Modeling elastic wave propagation in waveguides with the finite element method

This paper reports on the application of guided waves techniques to nondestructively determine the structural integrity of engineering components. Specifically, this research uses a commercial finite element (FE) code to study the propagation characteristics of ultrasonic waves in annular structures. In order to demonstrate the accuracy of the proposed FE technique, the propagation of guided waves in a flat plate is examined first. Next, the propagation of guided waves in thick ring structures is investigated. Finally, these FE results are compared to analytical and experimental results. The results of this study clearly illustrate the effectiveness of using the FE method to model guided wave propagation problems and demonstrate the potential of the FE method for problems when an analytical solution is not possible because of “complicated” component geometry.     

Accurate finite element modelling of guided wave scattering from irregular defects

Modelling the scattering of guided waves by defects in three dimensions (3D) can be challenging. The most popular way to achieve this is the finite element (FE) method, at the cost of high computational load, which generally leads to a compromise between the accuracy of the results and the computational time, even when the geometry of the scatterer is simple. In this paper, we describe a procedure aimed at calculating the scattering matrix of an irregular defect in the most efficient way. The use of a frequency domain hybrid model which combines the flexibility of FE modelling and the efficiency of an integral representation of the acoustic fields allows very accurate results to be obtained with low computational load. The modelling procedure that we propose includes optimization of the size of the absorbing region and that of the mesh elements, minimization of number of incident directions, and the study of a spatial filter to smooth the geometry of the defect prior to meshing. Finally, the scattering matrix of a representative example of an irregular corrosion patch is calculated using the optimized procedure. Energy balance criteria are implemented to check the accuracy of the results.     

Analysis of plate wave propagation in anisotropic laminates using a wavelet transform

A new approach is presented for the analysis of transient waves propagating in anisotropic composite laminates. The wavelet transform (WT) using the Gabor wavelet is applied to the time-frequency analysis of dispersive flexural waves in these plates. It can be shown that the peaks of the magnitude of WT in a time-frequency domain is related to the arrival times of the group velocity. Experiments were performed using a lead break as the simulated acoustic emission source on the surface of unidirectional and quasi-isotropic laminates. A method was developed to obtain the group velocity of the flexural mode as a function of frequency. Theoretical predictions were made using the Mindlin plate theory, which includes the effects of shear deformation and rotatory inertia. Our predictions on the dispersion of the flexural mode showed good agreement with the experimental results.     

An NDT guided wave technique for the identification of corrosion defects at support locations

Due to the large number of pipe supports over a piping run, a rapid reliable NDT system is needed to identify hidden corrosion defects at a pipe-support interface. To accomplish this, a system using Periodic Permanent Magnet (PPM) Electromagnetic Acoustic Transducers (EMAT׳s) to generate dispersive SH1 guided waves is implemented. For this study, both the effect of the support contact and a corrosion type defect are evaluated independently through finite element models and experiments utilizing a flat plate approximation. It was found that utilizing the SH1 plate wave near the inflection point or ‘knee’ of the dispersion curve yields a high sensitivity to gradual wall loss defects while experiencing a minimal effect from the support contact.     

A guided wave approach to defect detection under shelling in rail

Rail defects are responsible for many railroad accidents. Trains are derailed and serious consequences often occur. Traditional bulk wave testing wheel probes are often inadequate for finding all defects in a rail, especially under shelling. Shelling takes place as a result of wheel to rail Hertzian contact stresses that lead to surface and subsurface defects, as a result of high stresses below the surface of the rail. Guided waves can detect shelling, if so desired, by employing the proper mode and frequency. Guided waves can also detect transverse cracking under shelling by selecting a mode and frequency insensitive to the shelling, but sensitive to transverse cracking under the shelling. Special modes and frequencies can also be used to detect defects in the web or base of a rail. The guided wave methodology of using a hybrid analytical-FEM technique to accomplish the sensor design and subsequent scattering analysis is presented in this paper. Sensor design to generate rail boundary conditions via dispersion curve and wave structure analysis is illustrated along with a few static shots of animations of wave propagation and reflection from defects in a rail.     

