezoelectric elements for generation of torsional vibration mode and even expensive. Recently a magnetostrictive metal strip sensor was used for a generation of the torsional vibration modes in a pipe and this technique has shown several advantages for practical applications. This study investigated the applicability of a long-rang guided ultrasonic method to the detection of artificial notches even in the presence of various foreign objects.

Detection of impact damage in CFRP composites by thermosonics

This study investigated methods of reducing the electrical power required to detect defects by thermosonics, or vibro-thermography, and thus to reduce surface damage that can occur at the exciter attachment point during testing. The surface temperature rise over excited defects has been modelled to determine the heating requirements to visualise damage. A long pulse, low power excitation method has been found to produce satisfactory impact damage images whilst eliminating damage at the exciter attachment point. Thermosonics has been found to detect impact damage in composites more reliably than optically stimulated thermal NDE methods.

Reliable low-cost NDE of composite marine sandwich structures

The research summarized in this paper centered on the development of reliable low-cost NDE methods for marine composites, especially composite sandwich structures. It has involved the investigation of vibration-based mechanical identification of thick composite structures, low velocity impact and damage studies of foam-cored composite sandwich structures, as well as the development- and application-oriented investigation of various other NDE methods. Composite materials are being increasingly deployed into ship structures, and generally into both the civilian and military sectors of the US economy. It thus becomes essential to have rapid, low-cost and reliable methods for obtaining mechanical property data for these materials, and assessing damage in them. The research has sought to apply low-frequency vibration, ultrasonic, acoustic absorption, thermosonic and especially, acoustic emission methods to the characterization and integrity assessment as well as strain rate and nano-phasing behaviors of composite marine structures.

Assessment of damage detection methods in GRID-LOCK ® structures

Bonded metallic GRID-LOCK ® structures are being adopted for a variety of aerospace applications due to their structural efficiency and damage tolerance. However, effective damage detection in these structures is complicated by a lack of interior access. In this paper, the feasibility of various non-destructive evaluation (NDE) techniques for detecting the presence, location, and magnitude of damage in GRID-LOCK ® is investigated. Experiments conducted on a GRID-LOCK ® test structure reveal that vibration-based damage detection and optical nde methods are superior to ultrasonic C-scans in detecting disbond defects. Finite-element analysis (FEA) is used to interpret experimental results and highlight the advantages of alternate NDE approaches.

Magnetic field aberration induced by cycle stress

Magneto-mechanical effect has been causing people's growing interest because of its relevance to several technology problems. One of them is the variation of surface magnetic field induced by stress concentration under the geomagnetic field. It can be used as an innovative, simple and convenient potential NDE method, called as magnetic memory method. However, whether and how this can be used as a quantitative measurement method, is still a virginal research field where nobody sets foot in. In this paper, circle tensile stress within the elastic region was applied to ferromagnetic sample under geomagnetic field. Experiment results on the relation between surface magnetic field and elastic stress were presented, and a simple model was derived. Simulation of the model was reconciled with the experimental results. This can be of great importance for it provides a brighter future for the promising Magnetic Memory NDE method—the potential possibility of quantitative measurement.

NDE of Decayed Wood with Ultrasonic CT

Recently, a new ultrasonic CT system has been developed as the NDE method suitable for wood. In this system, several characteristics related to the ultrasound traveling in the woods such as the anisotropy of woods, existence of immature woods and ultrasound diffraction in the woods are considered during image composition process. In this study, we inspected the system that had been newly developed for the decayed wood specimen. We performed the ultrasound measurements to reconstruct the CT image. And then, we verified the distributions of each characteristic in the section by measuring the moisture content, specific gravity and compressive strength after cutting the measured specimen by small specimen again. The acquired results are compared with the CT image acquired in the same section. As a result, it is determined that it is possible to detect the locations and sizes of decayed parts in the wood by using the developed ultrasound CT system. In particular, the results show that the initial decays that the weight loss rate is less than 10% and the strength loss rate is less than 30% are also detectable.

Fretting fatigue and wear damage of structural components in nuclear power stations—Fitness for service and life management perspective

Fretting fatigue and wear problems have major economical and safety impact on the nuclear industry. This keynote paper provides examples of the fretting problems encountered in nuclear power stations and an overview of the methodologies used to assess their root cause, their potential effect on the integrity of structural components and the future damage projection for risk management. The limitations of existing models that are commonly used to predict fretting wear rate are discussed. A system approach to the fretting wear/fatigue problem allowed us to significantly improve the capability of predicting fretting damage through the recognition of the problem nonlinearity, and the effect of self-induced changes. The application of linear elastic fracture mechanics principles for predicting the fretting wear and fretting fatigue strength is demonstrated. The paper underlines the critical roles of the following two factors. First, the validation of the above mentioned methodologies, through experimental investigation of the long-term fretting wear and fatigue behavior of structural components under realistic operating conditions. Second, the qualification of in -situ measurements of fretting wear damage using nondestructive evaluation NDE and inspection methods.