Showing papers in "Quantitative Nondestructive Evaluation in 2006"
TL;DR: In this article, a model-based, broadband, four-point potential drop measurement technique has been used to characterize metal plates by means of a model•based broadband, broadband and four point potential drop technique.
Abstract: A study to characterize metal plates by means of a model‐based, broadband, four‐point potential drop measurement technique has shown that the relative permeability of alloy 1018 low‐carbon steel is complex and a function of frequency. A magnetic relaxation is observed at approximately 5 kHz. The relaxation can be described in terms of a parametric (Cole‐Cole) model. Factors which influence the frequency, amplitude and breadth of the relaxation, such as applied current amplitude, sample geometry and disorder (e.g. percent carbon content and surface condition), are considered.
70 citations
TL;DR: In this article, a method of modelling guided wave propagation with finite elements using frequency domain analysis and absorbing regions is presented, where regions are placed at the extremities of the models and can absorb guided waves, thus avoiding reflections from the boundaries.
Abstract: This paper presents a method of modelling guided waves propagation with Finite Elements using frequency domain analysis and absorbing regions. These regions are placed at the extremities of the models and can absorb guided waves, thus avoiding reflections from the boundaries. Guidelines for efficient definition of these regions are discussed. Typical case studies are presented in order to validate the process. This method will benefit the modelling of large problems such as the interaction of guided waves with complex 3D defects in complex 3D structures.
66 citations
TL;DR: In this paper, the authors investigated the heat generation mechanisms that occur during excitation of a specimen with high power ultrasound (20 kHz and above) in order to obtain stable and easy to interpret results.
Abstract: In this paper we investigate the heat generation mechanisms that occurs during excitation of a specimen with high‐power ultrasound (20 kHz and above). In order to obtain stable and easy to interpret results we use a set‐up with a tunable piezo instead of an ultrasound welding system commonly used and excite the specimens at their resonance frequencies. We will report the results of recent investigations which reveal several different mechanisms contributing to the overall thermal signal. Besides frictional effects at crack faces also thermoplastic heating may occur at crack tips. In materials with high sound attenuation the heating of the bulk material itself can be measured. In this case the detected infrared signal corresponds to local stress fields of the induced vibration.
55 citations
TL;DR: In this paper, the authors presented a field deployable ultrasonic inspection method which has the sensitivity of TTU methods, is noncontact, and does not require access to the back side of the part.
Abstract: Through transmission ultrasonic (TTU) methods are widely used for inspection of critical aerospace structural components because of their simplicity and the high sensitivity of TTU methods to bonding and delamination type defects. TTU inspection requires access to both sides of a component, which is generally not a problem before components are installed in final assemblies. However, limited access to the back side of a component on final assemblies usually precludes using TTU methods. Pulse echo (PE) methods are often used when only single side access is available, but PE ultrasonic methods have a limited penetration range from the outer surface, and do not have the sensitivity of TTU, particularly when surfaces are non‐parallel. Thus, structural assemblies are often disassembled when a thorough TTU inspection is required. The work presented here addresses the need for a field deployable ultrasonic inspection method which has the sensitivity of TTU methods, is non‐contact, i.e., couplant is not required, and does not require access to the back side of the part. These goals are accomplished by attaching a sparse array of ultrasonic transducers to the back side of a component or embedding them within the component. These transducers are excited to generate ultrasonic waves which propagate through the structure, and the resultant acoustic wavefields are imaged using a non‐contact, air‐coupled transducer. This ultrasonic wavefield imaging method is referred to as Acoustic Wavefield Imaging (AWI). Results are presented for a bonded aluminum plate specimen demonstrating that recorded wavefield images clearly show bonding flaws at internal interfaces.
53 citations
TL;DR: In this article, three different synthetically focused imaging algorithms for a linear array aperture: CSM (Common Source Method), SAFT (Synthetic Aperture Focusing Technique) and TFM (Total Focusing Method).
Abstract: Synthetically focused imaging has been used for some time in the NDE community. The techniques have primarily been directed towards imaging using bulk waves. There has recently been use of SAFT (Synthetic Aperture Focusing Technique) using guided waves in plates. Here, we review three different synthetically focused imaging algorithms for a linear array aperture: CSM (Common Source Method), SAFT and TFM (Total Focusing Method). The resolution of the different techniques is obtained from scalar diffraction theory and then validated by means of a low frequency (50kHz) steel plate experiment using PZT excitation and laser reception of the A0 mode. Imaging of through thickness slits parallel to the array is then discussed.
