Laser Ultrasonic Inspection Research Articles

Mechanical properties and damage characterization of 310S steel using laser ultrasonic inspection under extreme temperature condition

Abstract To evaluate the mechanical properties of 310S stainless steel under extreme working conditions, and realize the damage characterization function, in this study, the longitudinal wave and Rayleigh wave of materials are measured by the method of coaxial inspection on reverse surface and ipsilateral ectopic inspection, respectively, and surface cracks and internal voids are detected by the scanning laser source technology and transmission scanning detection, respectively. The detection methods are designed based on the energy distribution characteristics of Rayleigh wave and longitudinal waves in laser ultrasound. The laser ultrasonic detection systems coupled with a temperature loading device (high temperature: vacuum chamber; low temperature: refrigerating chamber) developed by the laboratory enable on-site monitoring of laser ultrasonic technology in harsh environments. Experimental results demonstrate that the error in mechanical properties (elastic modulus, shear modulus, Poisson ratio) is less than 5% over a wide temperature range (-180-1000°C). At 1000°C, surface cracks wider than 0.5mm and deeper than 1.9mm as well as internal hole defects larger than 1.0mm can be detected with an error rate below 5%.

A laser ultrasonic intelligent inspection method for metal surface defects based on digital twin model

To enhance the efficiency and intelligence of laser ultrasonic detection of metal surface defects, a detection method based on digital twins is proposed. Given the scarcity of defect samples and the difficulty of obtaining labeled data in practical applications, the finite element method is used to construct a laser ultrasonic metal defect detection model, effectively generating twin data for defect detection. With the help of generative adversarial networks, the distribution difference between twin data and real data is minimized. Additionally, by using continuous wavelet transform, one-dimensional ultrasonic signals are converted into two-dimensional time–frequency images for detection input. In terms of detection algorithm development, a residual network with an optimized global attention mechanism is introduced to ensure real-time synchronization and updates between the digital twin model and the actual laser ultrasonic detection equipment. Finally, experimental validation of the proposed method shows that it effectively addresses the issue of small sample sizes in defect detection and achieves high detection accuracy. Moreover, comparative experiments on crack defect migration tasks under different settings demonstrate the method’s good generalization ability.

Thin Copper Plate Defect Detection Based on Lamb Wave Generated by Pulsed Laser in Combination with Laser Heterodyne Interference Technique.

Thin copper plate is widely used in architecture, transportation, heavy equipment, and integrated circuit substrates due to its unique properties. However, it is challenging to identify surface defects in copper strips arising from various manufacturing stages without direct contact. A laser ultrasonic inspection system was developed based on the Lamb wave (LW) produced by a laser pulse. An all-fiber laser heterodyne interferometer is applied for measuring the ultrasonic signal in combination with an automatic scanning system, which makes the system flexible and compact. A 3-D model simulation of an H62 brass specimen was carried out to determine the LW spatial-temporal wavefield by using the COMSOL Multiphysics software. The characteristics of the ultrasonic wavefield were extracted through continuous wavelet transform analysis. This demonstrates that the A0 mode could be used in defect detection due to its slow speed and vibrational direction. Furthermore, an ultrasonic wave at the center frequency of 370 kHz with maximum energy is suitable for defect detection. In the experiment, the size and location of the defect are determined by the time difference of the transmitted wave and reflected wave, respectively. The relative error of the defect position is 0.14% by averaging six different receiving spots. The width of the defect is linear to the time difference of the transmitted wave. The goodness of fit can reach 0.989, and it is in good agreement with the simulated one. The experimental error is less than 0.395 mm for a 5 mm width of defect. Therefore, this validates that the technique can be potentially utilized in the remote defect detection of thin copper plates.

Calculations of Temperature and Thermo-Elastic Displacement Distributions Generated by Laser-Ultrasonic Inspection System

Calculations of temperature and thermo-elastic displacement distributions generated by laser-ultrasonic inspection system are performed. The goal is to determine temperature distributions created in the test objects by generation laser and the respective thermo-elastic displacement distributions causing the ultrasonic waves propagating within the test objects during laser-ultrasonic inspection of parts. The respective heat propagation equation and thermo-elastic equation of motion are solved analytically for 1D and 3D test objects with different geometries.

