Nonlinear Ultrasonic Method Research Articles

The measurement of weld morphology and inclusions using ultrasonics

The morphology of welding pool (fusion zone and heat affected zone) includes a variety of microstructural constitutions (the formation of new boundaries, pores, and textures) that can affect the integrity of welded joint. Currently, microscopic observations are the common method to characterize the welding pool, which is a destructive post-welding procedure, and requires a complex preparation process. In this paper, linear and non-linear ultrasonic methods are investigated to measure the weld morphology and the presence of porosity nondestructively in welds manufactured with gas tungsten arc welding (GTAW). The weld morphology and the presence of porosity are quantified using microscopic investigations. The measurements show that the ultrasonic energy obtained by linear ultrasonics and the acoustic nonlinearity parameter obtained by nonlinear ultrasonics have good correlation with the weld morphology. The relationship between the ultrasonic energy and the weld morphology is validated further with numerical models. The linear UT at 1 MHz is not sensitive to the presence of porosity as the resolution is limited by ultrasonic wavelength. However, nonlinear ultrasonics has better sensitivity to the inclusions due to sub-wavelength resolution.

Finite element simulation and experimental study of residual stress testing using nonlinear ultrasonic surface wave technique

Due to the influence of residual stress on material properties, residual stress measurement is particularly important for the optimization of structure designs and the condition monitoring of machines. In this paper, the influence of residual stress on materials nonlinearity was reported. Firstly, the propagation model of ultrasonic surface waves was established by the finite element method and the effect of frequency was studied to determine an appropriate frequency of acoustic nonlinearity parameters. Then, the different residual stresses were produced into specimen by adding internal stress in the material and the impact on acoustic nonlinearity parameter was studied. The results show that the nonlinearity of the material increases monotonously with the increase of residual stress. Experiments were done by using the nonlinear ultrasonic surface wave on three 7075Al alloy specimens under different residual stresses. Experimental data matched well with the simulation results. It has been shown that the nonlinear ultrasonic surface wave method has great potential in the residual stress detection and evaluation.

New nonlinear ultrasonic method for material characterization: Codirectional shear horizontal guided wave mixing in plate

A nonlinear ultrasonic method is proposed based on a group of newly discovered wave triplets where two primary codirectional shear-horizontal SH0 waves mix in a weakly nonlinear plate and generate a cumulative S0 Lamb wave at the sum frequency. Theoretical analyses show that any combination of two primary SH0 waves whose frequencies sum to the frequency at which the SH0 mode intersects the S0 Lamb wave mode results in an internally resonant secondary S0 Lamb wave. Moreover, the relationship between the frequency combination and the nonlinear Lamb wave generation efficiency is revealed, which guides further engineering applications. Finite element validations are carried out with the aid of a subtraction method for the nonlinear feature extraction. The cumulative effect of the generated S0 Lamb wave at the sum frequency as well as the influence of the frequency combinations on the nonlinear Lamb wave generation efficiency is confirmed. Experiments are performed to validate the proposed method as well as demonstrate its use for material characterization. The experiments require a gel filter to mitigate the influence of the undesired nonlinear sources. With the gel filter, the cumulative effect of the secondary S0 Lamb wave is verified and the corresponding slope is extracted and further used to characterize the material status of the fatigue samples. Results demonstrate the proposed method provides high sensitivity to early fatigue damage, which makes it promising for the further early damage detection applications.

Modulated high frequency excitation approach to nonlinear ultrasonic NDT

A novel approach for nonlinear ultrasonics is presented which relies on an amplitude modulated high-frequency excitation, as distinct from the conventional vibro-acoustic approach based on a low-frequency pumping wave in combination with a high-frequency probing wave. This amplitude modulated excitation contains three frequencies that interact in the presence of nonlinearity and transfers energy to frequencies not part of the original input. This effect can be exploited to detect and quantify damage-induced nonlinearity. This novel method is demonstrated experimentally for three distinct contact conditions, utilising different types of transducer and excitation signals of various amplitudes and frequencies. The performance is compared with the conventional vibro-acoustic technique. It is shown that the new technique has comparable performance to existing methods. Specialized measurement system design with superb sensitivity is also demonstrated. Unlike conventional cross-modulation based nonlinear ultrasonic techniques, this new method does not require two separate excitation sources. This represents a significant practical advantage which will greatly facilitate the implementation of nonlinear ultrasonic methods for a wide range of applications.

