Thermoelastic Regime Research Articles

Measurement of thickness and ultrasonic velocities based on imaging method using laser-generated ultrasonic waves at thermoelastic regime

Abstract This study introduces a simple method for determining thickness and ultrasonic velocities, including skimming longitudinal waves, longitudinal waves, and shear waves, utilizing laser-induced ultrasonic waves at the thermoelastic regime and the time-position imaging technique. Laser scanning was performed on two aluminum samples with thicknesses of 6 mm and 10 mm, generating data on the arrival time of multiple wave modes according to the separated distances of two laser spots. Using the least squares technique, the data obtained from the experiment were employed to fit theoretical curves of various waves, enabling the simultaneous determination of several material parameters. The proposed method exhibits high accuracy in determining thickness, with a maximum relative error of 1% for thicknesses up to 10 mm. Additionally, comparisons with theoretical calculations reveal maximum errors of 1 % for shear wave velocity and 0.8 % for longitudinal wave velocity. The ratio between the velocity of skimming longitudinal wave and bulk longitudinal wave of 0.95 was established, providing a prospective method to calculate longitudinal wave velocity through guided waves along the surface.

Laser Ultrasonic Technique for Simultaneous Measurement of Thickness, Slope, and Wave Velocities for Slope Plate in the Thermoelastic Regime

Laser Ultrasonic Technique for Simultaneous Measurement of Thickness, Slope, and Wave Velocities for Slope Plate in the Thermoelastic Regime

Simultaneous measurement of thickness and ultrasonic wave velocity using a combination of laser-generated multiple wave modes in the thermoelastic regime

ABSTRACT This paper introduces a reliable laser ultrasonic technique without damage to the surface in the thermoelastic regime that provides a one-side access method for simultaneously determining the thickness and ultrasonic wave velocity by employing multiple wave modes. Initially, this study determines the optimal separated distance between two laser beams to achieve an effective signal-to-noise ratio of the employed wave modes. Subsequently, a pre-established ratio of 0.95 for the velocity of skimming longitudinal waves and longitudinal waves enables the simultaneous calculation using a combination of the skimming longitudinal wave, reflected shear wave, and mode-converted longitudinal wave in a single measurement. Experiments were carried out on various plane plates made of aluminium alloy 6061, ranging in thickness from 4 to 20 mm, demonstrating a high accuracy in thickness measurement, with a maximum absolute error of 0.05 mm. Compare the results of ultrasonic wave velocity measurements with theoretical calculations, the maximum absolute error is 32 m/s and 80 m/s for longitudinal wave and shear wave, respectively.

Frequency-Resolved High-Frequency Broadband Measurement of Acoustic Longitudinal Waves by Laser-Based Excitation and Detection.

Optoacoustics is a metrology widely used for material characterisation. In this study, a measurement setup for the selective determination of the frequency-resolved phase velocities and attenuations of longitudinal waves over a wide frequency range (3-55 MHz) is presented. The ultrasonic waves in this setup were excited by a pulsed laser within an absorption layer in the thermoelastic regime and directed through a layer of water onto a sample. The acoustic waves were detected using a self-built adaptive interferometer with a photorefractive crystal. The instrument transmits compression waves only, is low-contact, non-destructive, and has a sample-independent excitation. The limitations of the approach were studied both by simulation and experiments to determine how the frequency range and precision can be improved. It was shown that measurements are possible for all investigated materials (silicon, silicone, aluminium, and water) and that the relative error for the phase velocity is less than 0.2%.

Structural optimization of photoacoustic transducer with PDMS/CSNPs nanocomposite for fatigue crack detection using laser-induced nonlinear surface waves

Laser ultrasonics is a promising non-contact inspection technique but faces challenges of low signal-to-noise and low amplitude under non-destructive thermoelastic regime. In this paper, a laser ultrasonic surface acoustic wave (SAW) modulation method and photoacoustic transducer (PAT) are proposed and combined with nonlinear wave mixing technique to inspect microscale fatigue crack. PATs comprised of candle soot nanoparticles and polydimethylsiloxane are systematically optimized and combined with a line-arrayed laser source to generate desired high-amplitude and pure fundamental SAWs. Two modulated SAWs with frequencies of 2.1 and 2.9 MHz are excited on a fatigued aluminum plate to generate nonlinear mixed components, and the ultrasonic responses over the fatigue crack regions are acquired with a scanning laser Doppler vibrometer. The localization of fatigue crack with microscale is eventually achieved by mapping the nonlinear parameter of the mixed components, which proves it a reliable and non-destructive technique to inspect the fatigue crack.