Detectability of corrosion damage with circumferential guided waves in reflection and transmission

There is an increasing interest in high frequency short range guided waves to screen or monitor for corrosion. This contrasts with long range guided waves (LRGWs) which screen pipes for large patches of corrosion and have been successfully used in corrosion management for the past twenty years. The fundamental setup described in this paper uses circumferential guided waves, which are excited at a single location on a pipe and travel around the pipe wall and are detected at the same location. The study uses a finite element model assisted method to evaluate the detection capability of two short range circumferential guided wave setups which use both the reflected and transmitted signals. The setups themselves consist of either an axial array of transducers, for monitoring, or a single transducer which axially scans a pipe. Both setups have an array or scan pitch between either adjacent transducers or measurements. The detection capability of the fundamental Lamb wave modes (A0 and S0) in both reflection and transmission have been compared, as well as a hybrid shear horizontal wave setup, which uses the SH0 mode in reflection and the SH1 mode in transmission. A sensitivity analysis was conducted using two separate methods to determine the probability of detection (POD) for either the reflection or transmission signals. Both methods determine a POD for a specific defect, noise level, and array or scan pitch. Probability images are produced which map the POD for a range of defect sizes. For the parameters investigated in this study, it was found that in transmission large diameter defects have a higher detectability, whereas deep, narrow diameter defects are more detectable in reflection. A generalised overview of the sensitivity of short range guided waves is presented by combining both the reflection and transmission PODs. The data fused sensitivity of the S0 and SH hybrid modes are given as 0.6% and 0.75% cross sectional area (CSA) respectively, allowing for the comparison with LRGWs. The A0 mode was excluded from the POD analysis because it was much less sensitive than the other two modes.     

Lamb wave propagation in composite laminates and its relationship with acousto-ultrasonics

The acousto-ultrasonic technique has many potential applications in the NDE of composite materials. However, problems of poor reproducibility and of the sensitivity of the results to precise instrument settings have restricted its application in industry The waves employed in acousto-ultrasonics are chiefly Lamb waves which propagate in the plane of the laminate. In this paper, Lamb wave propagation in composite laminates both with and without defects is investigated both numerically and experimentally. Acousto-ultrasonic parameters based on both predicted and measured responses are calculated, and tests are carried out using a commercially available instrument. The factors which lead to the poor reproducibility of acousto-ultrasonic results are discussed and possible improvements to the technique are proposed.     

Quantitative assessment of through-thickness crack size based on Lamb wave scattering in aluminium plates

The interaction of Lamb wave modes at varying frequencies with a through-thickness crack of different lengths in aluminium plates was analysed in terms of finite element method and experimental study. For oblique-wave incidence, both numerical and experimental results showed that the wave scattering from a crack leads to complicated transmission, reflection and diffraction accompanied by possible wave-mode conversion. A dual-PZT actuation scheme was therefore applied to generate the fundamental symmetrical mode (S₀) with enhanced energy to facilitate the identification of crack-scattered wave components. The relationship between crack length and the reflection/transmission coefficient obtained with the aid of the Hilbert transform was established, through which the crack length was quantitatively evaluated. The effects of wavelength of Lamb waves and wave diffraction on the properties of the reflection and transmission coefficients were analysed.     

Circumferential higher order guided wave modes for the detection and sizing of cracks and pinholes in pipe support regions

The non-dispersive propagation of ultrasonic guided wave higher order modes cluster (HOMC) traveling along the circumferential direction in a hollow cylinder and its interaction with defects in pipe support regions is reported. These circumferential guided waves were generated in mild steel (MS) pipe specimens containing artificially created axial notches (simulating axial cracks) and pinholes (simulating pinhole-like defects) of different sizes in order to simulate conditions such as cracking and corrosion under pipe supports. The characteristics of these guided waves were also studied as a function of parameters related to how they were generated; namely, using: (a) 2.25 MHz linear phased array transducer, (b) 2.25 MHz conventional circular transducer and (c) 1 MHz conventional circular transducer. These higher frequency modes were explored for their ability to detect and size defects. Because of access limitations to the pipe support region in actual field testing, the transducer was always placed at a fixed circumferential position and moved axially along the length of the pipe. The defect position along the circumference was ascertained from the time of flight while the defect size was estimated using the amplitude data. The signals obtained for all three transducer configurations are compared for their ability to locate, detect and size the above-mentioned defects. It was shown that at these relatively higher frequencies, the guided wave modes exhibit small dispersion and have the ability to provide improved imaging of small size defects throughout the cross-section of the pipe.     