Nonlinear Ultrasonic Detection Method for Delamination Damage of Lined Anti-Corrosion Pipes Using PZT Transducers

Lined anti-corrosion pipes are widely used in oil and gas, petrochemical, pharmaceutical industries. However, defects, especially delamination, may occur in the production and service of pipes which result in safety accidents. Based on nonlinear ultrasonic theory, this paper studied the delamination detection method using the nonlinear harmonics for lined anti-corrosion pipes. The response characteristics of the anti-corrosion pipe were obtained through a sweep sine response experiment and the preferred excitation frequency was determined. The Wavelet Packet transform and Hilbert–Huang transform is applied for signal process and feature extraction. Then, a series of experiments were carried out and the results were analyzed and discussed. The results showed that a second-order and third-order nonlinear coefficient increased with the delamination damage. The amplitude of second-harmonic is much stronger than the third-order one. The mean squared error of the nonlinear coefficient, which was processed by Wavelet Packet transform and Hilbert–Huang transform, is smaller than wavelet packet transform and Discrete Fourier transform or processed only Hilbert–Huang transform. The higher harmonics can describe the change of delamination damage, which means that the nonlinear ultrasonic detection method could use for damage detection of anti-corrosion pipe. The nonlinear higher-harmonic is sensitive to delamination damage. The nonlinear ultrasonic method has the potential for damage detection for lined anti-corrosion pipes.

Ultrasonic waves for materials evaluation in fatigue, thermal and corrosion damage: A review

Abstract Mechanical properties of materials tend to deteriorate over time and thus become responsible for cracks and malfunctions in mechanical components or civil structures. Non-destructive ultrasonic wave analysis has proven to be a successful investigation method for inspecting mechanical properties of materials. This review has highlighted the main results of research in the field of non-linear ultrasonic wave investigations for the inspection of fatigue damage, thermal damage and chemical damage. In all three cases, non-linear ultrasonic wave survey method was effective in detecting first-stage damage.

Nonlinear ultrasonic detection for evaluating fatigue crack in metal plate

In engineering structures, metal materials always endure fatigue cracks under long-term service. There has been a demand for developing a structural health monitoring method to evaluate micro-sized fatigue cracks, as cracking is considered as a precursor to structural failure. However, conventional linear-ultrasound-based technology is not sensitive to crack when it is barely visible in a metal medium. In this article, we present a nonlinear ultrasonic technology based on crack–wave interaction to investigate the growth of a fatigue crack. A breathing-crack model with a plastic zone around it was precisely established to reveal the change in the Lamb wave. The relative nonlinear parameter calculated from the fundamental and harmonic components of the Lamb wave showed linearly increasing with the growth of the fatigue crack. The relative nonlinearity was related to ultrasonic parameters, such as the cycle number and the excited frequency of the tone-burst signal. In addition, it was also related to the angle between the sensor and the crack rather than their distance. A set of experiments were conducted, demonstrating that the increasing trend of ultrasonic nonlinearity fits very well to the finite element analysis results. In conclusion, the nonlinear ultrasonic method that can be applied to the detection of micro fatigue cracks in metal plates is an effective structural health monitoring technique.

Debonding Detection in Hidden Frame Supported Glass Curtain Walls Using the Nonlinear Ultrasonic Modulation Method with Piezoceramic Transducers.

Debonding defects are common and they are the main reason for the failure of hidden frame supported glass curtain walls, which are widely used as an external enclosure and decorative structure. In this paper, a debonding detection method for hidden frame supported glass curtain walls is developed based on nonlinear ultrasonic modulation and piezoceramic transducers. First, the excitation frequency was determined according to the response characteristics. Then, empirical mode decomposition (EMD) was applied to extract the feature components. After discrete Fourier transform (DFT), the nonlinear coefficients were calculated to evaluate the debonding defect. Finally, the experimental setup was established and a series of experiments were carried out to verify the feasibility and effectiveness of the nonlinear ultrasonic modulation method. The nonlinear harmonics detection method was also investigated and it was compared with the nonlinear ultrasonic modulation method. The detection effect at different temperatures and impact were studied. The results showed that the nonlinear coefficient increases with the debonding length. The mean squared error (MSE) of the nonlinear ultrasonic modulation method was improved by 41% compared with the nonlinear harmonics method. The nonlinear ultrasonic modulation method can successfully detect debonding defects in hidden frame supported glass curtain walls at different temperatures and impact.

