Ultrasonic Bulk Waves Research Articles

Ultrasonic diffuse bulk wave passive array imaging of internal defects in a complex structure

Ultrasonic bulk wave inspection of defects in safety–critical components with complex external geometries, such as turbine blades is challenging. While ultrasonic phased array imaging can yield high-resolution subsurface images, a commercial phased array probe can hardly be mounted on irregular external boundaries to perform in-situ imaging. In fact, a component with irregular shapes, as a highly reverberant body, is capable of generating elastic random diffuse or coda wavefields. The diffuse wavefields can be utilized to reconstruct Green’s functions between any two passive receiving points. In this paper, an ultrasonic passive array imaging method using the diffuse reverberation resulting from complex boundaries is implemented to image internal defects. The method involves the utilization of active piezoelectric actuators to excite elastic diffuse waves within the component, which are received by a laser vibrometer scanning at multiple points. A passive full matrix capture (FMC) of array signals is extracted for defect imaging using the total focusing method. The proposed method is evaluated by the numerical simulations, and the effects of centre frequency, bandwidth, and source excitation methods on the imaging performance are investigated. An experiment using a turbine blade-like structure is conducted to further evaluate the imaging method.

A procedure for the identification of effective mechanical parameters of additively manufactured elements using integrated ultrasonic bulk and guided waves

The subject of the current work was a simple but robust novel two-stage procedure for the non-destructive determination of effective elastic constants using ultrasonic wave propagation. First, ultrasonic bulk wave velocities measured on cubic samples were used to calculate most of the elements of the stiffness matrix. Secondly, the remaining elements were determined using the dispersion curves of elastic guided waves measured on plate samples. Based on the complete stiffness matrix it was possible to calculate the complete set of effective elastic constants. The algorithm was verified for AM elements produced from PLA filament satisfying the conditions of transversely isotropic and orthotropic material models. For the transversely isotropic samples, Young’s moduli (E) varied from 2.6 to 2.9GPa, shear moduli (G) equalled between 0.9 and 1.2GPa, whereas Poisson’s ratios (ν) ranged between 0.20 and 0.32. In orthotropic sample the corresponding values were: E = 1.3–2.8GPa, G = 0.6–1.2GPa, and ν = 0.07–0.59. The results of the current study have been compared with references from the literature, giving satisfactory agreement.

Investigation on local monitoring paradigms of in-situ conformally fabricated piezopolymer coating-based array transducers: Ultrasonic bulk waves and local ultrasonic resonances

Realistic structures with complex features have been intricate challenges for structural health monitoring (SHM) with permanently installed transducers. Poor conformability of conventional rigid and brittle piezoceramic wafers is a typical issue of applications on surfaces with complex geometry. Moreover, the accompanied high-stress concentration requires high-sensitivity defect detection which is difficult to achieve with large-area monitoring methods like lamb waves at tens to hundreds kHz. Previously, in-situ conformally fabricated piezopolymer coating-based array transducers (PCATs) have been developed to build large-area, lightweight, flexible, and tunable Lamb wave networks. In this study, two novel local monitoring methods were investigated with PCATs, namely ultrasonic bulk wave array inspection and local ultrasonic resonance spectroscopy. For thick structures, ultrasonic bulk waves were generated and detected by PCATs with broadband operating frequencies (1–10 MHz) and flexible array parameters. Simulation tools and imaging algorithms of array inspection in non-destructive testing (NDT) were well implemented based on the analogous directivity pattern and normal-pressure coupling mechanism. As proof of concept, PCATs were applied on example structures with flat, concave, and convex surfaces for internal defect imaging. For thin-walled and/or multilayer structures, PCATs were used to measure local ultrasonic resonances, comparable to conventional non-contact methods. With negligible influence on local mechanical properties and broadband frequency response, multiple resonance peaks from 0 to 25 MHz were identified as zero group velocity (ZGV) Lamb modes and thickness vibration modes of host structures, which can be used as damage indices for local monitoring of corrosion, delamination, stiffness degradation, etc. Through embracing advanced NDT techniques with PCATs, high-sensitivity and quantitative local monitoring could be achieved with conformal networks, offering the possibility to integrate with large-area monitoring as multi-scale SHM for complex structures.

