This paper presents the development of an online stress monitoring technique based on Lamb-wave measurements and a convolutional neural network (CNN) for metallic plate-like structures under static and dynamic loadings. Monitoring stress levels in structural components is crucial because stress concentrations and stress redistribution can be precursors of structural damage and failure. Moreover, the stress variations owing to dynamic loadings are directly related to the fatigue life of a structure. First, an aluminum plate specimen is fabricated, and piezoelectric transducers are installed on the specimen. Different levels of static loading are applied to the specimen with a hydraulic loading machine, and the ultrasonic responses are obtained at each loading level. The applied loads (ground truths) are measured by a load cell built into the loading machine. Then, a CNN is designed and trained by defining the Lamb-wave time responses as the input and the measured stress levels as the output. The performance of the trained CNN is evaluated using the blind test data obtained from various static and constant-amplitude cyclic loading conditions. The uniqueness of this study lies in (1) the automated stress estimation without feature extraction and (2) the stress monitoring capability under constant-amplitude cyclic loadings up to a frequency of 5 Hz.

Online Stress Monitoring Technique Based on Lamb-wave Measurements and a Convolutional Neural Network Under Static and Dynamic Loadings

Measuring stress levels in loaded structures is crucial to assess and monitor their health, and to predict the length of their remaining structural life. However, measuring stress non-destructively has proved quite challenging. Many ultrasonic methods are able to accurately predict in-plane stresses in a controlled laboratory environment, but struggle to be robust outside, in a real world setting. That is because they rely either on knowing beforehand the material constants (which are difficult to acquire) or they require significant calibration for each specimen. Here we present a simple ultrasonic method to evaluate the in-plane stress in situ directly, without knowing any material constants. This method only requires measuring the speed of two angled shear waves. It is based on a formula which is exact for incompressible solids, such as soft gels or tissues, and is approximately true for compressible "hard" solids, such as steel and other metals. We validate the formula against virtual experiments using Finite Element simulations, and find it displays excellent accuracy, with a small error of the order of 1%.

An ultrasonic measurement of stress in steel without calibration: the angled shear wave identity

Magnetic, ultrasound and magnetoacoustic properties of low-carbon steel subjected to cold plastic deformation by rolling with different compression and subsequent flat pressing have been investigated. The hardness and coercive force are changing non-monotonically with deformation degree increase determined by cross-section change. Change of shape during flat pressing leads to the partial softening of the samples that lost flatness during rolling. The coercive force increases at low deformation degrees and has a maximum at 6% deformation degree and monotonously decreases with increasing of the deformation degree up to 20%. Peculiarities of hardness and magnetoacoustic properties are related to the change of dislocation structure and relaxation of longitudinal compressive stresses. The softening of the material can be detected by the amplitude of the parameter interrelated with the shape of the major hysteresis loop. The parameter proportional to the field of the maximum of magnetoacoustic emission can be a diagnostic parameter of the resulting hardening of steel under study as a result of two-stage plastic deformation.

Influence of the two-stage plastic deformation on the complex of the magnetoacoustic characteristics of low-carbon steel and diagnostics of its structural state

Based on the Lcr wave acoustoelastic effect, surface stress of laser cladding Fe314 alloy coating after ultrasonic impact treat (UIT) was measured, and relation between Lcr wave acoustoelastic coefficient and repetition rate of UIT was discussed in this paper. The laser cladding Fe314 alloy coating was prepared on surface of carbon steel and impacted with different repetition rates of UIT by using UIT-300 system. After that, the experimental system of stress measurement with Lcr wave, consisting of a ultrasonic wave generator, a digital oscilloscope and two Lcr wave transducers (including one transmitting transducer and one receiving transducer) with 2.5MHz frequency, was used to collect Lcr waves of coating with calibration test, and then the difference in time of flight between Lcr waves was calculated with the cross correlation theory. To explain experimental results, the microstructure and three dimensional surface topography of coating were observed by using SEM and LSCM. Results show that as repetition rate of UIT increases, the Lcr wave acoustoelastic coefficient increases, when it reaches 300%, the Lcr wave acoustoelastic coefficient also reaches the maximum, after that the Lcr wave acoustoelastic coefficient decreases irregularly as repetition rates of UIT increases further. Compared with the Lcr wave acoustoelastic theory, when stress is in the range of turning point stress, there is a deviation on experimental result, and that deviation becomes more obvious as stress and repetition rate of UIT increase. It is analyzed that the change of microstructure, deformation state and initial stress of coating caused by UIT and different repetition rate are main reasons.

Effect of ultrasonic impact treatment on surface stress evaluation of laser cladding coating by using critically refracted longitudinal wave

Recent Research Progress on Residual Stress Measurement Using Non-Destructive Testing

An improved ultrasonic method was developed theoretically and experimentally for plane stress measurement using critically refracted longitudinal (LCR) waves. Cruciform specimen method, combined with a digital image correlation (DIC) method, was applied to confirm the validity of this method. Based on the acoustoelastic theory, LCR waves with arbitrary detection directions were considered and relations between time-of-fight and biaxial principal stresses were described. To generate the LCR wave in cruciform specimen, a regular octagon polymethyl-methacrylate (PMMA) wedge was designed in accordance with Snell's law. Finally, validation experiments were performed under different step loads using cruciform specimens. In these experiments, the strain fields of cruciform specimens were extracted by DIC method and used to calculate the reference values of plane stress. By comparing the measured principal stresses with the reference values, the validation and precision of the improved ultrasonic method presented in this work were demonstrated.