A sensitivity-enhanced all-optical probe for non-contact laser ultrasonic inspection

Non-contact laser ultrasonic technique has been increasingly implemented for non-destructive inspections in harsh environments, high-temperature fields, and components having complex geometries. However, the poor signal-to-noise ratio and low amplitude of laser generated ultrasonic signals under a thermoelastic regime severely restrict its applications. Here, a sensitivity-enhanced all-optical probe was proposed for laser ultrasonic non-destructive testing. It consists of an optical sensor and an ellipsoidal acoustic cavity, where an optical sensor is placed at one focus of the cavity, and the detection point is set at another focus. The ultrasound signals are focused through the cavity and detected by the optical sensor. Side-by-side comparison experiments were carried out, and the results show that the probe can improve the signal amplitude by about 7.8 times compared to using a traditional optical sensor alone. The probe can make laser ultrasound detect defects with lower laser energy, which is of great significance to improve the efficiency of non-contact defect detection.

Adapting the Time-Domain Synthetic Aperture Focusing Technique (T-SAFT) to Laser Ultrasonics for Imaging the Subsurface Defects.

Traditional ultrasonic testing uses a single probe or phased array probe to investigate and visualize defects by adapting certain imaging algorithms. The time-domain synthetic aperture focusing technique (T-SAFT) is an imaging algorithm that employs a single probe to scan along the test specimen in various positions, to generate inspection images with better resolution. Both the T-SAFT and phased array probes are contact methods with limited bandwidth. This work aims to combine the advantages of the T-SAFT and phased array in a noncontact way with the aid of laser ultrasonics. Here, a pulsed laser beam is employed to generate ultrasonic waves in both thermoelastic and ablation regimes, whereas the laser Doppler vibrometer is used to acquire the generated signals. These two lasers are focused on the test specimen and, to avoid the plasma and crater influence in the ablation regime, the transmission beam and reception beam are separated by 5 mm. By moving the test specimen with a step size of 0.5 mm, a 1D linear phased array (41 and 43 elements) with a pitch of 0.5 mm was synthesized, and three side-drilled holes (Ø 8 mm-thermoelastic regime, Ø 10 mm and Ø 2 mm-ablation regime) were introduced for inspection. The A-scan data obtained from these elements were processed via the T-SAFT algorithm to generate the inspection images in various grid sizes. The results showed that the defect reflections obtained in the ablation regime have better visibility than those from the thermoelastic regime. This is due to the high-amplitude signals obtained in the ablation regime, which pave the way for enhancing the pixel intensity of each grid. Moreover, the separation distance (5 mm) does not have any significant effect on the defect location during the reconstruction process.

Simultaneous determination of Young's modulus and Poisson's ratio in metals from a single surface using laser-generated Rayleigh and leaky surface acoustic waves

Material elastic moduli are used to assess stiffness, elastic response, strength, and residual life. Ultrasound (US) measurements of propagation wave speeds (for longitudinal and shear waves) are now primary tools for non-destructive evaluation (NDE) of elastic moduli. Most US techniques measure the time-of-flight of through-transmission signals or reflected signals from the back wall. In both cases, an independently determined sample thickness is required. However, US methods are difficult for complex (non-flat) samples. When the local thickness is unknown, the propagation speed cannot be determined. On the other hand, the propagation speed of Rayleigh waves can be calculated without knowledge of sample thickness, but another independent measurement is still required to compute both Young's modulus and Poisson's ratio. We present a comprehensive theoretical background, numerical simulations, and experimental results that clearly show that when the material density is assumed known, both elastic constants of an isotropic metal sample can be determined with laser-ultrasound by tracking two types of surface propagating waves without any sample contact (both signal excitation and detection are performed optically). In addition to a conventional surface, or Rayleigh, acoustic wave, a leaky surface wave can also be launched with nanosecond laser pulses in the thermoelastic regime of excitation (i.e., without material ablation) close to the source that propagates along the sample surface with speed close to that of bulk longitudinal waves. Samples can be of arbitrary shape and their thickness need not be measured.

