Acta Mechanica Sinica最新文献

In this work, the fracture toughness of limestone was determined utilizing advantage edge-notched disc (END) type specimens. The normalized Phase I stress high transformational of limestone was calculated using a novel method. The samples’ Mode I (opening)-fracture toughness was accurately calculated using the flexible finite element method (FEM) to develop this helpful formula. The notched Brazilian discs (NBD) were used in a lab experiment to test the accuracy of the END procedure. To recreate these tests numerically and verify the correctness of the fracture toughness measurements, a discrete element analysis of the END tests was also performed. The findings of the numerical simulations and the fracture extension patterns derived from the experimental testing showed excellent agreement. The toughness levels determined by END studies, however, were less than those specified by NBD testing. This is because the notch in the END limestone sample has a uniform tensile stress distribution.

Conductor on round core (CORC) cables are superconductor rare earth-barium-copper-oxide (REBCO) based conductors that allow for high operating currents and have been becoming one of the most flexible high temperature superconducting (HTS) conductors to date. Nevertheless, due to the brittle nature of superconductor REBCO ceramics, the critical current exhibits a sensitive strain dependence, i.e., the critical current varies closely with strain, and especially raises a significant degradation once the strain exceeds a critical value. And, due to the complex deformation of tape winding and after cable stranding, firstly there is a practical challenge that how to accurately estimate the axial strain of the superconductor tape of cables under various deformations. In this paper, we developed the analytical models that can be used to accurately calculate the axial strain of the superconductor tape of CORC cables under the practical conditions of tape winding on the cable core, the cable bending, and twist deformations for stranding. The analytic model was validated by comparing the analytic result both with FEM numerical result and experimental data that the critical current of the superconducting tape varies with the core diameter of CORC cables. Further, on the analytic models, the axial strain, and the dependence on the core diameter and winding angle, the bending diameter, and the strand pitch were obtained respectively for CORC cables under tape winding, cable bending, and twist deformations. Particularly, the critical parameters, which are here defined at which the axial strain of the superconductor layer just reaches the critical strain, such as the critical core diameter, critical bending diameter, and critical pitch were determined. Thus, the analytic study of the strain of the superconductor REBCO tape of CORC cables will be helpful for CORC cables processing and further the design of high-level superconducting coils and magnets.

Piezoelectric metamaterials with shunt circuits have been widely studied for bandgap tuning. However, broadband vibration suppression is a great challenge in engineering applications. In this paper, a novel approach to address the challenge of achieving broadband vibration suppression in piezoelectric metamaterials with shunt circuits is presented. A piezoelectric supercell model containing multiple piezoelectric units is designed. In complex band structures, it is difficult to analytically couple multiple bandgaps to form a wider bandgap. An optimization method for a piezoelectric metamaterial beam with LR circuit is proposed to broaden the frequency range of vibration suppression. The electrical parameters of the LR circuit of the supercell are optimized by a genetic algorithm. Multiple locally resonant bandgaps are coupled to the Bragg bandgap by the optimization method. The attenuation rate can be customized, and the maximum bandwidth is obtained. It is verified that the optimized metamaterial can achieve vibration suppression in a wide frequency range by the transmissibility of the finite period metamaterial beam. Vibration suppression over a wide frequency range is verified by the finite element method. Finally, a synthetic circuit is used to simulate an adjustable inductor in an LR circuit, and the vibration suppression performance of the optimized metamaterial is experimentally verified. The experimental results show that the attenuation bandwidth of metamaterials is significantly broadened through optimization. The vibration suppression capability of wide frequency tunable is realized experimentally. This paper provides a novel way for broadband vibration suppression.