Ploughing detection in micromilling processes using the cutting force signal

A new method is presented for detecting the onset of ploughing during micromilling on the basis of the cutting force signal. The method utilises the variation in the peak values of the cutting force. Multiresolution analysis with the undecimated wavelet transform is used to detect the peak values of the cutting force signal. A ploughing-detection algorithm is developed, which compares the normalised variance in peak values with preset thresholds. The proposed method is verified by comparing the surface roughness of the sidewall under ploughing and normal cutting conditions in sidewall-machining experiments with a micromilling tool.     

Detection of a straight groove in a metal plate by acoustic scattering in water with applications to marine current turbines

The developed method is based on acoustic underwater scattering measurements and can be applied to characterize a defect in a blade of a marine current turbine. To simplify the study, the blade is replaced by a rectangular plate immersed in water having a groove opening out. The measurements are made on horizontal plane perpendicular to the long axis of the stainless steel plate. The frequency range of signal is between 50 and 400 kHz. The transducer is remote from the plate. The measurements are recorded on 360° with a 1° step. The experimental trajectories of the scattering signals are compared to the trajectories calculated from the group velocities of Lamb waves A and S₀. Spectra give also an information on groove position. The present study is the first step toward remote monitoring of blades on a marine current turbine.     

Experimental and simulation methods to study the Magnetic Tomography Method (MTM) for pipe defect detection

The Magnetic Tomography Method (MTM), a passive Non-Destructive Testing (NDT) technique based on the magneto-mechanical effect, has been claimed to be able to detect defects at large (>1 m) stand-off distances. In this study, the MTM signal was studied experimentally on a 4140-L80 pipe sample, in which a flat bottom defect was electrochemically generated. The Residual Magnetic Leakage Field (RMLF) signal was recorded using an annular array of AMR sensors. The experimental results show perturbations due to the defect (about 12 dB above noise level) only when the AMR sensors were positioned at a very low stand-off. The presence of ferromagnetic objects near the sensors could cause perturbations many times larger than that from a defect (about 18 dB above the noise level). A Finite Element (FE) model validated the experimental results. The model showed that there is a significant risk of false indications due to foreign ferromagnetic objects when measuring at a large distance from the pipe.     

小波变换在管道弱超声回波奇异性检测中的应用

研究了小波变换在细尺度下检测信号奇异性的原理。通过对样管中标准缺陷的超声回波分析试验,证明6层db5小波变换可以有效地分离出湮没在噪声中的微弱缺陷信号,并可以有效地分离出信号的孤立奇异点,准确定位缺陷出现时刻。     

小波分析在固体导弹发动机壳体多层界面缺陷检测中的应用

针对固体导弹发动机壳体多层界面胶结结构的超声检测信号,应用小波变换模极大值提取特征参数的方法,用奇异性指数(Lip指数)表征脱粘缺陷信号的特征,进行发动机壳体层间缺陷的识别。试验结果表明,该方法能有效实现固体导弹发动机壳体多层界面胶结结构缺陷的分析与识别,并对其它多层胶接结构质量的缺陷定位有一定的应用价值。     

钢板孔洞涡流检测的小波分析

小波的时频局部化和多尺度特性，使其特别适用于信号的奇异性和瞬态信号的检测以及强周期性噪声干扰下非平稳信息的提取。小波分析用于钢板孔洞测量的数据处理过程中，通过以最大信噪比为依据的自定义代价函数来获取小波包最优基函数，使重构信号的信噪比大幅度提高，从而确保了钢板孔洞的良好测量与分辨。     

Wave propagation in water-immersed adhesive structure with the substrates of finite thickness

