Vibro-acoustic Modulation Method Research Articles

Cross-correlation vibro-acoustic modulation method for damage detection

The vibroacoustic modulation (VAM) method distinguishes itself from other linear or nonlinear acoustic structure health monitoring methods due to its advantages of high sensitivity, relatively simple implementation, and the capability to detect defects in complex structures. However, the damaged index of VAM—modulation index, which is used for quantifying the damage severity, is highly influenced by the frequency of the probe wave. To increase the reliability and robustness of the VAM method, researchers modified it by replacing the single harmonic probe wave with a chirp signal. The instantaneous frequency of the chirp signal linearly increases with time, allowing the modified VAM method to access the broadband frequency range in a relatively short amount of time. However, the sideband that attributes to the acoustic nonlinearity of the defect would be overwhelmed by the broadband frequency content of the probe wave. Hence, a different algorithm is needed to process the received signal of the modified VAM method. Previous studies have employed short-time Fourier transform, wavelet transform, Hilbert transform, and synchronous demodulation for this purpose. Yet, these signal processing methods do not take into account affection of the phase difference between the probe and pump waves to the final result. To address this issue, this paper proposes using a cross-correlation (CC) approach. The performance of the proposed CC approach and existing methods were compared using simulation and experimental testing, evaluating their ability to detect acoustic nonlinearity.

Vibration acoustic modulation for bolt looseness monitoring based on frequency-swept excitation and bispectrum

The monitoring of bolt looseness is crucial to ensure the safety and reliability of structures. Prior studies have demonstrated that the vibro-acoustic modulation (VAM) method based on the nonlinear ultrasonic theory is sensitive to the early looseness of bolted connections. However, one limitation of the traditional VAM method is that the low frequency (LF) and high frequency (HF) for excitation should be specified in advance. The resonant frequency of the bolted structures changes after loosening, leading to inaccuracies in monitoring results if pre-specified excitation frequencies are used and not adapted to the new situation. To address this limitation, this paper improves the VAM method by using swept sine signals for both LF and HF excitations and relying on the bispectrum energy of the measured response to indicate the bolt pre-load. A steel bolted connection was fabricated and loaded on a universal testing machine to simulate different bolt pre-loads. Three low-cost lead zirconate titanate patches served as the LF actuator, HF actuator and sensor in the experiment. The experimental results demonstrate that the improved VAM method can evaluate the bolt looseness with better efficiency and robustness than the traditional VAM methods which use fixed frequencies as excitations. Therefore, the proposed method in this paper can potentially monitor the damages in complex structures based on nonlinear ultrasound theory.

Vibroacoustic modulation method with arbitrary probe wave frequency based on machine learning algorithms

ABSTRACT The vibro-acoustic modulation (VAM) method assesses the integrity of a structure using modulation of a high-frequency probe wave and a low-frequency pump wave and measuring the modulation index. A large body of studies found that the modulation index could be used for the defection of contact-type defects in different materials. However, it was also found that the performance of this method is dependent on the probe wave frequency selection. To enhance the robustness of this method, researchers made efforts to modify the VAM method raising the complexity of the equipment and related operator skills, creating needs for extra preparation or procedures. All of these limits VAM potential for industrial application. To develop a robust, efficient, and fast VAM-based non-destructive testing method, the machine learning algorithm was employed. The algorithm utilises amplitude of the multiple sidebands of the modulation spectrum as feature variables and the structural integrity as target variables for training and testing the machine learning model. The experimental result shows that the fatigue crack could be detected with an arbitrary probe wave frequency using the applied machine learning approach.

Physical explanation for vibro-acoustic modulation due to local and global nonlinearities in a structure and its experimental and numerical validation

This article introduces a simple physical explanation that demonstrates why a local or global nonlinearity in a structure causes amplitude modulation and phase modulation in the system response, when employing the vibro-acoustic modulation method for structural health monitoring (SHM) applications. The underlying major assumption and the explanation itself are validated experimentally and numerically. Afterwards, the explanation is used to demonstrate comprehensively why very different types of envelope functions can be observed in the system response, depending on the exciting ultrasonic carrier frequency ω. The beauty of the suggested explanation is that it does not only work for local nonlinearities, such as exhibited by defects, but also for any non-damage induced (local or global) nonlinearity in the system which can cause modulation as well. A physical understanding of cause and effect is of crucial importance here, because the modulation caused by non-damage induced nonlinearities might camouflage the effects of local damage that one is actually interested in during SHM applications.

