Feature-guided Waves Research Articles

Damage assessment in L-shaped bends by nonlinear feature guided waves

L-shaped bends are crucial lightweight structural components in aerospace manufacturing, renowned for their high specific strength and stiffness. However, the geometric configuration of these bends makes them susceptible to stress concentration in the bending zone. Effective technique for damage assessment of the bending region of L-shaped bends is highly demanded. This paper proposes a nondestructive testing method using nonlinear ultrasonic feature guided waves (FGWs) for early damage assessment of L-shaped bends. Semi-analytical finite element (SAFE) method and perfectly matched layer (PML) technique are employed to analyze FGW propagation in L-shaped bends. Three-dimensional finite element models are utilized to investigate the FGWs propagation in the bending region of L-shaped bends in a numerical perspective. Cumulative second harmonic generation (SHG) of FGWs is successfully observed by selecting suitable mode pairs based on synchronism and non-zero power flux conditions, which provides a promising alternative for early damage assessment in L-shaped bends. Experimental findings are consistent with the numerical ones. Subsequently, thermal damage and plastic deformation with varying gradients were introduced into the bending region of L-shaped bends. The use of SHG of selected FGW mode has effectively assessed the damage with an improved sensitivity, compared to conventional linear acoustic one. This investigation substantiates the potential of nonlinear FGWs in detecting early damage in L-shaped bends, offering promise for future industrial testing applications.

Propagation characteristics of circumferential feature guided waves propagation in a revolved topological waveguide

In our previous research, propagation characteristics and combined harmonic generation of feature guided waves (FGWs) in welded joints of plate structure, were analyzed theoretically and observed numerically. Circumferential ultrasonic guided waves find extensive applications in assessing rotating structures, highlighting the importance of investigating the propagation characteristics of circumferential feature guided waves (CFGWs) in such revolved topological waveguides. In this study, propagation characteristics of CFGW in revolved topological waveguides are investigated. Semi-analytical finite element (SAFE) method in conjunction with a perfectly matched layer (PML) within a cylindrical coordinate system is used to explore the modal and dispersion characteristics of CFGWs. Second-harmonic generation (SHG) of CFGWs is further analyzed and discussed both in theoretical and numerical manner. Cumulative SHG of selected CFGW mode pairs, propagation in the welded joint of the revolved topological waveguide, is observed successfully. The obtained results demonstrate that the energy trapping effect of CFGWs in the welded joint of revolved topological waveguide amplifies the generation of cumulative second harmonics, thereby enhancing the measurability of SHG of CFGW. This investigation reveals the potential of damage assessment in revolved topological waveguides by CFGWs.

Quantitative assessment of corrosion-induced wall thinning in L-shaped bends using ultrasonic feature guided waves

The utilization of L-shaped bends has become widespread due to their versatility and ease of application in various industrial fields. Nevertheless, the geometric configuration of these bends makes them susceptible to stress concentration in the bending zone, which can lead to wall thinning caused by accelerated corrosion. This poses a profound challenge to the bearing capacity and reliability of L-shaped bends and highlights the urgent necessity for an effective technique to assess corrosion-induced wall thinning in the bending zone. In this study, we propose a nondestructive testing (NDT) method based on ultrasonic feature guided waves (FGWs) for the evaluation of local corrosion-induced wall thinning in L-shaped bends. Propagation characteristics of FGWs in L-shaped bends were analyzed using the semi-analytical finite element (SAFE) method and perfectly matched layer (PML) technique. Time-reversal (TR) method was employed to address the dispersion and multi-modes phenomenon of the selected guided wave modes and improve the signal-to-noise ratio (SNR) of measured FGW signals. Enhanced SNR of two types of FGW modes signals at different frequencies were verified by experimental and numerical results. Obtained results reveal that FGWs can effectively detect corrosion-induced wall thinning inside L-shaped bends, with improving the signal reflection coefficient by TR method.

