Diffraction correction for precision surface acoustic wave velocity measurements

Measurement of attenuation in a sample with nonparallel surfaces

Ultrasonic attenuation in solids is an important characteristic of structural materials related to their microstructure. It is well known that attenuation measurements in solids require that the front and back surfaces of the sample be parallel. In this paper, an immersion method for attenuation measurement in a sample with nonparallel surfaces is developed. The maximization method is based on independent alignment of the transducer orientations for maximizing each of the multiply reflected signals received. The experiment is accompanied by a measurement model that provides a diffraction correction of the ultrasonic beam. The method is compared with that involvingthe normalization procedure, of the sequence of the reflected pulses, by the transducer directivity function.Those independent measurements are in reasonable agreement for the amplitudes of the first few reflected pulses. In general, the maximization method is advantageous for attenuation measurements by immersion experiments.

Laser-based ultrasound interrogation of surface and sub-surface features in advanced manufacturing materials

Structures formed by advanced manufacturing methods increasingly require nondestructive characterization to enable efficient fabrication and to ensure performance targets are met. This is especially important for aerospace, military, and high precision applications. Surface acoustic waves (SAW) generated by laser-based ultrasound can detect surface and sub-surface defects relevant for a broad range of advanced manufacturing processes, including laser powder bed fusion (LPBF). In particular, an all-optical SAW generation and detection configuration can effectively interrogate laser melt lines. Here we report on scattered acoustic energy from melt lines, voids, and surface features. Sub-surface voids are also characterized using X-ray Computed Tomography (CT). High resolution CT results are presented and compared with SAW measurements. Finite difference simulations inform experimental measurements and analysis.

Research on the influence of heat treatment on residual stress of TC4 alloy produced by laser additive manufacturing based on laser ultrasonic technique

During the laser additive manufacturing (LAM) process large temperature gradients can lead to high level of residual stress. The residual stress can have irreversible effects such as warping and cracking of parts during and post manufacturing. Heat treatment is an effective method to control and eliminate residual stress. In this paper, the TC4 parts are prepared by laser additive manufacturing, and the influence of heat treatment process on residual stress is researched. Laser ultrasonic technology, as an advanced nondestructive testing method, is applied to measure the residual stress under different heat treatment processes for the first time. The surface wave generated by laser is used to evaluate the residual stress. The results show that laser ultrasonic method can complete the in-situ evaluation of residual stress in additive manufacturing components. The residual stress in TC4 deposited specimen is large, the longitudinal stress is obviously greater than the transverse stress, and the maximum residual stress is about half of the yield strength. The residual stress increases gradually from the upper surface to the bottom layer near the substrate before heat treatment. After heat treatment, the residual stress is reduced to low stress level and a small compressive stress appears. The cooling rate and solution temperature are the main factors affecting the residual stress, and the residual stress increases with the increase of cooling rate and solution temperature. The effect of aging temperature and aging time on residual stress is not obvious. The study serves as useful guidelines for engineers to assessment and regulation of residual stress reasonably in LAM.

Time domain model and experimental validation of non-contact surface wave scanner

This paper presents a time-domain model for the prediction of acoustic field in an air-coupled, non-contact, ultrasonic surface wave scanner, which includes an air-coupled Emitter, the Propagation space, and an air-coupled Receiver (EPR). The computation is divided into three steps, with each step being modeled in the time domain by its spatio-temporal transfer function. The latter are then used in turn, to find the pulse response of the overall system. The model takes the finite size of the aperture receiver, the attenuation in both air and the tested solid sample, as well as the electric response of the emitter-receiver set h_e into account. The attenuation is characterized by a causal time-domain Green's function, allowing wideband attenuation of a lossy medium, obeying the power law αω=α₀ω^η,0⩽η⩽2, to be used. The model is implemented numerically using a Discrete Representation approach. It is then validated quantitatively by comparing the predicted acoustic field with experiment. The prediction error for three typical field features, the system's impulse response, the on-axis field distribution, and the directivity pattern, is globally smaller than 3%. In order to obtain this high level of accuracy in the model, the parameters characterizing the solid sample used during the experiment were measured experimentally, with a specifically developed experimental setup. Overall, the proposed model is approximately 100 times faster than 3D FEM with an equivalent spatio-temporal resolution. In parallel, a simplified model is proposed, which neglects the attenuation in air and assumes the emitter inclination angle to be perfectly adjusted. This approach makes it possible to further shorten the computational time by a factor of about ten, whilst maintaining good accuracy. Thanks to its computational efficiency, the proposed model can be used to formulate various recommendations concerning the scanner settings, in particular the inclination angles of the emitter and receiver, and their distance from the sample.