In previous theoretical studies on the adhesive structure, the substrate is mostly considered as a semi-infinite solid space and corresponding theoretical derivation is rarely related to the thickness of the substrate. In the paper, based on the transfer matrix method, we studied in the water-immersed trilaminar plate-like adhesive structure with the substrates of finite thickness and the perfect/sliding interfaces in the case of plane longitudinal wave incidence and deduced the expressions of reflection and transmission coefficients of longitudinal waves. In order to verify the accuracy and applicability of the deduced formula, this formula was firstly applied to the water-immersed steel–epoxy resin–steel adhesive structure with perfect interfaces and the calculated result was compared with the existing data. Taking the water-immersed aluminum–epoxy resin–aluminum adhesive structure with perfect/sliding interfaces as an example, the impacts of incident angle and frequency on the reflection and transmission characteristics of the longitudinal wave were then analyzed. Finally, the theoretical method was experimentally verified and the experimental results were well consistent with the numerical calculation results. The reflection and transmission coefficient curves of longitudinal wave showed the obvious resonance when the longitudinal wave was normally incident. Regardless of the substrate thickness, the frequency of acoustic waves or the incident angle, it can only confirm whether a sliding interface exists, but the interface to slide cannot be determined.     

Multi-directional adjacent wave subtraction and shifted time point mapping algorithms and their application to defect visualization in a space tank liner

We propose the multi-directional adjacent wave subtraction (MAWS), shifted time point (STP) mapping and variable time window amplitude (VTWA) mapping algorithms for crack visualization based on our finding that the processing direction of the adjacent wave subtraction with respect to crack orientation can dramatically affect crack visibility. The proposed MAWS, STP and VTWA mapping algorithms were implemented in a laser ultrasonic propagation imaging system and were applied in a nondestructive evaluation of an Al-alloy tank liner for space applications. A crack and void on the fusion zone of a weld were more effectively visualized using the proposed algorithms, and the crack could be visualized regardless of its orientation. The multi-directional processing results of the anomalous wave propagation imaging were visualized as video clips. The STP mapping algorithm, which maps the number of shifted time points to minimize the difference between adjacent waves, was developed to enhance the crack visualization capability of the system. Additionally, the multi-directional VTWA mapping algorithm performed data projection using the residuals of MAWS processing and provided stationary mapping results of crack-induced anomalous waves. The presented results demonstrate that the proposed visualization algorithms are promising techniques that do not overlook unknown crack orientations.     

Validation of a modified material model for use with shell elements to improve the predictive accuracy of the thickness distribution in superplastic forming of sheet metals

The automotive industry has recently begun using the superplastic forming (SPF) process to fabricate complex aluminum and magnesium alloy panels that cannot be formed at room temperature due to insufficient formability. One of the manufacturing problems encountered during SPF is excessive thinning in the form of a localized neck; which can lead to fracture. Localized necking can be predicted with the use of continuum elements in finite element analysis (FEA); however, the use of these elements in simulating SPF of large automotive panels is computationally intensive and often computationally prohibitive due to convergence issues. This paper examines the use of a modified material model (developed by engineers at Livermore Software Technology Corporation (LSTC) that can be used with conventional Belytschko-Tsay shell elements. This model considers normal stresses during SPF, which is needed to predict necking locations. The paper reports the results on investigating means for improving computational efficiency with this new formulation (i.e. element size, mass scaling, and adaptive meshing) and compares the performance of the normal stress element formulation with that of Belytschko-Tsay shell element in simulating the SPF process. The findings indicate that the newly developed formulation can be used for predicting localized thinning under SPF conditions.     

Stabilization of a surface plasmon resonance (SPR) optical fibre sensor with an ultra-thin organic film: application to the detection of chloro-fluoro-carbon (CFC)

Surface plasmon resonance (SPR) is a powerful technique for direct sensitive (bio) chemical detection. This phenomenon can be used to measure the refractive index of either bulk chemical samples or chemically sensing thin layers. In this work, a SPR fibre optic sensor has been developed. A 50 nm thick silver film is deposited by thermal evaporation onto the silica core of the optical fibre. To protect silver from oxidation, the evaporated silver film was covered with self-assembled monolayers (SAMs) of long-chain alkanethiols (1-octadecanethiol). To characterize these SAMs, silver films evaporated onto macroscopic glass surfaces as test samples and several techniques such as contact angle measurements (sessile drop method), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) were used. In the subsequent step, a chemically sensing thin layer (polyfluorosiloxane) was deposited onto the thiol surface. In such a configuration the SPR fibre optic sensor was able to detect a few percent of chloro-fluoro-carbon (CFC) vapours.