Sideband Peak Count in a Vibro-Acoustic Modulation Method for Crack Detection

This paper presents a new method of signal processing for vibro-acoustic modulation (VAM) methods in order to detect damage accumulation in steel samples. Damage in the tested samples was produced by cycle loading, which, with a small amplitude, was used as a pump wave to modulate an ultrasonic probe wave. Multiple sideband peaks were observed, which were used to characterize the modulation effect. We propose the effectiveness sideband peak number (SPN) method as an indicator of any damage accumulation when the load cycle is applied. Moreover, after comparing the SPN with the previously used modulation index (MI), we concluded that, for some of the samples, the SPN provided better damage indication than the MI. The presented results can be explained by a simple model of bilinear crack nonlinearity. This model demonstrates that the amplitude dependences of the sideband components on the pump and the probe wave amplitudes are very different from the quadratic crack model that is usually used for MI test explanation.

Proof-of-concept study of high-order sideband for bolt loosening detection using vibroacoustic modulation method

The significance of continuous loosening detection for bolted joints in engineering structures cannot be overemphasized. Vibroacoustic modulation (VAM) is a popular loosening detection method exploiting nonlinear features. Existing left and right sidebands can only reflect partial contact nonlinearity caused by loose bolted joint. In this paper, we proposed the concept of high-order sideband and established the theoretical model of high-order sideband for the first time. Multi-order sidebands were regarded to contain more information of contact nonlinearity and a novel loosening index (HSLI) integrating the first four high-order sidebands was developed. The VAM experiment was conducted to clearly display multi-order sidebands in the response spectrum for the concept validation and the good universality of high-order sideband was also proved by changing the material and roughness of clamped plates. Moreover, the experiment also found that the HSLI decreased almost linearly with the increase of tightening torques, demonstrating the excellent loosening detection performance compared with the existing nonlinear and linear loosening indexes. Taking a step further, the effects of input voltages of high-frequency and high-frequency excitations on high-order sidebands were discussed in the experiment. The proposed high-order sideband and nonlinear loosening index provide a reliable method of continuous evaluation of bolt loosening, as well as other contact-type damage, such as crack, debonding and bond-slip.

Full life-cycle monitoring and earlier warning for bolt joint loosening using modified vibro-acoustic modulation

Structural health monitoring (SHM) has played a crucial role in continuously assuring adequate integrity across equipment, especially early loosening estimation of bolt joints. Recently, the vibro-acoustic modulation (VAM) technique for bolt early loosening monitoring has received much attention due to its high sensitivity to contact-type defects. However, the currentearly loosening monitoringmostly focuses onthe bolt head and interconnecting componentlooseningwhile ignoring theearlier loosening stage. The central contribution of this work is to develop full life-cycle monitoring for bolt loosening and consequently achieve even earlier warning in the stage of thread loosening. Firstly, regarding the bolt joints in the sand spreading device of high-speed trains, a modified VAM method, in which the designed sensor layout is more applicable for engineering and a swept signal is applied as a probing wave to overcome the dependence of detection accuracy on the exact resonance frequency excitation, is conducted for the full life-cycle monitoring. Secondly, the mechanisms, performances and dependency on the applied torques of the loosening-induced nonlinearity are explored with the proposed nonlinear indexes, theoretically and experimentally. The results demonstrate that both the nonlinear elastic mechanism and the amplitude-dependent dissipation effect contribute to such nonlinearity. Further analysis suggests two valuable and not yet mentioned findings: 1) there is a thread loosening stage that exhibits sufficiently sensitive nonlinear modulation index variations to predict bolt early looseness before the bolt head and interconnecting componentloosening; 2) such nonlinear indexes are non–monotonically related to the torque levels in both the above two loosening stages, consistent with theory. Further, such full life-cycle monitoring is achieved by associating such nonlinear indexes and a linear transmitted energy index. All of the obtained results will aid in understanding the nonlinear symptoms under various bolt tightening levels and further improving the reliability of the VAM-based bolt early loosening monitoring.