Noncontact laser ultrasonic inspection of weld defect in lithium-ion battery cap

The continuous development of electric vehicles and electronic devices has increased the demand for lithium-ion batteries. In this study, a laser ultrasonic inspection system was developed for the noncontact and nondestructive inspection of the laser welding conditions of a cylindrical lithium-ion battery cap. An Nd: YAG pulse laser was used for Lamb wave generation on the battery cap. Lamb waves were measured from the front surface of the battery cap using a laser Doppler vibrometer. The bending stiffness of the battery cap decreased as the welding condition deteriorated, and the Lamb wave amplitude was amplified. Accordingly, an automated defect detection algorithm for inspecting welding conditions was developed. The effectiveness of the proposed inspection system was theoretically and experimentally evaluated using actual cylindrical battery caps. The battery cap weld inspection accuracy exceeded 89 %, correctly identifying seven defective and ten intact battery caps out of eight defective and 11 intact battery caps, respectively. The numerical simulation indicated that the Lamb wave amplitude increased at the weld joint, demonstrating consistency with the experiment results. The proposed system is promising for the in-line inspection of battery caps because it facilitates high-speed, noncontact, and nondestructive inspection. Furthermore, the proposed inspection system can detect invisible weld defects on the back surface of a battery cap by inspecting its front surface.

Image denoising for laser ultrasonic inspection of wire arc additive-manufactured components with a rough surface

ABSTRACT Laser ultrasonic testing (LUT) is a non-contact and non-destructive testing (NDT) technique which can detect the wire arc additive manufacturing (WAAM) component defect in hot state. However, the complex surface topography of the WAAM component induces a low signal to noise ratio (SNR) and a poor detection sensitivity. In this paper, a denoising method based on improved empirical mode decomposition with adaptive noise (ICEEMDAN) and Wavelet Packet Transform (WPT) was applied in the LUT of the WAAM component. All the signals in the LUT scan maps with low SNR were decomposed into a series of intrinsic mode functions (IMFs) using ICEEMDAN. The Pearson correlation coefficient of the original signal and the IMFs were calculated. The high frequency (HF) and low frequency (LF) IMFs with correlation coefficients under the threshold were denoised using WPT and deleted, respectively. The denoised IMFs and the IMFs without processing were stacked up to compose the denoised signal and plot the denoised scan maps. The denoising results show that the proposed method improved the LUT efficiency and sensitivity. Finally, the denoising method was applied and verified in the LUT of the WAAM nuclear power pipeline nozzle.

Laser ultrasonic inspection for mechanical properties of materials at high temperature

The measurement of mechanical properties of high-temperature resistant materials at high temperatures is particularly critical, and laser ultrasonic non-destructive testing technology is a prospective approach for measuring mechanical properties in high-temperature environments. Laser ultrasonic propagation models are constructed by finite element analysis, which reveals the effects of different excitation mechanisms on acoustic velocity measurements. It is found that the shear wave in the main propagation direction is hardly detected, and the distinction between shear waves and surface waves in the time domain signal is difficult. Based on the conclusions, the relationships between surface waves, longitudinal waves, and mechanical properties of materials are established by theoretical derivation. According to the propagation characteristics of surface waves and longitudinal waves, the eccentricity detection scheme of the same side and the concentricity detection scheme of the opposite side are, respectively, designed. The velocities of surface waves and longitudinal waves are measured, considering the thermal expansion coefficient and density changes with temperature. The mechanical properties of materials at different temperatures (25–1000 °C) are successfully calculated, and the experimental results are well in accordance with the reference values. It gives a reliable basis for efficient measurement of mechanical properties of materials at high temperatures.

Imaging features of different defects in metals using laser ultrasonic techniques

Different types of defects such as pores, porosity, cracks in different directions, have various impact on the load bearing capacity and fatigue strength of metal components used in the field of aerospace, rail transportation, electrical equipment, etc. Therefore, feature recognition and classification of defects contribute greatly to the performance evaluation of the metal components. In this paper, the imaging features of six typical defects in metals were obtained by using laser ultrasonic (LU) scan inspection techniques. First, the numerical models of LU inspection of different defects were established based on the discontinuous Galerkin finite element method (dG-FEM) in COMSOL/Multiphysics. Then, multiple groups of FE simulations were carried out. The generation and receiving lasers were moved along the scanning direction according to a fixed step during the simulation. All the simulated A-scan signals were stacked up and plotted in the B-scan and D-scan maps. At last, six samples with different artificial defects were manufactured for the LU inspection experiment. The defect imaging features in the experimental B-scan maps are consistent with the numerical results. Besides, the mathematical calculation methods of defect position, dimension and orientation were put forward.

