Analytical model of tilted driver–pickup coils for eddy current nondestructive evaluation

A driver-pickup probe possesses better sensitivity and flexibility due to individual optimization of a coil. It is fre- quently observed in an eddy current （EC） array probe. In this work, a tilted non-coaxial driver-pickup probe above a multilayered conducting plate is analytically modeled with spatial transformation for eddy current nondestructive evalua- tion. Basically, the core of the formulation is to obtain the projection of magnetic vector potential （MVP） from the driver coil onto the vector along the tilted pickup coil, which is divided into two key steps. The first step is to make a projection of MVP along the pickup coil onto a horizontal plane, and the second one is to build the relationship between the pr, ojected MVP and the MVP along the driver coil. Afterwards, an analytical model for the case of a layered plate is established with the reflection and transmission theory of electromagnetic fields. The calculated values from the resulting model indicate good agreement with those from the finite element model （FEM） and experiments, which validates the developed analytical model.

High-precision nondestructive evaluation of a thermal barrier coating based on perovskite quantum dot anion exchange

Thermal barrier coatings(TBCs)in gas turbine engines are used in expressly harsh environments;thus,assessing TBC integrity status is critical for safety and reliability.However,traditional periodic maintenance involves visual inspections of the TBCs,requiring the gas turbine to be decommissioned and partially dismantled.Most importantly,tiny defects or internal damages that easily cause coating failure cannot be identified.In this work,a new nondestructive evaluation(NDE)technique of TBCs based on quantum dot(QD)anion exchange is first explored internationally.By exchanging anions between the Cl ions and the CsPbBr_(3) QDs,the degrees of salt corrosion of the TBCs are evaluated.The resultant NDE technique shows that the colour of the TBCs obviously changes from green to blue,accompanied by a large blueshift(~100 nm)of the photoluminescence(PL)peak position.In addition,the relationship between the PL peak position and coating thermophysical properties indicates that the precision of this NDE technique may easily identify theμm-level of the thermal growth oxide(TGO)changes inside the TBCs.

Recent developments in photoacoustic imaging and sensing for nondestructive testing and evaluation

Photoacoustic(PA)imaging has been widely used in biomedical research and preclinical studies during the past two decades.It has also been explored for nondestructive testing and evaluation(NDT/E)and for industrial applications.This paper describes the basic principles of PA technology for NDT/E and its applications in recent years.PA technology for NDT/E includes the use of a modulated continuous-wave laser and a pulsed laser for PA wave excitation,PA-generated ultrasonic waves,and all-optical PA wave excitation and detection.PA technology for NDT/E has demonstrated broad applications,including the imaging of railway cracks and defects,the imaging of Li metal batteries,the measurements of the porosity and Young’s modulus,the detection of defects and damage in silicon wafers,and a visualization of underdrawings in paintings.

Terahertz Spectroscopic Characterization and Thickness Evaluation of Internal Delamination Defects in GFRP Composites

The use of terahertz time-domain spectroscopy(THz-TDS)for the nondestructive testing and evaluation(NDT&E)of materials and structural systems has attracted significant attention over the past two decades due to its superior spatial resolution and capabilities of detecting and characterizing defects and structural damage in non-conducting materials.In this study,the THz-TDS system is used to detect,localize and evaluate hidden multi-delamination defects(i.e.,a three-level multi-delamination system)in multilayered GFRP composite laminates.To obtain accurate results,a wavelet shrinkage de-noising algorithm is used to remove the noise from the measured time-of-flight(TOF)signals.The thickness and location of each delamination defect in the z-direction(i.e.,through-the-thickness direction)are calculated from the de-noised TOF signals considering the interaction between the pulsed THz waves and the different interfaces in the GFRP composite laminates.A comparison between the actual and the measured thickness values of the delamination defects before and after the wavelet shrinkage denoising process indicates that the latter provides better results with less than 3.712%relative error,while the relative error of the non-de-noised signals reaches 16.388%.Also,the power and absorbance levels of the THz waves at every interface with different refractive indices in the GFRP composite laminates are evaluated based on analytical and experimental approaches.The present study provides an adequate theoretical analysis that could help NDT&E specialists to estimate the maximum thickness of GFRP composite materials and/or structures with different interfaces that can be evaluated by the THz-TDS.Also,the accuracy of the obtained results highlights the capabilities of the THz-TDS for the NDT&E of multilayered GFRP composite laminates.