Flexible and high-intensity photoacoustic transducer with PDMS/CSNPs nanocomposite for inspecting thick structure using laser ultrasonics

Laser ultrasonic technique has been increasingly implemented for nondestructive testing and structural health monitoring. However, the poor signal-to-noise ratio (SNR) and low amplitude of laser generated ultrasonic signals under thermoelastic regime severely restrict its applications. In this paper, we propose a method for fabricating flexible, high-intensity and readily transplantable photoacoustic transducer (PAT) comprised of candle soot nanoparticles (CSNPs) and polydimethylsiloxane (PDMS), and utilize it to generate high SNR and amplitude ultrasonic signals for inspecting thick structures. A robotic-arm based layer-by-layer automatic scanning strategy is developed to realize candle soot nanoparticles (CSNPs) deposition with excellent homogeneity and thickness controllability, and the optimal thickness of PDMS/CSNPs nanocomposite layer to achieve high SNR and amplitude ultrasonic signal is obtained. In addition, a novel method for optimizing the PAT's structure to generate distinguishable and pure longitudinal ultrasonic signals is proposed, with amplitude over 100 times, and with center frequency (13.5 MHz) and −6 dB bandwidth (20.1 MHz) 29.8% and 35.8% higher than those generated without PAT. The optimized PAT is eventually combined with laser ultrasonic technique to successfully inspect and visualize the internal defects with various sizes (4, 2, 0.8 mm in diameter) of an aluminum component with thickness of 50 mm. With the merits of flexible and high-intensity nanocomposite PAT, the laser ultrasonics can be promisingly implemented for inspecting structure with large thickness and complex geometry under thermo-elastic mechanism.

Thermoelastic pulsed laser ablation of silver thin films with organic metal–SiO2 adhesion layer in water: application to the sustainable regeneration of glass microfluidic reactors for silver nanoparticles

The synthesis of metal nanoparticles (NPs) using microfluidic reactors has become a major method for limiting reagent consumption and achieve a precise control of the morphological properties. Failure in realizing the reproducibility of the results is mostly associated with the accumulation of metallic nanostructures on the walls of the microfluidic devices, periodically removed by acid treatment. In this study, we show that ns-pulsed laser ablation (PLA) in water can be a safe, effective, and green method for the regeneration of clogged microfluidic reactors. The effect of the laser-pulse fluence on the removal of metallic nanostructures was studied for the first time on silver (Ag) thin films with a thickness of 50 nm deposited over SiO2 substrates, using 3-mercaptopropyl trimethoxysilane as a chemical adhesion layer. As point of novelty, the experimental results show that at low fluence (F < 0.1 J cm−2), ablation is principally caused by delamination of the thin film associated with the thermoelastic force while thermal processes inducing phase conversion of the metal dominate at higher fluence. Low-fluence regimes are better suited for the single-pulse removal of the nanomaterial, whereas in high F regimes, we observed melting and recondensation of the metal on the SiO2 surface so that multiple pulse interactions were necessary for complete ablation of the thin film. For the delamination and the phase transformation processes, the threshold fluences were 3.7 × 10−2 and 7.0 × 10−2 J cm−2, respectively. The experimental setup in the thermoelastic PLA regime was applied to unclog glass microfluidic devices used for synthesizing citrate-stabilized AgNPs. Using this simple and easily achievable laser-scanning experimental configuration, we demonstrated that PLA in water is a reliable and efficient technique, with results comparable to acidic treatment in terms of efficiency and time necessary for the complete removal of the Ag nanomaterial.

Nondestructive characterization of aluminum grain size using a ring-shaped laser ultrasonic method

In this study, a nondestructive ring-shaped laser ultrasonic method with a thermoelastic excitation regime was used to determine the grain size of metal materials. This method was proposed in order to evaluate the quality of metal in a fast online nondestructive manner. Normally, laser ultrasonic is used to detect grain size in the ablation excitation regime. The laser excites high energy longitudinal waves but causes damage to the surface of metal materials. To achieve strict online nondestructive testing, the thermoelastic regime was used in this work. The ring-shaped laser was converted from a circular collimated laser by an axicon and irradiated on the surface of the aluminum sample to induce ultrasonic waves and enhance the signal amplitude. The directivity pattern was analyzed to find a suitable detection parameter by the finite element method before performing laser ultrasound experiments. Quantitative analysis of the converging waves with different deviations from the center via laser ultrasound experiments demonstrated the enhancement effect of signal energy using a ring-shaped laser. The issues of low signal energy and the generation of a directivity pattern were solved by this ring-shaped laser ultrasonic method aimed at nondestructive grain size inspection. Aluminum samples with different mean grain sizes were detected by ring-shaped laser ultrasonic technology. A grain size characterization model was built with mean grain sizes and ultrasonic signals. Laser-generated ultrasound technology in the thermoelastic regime is a promising online detection method and can be used to detect material properties nondestructively with a ring-shaped laser.