Optical polarization perturbed by shear strains of ultrasonic bulk waves in anisotropic semiconductors: Multiphysics modeling and optoacoustic validation

Characterization of lattice properties of monocrystalline semiconductors (MS) has been rapidly advanced. Of particular interest is the use of shear strains induced by optoacoustic-bulk-waves. However, this technique has been hindered owing to the lack of quantitative correlations between optoacoustic-bulk-waves-induced shear strains and anisotropic photoelasticity of MS. Motivated by this, a multiphysics model is developed to interrogate the coupling phenomena and interaction between optical polarization and shear strains in MS. With the model, perturbation to the polarization of a monochromatic laser beam, upon interacting with optoacoustic waves in MS, is scrutinized quantitatively. Experimental results are in agreement with those from the model, both revealing the polarization perturbed by shear strains quantitatively depends on the crystal orientation and crystal-structure-related symmetry, which are jointly governed by mechanical/photoelastic/optical anisotropies of MS. The approach has paved a new way for selectively acquiring high-sensitivity shear components of optoacoustic-ultrasonic-waves for in situ, high-definition characterization of anisotropic MS.

Ultrasonic defect detection of high-density polyethylene pipe materials using FIR filtering and block-wise singular value decomposition

Condition monitoring of high-density polyethylene (HDPE) pipes used for fluid and gas transfer is important for the safety of energy conservation and the environment. Ultrasonic phased array imaging methods provide a solution to detect and assess defects in HDPE pipes. However, ultrasonic bulk waves propagating in these viscoelastic media are strongly attenuated, resulting in reduced signal amplitude. In this study, a linear-phase Finite Impulse Response (FIR) filter is used to remove unwanted frequency components from the measured ultrasonic signals to improve the signal-to-noise ratio before applying the imaging algorithm of the total focusing method (TFM). Building upon this, a block-wise singular value decomposition (SVD) technique, which can adaptively determine the singular value cutoff threshold based on each block in the whole TFM image, is used to enhance the obtained TFM image quality. The performance of the combination of FIR filtering and block-wise SVD technique is validated by experimental data of HDPE pipe materials. Results demonstrate that the proposed approach generates good images to provide the detection and characterization of side-drilled holes in HDPE pipe materials.

Quantitative detection of rail head internal hole defects based on laser ultrasonic bulk wave and optimized variational mode decomposition algorithm

This paper proposes a quantitative rail head internal defect detection method combining mode conversion and time-of-flight based on the theory of laser ultrasonic bulk wave scattering and diffraction. The interaction between laser ultrasonic bulk wave and rail internal hole defects is modeled using finite element simulation, and the modal conversion of bulk wave encountered defects is investigated. The feasibility of localization and the quantitative method are both confirmed. Simultaneously, an Optimized Variational Modal Decomposition (OP-VMD) method is proposed to solve the problem of low SNR of bulkwave signals and to facilitate multi-mode wave discrimination. This method can extract the defect diffraction Shear Wave (S) signal from complex ultrasonic signals. Finally, experiments are performed on the defects of artificial internal holes in rail heads with varying burial depths and sizes. The experimental results show that the method can not only detect but also quantify the defects of internal holes in rail heads. The location relative errors are within 3%, and the quantitative relative errors are within 7%.

Thermo-acoustoelastic effect of Rayleigh wave: Theory and experimental verification

Previous studies showed that the thermally-induced ultrasonic bulk wave velocity change could be used to measure acoustoelastic coefficients and third-order elastic constants of elastic materials. This method is naturally immune from the ambient temperature effect and has improved sensitivity and a simpler test setup than the conventional acoustoelastic test. However, Rayleigh wave is preferred for thick components or structures with only one accessible surface. In this work, the thermo-hyperelastic constitutive equation, along with acoustoelastic theory, is used to derive the expression of the thermo-acoustoelastic coefficient (TAEC) of Rayleigh wave. The numerical relationship between the TAEC of Rayleigh wave and Murnaghan constants (l, m and n) are given for common metals. The TAEC expressions for Rayleigh wave and shear wave are similar, and both are dominated by the constant m. The TAEC of Rayleigh wave was measured on an aluminum 6061 specimen using the thermal modulation experiment in a temperature range of 22 ∼35 °C. The measured TAEC shows good agreement with the theoretical calculation. Then the third-order elastic constants were calculated based on TAECs of bulk waves and Rayleigh wave.