Investigation of energy trapping effect for nonlinear guided waves in a topographical structure

Cumulative second harmonic generation (SHG), where the amplitude of the generated second harmonic increases with propagation distance, is commonly utilized to enhance the measurability of nonlinear guided waves by providing a sufficient signal-to-noise ratio. Investigating and confirming the cumulative SHG of feature guided waves (FGWs) in topographical waveguides is crucial for accurately assessing material nonlinearity in such structures. This study examines the cumulative SHG of FGWs propagating in a typical topographic structure of a welded joint through both numerical simulations and experimental investigations. A theoretical model is developed using the semi-analytical finite-element (SAFE) method and perfectly matched layer (PML) to analyze the propagation characteristics of FGWs in a welded joint. The numerical and experimental results provide verification of the “trapping effect” of FGWs within the welded joint. Furthermore, the “cumulative effect” of SHG of FGWs is observed. Remarkably, both the experimental and numerical findings demonstrate that cumulative SHG of FGWs can be achieved by employing suitable mode selection and frequency tuning in the topographic structure, which aligns well with the theoretical predictions. This investigation contributes to advancing our understanding of cumulative SHG in FGWs propagated within topographical structures. These findings have practical implications for accurately characterizing material nonlinearity in topographic waveguides.

Validation of zero-group-velocity feature guided waves in a welded joint

Extensive research has been conducted on zero-group-velocity (ZGV) Lamb waves in elastic plates, demonstrating significant progress in the field of nondestructive testing. However, there is a scarcity of studies focusing on ZGV modes in complex structures. In this paper, we present our research investigating the presence of ZGV feature guided waves (FGWs) in a welded joint. Our approach follows a similar methodology used to study ZGV Lamb waves in elastic plates. By employing two-dimensional (2D) finite element (FE) modeling, we analyze the response spectra of the welded joint when subjected to a force source, revealing the occurrence of resonance in the response spectra. To investigate resonance modes in the welded joint, we employ the three-dimensional (3D) time-step FE method. By applying spatial 2D and short-time Fourier transforms to the received time-domain signals, we analyze the frequency content and spatial distribution of the signals. This analysis allows us to verify the existence of non-propagation and propagation modes in the welded joint. The non-propagation mode refers to the presence of signals with a zero wavenumber, indicating that they do not propagate or travel along the welded joint. These signals are typically associated with local resonances or vibrations within the welded joint itself. On the other hand, the propagation mode corresponds to signals with nonzero wavenumbers, suggesting that they propagate or travel along the welded joint. Furthermore, by further analyzing the propagation mode in the welded joint, similar to the analysis of ZGV modes in solid plates, we have observed that it also exhibits ZGV characteristics based on the wavenumber-frequency spectra. To further analyze acoustic field distributions at resonance frequencies, we utilize the semi-analytical finite element method in conjunction with the perfectly matched layer method. The results obtained from this analysis are consistent with those obtained from the 2D FE method and 3D time-step FE method, thereby confirming that propagation modes with ZGV characteristics at resonance frequencies correspond to FGWs, which we refer to as ZGV-FGWs. Through this step-by-step analysis, we ultimately establish the existence of ZGV-FGWs in the welded joint. This study introduces fresh ideas and serves as a point of reference for future research on ZGV-FGWs in complex structures.

A study on the wave propagation on weld joints by the use of feature-guided wave mixing

The concentration of the wave energy around certain features such as bends, stiffeners, and welds in plate-like structures has been identified as feature-guided waves (FGW). In practice, these features can be useful for assessing defects in or near them. The propagation characteristics of FGW propagating along a feature such as a welded joint have been extensively studied. However, most of them are based on the linear acoustic response of materials that generally results in the amplitude and phase variations of the input signal. Compared to linear methods, nonlinear ultrasonic techniques have shown to be effective in characterizing the microstructural changes in engineering materials. In this paper, a wave mixing method is employed to investigate the nonlinear acoustic response of FGWs propagating in a welded joint. The dispersion characteristics of weldguided modes are first revealed via the eigenfrequency analysis of an unbounded welded plate by using the finite element method. The nonlinear response of FWGs is studied through 3D finite element (FE) simulations. A pair of a weld-guided modes of different frequencies are identified and allowed to mix within the welded joint to generate second-order harmonic waves. The current study reveals that the mixing of FGWs could be effectively used for the evaluation of material nonlinearity in welded joints.