Ceramic composite armor, mainly composed of ceramic and backing layers, has been widely used in impact protection. However, the quantitative understanding and analysis for the role of the backing layer in improving the ballistic resistance of the ceramic composite armor system is still lacking. In this paper, by taking the B₄C/UHMWPE bi-layer armor system as an example, the enhanced mechanism of the UHMWPE layer in improving the ballistic resistance of the ceramic composite armor and the appropriate UHMWPE thickness are systematically studied theoretically. A theoretical model predicting the residual velocity of a bi-layer armor system is developed and verified. Specifically, the dissipated energy associated with plasticity, fracture and friction and the stored energy composed of the elastic strain energy and kinetic energy, is theoretically obtained, respectively. The theoretical results show that as the increase of the UHMWPE thickness, the dissipated energy monotonically increases, while the stored energy first increases and then decreases with the appearance of a turning point due to the dominant mechanism of the stored energy changing from the maximum stored energy of the system inherently to residual kinetic energy. Furthermore, for a given ballistic resistance, a reference value for the optimal UHMWPE thickness to lower the areal density is proposed according to the transition of the stored energy, which is related to the ceramic thickness, impact velocity and the mass of the projectile. The study in this paper helps guide the lightweight design of ceramic composite armor.

High temperatures in a gas turbine may lead to severe blade rubbing failure for the bladed thin drum rotor. It is essential to demonstrate such rubbing features. This paper established a bladed drum rotor model with blade rubbing induced by high temperatures. The analytical function of a coupled axial-radial temperature in the drum according to the actual thermal field analysis is obtained. The equations of motion for this rotor are derived. The dynamic model and its solution method are verified through the natural frequency comparison and the rub-impact response analysis. Thereafter numerical simulations are carried out. Results show that the heat at the turbine is transferred from its outer surface to its inner surface, then to the compressor’s inner surface along the axial direction, and finally from the compressor’s inner surface to its outer surface. This is a novel coupled axial-radial thermal effect for the gas turbine, which causes special axial and radial thermal gradients. The effect is induced by high temperatures in a gas turbine and intensifies a blade rubbing fault. Increasing the exhaust temperature aggravates the coupled axial-radial thermal effect, which causes more severe blade rubbing. Fortunately, introducing a lower temperature on the drum’s inner surface can prevent blade rubbing caused by this thermal effect.

Cellular foams with randomly distributed open pores are increasingly exploited in sound management applications, where the sound absorption coefficient (SAC) typically serves as a crucial acoustic parameter for performance evaluation and design optimization. Dependent upon the processing method, the pores in a cellular foam can be either fully open or semi-open and often exhibit fractal distribution features. To facilitate engineering applications, it is imperative to analytically predict the SACs of these foams. However, predicting analytically the SAC for foams poses a challenge. Therefore, this study proposes a simplified representative structure (RS) with semi-open or fully open pores to analyze the flow properties within the foam microscopically, while the fractal theory is applied to portray the randomly distributed pores. With the extent to which the pores are open characterized using a purposely introduced parameter called the open-pore degree, both viscous and thermal characteristic lengths of the RS are analytically obtained. Subsequently, built upon the classical Johnson-Champoux-Allard (JCA) model for sound propagation in porous media, an analytical model is developed to unify the RS with the fractal theory so that the SAC can be predicted as a function of key morphological parameters of the foam having fully/semi-open pores. Compared with existing experimental measurements and numerical simulation results, the proposed analytical model predicts well the key flow properties as well as the SAC of foams having either semi-open or fully open pore topologies. In the frequency range of 0–4500 Hz, a semi-open foam can better attenuate the sound wave relative to its fully-open counterpart having the same porosity. With the porosity fixed at 0.95, the overall SAC of semi-open foam is improved by 21.2%, 57.7%, and 75.8%, respectively, as its open-pore degree is reduced from 0.75 via 0.50 to 0.25.