47 citations
TL;DR: Terahertz NDE imaging under paint for corrosion is examined as a method to inspect for corrosion by examining the terahertz response to paint thickness and surface roughness as discussed by the authors.
Abstract: Corrosion under paint is not visible until it has caused paint to blister, crack, or chip If corrosion is allowed to continue then structural problems may develop Identifying corrosion before it becomes visible would minimize repairs and costs and potential structural problems Terahertz NDE imaging under paint for corrosion is being examined as a method to inspect for corrosion by examining the terahertz response to paint thickness and to surface roughness
46 citations
TL;DR: A quantitative sensor placement optimization method with covariance matrix adaptation evolutionary strategy (CMAES) and the reliability of this genetic and evolutionary optimization method is proved in this study.
Abstract: In structural health monitoring (SHM), sensor network scale and sensor distribution decisions are critical since sensor network performance and system cost are greatly affected. A quantitative sensor placement optimization method with covariance matrix adaptation evolutionary strategy (CMAES) is presented in this paper. A damage detection probability model is developed for ultrasonic guided wave sensor networks. Two sample problems are presented in this paper. One is for structure with irregular damage distribution probability, and the other is for an E2 aircraft wing section. The reliability of this genetic and evolutionary optimization method is proved in this study. Sensor network configurations with minimum missed‐detection probability are obtained from the results of evolutionary optimization. The tradeoff relationship between optimized sensor network performance and the number of sensors is also presented in this paper.
45 citations
TL;DR: In this article, the performance characteristics of surface-bonded piezoelectric sensors have been studied under accelerated exposure conditions typically found in an operational aircraft environment, including freeze-thaw, moderate heat levels, humidity, electrochemical attack, substrate bend and tensile strains, and dynamic vibration conditions.
Abstract: Significant progress has recently been reported in the area of integrated structural health monitoring, with many sensor systems being deployed in actual operational environments. A key question that needs to be addressed and answered with regard to successfully implementing structural health monitoring technologies in aerospace systems involves the long‐term operability, durability, and survivability of integrated sensor systems and their associated hardware. In this activity, the performance characteristics of surface‐bonded piezoelectric sensors have been studied under accelerated exposure conditions typically found in an operational aircraft environment. In particular, sensor performance was studied for freeze‐thaw, moderate heat levels, humidity, electrochemical attack, substrate bend and tensile strains, and dynamic vibration conditions. The sensor performance was characterized using displacement‐field imaging, pitch‐catch signal transmission, and pulse‐echo signal transmission. Evidence of general ...
36 citations
TL;DR: In this article, a number of tomographic and phased array methods have been proposed for generating two dimensional images of plate-like structures using sparse arrays of spatially distributed ultrasonic transducers.
Abstract: A number of tomographic and phased‐array methods have been proposed for generating two dimensional images of plate‐like structures using sparse arrays of spatially distributed ultrasonic transducers. The phased array differential approach is considered here whereby pulse echo and through transmission signals are recorded before and after localized damaged is introduced, and differenced signals are combined using a focusing rule to produce an image of the plate. The application is structural health monitoring where the transducers are permanently bonded to the structure. The quality of the image is affected by many factors such as the number and location of the transducers, the characteristics of the damage, the signal‐to‐noise ratio, presence of edge reflections, and anything unrelated to damage that may perturb the ultrasonic signals such as temperature changes and transducer bonding variations. Two methods for enhancing image quality are implemented and then evaluated as to their effectiveness. In the first method, the windowing function is changed in width prior to phased signal addition to yield the best image quality. In the second method, signals are envelope‐detected prior to phased signal addition to eliminate phasing artifacts. Results are reported for artificial defects introduced in aluminum plates.
36 citations
TL;DR: In this article, a new interferometric scheme that combines quadrature detection with detector arrays for multi-speckle processing has been presented for robust and sensitive detection of ultrasound in industrial environment.
Abstract: The new interferometric scheme that was presented at last year QNDE conference for robust and sensitive detection of ultrasound in industrial environment has been further improved. The proposed architecture combines quadrature detection with detector arrays for multi‐speckle processing. The quadrature is reached tanks to the random phase distribution of the speckle light and the parallel signal processing is simplified by using an approximated demodulation technique based on signal rectification. Results demonstrating this new principle of operation and its performances are presented.
