A novel laser-EMAT ultrasonic longitudinal wave resonance method for wall thickness measurement at high temperatures.

In this paper we propose a novel ultrasonic longitudinal wave resonance method for measuring the thickness of metal walls using a laser-electromagnetic acoustic transducer (Laser-EMAT). The method is based on the surface constraint mechanism (SCM) of the material and is expected to be capable of accurately detecting local thinning of metal walls in a non-contact manner and at high temperatures. Based on finite element analysis of laser-EMAT ultrasonic resonance measurement of aluminum alloy thickness, we investigated the effects of such key factors as SCM, irradiation parameters of laser source, and the size of EMAT receiving coil on the accuracy of thickness measurement (resonance frequency position) and on the amplitude of the resonance wave. Both numerical simulations and experiments are conducted to demonstrate that the measurement accuracy of the proposed method is not affected by SCM, irradiation laser source parameters, and EMAT receiving coil size, and that accurate detection of stepped aluminum plates with thickness thinning from 3.0 mm to 0.5 mm is achieved. Furthermore, we were able to perform rapid detection of aluminum thin plate thickness at 500 °C temperature with an EMAT lift-off of 5.0 mm and achieved a relative experimental error as small as 3.40 %. The results obtained in this study showed that the proposed method performed well in non-contact measurement of metal thinning in harsh environment of high temperature.

Highly Sensitive Detection of Microstructure Variation Using a Thickness Resonant Transducer and Pulse-Echo Third Harmonic Generation.

In nonlinear ultrasound testing, the relative nonlinear parameter is conveniently measured as a sensitive means of detecting and imaging overall variation of microstructures and damages. Compared to the quadratic nonlinear parameter (ß'), the cubic nonlinear parameter (γ'), calculated as the third harmonic amplitude divided by the cube of the fundamental amplitude, has generally a higher value, providing better sensitivity in nonlinear parameter mapping. Since the third harmonic amplitude is about two orders of magnitude lower than the fundamental amplitude, efficient excitation and highly sensitive reception of third harmonic is very important. In this paper, we explore an odd harmonic thickness resonant transducer that meets the requirements for pulse-echo third harmonic generation (THG) measurements. We also address the problem of source nonlinearity that may be present in the measured amplitude of the third harmonic and propose a method to properly correct it. First, we measure γ' for a series of aluminum specimens using the through-transmission method to observe the behavior of γ' as a function of specimen thickness and input voltage, and examine the effects of various corrections such as attenuation, diffraction and source nonlinearity. Next, we apply the odd harmonic resonant transducer to pulse-echo THG measurements of precipitation heat-treated specimens. It is shown that such transducer is very effective in generation and detection of fundamental and third harmonics under finite amplitude toneburst excitation. The highly sensitive detectability of γ' are presented as a function of aging time, and the sensitivity of γ' is compared with that of ß' and ß'2.

Measurement and In-Depth Analysis of Higher Harmonic Generation in Aluminum Alloys with Consideration of Source Nonlinearity.

Harmonic generation measurement is recognized as a promising tool for inspecting material state or micro-damage and is an ongoing research topic. Second harmonic generation is most frequently employed and provides the quadratic nonlinearity parameter (ß) that is calculated by the measurement of fundamental and second harmonic amplitudes. The cubic nonlinearity parameter (ß2), which dominates the third harmonic amplitude and is obtained by third harmonic generation, is often used as a more sensitive parameter in many applications. This paper presents a detailed procedure for determining the correct ß2 of ductile polycrystalline metal samples such as aluminum alloys when there exists source nonlinearity. The procedure includes receiver calibration, diffraction, and attenuation correction and, more importantly, source nonlinearity correction for third harmonic amplitudes. The effect of these corrections on the measurement of ß2 is presented for aluminum specimens of various thicknesses at various input power levels. By correcting the source nonlinearity of the third harmonic and further verifying the approximate relationship between the cubic nonlinearity parameter and the square of the quadratic nonlinearity parameter (ß∗ß), ß2≈ß∗ß, the cubic nonlinearity parameters could be accurately determined even with thinner samples and lower input voltages.