Enhanced air-coupled impact echo technique by phase analysis of signals from multiple sensors

This paper presents an air-coupled impact echo(IE)technique that relies on the phase spectrum of the collected data to find the frequencies corresponding to the reflections from delaminations.The proposed technique takes advantage of the fact that the IE compression wave is not a propagating wave,but it is the 1st order symmetrical(S1)mode Lamb wave at zero group velocity(S1-ZGV).Therefore,it searches the phase spectra of the data collected by multiple sensors to locate the frequency corresponding to the lowest phase difference.As a result,the technique reduces the effect of propagating waves,including the direct acoustic wave and ambient noise.It is named the Constant Phase IE(CPIE).The performance of the CPIE is experimentally compared with the regular amplitude spectrum-based IE technique and two other multisensor IE techniques.The CPIE shows a performance advantage,especially in a noisy environment.

Evaluation of fracture damage of SUS306 structural steel using ultrasonic testing

High frequency ultrasonic nondestructive testing was conducted using a direct contact method for SUS306 stainless steel treated by high temperature and fracture tensile tests. Reflected ultrasonic echoes were analyzed. The relationships between the ultrasonic attenuation coefficient, strength of backscattering wave and the elongation at break of the samples were obtained. The damages were evaluated by using these results together with the analysis of microstructure and mechanics of the tested material.

Nonlinear Electrical Impedance Tomography Method Using a Complete Electrode Model for the Characterization of Heterogeneous Domains

This paper presents an electrical impedance tomography(EIT)method using a partial-differential-equationconstrained optimization approach.The forward problem in the inversion framework is described by a complete electrodemodel(CEM),which seeks the electric potential within the domain and at surface electrodes considering the contact impedance between them.The finite element solution of the electric potential has been validated using a commercial code.The inverse medium problem for reconstructing the unknown electrical conductivity profile is formulated as an optimization problem constrained by the CEM.The method seeks the optimal solution of the domain’s electrical conductivity to minimize a Lagrangian functional consisting of a least-squares objective functional and a regularization term.Enforcing the stationarity of the Lagrangian leads to state,adjoint,and control problems,which constitute the Karush-Kuhn-Tucker(KKT)first-order optimality conditions.Subsequently,the electrical conductivity profile of the domain is iteratively updated by solving the KKT conditions in the reduced space of the control variable.Numerical results show that the relative error of the measured and calculated electric potentials after the inversion is less than 1%,demonstrating the successful reconstruction of heterogeneous electrical conductivity profiles using the proposed EIT method.This method thus represents an application framework for nondestructive evaluation of structures and geotechnical site characterization.

Wavelet analysis of ultrasonic A-scan signal of solid-state welded joints

In the ultrasonic nondestructive evaluation of the quality of solid state welded joints, such as friction bonding and diffusion bonding, the main difficulty is the identification of micro defects which are most likely to emerge in the welding process. The ultrasonic echo on the screen of a commercial ultrasonic detector due to a micro defect is so weak that it is completely masked by noise, and impossible to be pointed out. In the present paper, wavelet analysis (WA) is utilized to process A scan ultrasonic signals from weak bonding defects in friction bonding joints and porosity in diffusion bonding joints. First, perception of WA for engineers is given, which demonstrates the physical mechanism of WA when applied to signal processing. From this point of view, WA can be understood easily and more thoroughly. Then the signals from welding joints are decomposed into a time scale plane by means of WA. We notice that noise and the signal echo attributed to the micro defect occupy different scales, which make it possible to enhance the signal to noise ratio of the signals by proper selection and threshold processing of the time scale components of the signals, followed by reconstruction of the processed components.

VARIATION METHOD FOR ACOUSTIC WAVE IMAGING OF TWO DIMENSIONAL TARGETS

A new way of acoustic wave imaging was investigated. By using the Green function theory a system of integral equations,which linked wave number perturbation function with wave field, was firstly deduced. By taking variation on these integral equations an inversion equation,which reflected the relation between the little variation of wave number perturbation function and that of scattering field, was further obtained. Finally, the perturbation functions of some identical targets were reconstructed, and some properties of the novel method including converging speed, inversion accuracy and the abilities to resist random noise and identify complex targets were discussed. Results of numerical simulation show that the method based on the variation principle has great theoretical and applicable value to quantitative nondestructive evaluation.

