A new technique for measuring the acoustic nonlinearity of materials using Rayleigh waves

This note presents a procedure to generate nonlinear Rayleigh surface waves without having to drive the transmitting piezoelectric transducer at high voltages; driving at low voltages limits the excitation of the intrinsic nonlinearity of the piezoelectric transducer element, and enables an efficient measurement procedure to isolate inherent material nonlinearity. The capabilities of this proposed technique are demonstrated by measuring the material nonlinearity of aluminum alloy 2024 and 6061 plates with Rayleigh surface waves.     

装甲车车内声学响应分析

装甲车的振动噪声控制一直是理论和工程领域不断研究的课题,其车内噪声直接影响作战人员的作战水平,进行装甲车内部噪声的研究十分必要。首先分别建立并分析了装甲车车内空腔模型以及考虑结构和空气之间相互作用的声固耦合模型,通过对比耦合前后的声学模态振型图,得出车体结构对空腔声学特性有一定的影响;然后使用边界元法对整车声固耦合模型进行声学响应分析,得到驾驶员右耳处的声压分布情况;最后对声压峰值的频率点进行板件声学贡献量分析,为改善车内的声学特性提供了参考依据。     

Fracture mechanism characterization of cross-ply carbon-fiber composites using acoustic emission analysis

The sequence of microscopic fracture mechanisms in locally loaded cross-ply carbon-fiber composites was studied by analyzing acoustic emission (AE) signals in combination with the modal analysis of Lamb waves, using microscopic and ultrasonic examination of the specimen after load interruption. The first 70 AE events were analyzed, which were detected during the initial loading segment when the first sudden load drop and gradual load recovery were observed. Characteristics of the detected waves were compared with the S₀- and A₀-mode Lamb waves produced by a spot- or line-focused YAG laser. The internal damage progression of the composite specimen was determined to be the fiber fracture in the front lamina, transverse cracks in the mid-lamina, delamination and splitting.     

Wavelet-based AE characterization of composite materials

This paper addresses an application of recently developed wavelet-based signal processing technique on a composite fracture behavior study. Here, the wavelet methodology is introduced, and a complete procedure of wavelet-based acoustic emission (AE) analysis methods developed by the author are demonstrated. In this research, the AE signals from glass fiber reinforced (GFR) composites are collected during the standard quasi-static tensile testing and transformed by the Daubechies discrete wavelets using the programs developed by the author. Based on AE tests, the fracture behaviors are studied for the material. In the study of GFR composite fracture behavior, classical linear fracture mechanics method is used for comparison. The exponential constant m value used to determine the relationship between stress and stress intensity factor are compared relative to the classical fracture mechanics and the AE techniques. In order to demonstrate the advantage of the wavelet-based AE techniques, the conventional AE analysis is also provided side-by-side for comparison. The results verify that the wavelet-based method better approximates residual strength relative to the classical AE techniques.In this paper, the results of the wavelet-based AE analysis of glass fiber composites are presented. The results indicate that wavelets-based signal processing is an efficient tool in the analysis and the transformation of AE signals.     

Fundamentals of nondestructive materials characterization

An attempt is made to outline the term ‘materials characterization’. On the basis of the structure-to-property-relationships, a philosophy is proposed, adapted to systematical development of methods for nondestructive materials characterization. The state of the art reached for metals is reviewed and typical applications of macroscopic physical properties for nondestructive materials characterization are given. The materials characterization of ball bearing steel and cast iron by multiparameter materials characterization is discussed in detail.     

Characterization of micro-crack propagation through analysis of edge effect in acoustic microimaging of microelectronic packages

The miniaturization and three dimensional die stacking in advanced microelectronic packages poses a big challenge to their non-destructive evaluation by acoustic microimaging. In particular, their complicated structures and multiple interfaces make the interpretation of acoustic data even more difficult. A common phenomenon observed in acoustic microimaging of microelectronic packages is the edge effect phenomena, which obscures the detection of defects such as cracks and voids.In this paper, two dimensional finite element modelling is firstly carried out to numerically simulate acoustic microimaging of modern microelectronic packages. A flip-chip with a 140 µm solder bump and a 230 MHz virtual transducer with a spot size of 16 µm are modelled. Crack propagation in the solder bump is further modelled, and B-scan images for different sizes of micro-cracks are obtained. C-line plots are then derived from the simulated B-scan images to quantitatively analyze the edge effect. Gradual progression of the crack is found to have a predictable influence on the edge effect profile. By exploiting this feature, a crack propagation characterization method is developed. Finally, an experiment based on the accelerated thermal cycling test is designed to verify the proposed method.     