Combined harmonic generation of feature guided waves mixing in a welded joint

In our previous research, the second harmonic generation (SHG) of feature guided waves (FGWs) in a welded joint with material nonlinearity was analyzed theoretically and observed numerically. However, some drawbacks including strict generation criteria, inability of distinguishing nonlinearity, and being unable to localize local defects of SHG-based nonlinear ultrasonic technique, limit the practical testing and application. Wave mixing is a promising alternative to overcome these shortcomings. In this paper, we investigated another acoustic nonlinear response of FGWs, so-called combined harmonic generation (CHG) caused by FGWs mixing in a welded joint. Theoretical analysis of second-order combined harmonic waves induced by two FGWs mixing in a welded joint was firstly implemented based on semi-analytical finite element (SAFE) and perfectly matched layer (PML) methods. Based on the theoretical analysis, four FGW mode triplets, which satisfy the internal resonant conditions, were selected for the observation of CHG caused by FGW mixing in a welded joint from a numerical perspective. CHG induced by FGW mixing with the cumulative growth effect was successfully observed within the mixing zone, which is consistent with the theoretical prediction. This study provides a straightforward analysis not previously available of the process of CHG of FGWs propagating and mixing in a welded joint.

Inspection of nuclear assets with limited access using Feature Guided Waves

Resistance Seam Welding (RSW) is frequently used in many industries where low-cost mass manufacturing of gas/fluid-tight seals are required. One particular use case is in the fabrication of canisters to store nuclear material. Given the expected long lifetime and high replacement cost associated with nuclear infrastructure, it is common for an increase in the design life to be sought during operation. This leads to a need to integrate Non-Destructive Testing (NDT) techniques in a complex environment which frequently has accessibility issues. For the inspection of RSW nuclear canisters, one promising solution is to use Feature Guided Waves (FGWs) to rapidly screen the whole circumferential RSW joint from only partial circumferential access to ensure safe storage and targeted repackaging. FGWs can travel long distances while locally confining their energy to a topological feature such as a weld or stiffener. This paper explores such use of FGWs through both theoretical and practical experimental methods. To understand which FGW modes exist in the RSW joint, the Semi-Analytical Finite Element (SAFE) method was used, revealing four suitable wave modes. An anti-symmetric flexural RSW guided mode was down-selected due to its ease of excitation, intense energy concentration around the RSW, low dispersion and attenuation. Three-dimensional Finite Element (FE) simulations were conducted to explore the sensitivity of the flexural FGW to transverse cracks of differing dimensions. Experimental results on simplified and realistic geometries to transverse crack defects of ≥1 mm deep show the efficacy of using this method to efficiently screen nuclear canister RSW joints for such defects in-situ.

Theoretical and numerical investigations of the nonlinear acoustic response of feature guided waves in a welded joint

The propagation characteristics of feature guided waves (FGWs) in topographical waveguides, such as welded joints, have been widely investigated. However, most of these investigations analyzed linear acoustic characteristics. Considering the high sensitivity of nonlinear ultrasonic techniques in the nondestructive evaluation of microscopic defects in materials, such as identifying micro-defects at an early stage, it is particularly relevant to investigate the nonlinear acoustic response of FGWs in topographical waveguides. In this paper, we use a theoretical analysis and numerical simulation to investigate the second-harmonic generation (SHG) of FGWs in a welded joint. The theoretical framework is firstly expounded based on the reciprocity relation and modal expansion with the nonlinear semi-analytical finite element (SAFE) method and perfectly matched layers. Numerical models are implemented to predict the effect of SHG of FGWs in a welded joint. Two families of FGW propagation in a welded joint are analyzed. The results obtained demonstrate that mode conversion and the cumulative effect of the generated second harmonics can be observed in the welded joint. This investigation provides new physical insights into SHG for FGW propagation.

A Guided Wave Inspection Technique for Wedge Features.