Simulation of Layer Thickness Measurement in Thin Multi-Layered Material by Variable-Focus Laser Ultrasonic Testing

Thin multi-layered materials are widely used in key structures of many high technology industries. To ensure the quality and safety of structures, layer thickness measurement by non-destructive testing (NDT) techniques is essential. In this paper, a novel approach for the measurement of each layer's thickness in thin multi-layered material is proposed by using ring-shaped laser generated focused ultrasonic bulk waves. The proposed method uses a ring-shaped laser with a variable radius to generate shear waves with variable focus inside the structure. By analyzing the signal characteristics at the ring center when the laser radius varies from zero to maximum, the direct measurement of layer thickness can be realized, considering that only when the focal depth and the layer thickness satisfy the specific relationship, the reflected shear waves converge and form a peak at the ring center. This straightforward approach can increase the pulse-echo SNR and prevent the processing of aliasing signals, and therefore provides higher efficiency and accuracy for the layer thickness measurement. In order to investigate the feasibility of this method, finite element simulations were conducted to simulate the ring-shaped laser generated ultrasonic waves in multi-layered structure in detail. Following the principle of the proposed method, the layer thickness of a bi-layer and 3-layer structure were respectively measured using simulation data. The results confirm that the proposed method can accurately and efficiently measure the layer thickness of thin multi-layered material.

Inspection of Concrete/Steel/Concrete pipes by ultrasonic methods affected by corrosion

Concrete/Steel/concrete pipes are present in French nuclear powerplants. These pipelines are composed of three layers: an internal layer of centimeter concrete, a millimeter layer of steel and a layer of centimeter reinforced concrete. The pipelines located by the sea undergo corrosion of the steel part caused by the chlorides present in the seawater they transport. Nowadays, the diagnosis of corrosion is made with electrochemical measurements. But they provide a qualitative indicator about corrosion. In fact, the goal is to dispose of a Non-Destructive Technique capable of evaluating the extension of holes in the steel part with a minimum diameter of 1 cm and the steel thickness elsewhere with a 100 µm uncertainty. The inspection must be conducted from the outside of the pipe still working. Ultrasonic bulk waves, ultrasonic guided waves and vibration techniques are investigated. All of them are studied with a methodology from the lab to the field. In lab, specimens are manufactured and tested. The experimental results are compared to numerical results obtained with simulation tools like CIVA or Specfem2D. Then, the promising techniques are tested on a real concrete/steel/concrete pipe. For the bulk wave techniques, the major issue is to increase the frequency signal over 1 MHz to get a better resolution of the steel investigation. But at this frequency range, the multi-scattering phenomenon on concrete aggregates is prominent. First experimental results using pulse compression on lab samples are promising: holes in steel give a specific signature and compression techniques are used to get a better resolution in steel thickness after signal autocorrelation. For both guided wave and vibration techniques, simulations are used to determine modes sensitive to the steel thickness and Zero Group Velocity modes which will induce a strong characteristic frequency response. Experimental study is going on to reveal these modes on lab specimen.

Ultrasonic-based tensile force estimation for cylindrical rod at various temperature conditions

This study presents the development of a tensile force estimation technique based on the ultrasonic bulk wave for cylindrical rod in a suspension bridge anchor system. Three piezoelectric macro fiber composites (MFC) transducers are installed on the surface of a cylindrical rod specimen for the generation and sensing of ultrasonic bulk waves. Ultrasonic responses are obtained at various tensile force levels and temperature conditions, and the relationship between the time lag of the ultrasonic response and the tensile force is established. In particular, the temperature effect on the tensile force estimation is theoretically derived and compensated for field applications. The originality of this study includes (1) theoretical derivation and investigation of the relationship between an applied tensile force level and the time lag of an ultrasonic response considering the variations of temperature, density, and elastic modulus, (2) development of a tensile force estimation and temperature compensation technique without temperature measurement, and (3) application to the tensile force estimation of the rod structure commonly used in a suspension bridge anchor system. The performance of the developed tensile force estimation technique was examined based on mock-up experiments of the rod structure of a real suspension bridge anchor system.