29 citations
TL;DR: In this paper, the magnetic hysteresis properties and Barkhausen effect signals in a series of induction hardened steel rods were studied through measurements and model simulations, with the objective of developing the measurement techniques for nondestructive evaluation of case depth.
Abstract: The magnetic hysteresis properties and Barkhausen effect signals in a series of induction hardened steel rods were studied through measurements and model simulations, with the objective of developing the measurement techniques for nondestructive evaluation of case depth. It was inferred from the measured hysteresis loop that magnetization reversal proceeded in two stages which took place in the core and the case of the hardened rods. The case depths of the samples were estimated by considering the hysteresis loops as a weighed sum of signals from the case and the core. The results were in good agreement with the nominal case depths determined from the hardness depth profiles.
TL;DR: In this paper, a phased array focusing technique was developed to improve the ultrasonic guided wave inspection results by concentrating the energy onto a defect, which can increase the energy impinging onto the defects, reduce false alarm ratio, locate the defects and enhance the propagation distance of the guided waves.
Abstract: Ultrasonic guided wave inspection techniques are well‐known for the inability to scan a long range axial distance of a pipe from a single transducer position. A phased array focusing technique was developed to improve the ultrasonic guided wave inspection results by concentrating the energy onto a defect. Focusing can increase the energy impinging onto the defects, reduce false alarm ratio, locate the defects, and enhance the propagation distance of the guided waves. An ultrasonic system with n (n>1) individual excitation channels is required to achieve phased array focusing. When phased array focusing was carried out, time delays and amplitude factors were applied to control the input signals for each excitation channel. Different from the time delays for bulk wave linear array focusing, the time delays and amplitude factors for guided wave array focusing are non‐linear functions of the focal distance, the pipe size, excitation conditions, and the active frequency.A challenge of this technique is to focu...
TL;DR: In this paper, the authors utilized the physics of guided wave propagation for structural health monitoring (SHM) transducer designs and demonstrated the importance of wave mode selection for SHM applications.
Abstract: The work presented in this paper utilizes the physics of guided wave propagation for structural health monitoring (SHM) transducer designs. Both the theoretical and experimental studies illustrated the importance of guided wave mode selection for SHM applications. Guided wave mode control is realized with an annular array transducer design on a PVDF polymer piezoelectric film. A sample problem on a 1mm thick aluminum plate is presented. Numerical calculations of the wave structures and guided wave power flow distribution inside the plate provide quick guidelines for the wave mode selection in structural health monitoring. Experimental study illustrates the importance of mode control with the comparison of PVDF annular array transducers and PZT ceramic disc transducers. The characteristics of wave mode reflections to defect depth and the defect sizing effect are also discussed in this paper.
TL;DR: In this article, an ultrasonic piezoelectric transducer together with a plastic wedge was used for the experimental generation of the Rayleigh wave and measurements were performed to detect the fundamental wave as well as the second harmonic.
Abstract: This research studies higher harmonics of Rayleigh surface waves propagating in nickel base superalloys. Rayleigh waves are used because they carry most of the energy and travel along the surface of a specimen where fatigue damage is typically initiated. The energy concentration near the free surface leads to stronger nonlinear effects compared to bulk waves. An ultrasonic piezoelectric transducer together with a plastic wedge is used for the experimental generation of the Rayleigh wave. The detection system consists of a laser heterodyne interferometer. Measurements are performed to detect the fundamental wave as well as the second harmonic. The amplitude ratio is related to the nonlinearity parameter β which is typically used to describe changes in microstructure and investigate fatigue damage.
TL;DR: To validate the concept of the Smart Flexible Phased Array Transducer, prototypes have been integrated to detect flaws machined in mock-ups with realistic irregular 2D and 3D shapes and Inspections have been carried out on samples showing the enhancement performances of the " smart flexible phased array" and validating the mechanical and acoustical behaviors of these probes.