Characterization of the grinding process by acoustic emission

The properties of a ground surface can be estimated on-line during manufacturing based on the analysis of acoustic signals emitted by the grinding process. This possibility is demonstrated using an experimental system comprising an external grinding machine, a data acquisition unit and an artificial neural network. In the initial phase of system application, an empirical model of the grinding process is formed in the memory of the neural network by self-organized learning driven by empirical data consisting of the acoustic emission spectrum and a surface roughness correlation function. After learning, the system applies the model to estimate the correlation function of the surface profile from the input acoustic emission spectrum. For this purpose, non-parametric regression, based on the conditional average estimator, is utilized. Experiments were done on the grinding of hardened steel workpieces by a corundum wheel. During formation of the model, the surface profile and its correlation function were determined off-line, while in testing system performance the surface correlation function was estimated on-line from the acoustic emission spectrum. With respect to the estimation error, three characteristic periods of the process were observed corresponding to grinding with a newly dressed, slightly worn, and worn out wheel. The best estimation is obtained during grinding by a slightly worn wheel.     

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.     

Magneto-thermo-mechanical characterization of giant magnetostrictive materials

The variations of magnetization and magnetostriction with temperature and stress were investigated through the analysis of the effective field,induced by temperature and stress.A nonlinear magnetostrictive model of giant magnetostrictive materials was proposed.The proposed model can be used to calculate the magnetostrictive characterization of giant magnetostrictive materials in different temperatures and under different stresses.The coupling effects of axial stress,magnetic field,and temperature on the magnetostriction of a Terfenol-D rod were numerically simulated as well as experimentally tested.Comparison between the calculating and experimental results shows that the proposed model can better describe the magneto-thermo-mechanical characteristics of Terfenol-D rod under different temperatures and compressive stress.Therefore,the proposed model possesses an important significance for the design of magnetostrictive devices.     

Determination of the elastic properties of some coated materials by simulation of the analogue signal of the reflection acoustic microscope

We discuss in this work the elastic properties of monolayer materials (layer/substrate). We consider both cases of quick/slow and slow/quick systems. We calculate Young's modulus, shear modulus and Poisson's ratio of the following four systems: Cr/Fe, TiN/Fe, Al/Si and Al/SiC. We make it evident that the use of the simulated acoustic signal of the reflection acoustic microscope yields to the determination of the wave propagation velocity of different modes in the layer/substrate according to the frequency and the thickness of the layer. We demonstrate that Victorov relation is not applicable in this case. A new method, that we called ‘method of homogenization’ is proposed to resolve the problem of dispersion.For the considered materials, the calculations have been carried out in a large range of frequencies (50 MHz–3.5 GHz) and coating layer thickness (0.5–150 μm). We show that for the studied materials the elastic constants take the values of the material layer beyond a thickness of 30–40 μm.     

Experimental and numerical evaluation of electromagnetic acoustic transducer performance on steel materials

Electromagnetic Acoustic Transducers (EMATs) are an attractive alternative to standard piezoelectric probes in a number of applications thanks to their contactless nature. EMATs do not require any couplant liquid and are able to generate a wide range of wave-modes; however these positive features are partly counterbalanced by a relatively low signal-to-noise ratio and by the dependence of EMAT performance on the material properties of the test object. A wide variety of steel materials is employed in many industrial applications, so it is important to assess the material-dependent behaviour of EMATs when used in the inspection of different types of steel. Experimental data showing the performance of bulk shear wave EMATs on a wide range of steels is presented, showing the typical range of physical properties encountered in practice. A previously validated Finite Element model, including the main transduction mechanisms, the Lorentz force and magnetostriction, is used to evaluate the experimental data. The main conclusion is that the Lorentz force is the dominant transduction effect, regardless of the magnitude and direction of the bias magnetic field. Differently from magnetostriction, the Lorentz force is not significantly sensitive to the typical range of physical properties of steels, as a consequence the same EMAT sensor can be used on different grades of ferritic steel.     

Modal analysis of acoustic emission signals from CFRP laminates

As a result of its continuous and in situ detection capabilities, the acoustic emission (AE) technique is the prime candidate for damage monitoring in loaded composite structures. None of the AE analysis techniques used in laboratory studies has, however, proven to be capable of consistently dealing with the difficulties encountered in larger structures: large amount of data, the elimination of noise sources and the influence of wave propagation effects (attenuation, dispersion). This work will use the modal acoustic emission (MAE) technique as a more intelligent and efficient way of analysing AE results. AE waveforms obtained during tensile and bending testing of CFRP laminates will be presented. It will be demonstrated how taking into account the modal nature of AE waves can in future lead to more quantitative and accurate results.     