Vibro-acoustic modulation baseline-free non-destructive testing

Vibro-Acoustic Modulation method for detection and characterization of various structural and material flaws has been actively researched for the last two decades. Most of the studies focused on detection and monitoring of macro-cracks requiring well established baseline (no-damage) value of the modulation index. The baseline value is specific for a particular structure, measuring setup, and other factors and can't be established in many practical situations without a long term monitoring looking for a relative change in the Modulation Index. In this work, we propose and investigate a baseline-free Vibro-Acoustic Modulation method, which does not require monitoring of relative Modulation Index change, unlike conventional approach. It was hypothesized that the nonlinear mechanisms (and respective nonlinear response) of a structure are different for undamaged and damaged material. For example: material without damage or at early stages of fatigue have classic elastic or hysteretic/dissipative nonlinearity while damaged (cracked) material may exhibit contact bi-linear or Hertzian nonlinear mechanisms. These mechanisms yield different power law dependencies of Modulation Index (MI) as function of applied vibration frequency input amplitude, B: MI ~ B^b Thus, quadratic nonlinearity yields linear dependence, b =1, and Hertzian nonlinearity results in b<1. Other nonlinear mechanisms yield different power laws. Therefore, measuring power damage coefficient b instead of MI may offer testing without established reference value. It is also offer some insights into the nonlinear mechanisms transformation during damage evolution. This approach was experimentally investigated and validated.

Experimental and numerical looseness detection and assessment in flanged joints using vibro-acoustic modulation method

Flanged joints’ looseness is among the common causes for the failure of industrial structures with flanged joints the timely detection of which can prevent the imposition of heavy financial losses and in some cases injuries. There are conventional methods for detecting this fault and each of them has its own drawbacks. For instance, torque control methods have high error of measurement, impedance-based measurement methods, have high expenses, and vibration or ultrasonic methods lack accuracy due to the use of linear phenomena in fault detection. Vibro-Acoustic modulation method is one of the nonlinear fault detection methods that can detect and assess looseness of flanged joints with high precision through the measurement of the intensity of the vibrational and ultrasonic signals modulation applied to the structure. This paper was an attempt to investigate the efficiency of the Vibro-Acoustic modulation method in the detection and evaluation of flanged joints numerically and experimentally. For this, a laboratory sample composed of two pipes connected with flanged joints is employed. It is revealed that this method can detect bolt looseness with 12.5% precision. In addition, the effect of parameters such as ultrasonic and vibrational frequency, amplitude of applied torque, sensors and actuators position, as well as excessive increase of torque have been examined and the method precision in the studied structure has been estimated.

Looseness Detection and Assessment of Flange Type Joints Using Vibroacoustic Modulation Method

Flanged joints’ looseness is among the common causes for the failure of industrial structures with flanged joints such as wind turbine or transmission line pipes the timely detection of which can prevent the imposition of heavy, financial losses and in some cases property damage. Conventional methods for detecting this fault, such as torque control methods, have high error of measurement, or impedance-based measurement methods, have high expenses, or vibration or ultrasonic methods which lack accuracy due to the use of linear phenomena in fault detection. Vibroacoustic modulation method is one of the nonlinear fault detection methods that detect and evaluate looseness of flanged connection with high precision through the measurement of the intensity of the vibrational and ultrasonic signals modulation applied to the structure. Although the published papers in recent years have often identified the cracking, delamination, or corrosion and decay of parts using this method, in this paper the efficiency of the vibroacoustic modulation method in the detection and evaluation of flanged joints of simulated wind turbine towers has been numerically investigated by defining an index for modulation intensity and bond relaxation. Then, the effect of parameters such as ultrasonic and vibrational frequency, amplitude of ultrasonic and vibrational stimulation, sensors and actuators position, as well as preload force, on the method performance have been studied. Finally, in order to reduce the simulation time in ABAQUS software, the modeling of the neural network was performed using MATLAB software and the obtained results were compared with numerical results.

Monitoring of multi-bolt connection looseness using a novel vibro-acoustic method

Bolted connections are prone to losing their preloads with the increasing service life, thus inducing engineering accidents and economic losses in industries. Therefore, it is important to detect bolt loosening, while current structural health monitoring methods mainly focus on single-bolt joints, whose applications in industries are limited. Thus, in this paper, a novel vibro-acoustic modulation (VAM) method, is developed to detect looseness of the multi-bolt connection. Compared to traditional VAM, the proposed method uses linear swept sine waves for both low-frequency and high-frequency excitations, which avoids a priori knowledge of the structure. Moreover, the orthogonal matching pursuit method is applied to compress original modulated signals and exclude redundant features. Then, a new entropy, namely the Gnome entropy with acronym gEn, is proposed in this paper. According to simulation analysis, the gEn has better anti-noise capacity and fewer parameters than traditional entropy. Finally, after quantifying the dynamic characteristics of compressed signals to obtain feature sets through the gEn, we feed feature sets into a random forest classifier and achieve looseness detection of the multi-bolt connection. Moreover, the proposed method in this paper has great potential to detect other structural damages and provides guidance for further investigations on the VAM method.