Influence of residual stress on the free vibrations of an elastic solid; application to resonant ultrasound spectroscopy (RUS)

Resonant ultrasound spectroscopy (RUS) is a standard technique for experimentally measuring the elastic constants of anisotropic elastic materials. Traditionally, the influence of internal or residual stresses on the measurement of elastic constants is not taken into consideration. However, it is well-known that residual stress can impart a small variation in the phase velocity of ultrasonic bulk waves, which is known as the acoustoelastic effect. Thus, it is plausible that residual stress will influence the resonances of samples measured during RUS. In this presentation, the elastodynamic theory for describing free vibrations is reformulated by using a stress-dependent constitutive relationship. The model culminates in an eigenvalue problem with a stiffness matrix being dependent on the spatially varying residual stress field within the domain of the sample. The model is evaluated by considering a realistic residual stress distribution. Results show the degree at which frequencies of resonance depend on the residual stress. While accounting for residual stress is expected to improve the estimates of elastic constants, there is also potential to use RUS as a tool to assess and characterize the residual stress present in samples.

3D Modelling of the Scattering of the Fundamental Anti-Symmetric Lamb Mode (A0) Propagating within a Point-Impacted Transverse-Isotropic Composite Plate

The present paper deals with an effort to model impact damage in 3D-FE simulation. In this work, we studied the scattering behavior of an incident A0 guided wave mode propagating towards an impacted damaged zone created within a quasi-isotropic composite plate. Besides, barely visible impact damage of the desired energy was created and imaged using ultrasonic bulk waves in order to measure the size of the damage. The 3D-FE frequency domain model is then used to simulate the scattering of an incident guided wave at a frequency below an A1 cut-off with a wavelength comparable to the size of the damaged zone. The damage inside the plate is modeled as a conical-shaped geometry with decayed elastic stiffness properties. The model was first validated by comparing the directivity of the scattered fields for the A0 Lamb mode predicted numerically with the experimental measurements. The modeling of the impact zone with conical-shape geometry showed that the scattering directivity of the displacement field depends significantly on the size (depth and width) of the conical damage created during the point-impact of the composite with potential applications allowing the determination of the geometric characteristics of the impacted areas.

Evaluating the elastic properties of Al2O3–Al FGMs by longitudinal and transverse ultrasonic bulk waves velocity features

The present study aimed to investigate the elastic properties of functionally graded materials using ultrasonic bulk wave velocity features. To this end, three Al2O3–Al FGM samples were fabricated by spark plasma sintering method at three different temperatures (1050 °C, 1400 °C, and 1450 °C). In addition, two homogenous aluminum and alumina samples were fabricated to evaluate the properties of the initial materials using the initial powders. Ultrasonic pulse-echo technique was applied on fabricated samples in order to measure the longitudinal and transverse wave velocities. The elastic properties of the graded samples were obtained by wave velocity features and Archimedes density. The alumina and aluminum elastic modulus were measured as 394 and 70 GPa, respectively. The results indicated that increasing the sintering temperature from 1050 °C to 1400 °C and 1450 °C leads to considerable improvement in the average elastic modulus. The average elastic modulus of the graded bundle sintered at 1450 °C (308 GPa) was about 8.5% higher than that of the one sintered at 1400 °C (284 GPa). The microstructure analogies of samples illustrated micro-voids and phases agglomeration in all samples. The highest wave attenuation was observed in the graded sample with the lowest sintering temperature, i.e., the largest number of micro-voids caused the highest energy absorption in the graded bundle sintered at 1050 °C.

Comparison between elastic and porous elastic material models for sediment acoustical behavior in an aluminum-sediment double layer

In this paper, Lamb waves dispersion curves are extracted for a double layer aluminum-sediment (gypsum) plate, both numerically and experimentally. For this purpose, the gypsum elastic properties are approximately calculated using ultrasonic bulk waves method. Then, by using the porous elastic behavior model for the gypsum, as well as the elastic model, available in ABAQUS commercial software, the dispersion curves are calculated numerically in the frequency range of 100–200 kHz for the sedimentary plate. In order to validate the numerical dispersion curves, an experimental setup consists of a 1-mm-thick aluminum plate covered by a 1.5 mm thick sediment (gypsum) area, equipped with two piezoelectric disks has been provided. The experimental dispersion curves for two fundamental symmetric and anti-symmetric modes, S0 and A0, have been extracted, using the time of flight method, applied on filtered experimental signals. Results show that the relative error between porous elastic and experimental models is 5.26% and 3.56% while relative error between numerical elastic model and the experimental model is 5.9% and 4.3%, for S0 and A0 modes, respectively. It is concluded that for dry sediments, the elastic mechanical behavior is acceptable. However, in case of wet sediments, dispersion curves should be extracted using a porous elastic model before defect detection in sedimentary structures. The unique capability of the porous elastic model is the possibility of wet sediment consideration, for dispersion curves evaluation.

