Contact Acoustic Nonlinearity Research Articles

Analytical extension of Finite Element solution for computing the nonlinear far field of ultrasonic waves scattered by a closed crack

Nonlinear scattering of ultrasonic waves by closed cracks subject to contact acoustic nonlinearity (CAN) is determined using a 2D Finite Element (FE) coupled with an analytical approach. The FE model, which includes unilateral contact with Coulomb friction to account for contact between crack faces, provides the near-field solution for the interaction between in-plane elastic waves and a crack of different orientations. The numerical solution is then analytically extended in the far-field based on a frequency domain near-to-far field transformation technique, yielding directivity patterns for all linear and nonlinear components of the scattered waves. The proposed method is demonstrated by application to two nonlinear acoustic problems in the case of tone-burst excitations: first, the scattering of higher harmonics resulting from the interaction with a closed crack of various orientations, and second, the scattering of the longitudinal wave resulting from the nonlinear interaction between two shear waves and a closed crack. The analysis of the directivity patterns enables us to identify the characteristics of the nonlinear scattering from a closed crack, which provides essential understanding in order to optimize and apply nonlinear acoustic NDT methods.

Nonlinear ultrasonic technique for closed crack detection

The detection of cracks at the early stage of fracture is important in industrial structures in order to guarantee their structural safety. Ultrasound has been widely utilized in the field of nondestructive testing of materials. However, most of these conventional methods using ultrasonic characteristics in the linear elastic region are mostly sensitive to opened cracks but much less sensitive to such closed cracks. The nonlinear ultrasonic technique (NUT) based on the contact acoustic nonlinearity (CAN) has been considered as a promising method for the closed crack detection. However, most of the previous studies were limited to the modeling of the second-order harmonic wave generation at contacted interfaces and its verification by testing artificially contacted interfaces in the through-transmission method. In this study, we investigated experimentally the contact acoustic nonlinearity at a real crack by using the measurement system constructed in the reflection mode that permits the transducers to access the only single side of a test structure. Results showed that the magnitude of the second-order harmonic wave represented the existence of the closed area clearly and that the crack sizing performance was greatly improved by the combination of the linear and nonlinear ultrasonic techniques.

2D finite element modeling of the non-collinear mixing method for detection and characterization of closed cracks

The non-collinear mixing technique is applied for detection and characterization of closed cracks. The method is based on the nonlinear interaction of two shear waves generated with an oblique incidence, which leads to the scattering of a longitudinal wave. A Finite Element model is used to demonstrate its application to a closed crack. Contact acoustic nonlinearity is modeled using unilateral contact law with Coulomb׳s friction. The method is shown to be effective and promising when applied to a closed crack. Scattering of the longitudinal wave also enables us to image the crack, giving its position and size.

Advanced Fatigue Crack Detection Using Nonlinear Self-Sensing Impedance Technique for Automated NDE of Metallic Structures

This article reports the application of a nonlinear impedance technique under a low-frequency vibration to detect contact-type structural defects such as fatigue cracks. If the contact-type damage is developed within the structure due to the low-frequency dynamic load, the vibration can cause a nonlinear fluctuation of the structural impedance because of the contact acoustic nonlinearity (CAN). This nonlinear effect can lead to amplitude modulation and phase modulation of the current flow. The nonlinear characteristics of the structural impedance can be extracted by observing the coupled electromechanical impedance of a piezoelectric active sensor and utilizing nonlinear wave modulation spectroscopy. Experimentally, a low-frequency vibration was applied to a notched coupon at a certain natural frequency by a shaker, so that a nonlinear fatigue crack can be artificially formed at the notch tip. Then, the nonlinear features are extracted based on a self-sensing impedance measurement from a host structure under a low-frequency vibration. The damage metric was established based on the nonlinear fluctuation of the impedance due to the CAN.