Nonlinear elastic imaging of barely visible impact damage in composite structures using a constructive nonlinear array sweep technique

Linear and nonlinear ultrasound imaging methods highlight different damage features: the linear method detects large stiffness changes, while the nonlinear technique identifies small impedance mismatches, such as microcracks or closed delaminations. Typically, nonlinear ultrasound techniques detect damage/defects in materials by measuring higher order harmonics. These harmonics can be difficult to measure due to low magnitude and signal to noise ratios (SNR): hence large excitation amplitudes are needed, which can further complicate the reliability of these methods as equipment nonlinearities can be generated. To overcome these issues, exciting at specific frequencies, known as local defect resonances (LDR), produce a much larger displacements at the damaged regions. However, estimation of LDR is time-consuming, complex and not an easily automated process.A coupled baseline-free linear and nonlinear ultrasonic imaging approach is proposed, using a Constructive Nonlinear Array Sweep excitation and an image subtraction method for identifying damage in layered materials. The signal sweep method uses a narrow band frequency excitation to increase the probability of detection of a LDR frequency. The novel imaging approach was employed using laser vibrometry measurements in various complex composite structures to assess barely visible impact damage, critical for the aircraft industry. The results showed better estimation of impact damage when compared to classical linear or nonlinear ultrasonic methods leading to improved reliability of aircraft inspections.

Application of Wavelet and EEMD Joint Denoising in Nonlinear Ultrasonic Testing of Concrete

The health state of concrete is deteriorating during its service. Nonlinear ultrasonic detection based on the amplitude of the fundamental and the second harmonic is considered to be a powerful tool for the discovery of the microcrack in concrete. However, the research on processing the nonlinear ultrasonic signal is still insufficient. In order to highlight the real frequency domain components in the nonlinear ultrasonic signal, wavelet and ensemble empirical mode decomposition (EEMD) were joined to denoise the numerical and measured signal. The optimal wavelet base and the decomposition level were determined by the signal‐to‐noise ratios (SNRs). Then, the wavelet threshold denoising signal was decomposed by EEMD, omitting the high‐frequency components and ultimately achieving the desired denoising effect. The denoising result of the test signals demonstrates that this method is effective in denoising the details of the ultrasonic signal and improving the reliability and adaptability of the nonlinear ultrasonic testing. In this experiment, the concrete with the microcrack was tested by linear and nonlinear ultrasonic methods. Based on the variation regularity of the nonlinear ultrasonic coefficient β and velocity v, we can conclude that the nonlinear ultrasonic parameter β is more sensitive to the microcrack in concrete than the traditional wave velocity v. The nonlinear ultrasonic testing can be an important supplement to the current nondestructive testing technique of the concrete.

Phononic Crystal Waveguide Transducers for Nonlinear Elastic Wave Sensing

Second harmonic generation is one of the most sensitive and reliable nonlinear elastic signatures for micro-damage assessment. However, its detection requires powerful amplification systems generating fictitious harmonics that are difficult to discern from pure nonlinear elastic effects. Current state-of-the-art nonlinear ultrasonic methods still involve impractical solutions such as cumbersome signal calibration processes and substantial modifications of the test component in order to create material-based tunable harmonic filters. Here we propose and demonstrate a valid and sensible alternative strategy involving the development of an ultrasonic phononic crystal waveguide transducer that exhibits both single and multiple frequency stop-bands filtering out fictitious second harmonic frequencies. Remarkably, such a sensing device can be easily fabricated and integrated on the surface of the test structure without altering its mechanical and geometrical properties. The design of the phononic crystal structure is supported by a perturbative theoretical model predicting the frequency band-gaps of periodic plates with sinusoidal corrugation. We find our theoretical findings in excellent agreement with experimental testing revealing that the proposed phononic crystal waveguide transducer successfully attenuates second harmonics caused by the ultrasonic equipment, thus demonstrating its wide range of potential applications for acousto/ultrasonic material damage inspection.

Evaluation of Fire-Damaged Concrete: An Experimental Analysis based on Destructive and Nondestructive Methods

Fire damage to concrete causes contact-type defects that degrade its durability through impaired mechanical properties. Various nondestructive tests are used to evaluate defects induced by fire damage. Recently, nonlinear ultrasonic methods such as the nonlinear resonance vibration method and nonlinear modulation method have been introduced. These nonlinear methods are more sensitive to fire-induced contact-type defects than the linear ultrasonic method. This study involved an experimental analysis of the residual material properties of fire-damaged concrete, specifically, compressive strength, splitting tensile strength, and static elastic modulus. The residual material properties of 116 cylindrical concrete samples with various mix proportions and subjected to various heating temperatures were measured by a destructive method, and their nonlinearity parameters were measured by two nonlinear ultrasonic methods. Through regression analysis, correlated relationships that can facilitate the prediction of residual material properties of fire-damaged concrete using measured nonlinearity parameters were identified. In addition, the effect of fire damage on the mechanical strength of concrete was investigated by comparison with the relationships for undamaged concrete, and relationships for the evaluation of fire-damaged concrete were identified through regression analysis.