Abstract: The inspection of piping in nuclear plants is mainly performed in contact with ultrasonic wedge transducers During the scanning, the fixed shape of wedges cannot fit the irregular surfaces and complex geometries of components (butt weld, nozzle, elbow) The variable thickness of the coupling layer, between the wedge and the local surface, leads to beam distortions and losses of sensitivity Previous studies have shown that these two phenomena contribute to reduce the inspection performances leading to shadow area, split beam To improve such controls, a new concept of contact “Smart Flexible Phased Array” has been developed with the support of the French “Institut de Radioprotection et de Surete Nucleaire” The phased array is flexible to fit the complex profile and to minimize the thickness of the coupling layer The independent piezoelectric elements composing the radiating surface are mechanically assembled in order to build an articulated structure A profilometer, embedded in the transducer, measures the local surface distortion allowing to compute in real‐time the optimized delay laws and compensating the distortions of 2D or 3D profiles Those delay laws are transferred to the real‐time UT acquisition system, which applies them to the piezoelectric elements This self‐adaptive process preserves, during the scanning, the features of the focused beam (orientation and focal depth) in the specimen To validate the concept of the Smart Flexible Phased Array Transducer, two prototypes have been integrated to detect flaws machined in mock‐ups with realistic irregular 2D and 3D shapes Inspections have been carried out on samples showing the enhancement performances of the “Smart Flexible Phased Array” and validating the mechanical and acoustical behaviours of these probes
TL;DR: In this article, the authors developed the appropriate dispersion curves and wave structures for circumferential guided wave propagation behavior in a pipe coated with a viscoelastic material, and experiments utilizing a linear transducer array were conducted at low frequency for successfully detecting delamination and volumetric defects in tar coated pipe.
Abstract: Circumferential guided wave propagation behavior in a viscoelastic multi‐layered hollow cylinder is studied to provide a baseline for defect detection in tar coated pipelines. Theoretical work was carried out by developing the appropriate dispersion curves and wave structures for circumferential guided waves in a pipe coated with a viscoelastic material. Parameters that affect wave attenuation were investigated with some initial guidelines being established for improved penetration power. Low frequencies are suggested from both attenuation and detection depth points of view. Under this guidance, experiments utilizing a linear transducer array were conducted at a low frequency for successfully detecting delamination and volumetric defects in tar coated pipe. A study was carried out to find the appropriate features for defect detection in coated pipe and a test protocol based on this study is recommended and summarized.
TL;DR: In this paper, a semi-analytical FEM model is used to calculate the theoretical wave structures for a rod embedded in concrete, and a qualitative relationship between percent delamination and wave reflection energy is demonstrated.
Abstract: It is necessary to quantify the amount of delamination present in load bearing rock bolts, as small amounts may be deemed acceptable and large amounts may lead to catastrophic failure. In this work, a semi‐analytical FEM model is used to calculate the theoretical wave structures for a rod embedded in concrete. A qualitative relationship between percent delamination and wave reflection energy is demonstrated. It is shown that with the proper selection of inspection parameters, it is possible to inspect large lengths (10+ ft.) of embedded rod using an ultrasonic guided wave pulse‐echo technique.
TL;DR: In this paper, the slope of pulsed eddy current signals at the Lift-Off point of Intersection (LOI) depends strongly on the lift-off, and the behavior of the LOI slope is studied under varying test conditions.
Abstract: It is observed that the slope of pulsed eddy current signals at the Lift‐Off point of Intersection (LOI) depends strongly on the lift‐off. The behaviour of the LOI slope is studied under varying test conditions. The signal response is curve‐fitted to a circuit equivalent non‐linear model using the Levenberg‐Marquardt algorithm and the LOI slope is computed using the curve‐fitted parameters. This study demonstrates that the LOI slope can be used to accurately measure the non‐conductive layer thickness (e.g. paint, coatings, etc.) on a conductive substrate.
TL;DR: In this article, the authors used a simple rectangular plate to detect defects in a structural health monitoring (SHM) system, which is shown to work well in the short-term and good sensitivity to defects is demonstrated.
Abstract: Guided waves offer an attractive means of performing structural health monitoring (SHM). It is desirable for such a system to have maximum sensitivity with minimum sensor density. However, in all but the most simple structures the wave interactions become too complex for the time domain signals to be interpreted directly. One approach to overcoming this complexity is to subtract a baseline signal from the measured system thus enabling changes in the system to be identified. Two key issues must be addressed to allow this paradigm to become a reality. Firstly, the system must be sufficiently sensitive to small reflections from defects Secondly, it must be able to distinguish between benign changes and those due to structural defects. In this paper this subtraction approach is used to detect defects in a simple rectangular plate. The system is shown to work well in the short‐term and good sensitivity to defects is demonstrated. The performance degrades over the long‐term. The principal reason for this degradation is shown to be the temperature of the system.