102 変調した超音波の非線形効果による接触型損傷の検出(セッション1 損傷検知・信号処理)

This paper presents a concept of nonlinear ultrasonic-based damage detection method that may not require high-rate data sampling as well as baseline data in the healthy state. Recently, because of a serious problem that most of Japanese infrastructures become deteriorated in near future, efficient and non-labor-intensive inspection methodologies are eagerly required. In this study, we propose a method that utilizes a frequency mixture phenomenon referred to as "frequency down-conversion" which is induced by a contact acoustic nonlinearity associated with contact-type damages involved in the structure. Because of this nonlinearity, when an amplitude-modulated wave is input to the structure, a low frequency component which has the same frequency as the modulation signal is excited. The result of a preliminary experiment using a steel beam specimen shows the excitation of the low frequency component when the specimen has a simulated damage, and the dependency of the magnitude of that component on the modulation frequency.

Diagnosis of Nonlinear Elastic Properties of the Boundary of two Flat Rough Solids by Surface Acoustic Waves

The paper presents the results of experimental investigations of nonlinear elastic properties of boundary between two rough surfaces of solids. The study was carried out using surface acoustic waves (SAW). We investigated the effect of the static pressure applied to the boundary of two solids and the amplitude of the probing SAW on efficiency of second harmonic generation at the border of rough surfaces. Nonlinear acoustic parameter of the second order depending on the external pressure applied to the interface was calculated. Analysis of the experimental results based on the theory of contact acoustic nonlinearity.

Improvement of crack sizing performance by using nonlinear ultrasonic technique

The nonlinear ultrasonic technique (NUT) based on the contact acoustic nonlinearity (CAN) has been considered as a promising method for the closed crack detection. However, most of the previous studies were limited to the modeling of the second-order harmonic wave generation at contacted interfaces and its verification by testing artificially contacted interfaces in the through-transmission method. In this study, we investigated experimentally the contact acoustic nonlinearity at a real crack by using the measurement system constructed in the pitch-catch method that permits the transducers to access the only single side of a test structure. Results showed that the magnitude of the second-order harmonic wave represented the existence of the closed area clearly and that the crack sizing performance was greatly improved by the combination of the linear and nonlinear ultrasonic techniques.

A Reflective Nonlinear Acoustic Microscope to Contour the Quantitative Adhesion at a Bonded Solid-Solid Interface

We set up a reflective nonlinear acoustic microscope to contour the quantitative adhesion at a bonded solid-solid interface by a contact acoustic nonlinearity (CAN) method. The principle of the reflective nonlinear acoustic microscope is described. After the vibration amplitude of the incident, focusing wave at the bonded interface is calculated, the standard adhesion with a complete bonding state is established by the tension test, the reflective CAN parameter is calibrated, and the quantitative contour of the adhesion at the interface can be obtained. The experimental contours of two samples are also presented. Compared with the transmitted microscope, the reflective one is more convenient and more suitable for practical applications.

Estimation of damaged part in a beam structure based on nonlinear wave modulation

This paper proposes a damage localization method for a beam structure, which can discriminate a specific part of the structure involving contact-type damages such as cracks, delamination and loose bolt failures. When the structure is subjected to a low-frequency excitation, the scatter characteristics of high-frequency waves in the vicinity of the damage location may fluctuate in synchronization with the low-frequency vibration due to the contact acoustic nonlinearity. Thus, a high-frequency sinusoidal probe wave going through the damaged part undergo amplitude and phase modulation, and as a consequence, sideband-frequency waves emerge accompanied with the the probe wave. In this paper, it is derived that the damaged structural part can be discriminated by measuring in-coming and out-going wave components separately by a sensor array and evaluating the amplitude ratio between those components at the sideband frequency. Experiments using a beam specimen with a simulated damage is conducted to examine the validity of the proposed method.