Characterization of stress-dependent ultrasonic nonlinearity variation in concrete under cyclic loading using nonlinear resonant ultrasonic method

In this work, an experimental study was carried out to investigate the stress-dependent characteristics of concrete under cyclic loading, based on a nonlinear resonant ultrasonic method. Since concrete subjected to cyclic loading accompanies variations in the elastic and plastic characteristics of concrete in relation to microstructural changes, the adoption of a nonlinear ultrasonic approach is required, which has advantages for the evaluation of micro-cracks in concrete. In this experimental study, two types of loading were considered, namely, continuously increased and cyclic repeated, to identify their effects on the ultrasonic nonlinearity of concrete and the load-induced damage. An additional experiment on the exposure of concrete samples to high temperature was conducted to further investigate the effects of cyclic loading accompanied by an increased occurrence of micro-cracking. A comparison analysis was also performed on the experimental results, and the potential to monitor the stress history of concrete by using the nonlinear resonant ultrasonic method was evaluated.

Experimental Characterization of Impact Fatigue Damage in an Adhesive Bonding Using the Second Harmonics

An adhesive bonding is widely used in various industrial applications. In this paper, an application of non-destructive evaluation (NDE) technique based on the second harmonics for experimental characterization of impact fatigue damage in an adhesive bonding is presented. Adhesively bonded specimens made from AZ31 magnesium–aluminum alloy were firstly subjected to impact fatigue loading; the ultrasonic harmonics generated due to impact fatigue damage within the adhesive layer were thereafter measured. The acoustic nonlinearity parameter (ANP) based on the fundamental and second harmonics was thus obtained. The experimental results show that the normalized ANP, which is an indicator of material properties, increases with the impact fatigue life. Further discussion based on a theoretical model with different interfacial compression and tension stiffness was also conducted. The experimental results consist well with the theoretical ones. The results in this paper demonstrate that the nonlinear ultrasonic method based on the second harmonic generation technique can be used to characterize the impact fatigue damage in an adhesive bonding.

Damage detection of fatigue cracks under nonlinear boundary condition using subharmonic resonance

In recent years, the nonlinear ultrasonic technique has been widely utilized for detecting fatigue crack, one of the most common forms of damage. However, one of limitations associated with this technique is that nonlinearities can be produced not only by damage but also by various intrinsic effects such as boundary conditions. The objective of this paper is to demonstrate the application of a nonlinear ultrasonic subharmonic method for detecting fatigue cracks with nonlinear boundary conditions. The fatigue crack was qualitatively modeled as two elastic, frictionless half spaces that enter into contact during vibration and where the contact obeys the basic Hertz contact law. The nonlinear ordinary differential equation drawn from the developed model was solved with the method of multiple scales. The threshold of subharmonic generation was studied. Different threshold behaviors between the nonlinear boundary condition and the fatigue crack were found that can be used to distinguish the source of nonlinear subharmonic features. To evaluate the proposed method, experiments using an aluminum plate with a fatigue crack were conducted to quantitatively verify the subharmonic resonance range. Two surface-bonded piezoelectric transducers were used to generate and receive ultrasonic wave signals. The experimental results demonstrated that the subharmonic component of the sensing signal could be used to detect the fatigue crack and further to distinguish it from inherent nonlinear boundary conditions.

Ultrasonic Monitoring of the Interaction between Cement Matrix and Alkaline Silicate Solution in Self-Healing Systems.

Alkaline solutions, such as sodium, potassium or lithium silicates, appear to be very promising as healing agents for the development of encapsulated self-healing concretes. However, the evolution of their mechanical and acoustic properties in time has not yet been completely clarified, especially regarding their behavior and related kinetics when they are used in the form of a thin layer in contact with a hardened cement matrix. This study aims to monitor, using linear and nonlinear ultrasonic methods, the evolution of a sodium silicate solution interacting with a cement matrix in the presence of localized cracks. The ultrasonic inspection via linear methods revealed that an almost complete recovery of the elastic and acoustic properties occurred within a few days of healing. The nonlinear ultrasonic measurements contributed to provide further insight into the kinetics of the recovery due to the presence of the healing agent. A good regain of mechanical performance was ascertained through flexural tests at the end of the healing process, confirming the suitability of sodium silicate as a healing agent for self-healing cementitious systems.