Modeling ultrasonic wave fields scattered by flaws using a quasi-Monte Carlo method: Theoretical method and experimental verification.

The modeling and visualization of wave fields scattered by flaws can be helpful in terms of guiding the testing and evaluation of flaws using an ultrasonic nondestructive method. In this work, the ultrasonic scattering of wave fields from flaws with different shapes is modeled using a quasi-Monte Carlo (QMC) method and measured through experiments for verification. The incident wave fields generated by a transducer can be modeled using the Rayleigh integral expression and calculated using the QMC method. When the size of the flaw is much larger than the wavelength, the incident wave over the lit portion of flaw can be treated as the source for the scattering of wave fields, and these wave fields can also be modeled using the proposed QMC method. In this paper, water is treated as the material and an embedded solid component is considered as the flaw. Numerical examples and results are presented for flaws with different shapes and sizes, and the properties of these scattering wave fields are analyzed and discussed. Experiments are performed to measure the scattering wave fields using a needle transducer, and it is shown that the results agree with the simulations, thus verifying the proposed modeling method. The work presented here can assist in understanding the wave-flaw interaction and can help in optimizing ultrasonic nondestructive testing.

Sideband peak count-index technique for monitoring multiple cracks in plate structures using ordinary state-based peri-ultrasound theory.

This work presents a peri-ultrasound theory based on ordinary state-based peridynamics for modeling elastic waves propagating in three-dimensional (3-D) plate structures and interacting with multiple cracks. A recently developed nonlinear ultrasonic technique called sideband peak count-index (or SPC-I) is adopted for monitoring one or more cracks with thickness values equal to 0 mm (crack-free), 1, 2, and 4 mm. Three separate scenarios-one crack, two cracks, and four cracks in 3-D plate structures-are investigated. These cracks can be classified as thin and thick cracks depending on the horizon size, which is mentioned in peri-ultrasound theory. Computed results for all three cases show larger SPC-I values for thin cracks than for thick cracks and the case of no cracks. This observation is in line with the previously reported results in the literature and proves that the state-based peri-ultrasound theory can capture the expected nonlinear response of elastic waves interacting with multiple cracks without changing the cracks' surface locations artificially, and this is always needed in most of the other numerical methods. The proposed state-based peri-ultrasound theory is more flexible and reliable for solving 3-D problems, and the out-of-plane wave field can be obtained for engineering analysis.

Modeling ultrasonic wave fields using a Quasi-Monte Carlo method: Wave transmission through complicated interfaces.

The sound fields generated by ultrasonic transducers can be modeled using the Quasi-Monte Carlo (QMC) method with a high level of accuracy and efficiency from Zhang [J. Acoust. Soc. Am. 149(1), 7-15 (2021)]. In this work, this method is extended to simulate transmitted wave fields through complicated interfaces. When a wave propagates in two-layer media, the vibrating waves over the interface radiated by the transducer can be treated as the source for generating waves in the second medium, thus, a nested-form Rayleigh integral expression can be used as a model equation for the transmitted wave calculation. When the QMC method is used to solve the nested integral, pseudo-random samples for constructing the transducer and the interface are sampled separately and the transmitted wave fields are obtained using the final sample mean. Numerical examples and results are presented when the wave transmits normally or obliquely through planar or curved interfaces. The results indicate that the high level of accuracy and efficiency remains when the QMC method is used to model the transmitted wave fields. One important advantage is that wave fields can be well simulated using the QMC method when the wave transmits through a complicated interface as long as the interface can be constructed using pseudo-random samples.

Fast Fourier transform method for determining velocities of ultrasonic Rayleigh waves using a comb transducer.