Residual Stress in Laser Welding of TC4 Titanium Alloy Based on Ultrasonic laser Technology

Laser welding is widely used in titanium alloy welding due to its high energy density, small heat affected zone, and rapid processing ability. However, problems with laser welding, such as deformation and cracking caused by residual stress, need to be resolved. In this paper, the residual stress in laser welding of TC4 titanium alloy was studied using an ultrasonic laser. The residual stress in titanium alloy plates is considered a plane stress state. A pre-stress loading method is proposed and acoustoelastic coefficients are obtained. Based on the known acoustoelastic coefficients, the transverse and longitudinal residual stresses in laser welding are measured using an ultrasonic laser. The results show that longitudinal residual stress is greater than the transverse stress. The distribution regularity of the residual stress is similar to normal welding, but the tensile stress zone is much narrower. Then, the influence of heat input and welding speed on residual stress is discussed. With increasing heat input, the welding zone widens, and the peak value of the residual stress increases. A higher welding speed should be chosen when the welding power is constant. This research has important significance for the measurement and control of residual stress in the laser welding process.

Analysis of the transient surface wave propagation in soft-solid elastic plates

In dynamic elastography, the goal is to estimate the Young's modulus from audio-frequency wave propagation in soft-tissues. Within this frequency range, the shear wavelength is centimeter-sized while the compressional wavelength is meter-sized. Thus, the experimental data are usually collected in the near-field of the source. Near-field effects have been widely studied for bulk wave propagation. However, the near- and transient-fields of surface and guided waves have received less attention. In this work, the transient surface displacement field in soft-solid elastic plates in vacuum is analyzed. Due to the high Poisson's ratio, mode conversion has special characteristics in soft-solids. They are analyzed through this work where it is shown that the transient-field over the surface can be interpreted by tracing a few reflections. The authors show the existence of a critical distance needed for the formation of Rayleigh-Lamb modes. Below this distance, only direct surface waves propagate without contribution from reflected waves. Thus, the dispersion curve differs from that predicted by Rayleigh-Lamb modes. Instead, the authors propose a model based on the interference of surface waves, which agree with the experimental data. In addition, the conditions needed in order to retrieve the shear wave phase velocity from the surface field are given.

Measurement of Rayleigh Wave Beams Using Angle Beam Wedge Transducers as the Transmitter and Receiver with Consideration of Beam Spreading

A theoretical model, along with experimental verification, is developed to describe the generation, propagation and reception of a Rayleigh wave using angle beam wedge transducers. The Rayleigh wave generation process using an angle beam wedge transducer is analyzed, and the actual Rayleigh wave sound source distributions are evaluated numerically. Based on the reciprocity theorem and considering the actual sound source, the Rayleigh wave beams are modeled using an area integral method. The leaky Rayleigh wave theory is introduced to investigate the reception of the Rayleigh wave using the angle beam wedge transducers, and the effects of the wave spreading in the wedge and transducer size are considered in the reception process. The effects of attenuations of the Rayleigh wave and leaky Rayleigh wave are discussed, and the received wave results with different sizes of receivers are compared. The experiments are conducted using two angle beam wedge transducers to measure the Rayleigh wave, and the measurement results are compared with the predictions using different theoretical models. It is shown that the proposed model which considers the wave spreading in both the sample and wedges can be used to interpret the measurements reasonably.