TL;DR: In this paper, the role of Lamb wave dispersion in focusing is discussed and visualized and discussed, and it is demonstrated that the sound beam can be directed into areas of interest and focused.
Abstract: Laser vibrometer are used to study Lamb wave propagation in plate structures. Sound fields are visualized as intensity and snapshoot images. It is demonstrated, that the sound beam can be directed into areas of interest and focused. The role of Lamb wave dispersion in focusing is visualized and discussed.
TL;DR: In this article, a study dedicated to the modeling of the ultrasonic propagation in parts made of carbon fiber-reinforced epoxy composites (CFRP), which is of great interest for the aircraft industry, is presented.
Abstract: This paper describes a study dedicated to the modeling of the ultrasonic propagation in parts made of carbon‐fiber‐reinforced epoxy composites (CFRP), which is of great interest for the aircraft industry. In a previous article, we proposed to evaluate the field radiated into such components by means of a homogenization method coupled to the pencil model implemented in CIVA software. This homogenization is based on the follow‐up of the energy ray paths inside each ply of a pattern, and leads to effective stiffness constants through the building of a slowness surface. An improvement is proposed here which allows to take into account the inner refraction energy losses.
TL;DR: In this paper, a wavelet transform is used to convert the 1D time traces into 2D binary thumbprint images, which are then identified according to their unique 2D time-scale patterns in these wavelet thumbprints.
Abstract: We describe a signal processing technique for time‐domain reflectometry (TDR) detection of flaws in wiring. For subtle flaws the backscattered TDR voltage pulses are too slight to be identified by amplitude‐based peak‐detection methods. Here, a wavelet transform is used here to convert the 1D time traces into 2D binary “thumbprint” images. Flaws are then identified according to their unique 2D time‐scale patterns in these wavelet thumbprints. The method is demonstrated for RG58 coaxial cables with varying amounts of damage to the shielding.
TL;DR: In this paper, the applicability of a new method by nonlinear acoustics to detection of closed cracks was investigated, and numerical analysis and experiment were conducted on a specimen with a closed crack.
Abstract: Recently diffusion joining and friction stir joining have been used for a solid state bonding. They can be affected by impurity of the atmosphere, temperature etc, so a closed crack may arise. This research investigates applicability of a new method by nonlinear acoustics to detection of closed cracks. Numerical analysis and experiment were conducted on a specimen with a closed crack. As a result, higher harmonics appeared in received signals. The longer the crack was, the larger the amplitude of higher harmonics became. These results show a possibility of nondestructive testing by nonlinear acoustics.
TL;DR: In this paper, a photorefractive interferometer is used for ultrasonic measurements in coarse microstructure materials and at temperatures up to 1250 °C, where the generated waves extend to very low frequencies.
Abstract: Laser‐ultrasonics presents many advantages compared to conventional ultrasonics, but is, generally, considered as less sensitive. As a consequence, laser‐ultrasonics should not be adequate for ultrasonic measurements in coarse microstructure materials or measurements of large thicknesses. However, since the generated waves extend to very low frequencies, measurements in such conditions can be successfully performed if a photorefractive interferometer sensitive also to these low frequencies and properly balanced is used for detection. This is demonstrated by measurements of thicknesses up to 100 mm (4″) for various steel grades and at temperatures up to 1250 °C.
TL;DR: In this paper, air-coupled sensors are used to determine the thickness and find delaminations in concrete slabs with impact-echo approach, and the results show that the air coupled sensors work as effectively as contact impact•echo sensors when proper impactors are used.
Abstract: Air‐coupled impact‐echo method for NDT of concrete is presented. In this study, air‐coupled sensors are used to determine the thickness and find delaminations in concrete slabs with impact‐echo approach. Test results show that air‐coupled sensors work as effectively as contact impact‐echo sensors when proper impactors are used. The air‐coupled impact‐echo test is then carried out on a concrete slab containing several artificial delaminations and voids. Evaluation of the degree of delaminations is successfully conducted using the air‐coupled impact‐echo method.
