Laser Ultrasonic Measurements Research Articles

<i>In-situ</i> Laser Ultrasonics Measurements of Ferrite Formation during Stepped Cooling of A 0.1C-2Mn Dual Phase Steel

<i>In-situ</i> Laser Ultrasonics Measurements of Ferrite Formation during Stepped Cooling of A 0.1C-2Mn Dual Phase Steel

Non-contact measurements of residual stress distribution and grain size in titanium alloys with laser ultrasonic system

In this study, a laser ultrasonic method is proposed for nondestructive non-contact measurement of both the residual stress depth profile and grain size in Ti-6Al-4 V alloy processed by warm shot peening (WSP). The method utilizes a pulsed laser to generate a surface acoustic wave (SAW) along the cross-section of the sample, and a laser interferometer to detect SAW signals. Laser ultrasonic measurements were performed on the samples treated by WSP at different temperatures (32, 100, 200 and 300 °C). A theoretical model for calculating the residual stress profile and a model to correlate the center frequency shift and the mean grain size of the sample were presented. The results obtained by the laser ultrasonic method were compared with those obtained by X-ray stress measurement, center-hole drilling (CHD), and electron backscattering diffraction (EBSD). The proposed method can measure and locate the maximum compressive residual stress (CRS) in a peened surface layer with a high spatial resolution of approximately 30 µm, allowing for precise extraction of the residual stress profile. This method can also provide an estimation of the grain size through the same experiment. The developed method is promising for the quick and nondestructive measurement of the residual stress profiles and grain size of surface treated metallic materials.

Elastic moduli and refractive index of γ-Ge3N4.

Germanium nitride, having cubic spinel structure, γ-Ge3N4, is a wide band-gap semiconductor with a large exciton binding energy that exhibits high hardness, elastic moduli and elevated thermal stability up to approximately 700°C. Experimental data on its bulk and shear moduli (B0 and G0, respectively) are strongly limited, inconsistent and, thus, require verification. Moreover, earlier first-principles density functional calculations provided significantly scattering B0 values but consistently predicted G0 much higher than the so far available experimental value. Here, we examined the elasticity of polycrystalline γ-Ge3N4, densified applying high pressures and temperatures, using the techniques of laser ultrasonics (LU) and Brillouin light scattering (BLS) and compared with our extended first-principles calculations. From the LU measurements, we obtained its longitudinal- and Rayleigh wave sound velocities and, taking into account the sample porosity, derived B0 = 322(44) GPa and G0 = 188(7) GPa for the dense polycrystalline γ-Ge3N4. While our calculations underestimated B0 by approximately 17%, most of the predicted G0 matched well with our experimental value. Combining the LU- and BLS data and taking into account the elastic anisotropy, we determined the refractive index of γ-Ge3N4 in the visible range of light to be n = 2.4, similarly high as that of diamond or GaN, and matching our calculated value. This article is part of the theme issue 'Exploring the length scales, timescales and chemistry of challenging materials (Part 1)'.

High Temperature Characterization of the Stiffness Matrix of Different Steels

The aim of this work is to acquire material data at elevated temperatures for different chemical compositions and phases as basis for numerical simulations incorporating the effect of crystalline texture. Several samples; AISI 304, interstitially free, low carbon, martensite, and pearlite, were measured in a Gleeble® Thermal-Mechanical Simulator 3800 in conjunction with LUS (GLUS®) during annealing from room temperature up to a maximum temperature in the range between 675 and 1000 °C. The laser ultrasonic measurements contain both pressure (P) and shear (S) waves. The arrival time of at least the first S-wave echo and two or more P-wave echoes were used. From the measurements it was observed that there was a slight tilt of the sample and misalignment of the centres of the generation and detection laser beams. An inversion scheme was implemented estimating the nine stiffness coefficients as well as the sample tilt and alignment error. The estimated stiffness coefficients as a function of temperature were successfully obtained from the measurements. Most of the coefficients decrease with increasing temperature, whereas C12, C13, and C23 remain relatively constant. The implemented inversion scheme successfully compensates for sample tilt and laser misalignment.