Delamination Detection in Bimetallic Composite Using Laser Ultrasonic Bulk Waves

In this study, a method based on laser ultrasonic bulk waves is used to detect delamination in a bimetallic composite. For this purpose, several artificial delamination defects were created in a copper-aluminum sample using wire-electrode cutting. The research includes numerical simulation and experimental analysis. The propagation process of laser ultrasonic in Cu/Al bimetallic compo-site, the interaction between bulk waves and composite interface, and the effect of delamination defects on the ultrasound field were studied by numerical simulation. Suitable parameters and features were determined by numerical simulation, which provided a basis for the parameter se-lection of experimental research. The reflected shear waves from the composite interface can act as a sensitive feature to detect the delamination in Cu/Al bimetallic composites. The distance between the detection point and the excitation point was set to 2 mm to take into account the detection resolution and efficiency. The experimental results were in good agreement with the simulation results, and the C-scan image can intuitively show the location and size of delamination defects. The detection method based on laser ultrasonic bulk waves can effectively detect the delamination in Cu/Al bimetallic composite, which is suitable for the on-line detection of the rolling composite process.

Ultrasonic high frequency guided waves for testing of dissimilar metal welds

Abstract Ultrasonic testing of nickel-based dissimilar metal welds (DMWs) in nuclear power plants is always challenging due to its internal structures. Indeed, ultrasonic bulk waves can be severely scattered and attenuated because of coarse grains in DMWs and consequent difficulties such as splitting and skewing can be observed due to the anisotropic and inhomogeneous structure of the weld pool. Ultrasonic high frequency guided waves (HFGW) has the potential to reduce and overcome these issues and can offer a good compromise between the volume coverage and defect sensitivity for testing of DMWs. This work presents defect detection in DMW joints made by 12 mm thickness P91 and alloy 800 plates using HFGW. The generation and propagation characteristics of HFGW on both P91 and alloy 800 have been extensively studied by using 2D plane strain finite element modelling (FEM). For both experimental measurements and FEM, the input parameters such as the excitation frequency, incident angle and scan distance have been optimized with the help of Dispersion curves. It has been observed that the amplitude response of HFGW is more than two times on the P91 plate when compared to the alloy 800 plate. The interaction of HFGW with artificial defects made on the DMW and at the weld interfaces have been tested. It has been noticed that the HFGW can able to detect defect size down to ~ 10% of 12 mm wall thickness and 2 mm diameter side-drilled holes reliably from both sides of P91 and alloy 800.

Development of rotational pulse-echo ultrasonic propagation imaging system capable of inspecting cylindrical specimens

A rotational pulse-echo ultrasonic propagation imager that can inspect cylindrical specimens for material nondestructive evaluations is proposed herein. In this system, a laser-generated ultrasonic bulk wave is used for inspection, which enables a clear visualization of subsurface defects with a precise reproduction of the damage shape and size. The ultrasonic waves are generated by a Q-switched laser that impinges on the outer surface of the specimen walls. The generated waves travel through the walls and their echo is detected by a Laser Doppler Vibrometer (LDV) at the same point. To obtain the optimal Signal-to-Noise Ratio (SNR) of the measured signal, the LDV requires the sensed surface to be at a right angle to the laser beam and at a predefined constant standoff distance from the laser head. For flat specimens, these constraints can be easily satisfied by performing a raster scan using a dual-axis linear stage. However, this arrangement cannot be used for cylindrical specimens owing to their curved nature. To inspect the cylindrical specimens, a circular scan technology is newly proposed for pulse-echo laser ultrasound. A rotational stage is coupled with a single-axis linear stage to inspect the desired area of the specimen. This system arrangement ensures that the standoff distance and beam incidence angle are maintained while the cylindrical specimen is being inspected. This enables the inspection of a curved specimen while maintaining the optimal SNR. The measurement result is displayed in parallel with the on-going inspection. The inspection data used in scanning are mapped from rotational coordinates to linear coordinates for visualization and post-processing of results. A graphical user interface software is implemented in C++ using a QT framework and controls all the individual blocks of the system and implements the necessary image processing, scan calculations, data acquisition, signal processing and result visualization.