Actuation of atomic force microscopy microcantilevers using contact acoustic nonlinearities

A new method of actuating atomic force microscopy (AFM) cantilevers is proposed in which a high frequency (>5 MHz) wave modulated by a lower frequency (~300 kHz) wave passes through a contact acoustic nonlinearity at the contact interface between the actuator and the cantilever chip. The nonlinearity converts the high frequency, modulated signal to a low frequency drive signal suitable for actuation of tapping-mode AFM probes. The higher harmonic content of this signal is filtered out mechanically by the cantilever transfer function, providing for clean output. A custom probe holder was designed and constructed using rapid prototyping technologies and off-the-shelf components and was interfaced with an Asylum Research MFP-3D AFM, which was then used to evaluate the performance characteristics with respect to standard hardware and linear actuation techniques. Using a carrier frequency of 14.19 MHz, it was observed that the cantilever output was cleaner with this actuation technique and added no significant noise to the system. This setup, without any optimization, was determined to have an actuation bandwidth on the order of 10 MHz, suitable for high speed imaging applications. Using this method, an image was taken that demonstrates the viability of the technique and is compared favorably to images taken with a standard AFM setup.

Nonlinear Acoustics in Non-Destructive Testing - From Theory to Experimental Application

A growing interest in non-destructive testing methods based on nonlinear acoustics have been observed for the last ten fifteen years. The majority of methods in this area take their origin from the observation that fatigue damaged materials often behave like mesoscopic nonlinear materials (e.g. rocks) in which nonlinear phenomena have been observed for years. The most important phenomena include: higher harmonics generation, vibro-acoustic wave modulations, amplitude dependent resonance frequency shift and slow dynamic effects. All these phenomena result mainly from elastic wave interactions with contact-type defects. There is enough experimental evidence in the literature showing that these nonlinear effects are much more distinct in damaged materials than in intact ones. Despite the fact that many experimental techniques - based on nonlinear acoustic phenomena - have been developed for the last ten years, the physical mechanism of elastic wave interaction with damage materials still not clear. The main reason is the variety of possible nonlinear mechanisms involved. This includes: nonlinear elasticity and dissipation, contact acoustic nonlinearity based on herztian and rough surfaces contact theories and other effects such us adhesion, friction and thermoelasticity. This paper provides a short summary of various theoretical developments and examples of applications to damage detection in different materials.

Imaging of contact acoustic nonlinearity using synthetic aperture technique

The angle beam incidence and reflection technique for the evaluation of contact acoustic nonlinearity (CAN) at solid–solid contact interfaces (e.g., closed cracks) has recently been developed to overcome the disadvantage of accessing both the inner and outer surfaces of structures for attaching pulsing and receiving transducers in the through-transmission of normal incidence technique. This paper proposes a technique for B-mode imaging of CAN based on the above reflection technique, which uses the synthetic aperture focusing technique (SAFT) and short-time Fourier transform (STFT) to visualize the distribution of the CAN-induced second harmonic magnitude as well as the nonlinear parameter. In order to verify the usefulness of the proposed method, a solid–solid contact interface was tested and the change of the contact acoustic nonlinearity according to the increasing contact pressure was visualized in images of the second harmonic magnitude and the relative nonlinear parameter. The experimental results showed good agreement with the previously developed theory identifying the dependence of the scattered second harmonics on the contact pressure. This technique can be used for the detection and improvement of the sizing accuracy of closed cracks that are difficult to detect using the conventional linear ultrasonic technique.

Investigation of contact acoustic nonlinearities on metal and composite airframe structures via intensity based health monitoring

Nonlinear structural intensity (NSI) and nonlinear structural surface intensity (NSSI) based damage detection techniques were improved and extended to metal and composite airframe structures. In this study, the measurement of NSI maps at sub-harmonic frequencies was completed to provide enhanced understanding of the energy flow characteristics associated with the damage induced contact acoustic nonlinearity mechanism. Important results include NSI source localization visualization at ultra-subharmonic (nf/2) frequencies, and damage detection results utilizing structural surface intensity in the nonlinear domain. A detection metric relying on modulated wave spectroscopy was developed and implemented using the NSSI feature. The data fusion of the intensity formulation provided a distinct advantage, as both the single interrogation frequency NSSI and its modulated wave extension (NSSI-MW) exhibited considerably higher sensitivities to damage than using single-sensor (strain or acceleration) nonlinear detection metrics. The active intensity based techniques were also extended to composite materials, and results show both NSSI and NSSI-MW can be used to detect damage in the bond line of an integrally stiffened composite plate structure with high sensitivity. Initial damage detection measurements made on an OH-58 tailboom (Penn State Applied Research Laboratory, State College, PA) indicate the techniques can be transitioned to complex airframe structures achieving high detection sensitivities with minimal sensors and actuators.