Application of the acoustic resonance method to evaluate the grain size of low carbon steels

The velocities and attenuations of ultrasonic waves in low carbon steels were measured precisely and automatically by the acoustic resonance method using a Lorentz-type EMAT. The attenuations were measured from the decay of a signal from the resonant vibration after stopping the external excitation at the predetermined resonant frequency, while the velocities were calculated directly from the resonant frequencies and material thicknesses. It was not possible to predict the yield strength from the velocity measurements. There was a close relationship between grain size and attenuation in the specimens which consist of ferrite and pearlite. The attenuations measured at a frequency of about 5 MHz showed a good correlation with the average grain size and yield strength. The yield strength could be evaluated within the accuracy of ±50 MPa by the acoustic resonance method. The results would be used for on-line evaluation of the grain size and the mechanical strength of steels in industry.     

Inspection of power plant headers utilizing acoustic emission monitoring

Fossil power plant high energy, high temperature steam headers have been found to be susceptible to thermal fatigue assisted creep degradation. These mechanisms initiate and grow cracks in chrome molybdenum headers, from the bore hole edges and stub tube-to-header welds. Linking up of multiple cracks can lead to explosive expulsion of tubes and severe shorting of the header life. In order to extend the header life and operate safely, a better understanding of crack growth that may occur during specific plant operating conditions is needed. With that understanding, harsh operating conditions that may be causing excessive crack propagation and header damage can be curtailed. Acoustic emission monitoring of headers was performed to assist in identifying operating conditions that lead to header damage. This Electric Power Research Institute (EPRI) sponsored program found acoustic emission activity levels correlated to identified crack growth and analytically calculated stresses. Utilizing these results, draft EPRI guidelines have been developed to aid electric utilities in performing acoustic emission monitoring on superheater headers.     

On the characterization of continuous fibres fracture by quantifying acoustic emission and acousto-ultrasonics waveforms

The aim of the present work is the classification of the characteristics of elastic waves, which are generated by fibre failures during quasi-static tensile fibre bundle tests and captured by the use of acoustic emission (AE) method. In addition, elastic waves generated out of the fibre bundle gauge length and propagated through the bundle at different stages of the loading process using an acousto-ultrasonic (AU) technique are also classified. To this target, a large number of tests were conducted on organic, ceramic and carbon fibre bundles according to DIN 53942. An in-house developed analysis and quantification methodology of the captured AE and AU waveforms is proposed in order to identify the frequency content of the fibre failure event, to characterize the medium of propagation and to investigate the effect of the acquisition system on the monitored signals. In fact, the application of the proposed analysis on the results of the conducted mechanical tests leads to the development of a useful database, concerning the ranges of AE features and the representative sets of frequency values that correspond to fibre fracture. Furthermore, the proposed database offers valuable knowledge for the role of material parameters, such as fibre structure and properties, on the characteristics of the recorded waveforms, constituting in this way a valuable tool that enables a better understanding of elastic waves initiation and propagation through continuous fibres.     

Study and characterization by acoustic emission and electrochemical measurements of concrete deterioration caused by reinforcement steel corrosion

The durability of reinforced concrete structures becomes a matter of concern, due primarily to the increase of damage by the corrosion of steel reinforcements. This corrosion is not only related to the composition and to the procedure of concrete manufacturing (water/cement, sand/cement, etc.), but also to the aggressive agents as chlorides, carbon dioxide, etc. present in the surrounding medium (Cl⁻, CO₂, etc.). It is well known that the first kind of rebar corrosion (chloride) is more detrimental and that this process contains three basic components: chloride diffusion, electrochemical corrosion and concrete fracture. Therefore the early detection of possible degradation of structures by means of non-destructive testing is essential in order to ensure the functionality of these structures.

This paper presents the results of an experimental investigation on the use of acoustic emission during the corrosion of steel rebars embedded in mortar and immersed in sodium chloride solution. The process of corrosion is accelerated by various imposed potentials and is followed by acoustic emission coupled to electrochemical techniques. The experimental results show that electrochemical techniques can evaluate the corrosive character of the medium used. The acoustic emission showed an activity characteristic of the corrosion initiation phase and the corrosion propagation phase. Thus, it was significantly possible to highlight the acoustic signature of the concrete damage related to the porosity of the mortar and to chloride concentration. The results also show a perfect correlation between the evolution of the acoustic activity and the current of corrosion density.     