Correction to: Laser ultrasonic inspection of additive manufactured components

Correction to: Laser ultrasonic inspection of additive manufactured components

Multi-feature fusion imaging via machine learning for laser ultrasonic based defect detection in selective laser melting part

Laser ultrasonic (LU) testing is a promising technique that meets the constraints of additive manufacturing (AM) online monitoring, which is vital for the quality control of AM products and the practical application of AM. However, the scattering noise caused by surface roughness of AM parts and the ambient noise can generally result in defect imaging with low resolution. This is not conducive to defect detection. In terms of this problem, a multi-feature fusion imaging methodology is proposed instead of the traditional method containing a single feature, i.e., maximum amplitude within a set window of all A-scans. The most discriminative features in each A-scan are automatically extracted by principal component analysis (PCA). These features are subsequently fused by a trained artificial neural network (ANN) to obtain an evaluated value that determines whether a A-scan is the normal case or defective case (denoted by 0 and 1, respectively). Finally, a C-scan image with high signal to noise ratio (SNR) can be produced by plotting the binary dataset of 0 and 1. LU inspection on a selective laser melting (SLM) component with the pre-induced micro hole defects on the rough surface is performed to validate the effectiveness of the methodology. The defects are distributed in 2 rows and 6 columns where the diameter of one row is 100 μm and the other is 50 μm. It is demonstrated that the proposed methodology can detect and size all micro defects by the obtained high SNR C-scan.

Laser ultrasonic inspection of defects in wire arc additive manufactured samples with different surface profiles

The laser ultrasonic (LU) technique has the advantages of a wide frequency band, small non-detection zone, high resolution and non-contact compared to conventional ultrasonic testing, which makes it potential for on-line inspection of the wire arc additive manufacturing (WAAM) process. To investigate the influence of surface profile on the LU inspection of WAAM defects, multiscale analysis was adopted in the mathematical characterization of the sample surface profile. Finite Element (FE) models of LU inspection of WAAM defects with different surface profiles were established using COMSOL software. The propagation of acoustic waves and their interaction with surface profile and defects were numerically analyzed. The LU inspection methods, that is, superficial crack detected by through-transmission of Rayleigh wave and internal hole detected by pulsed-echo of shear wave were put forward. A set of WAAM samples with different surface topographies was manufactured by the robotic gas metal arc welding (GWAM) system through varying the process parameters and filler materials. LU inspection experiments were conducted to obtain B-scan images and A-scan signals of the defects in these samples. The experimental results indicate that surface roughness has a significant effect on the signal-to-noise ratio (SNR) of internal through hole detection, and surface waviness has a significant effect on the sensitivity of superficial crack detection. The experimental results are consistent with the numerical results, which validate the proposed FE model. The proposed FE model can be used for the quantitative study of the influence of WAAM sample profile on the LU inspection. Furthermore, it is also a guidance for the formulation of key process parameters in LU inspection, such as the maximum generating-receiving laser distance and the laser’s layout. At last, a robotic LU inspection system was built and preliminarily achieved the automatic inspection of a WAAM part. This research provides a theoretical basis for the on-line application of LU techniques in WAAM process monitoring.

Correlation between Laser-Ultrasound and Microstructural Properties of Laser Melting Deposited Ti6Al4V/B4C Composites

The presence of large microtextured clusters (MTC) composed of small α-phase crystallites with preferred crystallographic orientations in 3D printed near-α titanium alloys leads to poor mechanical and fatigue properties. It is therefore crucial to characterize the size of MTCs nondestructively. Ti6Al4V/B4C composite materials are manufactured using Laser Melting Deposition (LMD) technology by adding an amount of nano-sized B4C particles to the original Ti6Al4V powder. TiB and TiC reinforcements precipitating at grain boundaries stimulate the elongated α crystallites and coarse columnar MTCs to equiaxed transition, and microstructures composed of approximately equiaxed MTCs with different mean sizes of 11–50 μm are obtained. Theoretical models for scattering-induced attenuation and centroid frequency downshift of ultrasonic waves propagating in such a polycrystalline medium are presented. It is indicated that, the studied composite material has an extremely narrow crystallographic orientation distribution width, i.e., a strong degree of anisotropy in MTCs. Therefore, MTCs make a dominant contribution to the total scattering-induced attenuation and spectral centroid frequency downshift, while the contribution of fine α-phase crystallites is insignificant. Laser ultrasonic inspection is performed, and the correlation between laser-generated ultrasonic wave properties and microstructural properties of the Ti6Al4V/B4C composites is analyzed. Results have shown that the deviation between the experimentally measured ultrasonic velocity and the theoretical result determined by the Voigt-averaged velocity in each crystallite is no more than 2.23%, which is in good agreement with the degree of macroscopically anisotropy in the composite specimens. The ultrasonic velocity seems to be insensitive to the size of MTCs, while the spectral centroid frequency downshift is approximately linear to the mean size of MTCs with a goodness-of-fit (R2) up to 0.99. Actually, for a macroscopically untextured near-α titanium alloy with a relatively narrow crystallographic orientation distribution, the ultrasonic velocity is not correlated with the properties of MTCs, by contrast, the central frequency downshift is dominated by the size and morphology of MTCs, showing great potentials in grain size evaluation.