TL;DR: Algorithms are demonstrated that enable the reflector orientation and specularity to be probed and the signal to noise performance in experimental full matrix capture is addressed.
Abstract: The full matrix of data from an ultrasonic array contains time‐domain signals from every possible transmitter‐receiver combination. In addition to standard imaging algorithms such as B‐scans and sector scans, the information in the full matrix allows more exotic algorithms to be executed in post processing. Here, algorithms are demonstrated that enable the reflector orientation and specularity to be probed. The signal to noise performance in experimental full matrix capture is also addressed.
TL;DR: In this article, the authors describe their experience in applying air-coupled ultrasound to the inspection of a variety of composite structures, from honeycomb with thin composite facesheet to very thick solid laminates.
Abstract: The inspection of composite materials and structures with air‐coupled ultrasound has the obvious advantage that it is non‐contact, non‐contaminating, and free from couplants. However, the transmission efficiency from air to solid is extremely low due to the enormous difference in acoustic impedance. The development of more efficient airborne ultrasonic transducers over the years has made it possible, and even practical, to inspect composites with airborne ultrasound. It is now possible to drive newer, more efficient transducers with a portable ultrasonic flaw detector to inspect 2‐inch thick solid CFRP in air. In this paper we describe our experience in applying air‐coupled ultrasound to the inspection of a variety of composite structures, from honeycomb with thin composite facesheet to very thick solid laminates. General considerations for making airborne ultrasonic measurement in composite are given, and mechanism of transmission through honeycomb core, and resonance effects in transmitting through thick laminates will be described. NDE results of defects and damage in various composite structures will be presented.
TL;DR: For a long time, non-destructive residual stress assessment has been one of the greatest opportunities as well as the greatest challenges for the NDE community, and probably it will remain so in the foreseeable future as mentioned in this paper.
Abstract: For a long time, nondestructive residual stress assessment has been one of the greatest opportunities as well as one of the greatest challenges for the NDE community, and probably it will remain so in the foreseeable future. The most critical issue associated with nondestructive residual stress assessment seems to be that of selectivity. Numerous NDE methods have been found to be sufficiently sensitive to the presence of residual stress, but unfortunately also rather sensitive to other spurious variations that usually accompany residual stresses, such as anisotropic texture, microstructural inhomogeneity, plastic deformation, etc., which could interfere with, or even overshadow, the elastic strain caused by the sought residual stress. The only sufficiently selective NDE method that is more or less immune from these spurious effects is X‐ray diffraction measurement, which however does not have the required penetration depth in most applications unless high‐energy neutron radiation is used. It is timely for the community to sit back and ask where we are in this important area. This paper presents an overview of the various indirect techniques that have been used to measure residual stress in the past. It is shown that traditional techniques have a number of limitations, which have spurred several recent research programs. Some of the new techniques that are presently being examined in the NDE community are reviewed and the current status of these research efforts is assessed.
TL;DR: In this article, the authors showed that chaotic sound is more efficient than non-chaotic sound excitation for heating cracks, and they also presented the results of a finite element calculation in which chaotic sound occurs spontaneously.
Abstract: It has been previously shown experimentally that the use of chaotic sound, instead of a pure frequency, greatly enhances the heating, and hence the detectability of cracks using sonic infrared imaging (SIR). In this paper we show an example of the enhancement of crack heating through the use of chaotic sound. We also present the results of a finite element calculation, in which chaotic sound occurs spontaneously. This modeling confirms the experimental result that chaotic sound is more efficient than non‐chaotic sound excitation for heating the cracks.
TL;DR: In this paper, a non-destructive measurement of residual stresses, their anisotropy, and distribution as a function of depth is sought to verify initial process quality and residual stress retention over time.
Abstract: Low‐plasticity burnishing (LPB) is used to introduce deep compressive surface residual stresses that improve the durability of parts A non‐destructive measurement of residual stresses, their anisotropy, and distribution as a function of depth is being sought to verify initial process quality and residual stress retention over time Laser‐ultrasonic measurements of Rayleigh wave and surface skimming longitudinal wave (SSLW) velocities were used together to evaluate the magnitudes and directions of the two principal stresses independently of LPB‐induced texture variations The results agree with x‐ray measurements at the surface In addition, it was found that the laser‐ultrasonic pulse generation mechanism was surface‐process dependent