Crack Resistance Evaluation for 38KhN3MFA-Sh Structural Steel According to Fracture Characteristics and Elastic Wave Velocities

The results of studies on the effects of tempering temperature on the static strength and crack resistance of 38KhN3MFA-Sh steel are presented. The fractured surface texture has been studied after different heat treatment modes using electron fractography analysis. Relationships between the fracture characteristics and the critical stress intensity factor have been established. The effect of tempering temperature on the velocities of ultrasonic bulk waves has been investigated. A linear relationship between the velocity of elastic waves and the critical stress intensity factor of 38KhN3MFA-Sh steel has been found. The revealed relationship makes it possible to estimate the changes in crack resistance of steel using a nondestructive testing method with variations in the tempering temperature. The method of electron fractography has been used to analyze the fractures in the samples of structural high-quality 38KhN3MFA-Sh steel with a crack. The investigations of the fractured surface texture after different heat-treatment modes have shown that the microrelief is represented by flattened cone-shaped pits. The increase in the tempering temperature is accompanied by an increase in the diameter of flattened pit-cones on the fracture surface. A quadratic relationship between the crack resistance parameter and the diameter of the pits has been revealed. It has been shown that the contribution of the structural condition of the ferrite matrix to crack resistance is much more significant than the contribution of isolated carbides. The elastic wave velocities in steel have been measured; their values exhibit an increase with an increase in the tempering temperature. The strength and crack resistance parameters of the structural steel exposed to high-temperature tempering have been predicted on the basis of the values of transverse wave velocities. The deviations of the predicted values of crack resistance K1c and ultimate strength σu from the experimental values do not exceed 5.4 and 12.6%, respectively.

Improving sensitivity and coverage of structural health monitoring using bulk ultrasonic waves

Practical ultrasonic structural health monitoring systems must be able to deal with temperature changes and some signal amplitude/phase drift over time; these issues have been investigated extensively with low-frequency-guided wave systems but much less work has been done on bulk wave systems operating in the megahertz frequency range. Temperature and signal drift compensation have been investigated on a thick copper block specimen instrumented with a lead zirconate titanate disc excited at a centre frequency of 2 MHz, both in the laboratory at ambient temperature and in an environmental chamber over multiple 20°C–70°C temperature cycles. It has been shown that the location-specific temperature compensation scheme originally developed for guided wave inspection significantly out-performs the conventional combined optimum baseline selection and baseline signal stretch method. The test setup was deliberately not optimised, and the signal amplitude and phase were shown to drift with time as the system was temperature cycled in the environmental chamber. It was shown that the ratio of successive back wall reflections at a given temperature was much more stable with time than the amplitude of a single reflection and that this ratio can be used to track changes in the reflection coefficient from the back wall with time. It was also shown that the location-specific temperature compensation method can be used to compensate for changes in the back wall reflection ratio with temperature. Clear changes in back wall reflection ratio were produced by the shadow effect of simulated damage in the form of 1-mm diameter flat-bottomed holes, and the signal-to-noise ratio was such that much smaller defects would be detectable.

Generation of higher harmonics with the fundamental edge wave mode

The measurement and evaluation of nonlinear elastic properties represents a great interest in materials science and engineering. These measurements can be conducted using the internal resonance of higher harmonics generated with ultrasonic bulk, Rayleigh, and Lamb waves. However, these waves are spatially dispersive, making the evaluation very difficult. In this paper, we present the outcomes of an experimental study on the generation of higher harmonics with the fundamental mode of edge waves. This wave mode—an analog of the Rayleigh wave—propagates along the apex of an elastic plate, where the lateral plate surfaces naturally avoid spatial dispersion. In addition, this mode is weakly dispersive in the frequency domain enabling approximate internal resonance, which is required for the measurement of material nonlinearities. The current experimental results indicate that the second harmonic of the fundamental edge wave mode grows linearly with the propagation distance. The linear rate of accumulation of this harmonic can then be related to the nonlinear elastic properties as procured by previous theoretical studies.