Diffraction, attenuation, and source corrections for nonlinear Rayleigh wave ultrasonic measurements

This research considers the effects of diffraction, attenuation, and the nonlinearity of generating sources on measurements of nonlinear ultrasonic Rayleigh wave propagation. A new theoretical framework for correcting measurements made with air-coupled and contact piezoelectric receivers for the aforementioned effects is provided based on analytical models and experimental considerations. A method for extracting the nonlinearity parameter β11 is proposed based on a nonlinear least squares curve-fitting algorithm that is tailored for Rayleigh wave measurements. Quantitative experiments are conducted to confirm the predictions for the nonlinearity of the piezoelectric source and to demonstrate the effectiveness of the curve-fitting procedure. These experiments are conducted on aluminum 2024 and 7075 specimens and a β11(7075)/β11(2024) measure of 1.363 agrees well with previous literature and earlier work. The proposed work is also applied to a set of 2205 duplex stainless steel specimens that underwent various degrees of heat-treatment over 24h, and the results improve upon conclusions drawn from previous analysis.

Laser-ultrasonic surface wave dispersion measurements on surface-treated metals

Surface acoustic wave (SAW) velocity spectroscopy has been long considered to be one of the leading candidates for nondestructive characterization of surface-treated metals because of its ability to probe the material properties at different penetration depths depending on the inspection frequency. We developed a high-precision laser-ultrasonic technique to study the feasibility of SAW dispersion spectroscopy for residual stress assessment on shot-peened metals. This technique is capable of measuring SAW dispersion with a relative error of 0.1% over a frequency range from 2 to 15 MHz. Our experimental results obtained from shot-peened aluminum 2024-T351 samples indicate that the dispersion of the surface wave is a superposition of different effects of surface treatment in the material, including surface roughness, compressive residual stress, and cold work. Although the surface roughness induced component is often the dominating part of the overall dispersion, the experimental results also indicate that it is feasible to observe a perceivable change in the dispersion of the SAW when the specimen is heat-treated at different temperatures, which has no perceivable effect on the surface roughness. The part of the dispersion, which changes during annealing via thermal relaxation, is due to near-surface residual stresses and the decay of texture, although at high frequencies nonuniform grain coarsening could also play a significant role.

NDE of two-layered mortar samples using high-frequency Rayleigh waves

The Spectral Analysis of Surface Waves (SASW) is a popular technique in seismics for imaging the ground subsurface. It uses the dispersive properties of Rayleigh waves in a transversely homogeneous, multilayered medium. The SASW approach is being transposed into the civil engineering domain to characterize subsurface damage in concrete structures. Such a damage consists in a few millimeters thick surface layer with porosity slightly higher than in the sound material. It is induced by contact with moisture or chemicals at the surface of the structure and may facilitate penetration of aggressive agents. In this study, two-layered mortar samples are made to mimic concrete cover damage in real structures. The dispersive behavior of Rayleigh waves arises when the wavelength is comparable to the thickness of the first layer. Given the small thickness of this layer, it requires increasing the frequency up to several hundreds of kHz, which means high attenuation and low signal-to-noise ratio. Rayleigh waves with 0.5 MHz central frequency are generated into the samples by the wedge method. Phase velocity dispersion curves are obtained by broadband phase spectroscopy from signals received at various distances from the source. The signal processing is first validated on simulated signals with known dispersion law. Then, the measured dispersion curves are compared with the theoretical curve for a two-layered medium, following Haskell's approach. The measured curve displays the general behavior expected from theory. However, a three-layered, visco-elastic model would be necessary to get a better fit and to estimate more accurately the parameters of each layer.

Beam and phase effects in angle-beam-through-transmission method of ultrasonic velocity measurement

The accuracy of a plane wave approximation for phase velocity measurements in isotropic and anisotropic material using the angle-beam-through-transmission method has been investigated numerically and experimentally. In this method the velocity is measured in different propagation directions as a function of incidence angle. The effect of two factors on the measurement accuracy have been discussed: intrinsic phase shift of the transmitted signal through a fluid-solid interface and beam diffraction due to the finite beam size of receiver and transmitter. It is shown that the interface-induced phase shift can introduce an error in time delay measurements of the shear wave after the first critical angle and that this time delay error can be accurately corrected for. Numerical results obtained by a time-domain beam model show that except at the critical angles, the finite width of the transmitter and receiver only affects the amplitudes of the transmitted signals and has almost no effect on the measured zero-cross time delay; therefore the plane wave approximation for obtaining phase velocity from the measured time delay data by this method and the plane wave interface-induced phase correction are fully applicable.