Grain size characterization of Ti-6Al-4V titanium alloy based on laser ultrasonic random forest regression.

In this paper, a random forest regression (RFR) rain size characterization method based on a laser ultrasound technique is investigated to predict the grain size of titanium alloy (Ti-6Al-4V). The longitudinal wave velocity of the ultrasound signal and the attenuation coefficient at different frequencies are used as the input and the grain size is used as the output. An RFR algorithm was used to develop a grain size prediction model. Meanwhile, the grain size calculation model based on conventional scattering attenuation was established by calibrating the n value in the classical scattering theory using the attenuation coefficients at different frequencies of ultrasonic signals. The results show that the RFR algorithm is feasible for the grain size characterization of titanium alloys.

Weld quality inspection of small-diameter thin-walled pipes by a laser ultrasonic method.

Defect inspection of small-diameter thin-walled pipes is a difficult problem in the field of nondestructive testing. In this paper, a new detection method based on laser ultrasonics and guided circumferential wave technology is proposed and used to inspect the defects in pipes. First, a theoretical model based on the theory of acoustic propagation in solids is proposed for the small-diameter thin-walled pipes. The dispersion characteristics of the guided circumferential waves of thin-walled pipes are obtained by numerical simulation. Second, a laser ultrasonic system is constructed and used to detect the flaws in the small-diameter thin-walled pipes. Finally, the laser ultrasonic system is used to analyze the welding quality of stainless steel pipes with a diameter of 4.20 mm and a wall thickness of 0.30 mm. The experimental results are in agreement with the theoretical analysis, which demonstrates the reliability and practicability of the laser ultrasonic method in the weld quality inspection of small-diameter thin-walled pipes.

Rapid measurement of the fourth-order texture coefficient by laser ultrasonic surface acoustic waves based on a neural network expert system.

In anisotropic materials, texture components and orientation density directly affect the surface acoustic waves' (SAW) velocity dispersion and SAW velocity variation. In this paper, a texture feature recognition and analysis system for a neural network is constructed based on the corresponding characteristics of texture components and orientation density and SAW velocity dispersion and variation by combining laser ultrasound technology with the partial texture analysis method, which is used for the identification and analysis of texture type and feature. At the same time, based on the relationship between surface wave velocity and the fourth-order texture coefficient, an expert system for accurate prediction of the fourth-order texture coefficient is constructed. Then, the fourth-order texture coefficients predicted by the neural network expert system (NNES) are compared with the texture coefficients measured by electron backscattered diffraction. The results show that the NNES can not only quickly identify and analyze texture features, but also accurately predict the fourth-order texture coefficients.

Analytical and numerical modeling of nonlinear lamb wave interaction with a breathing crack with low-frequency modulation.

To characterize fatigue crack, an analytical calculation and finite element (FE) simulation of Lamb wave propagating through the region of a breathing crack in a two-dimensional(2D) isotropic plate were studied. Contact surface boundary conditions between the two surfaces of the vertical crack were considered to study contact acoustic nonlinearity (CAN) from the breathing contact crack in conjunction with the modal decomposition method, Fourier transform, and variational principle-based algorithm. Reflection and transmission coefficients in the fundamental frequency and second harmonic frequency were calculated and analyzed quantitatively. Different ratios of incident wave amplitude to crack width were studied to calculate CAN results related to micro-crack width. In addition, a low-frequency (LF) vibration(10 Hz) excitation was introduced to perturb the free surface vertical crack to close, and an interrogating Lamb wave(1 MHz) was used to study crack-related CAN in different conditions for interpreting the modulation mechanism. The contact boundary conditions between two surfaces of vertical crack were set which were dynamically changed due to the low frequency modulation. The clapping effects when the crack closed due to the modulation of the contact boundary conditions between the crack surfaces were studied and analyzed to get the quantitative correlation between CAN and LF modulation. The results obtained from the analytical model were compared with those from the FE simulation, showing good consistency. Knowledge of these effects is essential to correctly gauge the severity of surface cracks in the plate, which can be spotlighted in its application to quantitative evaluation of micro fatigue cracks in structural health monitoring(SHM).

The effect of copper grain size on laser ultrasonic backscattered signal.

Grain size has an essential influence on the serviceability of metallic materials. In this paper, a noncontact laser ultrasonic testing platform is built to study the effect of copper grain size on the laser ultrasonic backscattered signal. According to the correlation between grain size and ultrasonic wavelength, the ultrasonic scattering by copper grains in the experiment contains not only Rayleigh scattering but also the transition region from Rayleigh scattering to stochastic scattering. Using time-frequency analysis, the influence of copper grain size on the characteristic parameters of backscattering was explored, and a prediction model of grain size was established, which was compared with the prediction model based on the attenuation method to verify the accuracy of the backscattering model. The results show that the backscattered signal can adequately characterize the grain size information and laser ultrasonics is a method that can realize on-line detection of grain size.