Determining longitudinal and transverse elastic wave attenuation from zero-group-velocity Lamb waves in a pair of plates

A method for the determination of longitudinal and transverse bulk acoustic wave attenuation from measurements of the decay-rate of two independent zero-group-velocity resonances in a couple of matched plates is presented. A linear relation is derived, which links the bulk-wave attenuation coefficients to the decay-rate of plate-resonances. The relation is used to determine the acoustic loss of tungsten at GHz frequencies from noncontact laser-ultrasonic measurements in plates with thicknesses of about 1 µm. The longitudinal and transverse attenuation was found to amount to 1918 m-1 and 7828 m-1 at 2.16 GHz and 3265 m-1 and 12181 m-1 at 2.46 GHz. The presented approach is validated with calculated responses to a thermoelastic source, and the accuracy of the obtained attenuation values is estimated to be in the range of 10%.

Significance of Annealing Twins in Laser Ultrasonic Measurements of Grain Size in High-Strength Low-Alloy Steels

In this study, we demonstrate the significance of austenite annealing twin boundaries when calibrating laser ultrasonic measurements for gauging austenite grain size in situ during the thermomechanical processing of high-strength low-alloy steels. Simple calculations show how differences in twinning density can lead to errors in grain size measurements if twins are disregarded during calibration and the method is used for a broad range of steels. Conversely, when calibration is performed using alloys with a metastable austenite microstructure at room temperature, the same calibration is suitable for a broad range of HSLA steels, provided that annealing twins are taken into account. Since light optical microscopy does not allow the characterization of annealing twins in low-alloy steel, the verification of the laser ultrasonic results was conducted using the novel approach of comparing the twinned grain sizes obtained using the ultrasonic method in low-alloy steels with the austenite grain maps reconstructed from martensite orientation maps measured using electron backscatter diffraction. Finally, we show how differences in twinning density occur even for alloys with a roughly similar stacking fault energy, further highlighting the importance of annealing twins in the calibration of laser ultrasonic measurements for industrial use.

Laser-Ultrasound-Based Grain Size Gauge for the Hot Strip Mill

The paper summarizes the creation of a robust online grain size gauge for a hot strip mill. A method and algorithm for calculating the grain size from the measured ultrasonic attenuation is presented. This new method is self-calibrating, does not rely on a geometrical reference sample and can cope with the effects of diffraction on the attenuation. The model is based on 52 quenched samples measured with more than 23,000 laser ultrasonics shots and has a correlation coefficient R2 of 0.8. Typical online laser ultrasonic measurements from the hot strip mill and the calculated grain size versus length are presented for a couple of steel strips.

Laser Ultrasonic Measurements of Phase Transformation Kinetics in Lean Ti–Mo Alloys

Laser Ultrasonic Measurements of Phase Transformation Kinetics in Lean Ti–Mo Alloys

In situ laser-ultrasonic monitoring of Poisson’s ratio and bulk sound velocities of steel plates during thermal processes

We demonstrate the in situ measurement of zero-group-velocity Lamb waves and thickness-resonances in steel sheet samples using laser-based ultrasound, while changing the sample temperature from ambient to 1000 ∘C. The ratio of resonance frequencies yields the temperature dependent Poisson’s ratio of its material without requiring knowledge on the sheets’ thickness. Changes in the temperature derivative of the determined Poisson’s ratio reflect changes in the microstructural constituents. We show that additional resonances present in the response spectrum can be used to cross-check the evaluation. With additional knowledge on the thickness, longitudinal and transverse sound velocities are determined as well. We find good qualitative agreement between the temperature dependence of the measured quantities and reference data obtained from dilatometry and thermodynamic material simulations. The determination of the Poisson’s ratio and transverse sound velocity complements previous investigations limited to the longitudinal sound velocity. This is particularly evident above the Curie temperature during the ferrite to austenite transformation and could be beneficial to disentangle transformations with more than two microstructural constituents involved. Finally, the limitations posed by elastic anisotropy introduced by rolling texture, and experimental uncertainties due to temperature inhomgeneities are discussed. We conclude that the laser-ultrasonic measurement of plate resonances provides a robust, contact-free, non-destructive, and in situ determination of Poisson’s ratio.

Spatial Dependence of Dynamic Nonlinear Rock Weakening at the Alpine Fault, New Zealand

AbstractWe perform laser ultrasonic measurements to investigate the spatial dependence of dynamic nonlinear weakening in rocks from the Alpine Fault, New Zealand. Rocks outside the damage zone display no nonlinear weakening. Within the damage zone (<30 m from the fault), cataclasites present a 3% reduction in shear modulus from wave amplitudes inducing 1–2 microstrain at atmospheric pressure. This nonlinear elasticity decreases with a characteristic pressure between 1 and 2.5 MPa. We show that rock weakening is therefore strongest in the near surface. However, this significant elastic nonlinearity in cataclasites at low strains confirms that rock weakening may play an important role in earthquake processes, such as fault weakening, triggering of slip, rupture propagation, and coseismic velocity decreases.