Characterization of Automotive Aluminum AA356 by Photo Thermal Radiometric Spectroscopy

Photothermal radiometric spectroscopy was employed for characterizing an aluminum automotive piston with a steel insert. Amplitude and phase thermal property differences allowed the mapping of both metals. Thermal images were generated by using table stages x-y movement. The thermal wave penetrated more deeply in aluminum than in steel. Results demonstrated that this relatively unknown, non-intrusively, non-contact, and non-destructive technique can be used for characterizing a wide variety of metals and other materials.

Characterization of surface damage of a solid plate under tensile loading using nonlinear Rayleigh waves

This letter reports experimental observation of a direct correlation between the acoustic nonlinearity parameter (NP) measured with nonlinear Rayleigh waves and the accumulation of plasticity damage in an AZ31 magnesium alloy plate specimen.Rayleigh waves are generated and detected with wedge transducers,and the NPs are measured at different stress levels.The results show that there is a significant increase in the NPs with monotonic tensile loads surpassing the material’s yielding stress.The research suggests an effective nondestructive evaluation method to track the surface damage in metals.

Magnetic Properties of Thermally Aged Fe-Cu Alloys with Pre-deformation

Magnetic properties of thermally aged Fe-Cu alloys with pre deformation have been evaluated to improve the understanding of using magnetic technology for the nondestructive evaluation （NDE） of irradiation embrittlement in reactor pressure vessel （RPV） steels. Fe-Cu alloys with and without pre-deformation were thermally aged at 500 ℃ and the changes in microstructure, mechanical properties and magnetic properties were determined. It is found that the strain-induced dislocations recover and the Cu-rich particles precipitate during the aging process, and the magnet- ic properties variation depends on the combined influence of these two factors. From the point of view of NDE, a fully tempered or annealed microstructure is favorable before RPV is put into service. These results improve the un- derstanding of magnetic property evolution in actual RPV steels and help to develop NDE theory for irradiation embrittlement.

Experimental and Numerical Study on Infrared Thermal Wave Imaging

The fast development of thermal wave imaging has led to the emergence of a novel technique of nondestructive evaluation (NDE) for thermophysical properties and subsurface defects of solids recently. This paper presents the experimental reserch on infrared thermal waving which can be used to measure thermal diffusivity and local heat transfer coefficient of thin materials. Numerical modeling of subsurface defect detecting is also performed for pulse echo thermal wave imaging.

Acoustic Nonlinearity Parameters Due to Microstructural Defects

This study presents a general approach to derive the acoustic nonlinearity parameters induced by various types of dislocation configurations including dislocation strings （monopoles）, dislocation dipoles, dislocation pileups and extended dislocations. It is found that expressions of the acoustic nonlinearity parameter induced by such a variety of dislocation con- figurations share a common mathematical form. They are all scaled with （Lch/b）n, where Lch is a characteristic length of the dislocation configuration, b is the magnitude of the Burgers vector, and n is either 3 or 4. Semiquantitative analysis is presented to compare the magnitudes of the acoustic nonlinearity parameters among different types of dislocation configurations.

Microstructural designs of plate-type elastic metamaterial and their potential applications: a review

Elastic metamaterials are of growing interest due to their unique effective properties and wave manipulation abilities.Unlike phononic crystals based on the Bragg scattering mechanism,elastic metamaterials(EMMs)are based on the locally resonant(LR)mechanism and can fully control elastic waves at a subwavelength scale.Microstructural designs of EMMs in plate-like structures have attracted a great deal of attention.In this paper,the recent advances in the microstructural designs of LRbased EMM plates are reviewed.Their potential applications in the fields of low frequency guided wave attenuation,wave manipulation and energy trapping at a subwavelength scale,and structural health monitoring are discussed.

Mixed Frequency Ultrasound Phased Array

A mixed frequency ultrasonic phased array (MPA) was developed to improve the focus, in which the element excitation frequencies are not all the same as in a normal constant frequency phased array. A theoretical model of the mixed frequency phased array based on the interference principle was used to simulate the array’s sound distribution. The pressure intensity in the array focal area was enhanced and the scanning area having effective contrast resolution was enlarged. The system is especially useful for high in- tensity focused ultrasound (HIFU) with more powerful energy and ultrasound imaging diagnostics with im- proved signal to noise ratios, improved beam forming and more uniform imaging quality.