A nonlinear ultrasonic method for real-time bolt looseness monitoring using PZT transducer–enabled vibro-acoustic modulation

A real-time nonlinear ultrasonic method based on vibro-acoustic modulation is applied to monitor early bolt looseness quantitatively by using piezoceramic transducers. In addition to the ability to detect the early bolt looseness, a major contribution is that we replaced the shaker, which is commonly used in a vibro-acoustic modulation method, by a permanently installed and low-cost lead zirconate titanate patch. In vibro-acoustic modulation, when stimulating two input waves with distinctive frequencies, namely the high-frequency probing wave and the low-frequency pumping wave, the high-frequency probing wave will be modulated by the low-frequency pumping wave to generate sidebands in terms of bolt looseness. Thus, the influence of low-frequency voltage amplitudes on the modulation results, which is ambiguous in previous research, is also analyzed in this article. The results of experiment demonstrated that the lead zirconate titanate–enabled vibro-acoustic modulation method is reliable and easy to implement to identify the bolt looseness continuously and quantitatively. In addition, low-frequency amplitudes of actuating voltage should be selected in a reasonable range. Finally, we compared the vibro-acoustic modulation method with the time-reversal method based on the linear ultrasonic theory, and the result illustrates that vibro-acoustic modulation method has better performance in monitoring the early bolt looseness.

New entropy-based vibro-acoustic modulation method for metal fatigue crack detection: An exploratory study

Metal fatigue is a typical industrial failure, and investigations on monitoring of fatigue cracks are required. Therefore, in this study, we developed an exploratory study by proposing a new vibro-acoustic modulation (VAM) method to monitor fatigue crack. To simulate different fatigue crack width, a mechanical auxiliary device was designed, and a surface profiler was employed to measure crack width. Compared to traditional VAM, we replaced the shaker through the PZT transducer, and both low frequency (LF) and high frequency (HF) excitations were based on linear swept sine waves. Then, a new entropy-based damage index (DI) using the multiscale permutation entropy (MPE) was developed to characterize the crack width. According to experimental results, there was a one-to-one relationship between the DI and the crack width, particularly, without saturation. Therefore, we demonstrated the capacity of the new VAM method to monitor crack propagation process, which can guide future investigations.

Optimal pump excitation frequency for improvement of damage detection by nonlinear vibro acoustic modulation method in a multiple scattering sample

We present a method to systematically optimize nonlinear damage detection in multiple scattering media by the nonlinear Vibro-Acoustic Modulation (VAM) technique. The latter consists here of exciting a medium simultaneously with a high frequency ultrasonic sinusoidal burst and with a low frequency continuous sinusoidal wave. Modulation of the high frequency (probe) by the low frequency (pump) is made possible by the presence of nonlinear scatterers, i.e. cracks, defects. A signal processing technique consisting of a closed loop system drives the automatic adaptation of the pumping frequency, yielding to the optimization of the nonlinear modulation (NM) of the output probing coda signal without a priori information on the medium and the scatterers. The correlation coefficient between a reference output probe signal without the pumping wave and an output modulated probe signal with a pumping wave was considered as our cost function. A multiple scattering solid beam where nonlinear scatterers can be controllably added or removed is designed and tested. The first step of this study is an empirical search of the correlation coefficient dependency on the pumping frequency to verify the performances of the proposed method. Then the implemented optimization algorithm based on genetic algorithm (GA) is used to find automatically the optimal pumping frequency. The obtained optimization results show a good agreement with the empirical study. Moreover, the genetic algorithm allowed to find the optimal pump frequency adapted to each configuration of nonlinear scatterers. This relatively fast search of the optimal nonlinear response could be extended to nonlinear scatterer imaging applications using the information on the resonant modes spatial shapes together with the associated optimal response.