A convenient, accurate and precise method is proposed to determine velocities of ultrasonic Rayleigh waves in different materials by extracting central frequencies of signals, which are measured by a comb transducer and converted to the frequency domain using the fast Fourier transformation (FFT). The velocities can be calculated as cr = fl, where f is the central frequency of the wave signal and l is the teeth spacing or period of the comb transducer. The experimental measurements are easy to do, as long as the Rayleigh wave reflected from the standard reflectors are measured using one comb transducer, without knowing the wave propagation distances and times. Results show that the proposed technique has a high level of precision, as the central frequencies are very stable. The same comb transducer is used to measure the Rayleigh wave velocities in different materials where the velocities vary from 2100 m/s to 3400 m/s. Comparison of the experimental results with those measured using the time-of-flight method showed a high level of accuracy - all relative errors were found to be less than 1%.

Design and Application of Partial Immersion Focused Ultrasonic Transducers for Austenitic Weld Inspection.

Austenitic stainless steel is a widely used material in the industry, and the welding technique enables stainless steel components to have different shapes for different applications. Any flaws in the weld will degrade the performance of the austenitic component; thus, it is essential to ultrasonically and nondestructively test flaws in welds to ensure service safety. Recently, weld inspection has been performed using contact transducers, but missed detections or false positives for flaws in welds usually occur due to a poor coupling condition in the detection, a low signal-to-noise ratio, and instantaneous noises. In this study, a partial immersion focused (PIF) ultrasonic transducer is designed and used for austenitic weld inspection to address the above issues. The detailed design and manufacture of the PIF transducer are described, and the advantages of the transducer are shown by comparing the results detected using different kinds of transducers. In addition, in order to suppress false positives, a B-image method optimized using a time-dependent threshold is proposed. Experiments are performed to detect flaws in a welded specimen. All the artificial flaws are evaluated using the developed transducer and the proposed method, but minor flaws are mis-detected when planar transducers are used, verifying the method proposed in this paper.

Determining the Responsivity of Air-Coupled Piezoelectric Transducers Using a Comparative Method: Theory and Experiments.

The responsivity of an ultrasonic transducer is an important parameter in evaluating its effective frequency band, the electroacoustic conversion efficiency, and the measurement capability of the system. The determination of the responsivity of a traditional immersion or contact piezoelectric transducer has been well investigated. However, due to the high attenuation of waves propagating in air and the large acoustic impedance mismatch between the active piezoceramic material and the load medium, there are few reports of the calibration of an air-coupled piezoelectric transducer. In this work, we present a comparative method of measuring the responsivity of an air-coupled transducer: the air-coupled transducer is used to receive a broadband pulse signal to evaluate its frequency spectrum, and a toneburst signal with known vibration displacement is measured by the air-coupled transducer in order to calibrate the amplitude of the responsivity. The effects of transmitter responsivity, input pulse characteristics, attenuation, and diffraction are taken into account to improve the accuracy of the responsivity determination. In addition, the measurement of the amplitude of the responsivity by comparing the measured displacements avoids the complicated task of characterizing the effects of electrical equipment. The determined responsivity is checked by comparing the measured displacements using different methods at different frequencies in order to evaluate its frequency spectrum and by measuring the nonlinearity parameters of the material to evaluate its amplitude. The agreement between results obtained using different methods demonstrates that the calibrated responsivity of the air-coupled transducer is valid, and the proposed method is effective.

Absolute Measurement of Material Nonlinear Parameters Using Noncontact Air-Coupled Reception.

Nonlinear ultrasound is often employed to assess microdamage or nonlinear elastic properties of a material, and the nonlinear parameter is commonly used to quantify damage sate and material properties. Among the various factors that influence the measurement of nonlinear parameters, maintaining a constant contact pressure between the receiver and specimen is important for repeatability of the measurement. The use of an air-coupled transducer may be considered to replace the contact receiver. In this paper, a method of measuring the relative and absolute nonlinear parameters of materials is described using an air-coupled transducer as a receiver. The diffraction and attenuation corrections are newly derived from an acoustic model for a two-layer medium and the nonlinear parameter formula with all corrections is defined. Then, we show that the ratio of the relative nonlinear parameter of the target sample to the reference sample is equal to that of the absolute nonlinear parameter, and this equivalence is confirmed by measurements on three systems of aluminum samples. The proposed method allows the absolute measurement of the nonlinear parameter ratio or the nonlinear parameter without calibration of the air-coupled receiver and removes restrictions on the selection of reference samples.