Pulsed-laser source characterization in laboratory seismic experiments

The present study aimed to characterize the properties of a laser-generated seismic source for laboratory-scale geophysical experiments. This consisted of generating seismic waves in aluminum blocks and a carbonate core via pulsed-laser impacts and measuring the wave-field displacement via laser vibrometry. The experimental data were quantitatively compared to both theoretical predictions and 2D/3D numerical simulations using a finite element method. Two well-known and distinct physical mechanisms of seismic wave generation via pulsed-laser were identified and characterized accordingly: a thermoelastic regime for which the incident laser power was relatively weak, and an ablation regime at higher incident powers. The radiation patterns of the pulsed-laser seismic source in both regimes were experimentally measured and compared with that of a typical ultrasonic transducer. This study showed that this point-like, contact-free, reproducible, simple-to-use laser-generated seismic source was an attractive alternative to piezoelectric sources for laboratory seismic experiments, especially those concerning small scale, sub-meter measurements.

Thermoelastic modeling of laser-induced generation of strong surface acoustic waves

Short pulse laser irradiation of a substrate can generate pulses of surface acoustic waves (SAWs) capable of propagating long distances along the surface of the irradiated substrate. In this work, we use thermoelastic modeling of the generation of SAWs on a Si substrate to explore the effect of irradiation parameters, i.e., pulse duration, laser spot size, absorption depth, and spatial profile of the laser energy deposition, on the strength of the SAWs. A particular goal of this study is to establish the optimum conditions for maximizing the strength of the surface waves generated in the nonablative, thermoelastic irradiation regime. The simulations demonstrate that the highest strain amplitude of the laser-generated SAWs can be achieved for a laser spot size comparable to the characteristic length of the SAW propagation during the laser pulse. The amplitude of SAWs increases with the increase in the characteristic laser energy deposition depth, and laser pulses with sharper spatial energy deposition profiles (flat-top laser beams) produce stronger SAWs. For the optimal set of irradiation parameters, the strain amplitude of a SAW generated in Si in the thermoelastic regime can reach the levels of 10−4–10−3, which are sufficiently high for causing nonlinear sharpening of the wave profile and the formation of a shock front during the wave propagation from the laser spot. The computational predictions suggest the feasibility of a continuous generation of strong nonlinear pulses of SAWs, which may be utilized for driving the surface processes in thin film deposition, growth of two-dimensional materials, heterogeneous catalysis, and other applications.

Self-reference broadband local wavenumber estimation (SRB-LWE) for defect assessment in composites

Local wavenumber estimation (LWE) applied to a full wavefield response is a powerful approach for detecting and characterizing defects in a composite structure. However, the narrowband nature of the traditional LWE techniques brings several challenges for application on actual test cases.This study proposes a self-reference broadband version of the LWE technique. The broadband vibrations are injected using low-power piezoelectric actuators (sine sweep signal) or using pulsed laser excitation in the thermoelastic regime. The out-of-plane velocity response of the surface is recorded using an infrared scanning laser Doppler vibrometer. The dispersive Lamb wave behavior, corresponding to the damage-free base material, is identified from the broadband vibrational response. Using the identified dispersion curves (i.e. self-reference approach), a Lamb mode passband filter bank in the wavenumber-frequency domain is constructed. Searching for the maximum bandpower density in function of the assumed material thickness provides a robust estimate of the effective local thickness of the tested component, and as such yields a detection and evaluation of damage.The performance of the self-reference broadband LWE algorithm is demonstrated on aluminum plates with various flat bottom holes, as well as on cross-ply CFRP aircraft components with a stiffener disbond and barely visible impact damage. Compared to the traditional narrowband LWE approaches, the proposed self-reference broadband LWE method allows a higher level of automation, removes the need for a priori knowledge on the material and/or defect properties, and results in an improved characterization of defects.

Detection of disbonds in adhesively bonded aluminum plates using laser-generated shear acoustic waves

Adhesively bonded metals are increasingly used in many industries. Inspecting these parts remains challenging for modern non-destructive testing techniques. Laser ultrasound (LU) has shown great potential in high-resolution imaging of carbon-reinforced composites. For metals, excitation of longitudinal waves is inefficient without surface ablation. However, shear waves can be efficiently generated in the thermo-elastic regime and used to image defects in metallic structures. Here we present a compact LU system consisting of a high repetition rate diode-pumped laser to excite shear waves and noncontact detection with a highly sensitive fiber optic Sagnac interferometer to inspect adhesively bonded aluminum plates. Multiphysics finite difference simulations are performed to optimize the measurement configuration. Damage detection is performed for a structure consisting of three aluminum plates bonded with an epoxy film. Defects are simulated by a thin Teflon film. It is shown that the proposed technique can efficiently localize defects in both adhesion layers.