Image processing of Casting defects based on Convolutional neural network

At present, there are numerous losses caused by corrosion cracking of metal castings in engineering in China. In order to detect the possible defects of metal castings in engineering, the laser ultrasonic vision inspection technology is used to image the castings, and then the identification efficiency is low. In order to process these images efficiently and quickly, convolutional neural network image processing technology is introduced. According to the actual needs, a convolutional neural network architecture is designed to recognize images, and whether the architecture meets the requirements is verified. Experimental results show that the performance of the architecture meets the design requirements. Under the same conditions, this structure provides a solution for casting defect detection combined with artificial intelligence.

Analysis on ultrasonic waves generated in anisotropic carbon fiber reinforced plastic laminate by laser incidence from various directions

Laser-ultrasonic waves are used in nondestructive inspections because they can be excited on arbitrarily shaped surfaces by irradiation of a pulsed laser from a distance. During the non-contact scanning of a laser, the waveform of excited ultrasonic wave is changed because the incident angle varies substantially. The change of the waveform will be more obvious for carbon fiber reinforced plastics (CFRP) laminates that have anisotropic properties. In order to guarantee the inspection, in this paper, we investigated the generation of ultrasonic waves in CFRP laminates by a laser irradiation with various incident angles and azimuth angles. Firstly, we constructed a simulation model: the thermal force field caused by the laser absorption is derived from a heat conduction equation, and it is introduced into an FEM software for analyzing the propagation behavior of generated ultrasonic waves. Then, the simulation result is compared with an experiment result. Through the FEM analysis, we found that the amplitude of the excited waves has an obvious directivity because of the strong anisotropy in the thermo-elastic property of the CFRP. In addition, the change in propagation behavior in the case of a small incident angle of 30° was found to be inconspicuous. However, when the incident angle of the laser beam is larger than 60°, the directivity pattern changes depending on the azimuth angle. Thus, it is important to control the incident angle and direction when a laser ultrasonic inspection is applied to CFRP structures.

Research on laser ultrasonic surface defect identification based on a support vector machine.

To solve the problem of difficult quantitative identification of surface defect depth during laser ultrasonic inspection, a support vector machine-based method for quantitative identification of surface rectangular defect depth is proposed. Based on the thermal-elastic mechanism, the finite element model for laser ultrasound inspection of aluminum materials containing surface defects was developed by using the finite element software COMSOL. The interaction process between laser ultrasound and rectangular defects was simulated, and the reflected wave signals corresponding to defects of different depths under pulsed laser irradiation were obtained. Laser ultrasonic detection experiments were conducted for surface defects of different depths, and multiple sets of ultrasonic signal waveform were collected, and several feature vectors such as time-domain peak, center frequency peak, waveform factor and peak factor were extracted by using MATLAB, the quantitative defect depth identification model based on support vector machine was established. The experimental results show that the laser ultrasonic surface defect identification model based on support vector machine can achieve high accuracy prediction of defect depth, the regression coefficient of determination is kept above 0.95, and the average relative error between the true value and the predicted value is kept below 10%, and the prediction accuracy is better than that of the reflection echo method and BP neural network model.

Measurement Capability of Laser Ultrasonic Inspection System for Evaluation of Ball-Grid Array Package Solder Balls

Abstract Laser ultrasonic inspection is a novel, noncontact, and nondestructive technique to evaluate the quality of solder interconnections in microelectronic packages. In this technique, identification of defects or failures in solder interconnections is performed by comparing the out-of-plane displacement signals, which are produced from the propagation of ultrasonic waves, from a known good reference sample and sample under test. The laboratory-scale dual-fiber array laser ultrasonic inspection system has successfully demonstrated identifying the defects and failures in the solder interconnections in advanced microelectronic packages such as chip-scale packages, plastic ball grid array packages, and flip-chip ball grid array packages. However, the success of any metrology system depends upon precise and accurate data to be useful in the microelectronic industry. This paper has demonstrated the measurement capability of the dual-fiber array laser ultrasonic inspection system using gage repeatability and reproducibility analysis. Industrial flip-chip ball grid array packages have been used for conducting experiments using the laser ultrasonic inspection system and the inspection data are used to perform repeatability and reproducibility analysis. Gage repeatability and reproducibility studies have also been used to choose a known good reference sample for comparing the samples under test.