Dynamic load identification plays a crucial role in structural design and optimization. The majority of current studies are focused on deterministic structures. However, the structural parameters of actual engineering structures are unknown. It is essential to investigate the issue of dynamic load identification for uncertain structures since the existence of uncertain parameters can lead to errors between load identification results and actual load values. Therefore, in this paper, we propose a data-driven dynamic load identification method for structures containing some uncertain parameters. To start, the uncertain parameters are characterized by a set of closed interval vectors. Then a convolutional neural network (CNN) is introduced for the reconstruction of the interval of unknown load. Combining the interval analysis theory with Taylor expansion, the upper and lower boundaries of the supervised loads are obtained and used as training samples. Finally, the trained CNN model directly identifies the boundaries of the unknown load interval. The simulation results demonstrate that the proposed method has great accuracy in load identification and has good robustness to noise. We construct a simply supported beam structure for experiments to further validate the feasibility of the proposed method in engineering. Additionally, we discuss the effect of measurement point distribution and number of samples on the identification accuracy, which is beneficial for applications in engineering practice.

A quasi-one-dimensional numerical model is developed to provide the internal ballistics information of a throttling segregated fuel-oxidizer system (SFOS). The present throttling SFOS is capable of regulating its thrust by adjusting the opening radius of a throttle valve mounted between the head-end fuel-rich chamber and the aft-end oxygen-rich chamber. The numerical model employs a simplified reaction mechanism to describe the chemical non-equilibrium processes and considers mass addition, wall friction, and propellant surface regression in the combustion chambers. With this numerical model, the internal flow parameter distributions and the performance of the throttling SFOS are demonstrated. The steady operation results show that when the throttle valve opening radius is adjusted from 10.5 mm to 1.4 mm, the motor thrust can be increased from 121.82 N to 250.60 N, which is a 206% thrust promotion. It validates the conception of throttling SFOS. The flow parameters also suggest that the function of the throttle valve can only be manifested when the valve opening radius is quite small. The dynamic operation results reveal that the performance histories of the throttling SFOS experience slight anti-regulations at the end of the valve actuation, which deserves extra protective measures. A theoretical prediction of the thrust regulation ability of the throttling SFOS is provided. It suggests that the thrust regulation ability is limited by the fuel-rich chamber pressure and the initial mass flow rate ratio, and a compromise has to be made among multiple parameters to achieve a reasonable thrust regulation ratio. Finally, the grain arrangement is tentatively discussed. It shows that, based on the present modeling conditions, the fuel-oxygen grain arrangement is superior in its thrust regulation ability than the reversed oxygen-fuel grain arrangement.

The burgeoning data-driven techniques endow large potential to predict fairly practical or complex dynamical systems in various fields through massive data. Lévy noise, a more universal and intricate fluctuation model comparing with Gaussian white noise, is widely employed in many non-Gaussian cases to mimic bursting or hopping. In this manuscript, we present a systematic data-driven method to identify the most probable exit trajectory of a system that is perturbed both by Gaussian white noise and non-Gaussian Lévy noise. The main theoretical and numerical conceptions involve a set of extended Kramers-Moyal formulas and the Kolmogorov forward equation in classic dynamical systems theory as well as a supervise learning theory to solve the fitting problems by using the Cross Validation. We then give two examples to show the feasibility in detail, and do a brief bifurcation analysis for the most probable exit trajectory. The above approach will serve as a numerical correspondence to as well as verification for the relative theoretical research, and provide a referential resolution to the numerical identification of more transition indicators of this complex system, which is more general than the Gaussian diffusion process.

Twist-induced lattice misalignment of bilayer graphene leads to moiré patterns that generate electronic flat bands with strongly correlated electronic states, but it still requires a sophisticated process to precisely control the twist angle. Here, we propose a different way to generate hexagonal moirés in bilayer graphene by uniaxially stretching the two layers along two distinct armchair directions, respectively. The heterostrain-induced moiré gives rise to flat bands near the Fermi level due to the deformation-induced equivalent misaligned angle between two graphene layers, featuring an electronic equivalence of twisted bilayer graphene. We demonstrate the flat bands at a heterostrain of 2.1%, equivalent to the first magic angle of 1.05°. Yet, a slight shift of Dirac point from K point due to the uniaxial strain splits the flat bands into two van Hove singularity peaks that are separated by 18 meV and located above and below the Fermi level, respectively. Our results suggest a potential way to control the electronic strong correlation in bilayer graphene of natural stacking.