Modeling of wave fields generated by ultrasonic transducers using a quasi-Monte Carlo method.

The sound fields generated by ultrasonic transducers are modeled using the quasi-Monte Carlo (QMC) method, which is found to overcome the conflict between accuracy and efficiency that occurs in existing wave field calculation methods. The RI equation, which is frequently used as a model equation in ultrasonic field calculation, is used here as an exact method and for comparison purposes. In the QMC method, the judgment sampling method and Halton sequence are used for pseudo-random sampling from the sound source, and then the sound field distributions are found by solving the integral solution using the sample mean. Numerical examples and results are presented when modeling unfocused, focused, and steered and focused beam fields. The accuracy and efficiency of the QMC method are discussed by comparing the results obtained using different modeling methods. The results show that the proposed method has a high level of efficiency due to the nature of the QMC algorithm and a high level of accuracy because no approximation is required. In addition, wave fields can be modeled with the QMC method as long as sound sources can be effectively pseudo-randomly sampled, allowing the proposed method to be applied to various types of transducers.

Psychological impact of COVID-19 outbreak on frontline nurses: A cross-sectional survey study.

AIMS AND OBJECTIVES: This study aimed to portray the prevalence and associated factors of psychological distress among frontline nurses during COVID-19 outbreak. BACKGROUND: The COVID-19 outbreak has posed great threat to public health worldwide. Nurses fighting against the epidemic on the frontline might be under great physical and psychological distress. This psychological distress was predominantly described as sleep disturbance, symptoms of anxiety and depression, post-traumatic stress, inability to make decisions and even somatic symptoms. DESIGN: Cross-sectional study. METHODS: Frontline nurses from designated hospitals for COVID-19 patients were invited to complete an online survey by convenience sampling, and the survey included six main sections: the General Health Questionnaire, the Perceived Social Support Scale, the Simplified Coping Style Scale, the Impact of Event Scale-Revised, socio-demographic, occupation and work history. Multiple logistic analysis was used to identify the potential risk factors of psychological distress. The study methods were compliant with the STROBE checklist. RESULTS: Of the 263 frontline nurses, 66 (25.1%) were identified as psychological distress. Multiple logistic analysis revealed that working in emergency department, concern for family, being treated differently, negative coping style and COVID-19-related stress symptom were positive related to psychological distress. Perceived more social support and effective precautionary measures were negatively associated with psychological distress. CONCLUSIONS: The study demonstrated that COVID-19 had a significant psychological impact on frontline nurses. Early detection of psychological distress and supportive intervention should be taken according to the associated factors to prevent more serious psychological impact on frontline nurses. RELEVANCE TO CLINICAL PRACTICE: This study highlighted that the frontline nurses were suffering from varying degrees of psychological distress, which needed early screening and supportive intervention for preventing more serious psychological impact on frontline nurses. Beside, more specific measurement should be combined with the GHQ-12 to assess the varying degrees of psychological distress in frontline nurses.

Characterizing Microstructural Evolution of TP304 Stainless Steel Using a Pulse-Echo Nonlinear Method.