Femtosecond laser ultrasonic inspection of a moving object and its application to estimation of silicon wafer coating thickness

In this study, an ultrasound generation and sensing system using a femtosecond laser is developed specifically for noncontact inspection of a moving object. In the developed femtosecond laser ultrasonic system, a laser pulse source is divided into pump and probe laser pulses. Using a pump laser pulse with a subpicosecond duration, ultrasounds with ultrashort wavelengths (micrometer to tens of nanometers) are generated up to THz. Then, the resulting ultrasounds are measured using a probe laser pulse based on reflectometry at a sampling frequency of up to 1.5 THz. The developed system is used to generate and measure ultrasounds from a silicon wafer while the wafer is moving in a horizontal direction. Because of the ultrashort pulse duration of the probe and pump laser pulses, the contact time of these pluses with respect to a moving object is extremely short (subpicosecond), and the distortion of ultrasounds due to object motion is minimized. Ultrasounds are measured from the silicon wafer in both pulse-echo and pitch-catch modes, and it is validated that the ultrasounds acquired from a moving condition of the silicon wafer are in good agreement with those obtained from a stationary condition. Then, the thickness of a submicrometer coating layer deposited on the silicon wafer was successfully estimated while the silicon wafer was moving up to 20 mm/s.

Correlation Studies Between Laser Ultrasonic Inspection Data and Finite-Element Modeling Results in Evaluation of Solder Joint Quality in Microelectronic Packages

Abstract Laser ultrasonic inspection (LUI) is a nondestructive and noncontact technique to evaluate the quality of solder ball interconnections in area-array micro-electronic packages. Dual-Fiber Array Laser Ultrasonic Inspection System was demonstrated identifying defects and failures in chip-scale packages, ball grid array packages, and flip-chip ball grid array packages. The location and severity of the defects and failures in packages have been identified accurately using this system. Further, it is important to establish the correlation between LUI results and the severity of the failures for failure mode analysis, which will enable us to eliminate the need for destructive testing and allow the study of failure evolution in a given sample under continued reliability testing. This paper discusses correlation studies between experimental LUI results and finite-element simulation results from the flip-chip ball grid array packages subjected to thermal cycling reliability testing. The correlation equations will help in predicting the severity of the failures at a given number of thermal cycles based on LUI results. Furthermore, the life of the micro-electronic packages can be predicted accurately from LUI results at a fewer number of thermal cycles.

A New Laser Ultrasonic Inspection Method for the Detection of Multiple Delamination Defects.

Delamination is one of the most common types of defects for carbon fiber reinforced plastic (CFRP) composites. The application of laser techniques to detect delamination faces difficulties with ultrasonic wave excitation because of its low thermal conductivity. Much of the research that can be found in the literature has only focused on the detection of a single delamination. In this study, aluminum foil was pasted onto the surface of the composite so that it was vulnerable to ablation and could acquire a usable signal. Using a fully noncontact system with laser excitation at a fixed point and a scanning laser sensor, the effects of different aluminum foil sizes and shapes on the wavefield were studied for the composites; we decided to use a rectangle with 3 mm length and 5 mm width for laser excitation experiments. Wavefield characteristics of the composite plates were analyzed with single- and multi-layered Teflon inserts. Taking the time window for standard ultrasonic testing as a reference, the algorithms for localized wave energy with appropriate time windows are presented for the detection of single and multiple defects. The appropriate time window is meaningful for identifying each delamination defect. The algorithm performs well in delamination detection of the composites with one or multiple Teflon inserts.

Evaluation of the Quality of BGA Solder Balls in FCBGA Packages Subjected to Thermal Cycling Reliability Test Using Laser Ultrasonic Inspection Technique

In modern integrated circuit (IC) packaging, it is difficult to perceive and evaluate the quality of the solder ball interconnections due to their complex hidden physical configuration and miniaturization. Laser ultrasonic inspection (LUI) technique is a novel, nondestructive, and noncontact method that has achieved great success in evaluating the quality of interconnections in chip-scale and ball-grid array (BGA) packages. In this article, an enhanced dual-fiber array LUI system is used to evaluate the quality of BGA solder balls in advanced industrial flip-chip BGA packages subjected to thermal cycling reliability tests. LUI results are validated with electrical testing, scanning electron microscope (SEM), and dye-and-pry results. LUI results are well correlated with the intermetallic compound cracks in BGA solder balls observed using SEM, and dye-and-pry. This study demonstrates the feasibility of using the LUI system for one of the modern IC packages and the potential to use the LUI system for future advanced IC packages such as 3-D packaging.