Fully noncontact measurement of inner cracks in thick specimen with fiber-phased-array laser ultrasonic technique

To realize a fully noncontact ultrasonic testing method for inner cracks inspection in thick metal specimen, a phased array laser ultrasonic testing system with using a compact optic fiber array bundle and a laser interferometer is developed in this study. The focusing and steering of the shear wave and longitudinal wave generated with seven fiber-phased-array laser sources in thermoelastic regime is investigated by a numerical simulation and validated by the experiment. A non-contact measurement of the inner-surface cracks by both the angle-beam testing method and time-of-flight diffraction method with the fiber-phased-array laser ultrasonic technique have been studied.

Surface circular-arc defects interacted by laser-generated Rayleigh wave

In this work, the finite element method (FEM) is used to investigate the propagation of laser-generated Rayleigh wave along the material surface at the quarter-arc transition surface under the thermoelastic regime, and to establish the relationship between the circular-arc radius and the time domain characteristics of reflected and transmitted Rayleigh waves. The simulation shows that the amplitude of the reflected Rayleigh wave decreases whereas the amplitude of the transmitted Rayleigh wave increases as the radius increases, which is significantly different from the well-studied interaction of Rayleigh waves with the perpendicular transition surface. By introducing the circular-arc defects which are easily formed in some engineering components during the material surface quenching, we find that the depth gauging of the surface circular-arc defects is more accurate in comparison to the surface rectangular defects based on the arrival time of the transmitted Rayleigh wave. This is further verified by the corresponding experimental results. These foundings are of practical values for detecting the depth of the arc defect quantitatively by the laser ultrasonic technique.

Feasibility Studies and Advantages of Using Dual Fiber Array in Laser Ultrasonic Inspection of Electronic Chip Packages

Fabrication and inspection techniques of electronic packages are two key factors influencing a chip’s success in post-Moore’s law era. As the electronic packaging industry rapidly evolves to cope with modern demands, a versatile inspection method for present and future packages is required. One approach is the development of a fiber array laser ultrasonic inspection system as presented in this paper. The new system uses two laser excitation points, allowing higher total energies to be delivered to the chip’s surface without exceeding the thermoelastic regime and causing damage. These beams induce strong ultrasonic waves, which can penetrate deeper and farther into and across a package. As a result, larger and thicker packages can be nondestructively inspected than was previously possible. Flip-chip ball grid array packages of size 52.5 mm $\times52.5$ mm from Cisco were evaluated using this system and compared against a single-beam system. Benefits of the new system include an increase in vibration amplitude due to high laser power, improved signal-to-noise ratio (SNR), an increase in resolution/sensitivity to measure microcracks, and high speed. The new dual-fiber-array system can administer 400-mW power safely, and as a result, the peak-to-peak amplitude of the generated ultrasound is three times that of ultrasound generated from a single laser beam. The SNR of 8:1 is achieved on the current Cisco package with double excitation point sources compared to the SNR of 1:1 that was obtained with a single excitation point source.

Improvement of focused ultrasonic beam generated by laser phased array: Theoretical analysis

To improve the focused ultrasonic beam induced by laser phased array (LPA), the superiority of an improved LPA distribution with conjunction of geometric attenuation and directivity functions of the stimulated ultrasonic beams are investigated theoretically instead of only considering the directivity function. Numerical simulations for the generation of focused longitudinal waves in the thermoelastic regime were implemented to reveal the advantages of the improved LPA design. It is shown that the amplitude of the focused beam increased by 42.1%, and the rise time reduced by 25.0%, as well as spatial sizes narrowed by 50.6% and 41.9% in longitudinal and transverse directions, respectively. In addition, the thermal expansion superposition caused by adjacent laser pulses, which deteriorates the focusing features of the focused beam, should be avoided by setting proper spacing of LPA.

Inspection of cracks with focused angle beam laser ultrasonic wave

In this work, in order to improve the detecting ability of laser ultrasonic testing technique for inner cracks in metals, an arc-line-focused laser source is proposed to generate focused angle-beam bulk waves in thermoelastic regime. The arc-line-focused laser source is achieved by passing an expanded and collimated laser beam through a convex and axicon lens off-center. The distribution of the shear waves generated by the arc-line laser source were detected by a laser interferometer to evaluate the focal area. Finally, the measurement of artificial cracks at the bottom surface of aluminum specimen with using the focused angle-beam bulk waves is investigated by experiment.