Fatigue crack monitoring of riveted aluminium strap joints by Lamb wave analysis and acoustic emission measurement techniques

Statistics show that fatigue crack development comes first and foremost as a damage source in aerospace metallic structures. Currently, widespread methods are available to inspect these structures, but they are quite time-consuming, costly and require the structural system to be idle. Next, attempts to develop damage detection integrated systems are paramount for the safety and cost of such structures. This paper describes an investigation into the feasibility of using an integrated system based on Lamb waves in order to assess the integrity of riveted aluminium joints during cyclical loading. In this experimental analysis, Lamb waves are excited and received outside the joint area using piezoelectric transducers coupled onto the plates. The detected damage is cracks in joint resulting from fatigue loading. The collected signals on the piezoelectric transducers are analysed using Hilbert transform and time–frequency analysis. It is shown that the final interpretation of Lamb wave analysis may provide a means of sizing the defects and following the crack development. In addition to that, an acoustic emission system is used jointly with the Lamb wave analysis in order to discuss results and damage development. Finally, it is demonstrated that both methods can work together and the results obtained are in good agreement.     

Wavelet transform analysis of acoustic emission in monitoring friction stir welding of 6061 aluminum

In this paper, acoustic emission (AE) signals are detected and preliminarily analyzed in order to investigate the possibility of applying the AE technique for the in-process monitoring of an entire friction-stir-welding (FSW) process. Experimental tests are carried out using a high-speed rotating tool traversing on two, butted 6061 aluminum alloy plates with three equally spaced gaps made of two notches aligned along the butting joint of the parts. The wavelet transform (WT) is used to decompose the AE signal into various discrete series of sequences over different frequency bands. There are significant sudden changes in the band energy at the moment when the probe penetrates into and pulls out of the weld joint, as well as when the shoulder makes contact with or detaches from the plates. The band energy variation during the traversing of the tool over the defected region reflects the existence, location, and size of the weld defects. A three-dimensional representation of band energy vs time and scale gives valuable information on the potential weld defects during friction stir welding. Coupled with a contour mapping, the representation can be effectively utilized for monitoring the transient welding state and quickly identifying gap defects.     

Higher harmonic analysis of ultrasonic signal for ageing behaviour study of C-250 grade maraging steel

Higher order harmonic analysis of ultrasonic signal has been applied to study the precipitation kinetics of 18Ni maraging steel. The material has been solution annealed (SA) at a temperature of 1088 K for 1 h and then air quenched. Ageing of SA materials have been done at 728 K for different durations of 10 min to 100 h and air cooled. It has been found that the measurement parameter of this new technique increases with the formation of finer precipitates at the initial stage of ageing till 12 h and then decreases at longer holding time with the coarsening of precipitates.     

A critical analysis of effectiveness of acoustic emission signals to detect tool and workpiece malfunctions in milling operations

The industrial demands for automated machining systems to increase process productivity and quality in milling of aerospace critical safety components requires advanced investigations of the monitoring techniques. This is focussed on the detection and prediction of the occurrence of process malfunctions at both of tool (e.g. wear/chipping of cutting edges) and workpiece surface integrity (e.g. material drags, laps, pluckings) levels. Acoustic emission (AE) has been employed predominantly for tool condition monitoring of continuous machining operations (e.g. turning, drilling), but relatively little attention has been paid to monitor interrupted processes such as milling and especially to detect the occurrence of possible surface anomalies.This paper reports for the first time on the possibility of using AE sensory measures for monitoring both tool and workpiece surface integrity to enable milling of “damage-free” surfaces. The research focussed on identifying advanced monitoring techniques to enable the calculation of comprehensive AE sensory measures that can be applied independently and/or in conjunction with other sensory signals (e.g. force) to respond to the following technical requirements: (i) to identify time domain patterns that are independent from the tool path; (ii) ability to “calibrate” AE sensory measures against the gradual increase of tool wear/force signals; (iii) capability to detect workpiece surface defects (anomalies) as result of high energy transfer to the machined surfaces when abusive milling is applied.Although some drawbacks exist due to the amount of data manipulation, the results show good evidence that the proposed AE sensory measures have a great potential to be used in flexible and easily implementable solutions for monitoring tool and/or workpiece surface anomalies in milling operations.