Characterization of thermal damage in sandstone using the second harmonic generation of standing waves

This paper investigates thermal damage in sandstone using linear and nonlinear ultrasonic methods. The nonlinear ultrasonic method is based on the higher harmonic generation in non-resonant standing waves; the nonlinearity parameter is defined as the ratio of the second harmonic amplitude to the square of the fundamental amplitude. This parameter is highly sensitive to the damage evolution/microstructural changes in sandstone. The longitudinal ultrasonic velocity and dynamic elastic modulus are also measured as representative linear ultrasonic parameters. Both the linear and nonlinear parameters exhibit a peculiar reverse trend in the temperature range 100–150℃. This effect is thought to be due to the filling of pre-existing microcracks by the clay component in sandstone, which swells and migrates with the temperature elevation. The nonlinearity parameter shows an accelerated increase at temperatures higher than 200℃. Variations of the wave velocity and elastic modulus are up to 49%, while the nonlinearity parameter increases as much as 19 times during the thermal process. The nonlinearity parameter is more sensitive to the microstructural changes in sandstone (microcracking and clay swell and migration) due to thermal loading than the linear parameters, and thus the present nonlinear ultrasonic technique can serve as a promising tool for characterizing damage progress in rock materials.

Application of Nonlinear Ultrasonic Method for Monitoring of Stress State in Concrete

As the lifespan of concrete structures increases, their load carrying capacity decreases owing to cyclic loads and long-term effects such as creep and shrinkage. For these reasons, there is a necessity for stress state monitoring of concrete members. Particularly, it is necessary to evaluate the concrete structures for behavioral changes by using a technique that can overcome the measuring limitations of usual ultrasonic nondestructive evaluation methods. This paper proposes the use of a nonlinear ultrasonic method, namely, nonlinear resonant ultrasonic spectroscopy (NRUS) for the measurement of nonlinearity parameters for stress monitoring. An experiment compared the use of NRUS method and a linear ultrasonic method, namely, ultrasonic pulse velocity (UPV) to study the effects of continuously increasing loads and cyclic loads on the nonlinearity parameter. Both NRUS and UPV methods found a similar direct relationship between load level and that parameter. The NRUS method showed a higher sensitivity to micro-structural changes of concrete than UPV method. Thus, the experiment confirms the possibility of using the nonlinear ultrasonic method for stress state monitoring of concrete members.

Evaluation of crack parameters by a nonlinear frequency-mixing laser ultrasonics method

The local evaluation of several parameters of a crack is realized with a nonlinear laser ultrasonic method. The method is based on the sample excitation by two laser beams, independently intensity modulated at two cyclic frequencies ωH and ωL (ωH≫ωL) and on the detection of nonlinear frequency-mixing ultrasonic components at frequencies ωH±nωL (n an integer). Frequency-mixing is a nonlinear process originating from the modulation of the crack state at low frequency ωL by laser-induced thermo-elastic stresses, which causes in turn the modulation of the acoustic waves at frequency ωH reflected/transmitted by the crack. We carry experiments with increasing laser power and observe a non-monotonous variation in the amplitude of up to 6 nonlinear sidelobes. We also improve a previously introduced theoretical model which leads to interpreting the experimental observations by the combined action on the crack of the thermo-elastic waves at low frequency ωL and of the stationary thermo-elastic stresses at ω=0. The latter are induced by the average laser power absorbed by the sample. While thermo-elastic wave can periodically modulate the parameters of the crack up to its periodic opening/closing, the stationary heating could cause complete local closure of the crack. By fitting the experimental amplitude evolutions for all monitored sidelobes with the theoretically predicted ones, various local parameters of the crack are extracted, including its local width and effective rigidity.

Evaluation of freezing and thawing damage of concrete using a nonlinear ultrasonic method

Freezing and thawing cycles induce deterioration and strength degradation of concrete structures. This study presumes that a large quantity of contact-type defects develop due to the freezing and thawing cycles of concrete and evaluates the degree of defects based on a nonlinearity parameter. The nonlinearity parameter was obtained by an impact-modulation technique, one of the nonlinear ultrasonic methods. It is then used as an indicator of the degree of contact-type defects. Five types of damaged samples were fabricated according to different freezing and thawing cycles, and the occurrence of opening or cracks on a micro-scale was visually verified via scanning electron microscopy. Dynamic modulus and wave velocity were also measured for a sensitivity comparison with the obtained nonlinearity parameter. The possibility of evaluating strength degradation was also investigated based on a simple correlation of the experimental results.