In-process monitoring of welding quality by robotic laser ultrasonic measurement system using microchip laser

ABSTRACT Laser ultrasonic technique (LUT) is applicable even in a high temperature state such as during welding because the laser is irradiated remotely without contact. Recent studies have reported that LUT is applicable during welding, and it is effective for in-process monitoring. However, in the previous studies, there was a problem with the portability of the measurement device especially in a generation laser, so the scope of its application was limited. In this study, we constructed a robotic measurement system of laser ultrasonic with a small microchip laser mounted on a robotic arm. Microchip laser, which is a prototype product (1064 nm, 20 mJ/pulse, 1.5 ns, 20 Hz), was used to generate ultrasonic, and a laser interferometer (532 nm, 1 W) was used to detect the surface micro-vibration caused by the arrival of ultrasonic wave and was scanned using an uniaxial stage. After acquiring the B scope, which is composed of a generation point and multi-detection points, the synthetic aperture focussing technique (SAFT) is applied. SAFT image obtained by in-process measurement was evaluated to confirm the presence or absence of welding defect indication. Then, during gas metal arc (GMA) welding process in the mid-pass of a multi-layer welding for a single bevel groove, we attempted to detect welding defects generated by introducing slits before welding. As a result, in the second pass, defects were indicated in all areas where defects existed, and no indications were found in areas without defects. Therefore, it was shown that the robotic laser ultrasonic measurement system constructed in this study can detect defects in the process of welding. In conclusion, we were able to show the possibility of evaluating the welding quality in the process of welding even at the construction site by selecting the appropriate generation position and detection range according to the required accuracy and laser specifications.

Mechanical properties estimation of additively manufactured metal components using femtosecond laser ultrasonics and laser polishing

Directed energy deposition (DED), also known as laser cladding, is one type of additive manufacturing where a high-power laser combined with a coaxial powder delivery system is used to produce metal components layer-by-layer. Despite many advancements in DED, little work has been accomplished in online estimation of mechanical properties, such as Young's modulus and Poisson's ratio, during the DED process. In this study, an online mechanical properties estimation technique was developed for the DED process using femtosecond laser ultrasonics and laser polishing. The unique contributions of this study are the following: (1) the development of a fully noncontact and nondestructive technique for the estimation of Young's modulus and Poisson's ratio using a femtosecond laser, (2) layer-by-layer estimation of mechanical properties accomplished by achieving micrometer-level spatial resolution, (3) laser ultrasonic measurement from a deposited metal layer using laser polishing instead of mechanical polishing, and (4) potential for online operation during the DED process. The performance of the proposed technique was examined using specimens fabricated under various DED operation conditions. The Young's modulus and Poisson's ratio values estimated by the proposed technique were consistent with those obtained from independent tensile tests performed after the completion of the DED process.

In-process monitoring of welding quality by robotic laser ultrasonic measurement system using microchip laser

Laser Ultrasonic Technique (LUT) is applicable even in a high temperature state such as during welding because the laser is irradiated remotely without contact. Recent studies have reported that LUT is applicable during welding and it’s effective for in-process monitoring. However, in the previous studies, there was a problem with the portability of the measurement device especially in a generation laser, so the scope of its application was limited. In this study, we constructed a robotic measurement system of laser ultrasonic with a small microchip laser mounted on a robotic arm. Microchip laser, which is prototype product (1064nm, 20mJ/pulse, 1.5ns, 20Hz), was used to generate ultrasonic, and a laser interferometer (532nm, 1W) was used to detect the surface micro vibration caused by arrival of ultrasonic wave and was scanned using a mechanical stage. After acquiring the B-scope, which is composed of a generation-point and multi detection-points, the synthetic aperture focusing technique (SAFT) is applied. SAFT image obtained by in process measurement was evaluated to confirm the presence or absence of welding defect indication. Then, during Gas Metal Arc (GMA) welding process in the mid-pass of multi-layer welding for a single bevel groove, we attempted to detect welding defects generated by introducing slits before welding. As a result, in the second pass, defects were indicated in all areas where defects existed, and no indications were found in areas without defects. Therefore, it was shown that the robotic laser ultrasonic measurement system constructed in this study can detect defects in process of welding. In conclusion, we were able to show the possibility of evaluating the welding quality in process of welding even at the construction site by selecting the appropriate generation position and detection range according to the required accuracy and laser specifications.