Estimation of Damage Location of Beam Subjected to Impact Hammer Excitation Based on Nonlinear Wave Modulation Phenomenon

This paper concerns the estimation method of damage location based on the nonlinear wave modulation demodulation which has attracted much attention in the field of structural health monitoring. When a structure containing a contact-type damage is subjected to a low-frequency vibration, a high-frequency wave which propagates through the damage may undergo amplitude and phase modulation at the damage location because the scatter characteristics of high-frequency waves in the vicinity of the damage may fluctuate in synchronization with the low-frequency vibration because of the contact acoustic nonlinearity. Since this modulation propagates through the structure with the group velocity, it may be possible to estimte damage location by measuring the modulation and performing an inverse analysis based on the wave propagation model. In this paper, a mathematical model of the modulation propagation on a Timoshenko beam is constructed, and by considering the damage as a source of the modulation, an estimation algorithm of damage location which utilizes the time difference between the modulated waves measured by sensor arrays located at both sides of the damage is proposed, and examined experimentally.

경사입사 초음파에서 계면 접촉 음향 비선형성에 의해 발생한 고조파의 영상화

본 연구에서는 닫힘균열의 조기발견을 위해서 경사입사 초음파를 이용하여 계면 접촉 음향 비선형성에 의해 발생한 고조파를 영상화하고자 한다. 알루미늄 시편 두 개를 접촉하여 고체-고체 접촉계면을 만들어 닫힘균열을 모사하였고, 접촉계면 압력에 따른 초음파의 음향 비선형을 영상화하였다. 영상화 기법으로는 sampling phased array(SPA)와 synthetic aperture focusing technique(SAFT) 기법을 사용하였다. 접촉계면 압력이 증가하면서 기본 주파수의 성분은 감소하였고 2차 고조파 성분은 증가하다가 닫힘균열로 모사된 부분에서 가장 큰 값을 나타내고 이후에는 감소하였다. 결과적으로 접촉계면에서 음향 비선형성에 의해 발생한 고조파를 영상화 하는 것이 가능하며 이는 초기균열의 조기발견에 적용 가능할 것으로 기대 된다. The objective of this study is to image the harmonic wave generated by contact acoustic nonlinearity in obliquely incident ultrasonic wave for early detection of closed cracks. A closed crack has been simulated by contacting two aluminum block specimens producing solid-solid contact interfaces and then acoustic nonlinearity has been imaged with contact pressure. Sampling phased array(SPA) and synthetic aperture focusing technique(SAFT) are used for imaging techniques. The amplitude of the fundamental frequency decreased with appling pressure. But, the amplitude of second harmonic increased with pressure and was a maximum amplitude at the simulation point of closed crack. Then, the amplitude of second harmonic decreased. As a result, harmonic imaging of contact acoustic nonlinearity is possible and it is expected to be apply for early detection of initial cracks.

A quantitative imaging of bonding strength at the bonded solid-solid interface obtained by nonlinear test

As well known, when a longitudinal wave propagates through an interface with micro-cracks and micro-defects between solids, the contact acoustic nonlinearity (CAN) will be generated dramatically and the nonlinear parameter can be used to contour the bonding state of the interface. However the contour can only show the relative state of bonding strength and can not be used to judge weather the multilayered composite materials in use is safe because the safe judgment is not by the relative state while by the absolute value of the bonding strength. Therefore characterization of quantitative bonding strength at the interface is very important for judging a multilayered material in safe use. In this paper, how to get the quantitative bonding strength from the CAN parameter is studied. After the vibration amplitude of incident focusing wave at the bonded interface was calculated, the standard bonding strength with complete bonding state was established by tension test and CAN parameter is calibrated, the quantitative imaging of the bonding strength is obtained by CAN microscope in experiments. From the imaging, the positions with weak bonding strength could be easily located, which can be used to decide whether the material could be employed continuously.