Tube/Pipe (TP) 304 stainless steel has been widely used in industry, but a change in its microstructures may endanger its service safety, and it is essential to evaluate its microstructural evolution. In this work, a pulse-echo nonlinear method is proposed to characterize the microstructural evolution of the TP304 stainless steel. The detailed pulse-echo nonlinear experimental process is presented, and it is shown that the absolute nonlinear parameter can be determined when the effect of attenuation is taken into account. The microstructural evolution of TP304 stainless steel is artificially controlled by annealing treatments before it is evaluated by using nonlinear ultrasonic method and metallographic method. The results show that the grain sizes increase as the annealing time increases, which leads to the performance degradation of the TP304 steel and an increase in the nonlinear parameters, with the reason discussed considering the variation in the microstructure. The present pulse-echo nonlinear method is easier to conduct than the traditional transmission-through method and the absolute nonlinear parameter can be determined for quantitative characterization. The variation in determined nonlinear parameters provides a reference to evaluate the microstructural evolution of TP304 stainless steel.

Simultaneously Determining Sensitivity and Effective Geometrical Parameters of Ultrasonic Piezoelectric Transducers Using a Self-Reciprocity Method.

Determination of the sensitivity of a transducer is essential in evaluating its central frequency and effective bandwidth, its electroacoustic conversion capability, or the measurement ability of an ultrasonic test system. In this work, a calibration method based on self-reciprocity is proposed for the determination of transducer sensitivity, which can be applied to both planar and focused transducers. The two-port electrical network of the experimental setup is analyzed, and a simplified measurement procedure is described in which the "impedance mismatch" problem is solved, and only input and output currents are needed. An acoustic transfer function is introduced, both to reduce the effects of wave energy loss on the determination of transducer sensitivity and to help extract the effective geometrical parameters of the transducer through these measured output current signals. The effects of diffraction, attenuation, and effective geometrical parameters on sensitivity determination are discussed in this work; the experimental results show that when all these factors are taken into account, accurate sensitivities for both planar and focused transducers can be obtained at various experimental distances.

Modeling Flaw Pulse-Echo Signals in Cylindrical Components Using an Ultrasonic Line-Focused Transducer with Consideration of Wave Mode Conversion.

Investigations on flaw responses can benefit the nondestructive testing of cylinders using line-focused transducers. In this work, the system function, the wave beam model, and a flaw scattering model are combined to develop an ultrasonic measurement model for line-focused transducers to predict flaw responses in cylindrical components. The system function is characterized using reference signals by developing an acoustic transfer function for line-focused transducers, which works at different distances for both planar and curved surfaces. The wave beams in cylindrical components are modeled using a multi-Gaussian beam model, where the effects of wave mode conversion and curvatures of cylinders are considered. Simulation results of wave beams are provided to analyze their propagation behaviors. The proposed ultrasonic measurement model is certified from good agreement between the experimental and predicted signals of side-drilled holes. This work provides guidance for evaluating the detection ability of line-focused transducers in cylindrical component testing applications.

[Clinical observation of rapid massage at Shuidao (ST 28) to prevent postpartum urinary retention].

OBJECTIVE: To explore the clinical effect of rapid massage at Shuidao (ST 28) to prevent puerpera from postpartum urinary retention. METHODS: A total of 200 puerpera giving birth through vagina were enrolled and divided into an observation group and a control group according to the random number table method, 100 cases in each group. In the observation group, rapid massage at Shuidao (ST 28) was applied. In the control group,there was no intervention and urinated naturally. The traditional Chinese medicine syndrome scale was used to evaluate poor sense of urination, and record puerpera with or without postpartum urinary retention, the poor sense of first urination, the first time of urination, the amount of first urination and postpartum hemorrhage. RESULTS: The poor sense of first urination in the observation group was significantly lower than that in the control (P<0.001), the first time of urination in the observation group was significantly earlier than that in the control group (P<0.001), the amount of first urination in the observation group was significantly more than that in the control group (P<0.001), the observation group was superior to the control group to prevent postpartum urinary retention (P<0.05). CONCLUSION: Rapid massage at Shuidao (ST 28) can reduce the poor sense of first urination, significantly advance the time of spontaneous urination, significantly increase the amount of first urination and effectively prevent postpartum urinary retention.

Application of Fresnel Zone Plate Focused Beam to Optimized Sensor Design for Pulse-Echo Harmonic Generation Measurements.