Picosecond laser ultrasonic measurements of interlayer elastic properties of 2H-MoSe2 and 2H-WSe2

We have used the ultrafast pump-probe technique known as picosecond ultrasonics to generate and detect coherent acoustic phonon pulses with frequencies reaching 40 GHz in exfoliated crystals of MoSe2 and WSe2 on Si and sapphire substrates. We report picosecond time-resolved reflectivity data from samples ranging from 180 nm to 920 nm in thickness and compare our results to a 1D simulation of strain-induced changes in the optical reflectivity. We find the longitudinal sound velocity along the c-axis (the interlayer direction) to be 2800 m/s ± 40 m/s for MoSe2 and 2510 m/s ± 60 m/s for WSe2. We also report the measured lifetime of longitudinal acoustic phonons approaching 40 GHz to be 0.85 ± 0.2 ns and compare this value with predictions of relaxation damping and 3-phonon models, as well as discuss its relationship to the predicted thermal conductivity of MoSe2.

Quantitative characterization of laser ultrasonic based on energy loss and resonance phenomenon

A novel method is proposed for quantitative characterization of cracks by laser ultrasound. It is found that cracks with different depth will have different feedbacks to sound waves based on crack closure effect, energy loss theory and resonance phenomenon. Therefore, damage sensitive characteristic parameters can be extracted from the acoustic waves and used to quantitatively characterize the crack size, including the waveform duration, the reflection energy coefficient and frequency spectral area. In this paper, cracks with gradient depth are used as experimental objects, and the relevant data is obtained through simulation and experiments. The defects are positioned, and the positioning error is less than 3%. The study found that as the crack depth increases, the waveform duration and the reflection energy coefficient gradually decrease, and the frequency spectral area first increases and then decreases. When the crack depth is larger than 0.9mm, characteristic parameters tend to be fixed values. When the crack depth is 0.5mm-0.7mm, abrupt phenomena occurred in both the reflection energy coefficient and the frequency spectrum area due to resonance phenomenon. By analyzing the corresponding relationship between damage sensitivity characteristic parameters and depth, the ability of laser ultrasonic quantitative measurement can be improved.

Application of Laser-Ultrasonic Technique of Acoustic Impedance Measurement with Signals Detection by Backward-Mode Scheme for Porosity Content Evaluation in CFRP Laminates

The aim of this work is the development and experimental implementation of the laser-ultrasonic method for CFRPs porosity assessment with one-side access to an investigated object. The scheme with the laser thermooptical generation and backward-mode piezoelectric detection of longitudinal acoustic waves is used. The proposed method is based on the measurement of the acoustic impedance of a composite specimen by using the value of the integral of the ultrasonic pulse reflected from an immersion layer-specimen interface. The existence of the relationship between the porosity content, ultrasound velocity, and material’s density allows one to assess the porosity of CFRPs by the measured acoustic impedance. The proposed method does not require plane parallelism of the input and output surfaces of the studied objects, as well as measuring the thickness of the object. This allows studying CFRP structures with complex shapes. This method may be of use for various types of composite materials, regardless of acoustical waves propagating features related to a periodical material’s structure. CFRP specimens with three reinforcement schemes and different average volume porosity contents were investigated. It is shown that the local porosity distribution in the studied CFRPs is nonuniform along the fiber stacking plane. The porosity content obtained by the laser-ultrasonic method was checked using the X-ray computer tomography. The porosity values, averaged from the laser-ultrasonic measurement results, coincide with the X-ray tomography data within the error limits. The method can be useful for quality control of obtained composite structures with the aim of modernization of technology processes and selecting optimal production conditions and also for the detection of changes in composite structures during their operation or during fatigue testing processes.

Use of in-situ laser-ultrasonics measurements to develop robust models combining deformation, recovery, recrystallization and grain growth

The hot deformation and subsequent annealing of two C-Mn steels have been investigated using a Gleeble 3500 simulator. The flow stress during deformation and subsequent relaxation stress were recorded. The grain size evolution was simultaneously evaluated by the in-situ laser-ultrasonic (LUS) measurements. The LUS data allowed clear identification of the dominant annealing phenomena. Excellent correlation was obtained between stress relaxation and laser-ultrasonic data, which provided additional confidence in the identification of the processes taking place. The solute drag effect of niobium led to the delayed onset and completion of recrystallization. Lower temperature generally slowed down the kinetics. Robust models combining deformation, recovery, recrystallization and grain growth have been developed and validated against the experimental data. The onset of recrystallization can be predicted precisely, as well as the time for recrystallization completion. As can be seen, the models are in good agreements with both the mechanical (stress evolution) and grain-size (laser-ultrasonic) data.