Steel Material Degradation Assessment Via Vibro-Acoustic Modulation Technique

The increasing probability of collapse in defective structures owing to aging is one of the major issues in transportation. Therefore, different methodologies that are capable of monitoring structural components have been used to identify defects and predict failure. Among these methods, the non-linear vibro-acoustic modulation (VAM) technique has been implemented for many years in a variety of industries, such as aerospace. This method utilizes the effect of nonlinear interactions between a high frequency ultrasonic wave (carrier signal) and a much lower frequency structural vibration (modulating signal). This interaction takes place at nonlinear interfaces (cracks, bolted connections, delaminations, etc.) manifesting itself in the spectrum as side-band components around the carrier frequency. In this study, the VAM method was investigated as a non-destructive evaluation (NDE) method for fracture critical members (FCMs) in steel bridges. The results of the experimental studies revealed that using the VAM technique on test specimens during the tension only fatigue tests would provide some useful information on the existence of micro-cracks and on failure prediction. The use of the VAM technique for center-notched rectangular test specimens of structural steel under low-amplitude fatigue loading at a frequency of 10 Hz is capable of predicting the failure at 70–80% of the fatigue lifetime of the specimen. Moreover, in this investigation utilizing fatigue cycling as a modulating signal was successfully substituted for the conventional utilization of resonance structural bending vibrations.

Bolt early looseness monitoring using modified vibro-acoustic modulation by time-reversal

Abstract Structural health monitoring (SHM) of bolted joints has played a vital role in estimation of bolt looseness and prediction of residual service life of bolted connections, thus saving money and significantly improving the efficiency of maintenance routines across industries. In the past decades, several SHM methods, particularly acoustic/ultrasonic methods, have been used to identify the health status of bolted connections. Compared to the linear ultrasound techniques such as active sensing, the vibro-acoustic modulation (VAM) method that is based on nonlinear ultrasonic features has proven its efficiency in bolt early looseness monitoring; however, some drawbacks impede its practical use. The main contribution of this paper is to develop a modified VAM (MVAM) that can circumvent existing problems with practical implementation and provide higher sensitivity. First, the shaker used in the traditional VAM was replaced by a piezoceramic transducer to improve its practicality. Moreover, instead of sine waves, linear swept sine signals were used for both low-frequency (LF) pump vibration and high-frequency (HF) probe wave. In other words, no a priori knowledge of the structural condition is needed, which further broadens the scope of application. Subsequently, the time reversal (TR) method was applied to overcome problems including signal energy dissipation and low signal-to-noise ratio (SNR) in traditional VAM. Moreover, the noise-assisted multivariate empirical mode decomposition (NA-MEMD) and multiscale multivariate sample entropy (MMSE) were used to develop a new damage index (DI) for bolt early looseness monitoring. Finally, multiple repeated experiments were conducted to verify the accuracy of the proposed method and its ability to simplify bolt early looseness monitoring in terms of practical operation, by comparing the proposed MMSE-based DI with nonlinear DI of traditional VAM method.

A vibro-acoustic modulation method for the detection of delamination and kissing bond in composites

In this paper we employ a nonlinear acoustic method, namely, the vibro-acoustic modulation method, for the detection of delamination and kissing bond in composites. Both a (large-amplitude and low-frequency) pumping wave and a (low-amplitude ultrasound) probing wave are used to vibrate the structure. Permanently bonded piezoceramic transducers are used for both excitation and measurement. A side-band ratio is used to estimate the extent of nonlinear interactions between the two input waves in the structure. Current results show that the proposed method successfully differentiates the intact specimens from both the delamination and the kissing bond specimens used in this study.

Analysis of Vibroacoustic Modulations for Crack Detection: A Time-Frequency Approach Based on Zhao-Atlas-Marks Distribution

The vibro-acoustic modulation (VAM) technique is probably the most widely used nonlinear method for crack detection. The VAM method is based on the effect of modulation of high-frequency acoustic waves by a low-frequency vibration. The intensity of the modulation is related to the severity of the damage and has been used so far as a damage index. The damage index simply based on the amplitude of the first side bands in the spectral domain often leads to controversial results about the severity of the damage. In this work, the nonlinear characteristics of the vibro-modulation were systematically investigated by employing time-frequency analysis based on the Zhao-Atlas-Marks (ZAM) distribution. The results of the analysis show that the amplitude of the sideband components is modulated by the low frequency vibration and the modulation amplitude depends on the size of the crack. Based on the obtained results, a new damage index was defined in relation to the strength of the modulation. The new damage index is more sensitive and robust and correlates better with crack size compared to the index based on the amplitude of the sidebands.