Harmonic generation of an obliquely incident ultrasonic wave in solid–solid contact interfaces

The conventional acoustic nonlinear technique to evaluate the contact acoustic nonlinearity (CAN) at solid–solid contact interfaces (e.g., closed cracks), which uses the through-transmission of normally incident bulk waves, is limited in that access to both the inner and outer surfaces of structures for attaching pulsing and receiving transducers is difficult. The angle beam incidence and reflection technique, where both the pulsing and receiving transducers are located on the same side of the target, may allow the above problem to be overcome. However, in the angle incidence technique, mode-conversion at the contact interfaces as well as the normal and tangential interface stiffness should be taken into account. Based on the linear and nonlinear contact stiffness, we propose a theoretical model for the reflection of an ultrasonic wave angularly incident on contact interfaces. In addition, the magnitude of the CAN-induced second harmonic wave in the reflected ultrasonic wave is predicted. Experimental results obtained for the contact interfaces of A16061-T6 alloy specimens at various loading pressures showed good agreement with theoretical predictions. Such agreement proves the validity of the suggested oblique incidence model.

106 非定常低周波振動を用いた非線形波動変調現象に基づく損傷位置の推定(セッション2 同定技術)

This study concerns damage localization method based on nonlinear elastic wave modulation spectroscopy. When fragments of a contact-type damage is rupping and crapping in synchronization with the low-frequency vibration, a high-frequency wave which propagates through the damage may undergo amplitude and phase modulation at the damage location because the scatter characteristics of high -frequency waves in the vicinity of the damage may fluctuate in synchronization with the low-frequency vibration because of the contact acoustic nonlinearity. Since thi modulation propagates through the structure with the group velocity, it may be possible to estimate damage location by measurering the modulation and performing an inverse analysis based on the wave propagation model. In this paper a mathematical model of the phase modulation propagation on a Timoshenko beam is constructed, and by considering the damage as a source of the modulation wave, a localization algorithm which utilizes the time difference between the demodulated waves measured by sensor arrays located at both sides of the damage is proposed, and examind experimentally.

Nondestructive Characterization of Quantitative Bonding Strength at a Bonded Solid-Solid Interface

We propose a nondestructive method to characterize the quantitative bonding strength at a bonded solid-solid interface by a contact acoustic nonlinearity (CAN) microscope. The principle of the CAN microscope is briefly described. The vibration amplitude of the incident focusing wave at the bonded interface is calculated, the standard bonding strength with a complete bonding state is established by the tension test, and the CAN parameter is calibrated. The quantitative contour of bonding strength at the interface could be obtained. The experimental contours of two samples are also presented.

Nonlinear piezoelectric impedance modulation induced by a contact-type failure and itsapplication in crack monitoring

In this paper, a self-sensing, sensitive and baseline-free structural health monitoringmethodology is developed, which aims to detect and characterize local structuralfailures of contact type, i.e. failures which arise along with the generation, growthand/or changes of imperfect solid–solid interfaces. The nonlinear piezoelectricimpedance modulation (NPIM) method presented is based on the ideas of two existingdamage detection principles; a piezoelectric impedance-based methodology anda nonlinear wave modulation spectroscopy. It uses a single piezoelectric activesensor bonded on the structural surface, which is driven by a high-frequencyharmonic voltage source. When the structure including a contact-type failureis subjected to a low-frequency dynamic load, the induced structural vibrationcauses a fluctuation of the scattering conditions for the high-frequency elasticwaves at the failure because of the contact acoustic nonlinearity (CAN). Thisnonlinear effect of vibro-acoustic interaction yields a significant fluctuation inthe driving-point impedance in the high-frequency range, which may lead to amodulation of the coupled electromechanical impedance (or admittance) of thepiezoelectric active sensor. Therefore, if the sensor is driven by a fixed amplitudehigh-frequency harmonic voltage source, modulation of the coupled admittance can beobserved as the amplitude modulation and phase modulation of the current flowingthrough the sensor. A simplified modeling study leads to the definition of a damageevaluation index that assesses the intensity of the stiffness fluctuation caused by theCAN. Experiments using cracked beam specimens are conducted to show how theNPIM can be observed and to examine the performance of the proposed method.