In nonlinear acoustic measurements involving reflection from the stress-free boundary, the pulse-echo method could not be used because such a boundary is known to destructively change the second harmonic generation (SHG) process. The use of a focusing acoustic beam, however, can improve SHG after reflection from the specimen boundary, and nonlinear pulse-echo methods can be implemented as a practical means of measuring the acoustic nonlinear parameter (ß) of solid specimens. This paper investigates the optimal sensor design for pulse-echo SHG and ß measurements using Fresnel zone plate (FZP) focused beams. The conceptual design of a sensor configuration uses separate transmission and reception, where a broadband receiver is located at the center and a four-element FZP transmitter is positioned outside the receiver to create a focused beam at the specified position in a solid sample. Comprehensive simulations are performed for focused beam fields analysis and to determine the optimal sensor design using various combinations of focal length, receiver size and frequency. It is shown that the optimally designed sensors for 1 cm thick aluminum can produce the second harmonic amplitude and the uncorrected nonlinear parameter corresponding to the through-transmission method. The sensitivity of the optimal sensors to the changes in the designed sound velocity is analyzed and compared between the odd- and even-type FZPs.

Acoustic nonlinearity parameter measurements in a pulse-echo setup with the stress-free reflection boundary.

This paper describes the acoustic nonlinearity parameter (ß) determination for fluids using a pulse-echo method with the stress-free boundary. A newly derived ß formula requires the measurement of the fundamental and second harmonic displacements with appropriate corrections for attenuation, diffraction, and boundary reflection. Measurements are composed of two steps: receiver calibration and harmonic generation. The ß values calculated for water at several distances between the planar transducer and the water-air interface are in good agreement with literature, providing a validation for the method.

Experimental investigation of material nonlinearity using the Rayleigh surface waves excited and detected by angle beam wedge transducers.

Angle beam wedge transducers are widely used in nonlinear Rayleigh wave experiments as they can generate Rayleigh wave easily and produce high intensity nonlinear waves for detection. When such a transducer is used, the spurious harmonics (source nonlinearity) and wave diffraction may occur and will affect the measurement results, so it is essential to fully understand its acoustic nature. This paper experimentally investigates the nonlinear Rayleigh wave beam fields generated and received by angle beam wedge transducers, in which the theoretical predictions are based on the acoustic model developed previously for angle beam wedge transducers [S. Zhang, et al., Wave Motion, 67, 141-159, (2016)]. The source of the spurious harmonics is fully characterized by scrutinizing the nonlinear Rayleigh wave behavior in various materials with different driving voltages. Furthermore, it is shown that the attenuation coefficients for both fundamental and second harmonic Rayleigh waves can be extracted by comparing the measurements with the predictions when the experiments are conducted at many locations along the propagation path. A technique is developed to evaluate the material nonlinearity by making appropriate corrections for source nonlinearity, diffraction and attenuation. The nonlinear parameters of three aluminum alloy specimens - Al 2024, Al 6061 and Al 7075 - are measured, and the results indicate that the measurement results can be significantly improved using the proposed method.

Improvement of pulse-echo harmonic generation from a traction-free boundary through phase shift of a dual element transducer.

The practical implementation of nonlinear ultrasonic technique has been limited to the through-transmission setup for measuring the second harmonic component induced by the nonlinearity or microstructural changes of test material. A more practical technique such as the pulse-echo testing has been ruled out because a traction-free reflecting boundary destructively alters the nonlinear generation process. A focusing acoustic beam or rigid boundary condition was often employed for biomedical imaging and fluid nonlinearity in the pulse-echo inspection. In this article, we further explore a more general and efficient method to improve the generation of the second harmonic component in the pulse-echo mode with traction-free surface. A dual element planar transdcer with optimal phase shift of the input signal in one element relative to another is proposed for this purpose. The validity of the phase shift concept is confirmed by comparing the enhanced generation of second harmonic amplitudes and the resulting nonlinear parameters with the rigid-boundary case equivalent to the conventional through-transmission setup.