Numerous engineering components feature prismatic wedge-like structures that require nondestructive evaluation (NDE) in order to ensure functionality or safety. This article focuses on the inspection of the wedge-like seal fins of a jet engine drum, though the capabilities presented will be generic. It is proposed that antisymmetric flexural edge modes, feature-guided waves localized to the wedge tips, may be used for defect detection. Although analytical solutions exist that characterize the ultrasonic behavior of ideal wedges, in practice, real wedges will be irregular (containing, for example, truncated tips that are built onto an associated structure or have nonstraight edges), and therefore, generic methodologies are required to characterize wave behavior in nonideal wedges. This article uses a semianalytical finite-element (SAFE) methodology to characterize the guided waves in wedge-like features with irregular cross sections to assess their suitability for NDE inspection and compare them with edge modes in ideal wedges. The science and methodologies required in this article are necessary to select an appropriate operating frequency for the particular application at hand. In addition, this article addresses the practical challenge of excitation and detection of flexural edge modes by presenting a piezoelectric-based dry-coupled transducer system suitable for pulse-echo operation. This article, therefore, presents the scientific basis required for industrial exploitation, together with the practical tools that facilitate use. The study concludes with the experimental demonstration of the edge wave-based inspection of a seal fin, achieving a signal-to-noise ratio of 28 dB from a 0.75-mm radial tip defect.

Fiber bragg grating based detection of part-thickness cracks in bent composite laminates using feature-guided waves

Composite structures with bends are widely used in aerospace and industrial sectors. However health monitoring of such structures is challenging due to their complex topographical features. Recent literature shows that bends in composite laminates can confine and guide ultrasonic energy along their length, known as feature-guided waves (FGW). This article demonstrates a fiber Bragg grating based technique using FGW modes for defect detection and identification in bent composite laminates. In addition, the effects of defect depth and excitation frequency on the FGW mode reflection coefficient are reported using 3D finite element simulations. Physical insight into the reflection behavior is discussed based on an analysis of mode interaction with part-thickness cracks.

Detection of damage in welded joints using high order feature guided ultrasonic waves

Abstract Topographical features such as welds, stiffeners, and bends in plate-like structures have been identified as special local waveguides, which are able to render ultrasonic wave energy preferentially guided along themselves. Such feature guided waves (FGW) at low frequencies have been reported in the literature as an effective screening tool for long-range features, which, however, can only detect relatively large defects. In this study, the existence of high order FGW modes, categorized into multiple wave families, are revealed via the modal analysis of an unbounded welded plate at high frequencies, by using the semi-analytical finite element (SAFE) approach. These FGWs exhibit strong mode confinement in specific regions of the weld, which can potentially offer enhanced interrogation of the localized area in long welds and provide improved detection sensitivity to small defects. A series of high order shear horizontal (SH) type weld-guided modes are investigated in both three-dimensional finite element (3D FE) simulations and experiments, for detecting axial and transverse cracks of sub-wavelength size in the weld bead. A new superposition technique is proposed to coherently enhance the amplitude of the selected mode in a multi-modal waveform, leading to an attractive single-mode inspection capability at high frequencies. Both small cracks are identified numerically and experimentally in the superposed reflection of the SH 1 -like weld-guided wave, and results are in close agreement. The feasibility of using high order weld-guided modes for inspecting small defects inside the weld has been demonstrated.

Feature guided wave inspection of bond line defects between a stiffener and a composite plate

Adhesive bonding is widely used in aerospace composite structures. A continuous well-cured bond can offer good joint strength and improved fatigue and impact resistance, and is therefore crucial to the performance of the entire structure. This paper explores the feasibility of using feature guided waves (FGW) for rapid screening of the bond line between a stiffener and a carbon fiber reinforced polymer (CFRP) composite panel. Such FGWs are capable of focusing the wave energy along the stiffener and the bond layer, with limited radiation to the adjacent plate. The Semi-Analytical Finite Element (SAFE) approach is employed to understand the modal properties of FGWs that exist in the structure, and criteria are suggested to choose proper mode-frequency combination that is sensitive to adhesive defects. A shear horizontal type FGW mode is identified to be well suited, as it is easy to excite, and propagates with little dispersion and relatively low attenuation, while it retains sufficient energy around the bond layer. Both 3D Finite Element (FE) simulations and experiments are performed to study the interaction of the selected FGW mode with defects in the adhesive bond, and the results show excellent agreement. The reflection behavior and the wave-defect resonance phenomenon are investigated, which demonstrate the capability of the FGW for the bond line inspection.