An improved ultrasonic method for plane stress measurement using critically refracted longitudinal waves

Numerical and experimental analysis of nanosecond laser ablation of SiC

Ultrasonic testing is one of the most commonly used non-destructive (NDT) methods to detect porosity in carbon fiber reinforced polymer (CFRP) material. However, the ultrasonic testing requires well-trained technicians and their high concentration during the testing process to avoid human errors. The artificial neural network (ANN) models provide an efficient and accurate way to reduce testing time and effort and reduce the human factor and uncertainty during ultrasonic testing. In this work, CFRP samples with 12 different kinds of porosity levels and thicknesses were prepared for the experiment. X-ray CT testing and through-transmission ultrasonic (TTU) testing are applied to measure the material porosity. Based on the porosity data obtained by these two testing methods, a backpropagation (BP) neural network was developed and trained to analyze and predict the sample porosity. In our experiment, the accuracy of our ANN-based method can reach up to 97.22%, and the average prediction result is 88.02%. Our experimental results demonstrate that our new method is quite promising to predict porosity testing results in CFRP materials.

Ultrasonic Signal Classification and Porosity Testing for CFRP Materials Via Artificial Neural Network

The structure, vibrational density of states, and transport coefficients of liquid alumina were studied using molecular dynamics simulations. At the temperature of 2500 K, 3000 K, 3500 K, and 4000 K, systems with three different densities were constructed, respectively, including the configurations with densities of 2.81 g/cm3 and 3.17 g/cm3, and the relaxed ones with nearly zero pressure at each temperature. With the changes in temperature or density, the transformations on the structural, vibrational and transport properties were discussed. The Born–Mayer–Huggins type of atomic interactions was used, with newly optimized parameters. The analysis of the interatomic correlations indicated that the short-range order of liquid alumina was mainly constructed by AlO4 tetrahedra, also a certain number of AlO3 and AlO5 was present. Meanwhile, the structural transitions on the elemental units occurred as either the temperature or density increased. Two primary frequency bands were observed in each vibrational density of states spectrum, with the higher frequency bands produced by the O atom vibrations, and the lower frequency ones generated by the Al atom vibrations. Self-diffusion coefficients were estimated using the linear behavior of the mean-squared displacement at long time, while by using the Green–Kubo relation during equilibrium molecular dynamics simulations, thermal conductivities and viscosities were calculated. Significantly, the viscosity at 2500 K with a density of 2.81 g/cm3 was equal to 25.23 mPa s, which was very close to the experimental finding.

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Molecular Dynamics Study on Structure, Vibrational Properties, and Transport Coefficients of Liquid Alumina

Structural, vibrational and transport properties of liquid alumina at 2500 K and amorphous alumina at 300 K were studied by molecular dynamics simulations using an empirical Born-Mayer-Huggins potential with the recently optimized parameters. The investigations were conducted for the predicted densities at almost zero pressure, as well as the experimentally reported densities of 2.81 g/cm3 and 3.175 g/cm3. A detailed examination of the interatomic correlations showed that for both liquid and amorphous alumina, the short-range order was dominated by the slightly distorted (AlO4)5− tetrahedra. Vibrational density of states (VDOS) was obtained from the Fourier transform of the velocity autocorrelation functions (VACF), which exhibited broader ranges for the liquid phases compared with those for the amorphous phases. Each VDOS spectrum was divided into two primary frequency bands for both liquid and amorphous alumina. Thermal conductivities (κ) and viscosities (η) were estimated respectively through the heat-current autocorrelation functions (HCACFs) and stress autocorrelation functions (SACF) by the equilibrium molecular dynamics (EMD) simulations using the Green-Kubo relation. And the results were shown to be consistent with the experimental data, especially that κ was equal to 2.341 ± 0.039 Wm−1K−1 for amorphous alumina at 2.81 g/cm3 and 300 K, η was equal to 0.0261 ± 0.0017 Pa·s and 0.0272 ± 0.0018 Pa·s for the liquid phases at 2500 K with densities of 2.81 g/cm3 and 2.863 g/cm3, respectively. Mean squared displacements (MSDs) were employed for the self-diffusion coefficients (D) estimation.

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Ya Deng

Papers

An improved ultrasonic method for plane stress measurement using critically refracted longitudinal waves

Numerical and experimental analysis of nanosecond laser ablation of SiC

Ultrasonic Signal Classification and Porosity Testing for CFRP Materials Via Artificial Neural Network

Molecular Dynamics Study on Structure, Vibrational Properties, and Transport Coefficients of Liquid Alumina

Structural, vibrational and transport properties of liquid and amorphous alumina: A molecular dynamics simulation study