Nonlinear resonance decomposition for weak signal detection.

This paper attempts to investigate the behaviors of coupling stochastic resonance (CSR) subject to α-stable noise and a periodic signal by using the residence-time ratio. Then, a nonlinear resonance decomposition is designed to successfully enhance and detect weak unknown multi-frequency signals embedded in strong α-stable noise by decomposing the noisy signal into a series of useful resonant components and a residue, where the residence-time ratio, instead of the output signal-to-noise ratio and other objective functions depending on the prior knowledge of the signals to be detected, can optimize the CSR to enhance weak unknown signals. Finally, the nonlinear resonance decomposition is used to process the raw vibration signal of rotating machinery. It is found that the nonlinear resonance decomposition is able to decompose the weak characteristic signal and its harmonics, identifying the imbalance fault of the rotor. Even the proposed method is superior to the empirical mode decomposition method in this experiment. This research is helpful to design the noise enhanced signal decomposition techniques by harvesting the energy of noise to enhance and decompose the useful resonant components from a nonstationary and nonlinear signal.

Analysis for angular dispersions of surface acoustic wave velocities in BCC crystals.

Based on the propagation theory of acoustic waves in anisotropic media, the angular dispersion curves of SAW velocities propagated in some unusual oriented, such as (016)- and (331)-oriented, Body-Centered Cubic (BCC) single crystals are simulated using an optimized numerical method. Meanwhile, the angular dispersion curves of SAW velocities propagated in these unusual oriented BCC single crystals are measured by Impulsive Stimulated Scattering (ISS) laser ultrasonic method, which show that the measurement results are in good agreement with those of the simulations. By the optimized numerical method, the relationships between the temperature and the dispersion curves of SAW velocity propagated in these single crystals are also simulated.

Texture in steel plates revealed by laser ultrasonic surface acoustic waves velocity dispersion analysis.

A photoacoustic, laser ultrasonics based approach in an Impulsive Stimulated Scattering (ISS) implementation was used to investigate the texture in polycrystalline metal plates. The angular dependence of the 'polycrystalline' surface acoustic wave (SAW) velocity measured along regions containing many grains was experimentally determined and compared with simulated results that were based on the angular dependence of the 'single grain' SAW velocity within single grains and the grain orientation distribution. The polycrystalline SAW velocities turn out to vary with texture. The SAW velocities and their angular variations for {110} texture were found to be larger than that the ones for {111} texture or the strong γ fiber texture. The SAW velocities for {001} texture were larger than for {111} texture, but with almost the same angular dependence. The results infer the feasibility to apply angular SAW angular dispersion measurements by laser ultrasonics for on-line texture monitoring.

[Antigenicity and physicochemical properties of two human NR1a polypeptides related to activation of NMDA receptor].

OBJECTIVE: To investigate the distribution of antigenic sites in two human NR1a polypeptides related to activation of N-methyl-D-aspartate receptor (NMDAR) and their physicochemical properties. METHODS: The amino acid sequences of two polypeptides, P1, a region containing 151 amino acid residues preceding the first transmembrane domain of the human NR1a, and P2 with 144 residues following the third transmembrane domain, were obtained from protein database by GOLDKEY software (4.0 version). Four parameters including Hopp-Woods and Kyte hydrophilicity, Janin accessibility, Karplus-Schulz flexibility, and Welling antigenicity were used to determine the antigenic sites, and Prosite programme and Chou-Fasman method were employed to analyze their related sequence motifs and the secondary structures. Finally, comparison of the comprehensive predictions with some of the available experimental information was made. RESULTS: There were about six and seven antigenic sites containing 8 approximately 15 residues in the P1 and P2 polypeptides respectively. The antigenic sites in P1 were mainly located in the amino terminal, but the ones in P2 were dispersed rather uniformly. Many sites in P2 polypeptide including some residues in its initial part, the amino terminal, showed higher hydrophilicity, accessibility, and antigenicity than those in P1. In addition, P1 and P1 were also different in the primary and secondary structure. P1 contained more cysteine residues and was rich in random coils, while P2 contained more aromatic residues and exhibited mainly helical structures. CONCLUSION: Both human NR1a polypeptides related to activation of NMDA receptor, P1 and P2, have a certain amount of antigenic sites. Compared with P1, P2 may be of higher antigenicity, and may be more easily used as a molecular target in immunization intervention to control the activation of NMDAR.