Laser ultrasonic measurement of recrystallization and grain growth in an L605 cobalt superalloy

Recrystallization and grain growth following hot deformation of an L605 cobalt superalloy was measured in real time using laser ultrasonics. Ex-situ microstructure characterization of quenched samples in combination with double hit tests was used to evaluate the potentials and limitations of the laser ultrasonic method for these in-situ measurements. Significant grain refinement occurs during recrystallization and was recorded with the ultrasonic attenuation using a calibration previously developed for Co superalloys including L605. At higher deformation temperatures complete recrystallization takes place and the recrystallized grain size is independent of temperature. At lower deformation temperatures, recrystallization remains incomplete which can be deduced from the apparently larger grain size obtained from attenuation measurements than in the cases of complete recrystallization. The present approach has the potential to accelerate the development of thermomechanical processing paths through real time measurements of microstructure evolution in Co superalloys as well as other metals and alloys.

Noncontact spatiotemporal strain mapping of composite multiferroic cylinders

Recently, the reinvigoration of research interest in in strain-mediated composite multiferroic structures resulted in translational magnetoelectric devices with superior performance than their traditional counterparts. Full-filed measurements and visualization of mechanical strain, with a focus at the interface, offer new possibilities for the advancement of magnetoelectric devices regardless of the material system. Here, we report spatiotemporal strain maps of composite magnetoelectric cylinders using a novel approach combining stable laser ultrasonic vibrometer and 2D motion system measurement technique. The physical measurement technique is augmented with a robust analysis algorithm that capable of effectively and consistently detect the interface. With the spatiotemporal maps, we forecast additional utility through the concurrent extraction of polarization and magnetization maps; hence highlighting the complex interactions between the involved order parameters. Understanding the converse magnetoelectric response in strain-mediated composite multiferroics is essential for the development of devices such and motors, antennas, and wireless power transferors. Given the dependence of the converse magnetoelectric response on the strain generation and transduction, mapping the mechanical response is essential for advancement of efficient devices. This mapping approach is challenging due to the dynamic nature of the response as well as the magneto-electro-mechanical coupling. Spatiotemporal maps of in-plane and out-of plane displacement and strain components were measured concurrently off the surface of a composite structure. To generate spatiotemporal strain maps, a laser-ultrasonic vibrometer mounted on a 2D gantry system was employed to map the in-plane and out-of-plane displacement components of a strain-mediated, concentric multiferroic composite cylinder. The strain maps provided insights on the behavior of the strain mediation mechanisms within the investigated composite structure including amplification of the strain due to the disparity in the mechanical properties of the constituents, which will result in a non-uniform distribution of magnetization and polarization responses. Additionally, the method to calculate the strain maps was shown to be robust for different spatial measurement resolutions.

LASER-ULTRASONIC METHOD OF ACOUSTIC IMPEDANCE MEASUREMENT FOR QUANTITATIVE POROSITY ESTIMATION OF CROSS-PLY CARBON FIBER REINFORCED PLASTIC MATERIALS

A method of measuring the acoustic impedance of carbon fiber plastics based on the laser optoacoustic effect is proposed and experimentally realized. Measurement of the acoustic impedance of the studied composite is made by the value of the antiderivative of ultrasonic pulse reflected from the interface between the immersion liquid and the sample. A method for determining the porosity of a material by the measured value of the acoustic impedance, based on the dependence of the material density and the velocity of propagation of longitudinal acoustic waves in it on its porosity, is presented. Porous samples of crossply reinforced carbon plastics with three types of carbon fiber lay-up schemes were studied. It was found that the studied carbon fiber plastics have a non-uniform distribution of local porosity in the plane of carbon fabric stacking. It is also shown that the variation of the local porosity in the sample depends on the fiber laying scheme. It is shown that the porosity value obtained by X-ray computed tomography coincides with the results of laser-ultrasonic measurements. The advantage of the proposed method is the possibility of rapid diagnosis of porosity with one-way access to the object under study without measuring its dimensions and mass, which can be used for composite structures of complex shape.