Damage detection in quasi-isotropic composite bends using ultrasonic feature guided waves

Complex-shaped composite components have been extensively incorporated as reinforcing structures in the aerospace industry. Various types of damages can be initiated in these structures due to the stress concentration and out-of-plane impacts during the in-service use, which have to be detected timely in case they propagate at subsurface laminae and ultimately lead to catastrophic failure. This paper explores the feasibility of using ultrasonic feature guided waves (FGW) for rapid screening of typical 90° bends made of quasi-isotropic composite laminates. Such FGWs are capable of focusing the propagation energy along the feature, with limited leakage to the adjacent plate. Modal studies of the composite bent plate are carried out by applying the Semi-Analytical Finite Element (SAFE) method, revealing properties of the FGWs that exist in the structure. A shear horizontal type bend-guided mode has been identified as a promising candidate. The mode is almost non-dispersive and non-leaky with strong energy confinement in the bend region, which is attractive to be applied as a screening tool for composite bends. Both 3D Finite Element (FE) simulations and experiments are performed to study the interaction of the identified FGW mode with different defects occurred in the bend region such as the interlaminar delamination and the transverse crack, showing good agreement. The wave-defect resonance phenomenon and the reflection behavior are investigated for localizing these two types of defects, and the potential of the FGW for efficient damage detection in composite bends is well demonstrated.

Feature-guided wave-based health monitoring of bent plates using fiber Bragg gratings

Harnessing of ultrasonic guided waves confined in local features such as bends and welds, known as feature-guided waves, has emerged as a promising technique for non-destructive testing and structural health monitoring of industrial and aerospace structures. This article introduces a fiber Bragg grating based technique which uses feature-guided waves to detect anomalies or defects in plate structures with transverse bends. We are able to obtain good consistency between simulation and experimental results, both in the case of defect-free bent plates and those with transverse defects. Such results establish fiber Bragg gratings as a viable alternative to conventional techniques for structural health monitoring of bent plates.

Shear horizontal feature guided ultrasonic waves in plate structures with 90° transverse bends

Antisymmetric and symmetric Lamb-type feature guided waves (FGW) have recently been shown to exist in small angle plate bends. This paper reports Semi-Analytical Finite Element (SAFE) method simulations revealing the existence of a new family of Shear Horizontal (SHB) type of FGW mode in 90° bends in plate structures. Mode shapes and velocity dispersion curves are extracted, demonstrating the SH-like nature of a bend-confined mode identified in studies of power flow across the bend. The SHB mode is shown to have reduced attenuation in the higher frequency range, making it an ideal choice for high-resolution inspection of such bends. Further modal studies examine the physical basis for mode confinement, and argue that this is strongly related to FGW phenomena reported earlier, and also linked to the curvature at the bend region. Wedge acoustic waves discussed widely in literature are shown as arising from surface-limiting of the SHB mode at higher frequencies. The results are validated by experiments and supported by 3D Finite Element (FE) simulations.

Feature-guided waves for monitoring adhesive shear modulus in bonded stiffeners

Adhesively bonded stiffeners are employed in aerospace applications to increase structural stiffness. The potential of feature-guided wave modes for the verification of adhesion and curing state in difficult-to-access regions has been investigated. The properties of guided wave modes propagating along a T-shaped stiffener bonded to an aluminium plate were calculated using the Semi-Analytical Finite Element (SAFE) method. Feature-guided modes dominated by shearing motion were identified to be well suited, with energy concentrated at the stiffener and bond line, limiting energy radiation into the plate and thus maximising inspection length. The influences of the bond line stiffness and thickness on the guided wave behaviour were investigated using SAFE and 3D Finite Element calculations, and found to be significant. Experiments were conducted to measure the properties of the guided waves during the curing of an epoxy joint attaching a stiffener to a plate. The feature-guided mode was excited using a piezo-electric shear transducer and measured using a laser interferometer. The measured phase speed changed significantly during curing. The frequency dependency was found to match well with the SAFE calculations for a variation of the shear (Coulomb) modulus of the adhesive. The potential of the feature-guided shear wave mode for bond line inspection and monitoring has been shown and the choice of guided wave mode and frequency range for good sensitivity to the bond line state discussed.