Acta Mechanica Solida Sinica最新文献

This paper proposes a novel numerical solution approach for the kinematic shakedown analysis of strain-hardening thin plates using the C¹ nodal natural element method (C¹ nodal NEM). Based on Koiter’s theorem and the von Mises and two-surface yield criteria, a nonlinear mathematical programming formulation is constructed for the kinematic shakedown analysis of strain-hardening thin plates, and the C¹ nodal NEM is adopted for discretization. Additionally, König’s theory is used to deal with time integration by treating the generalized plastic strain increment at each load vertex. A direct iterative method is developed to linearize and solve this formulation by modifying the relevant objective function and equality constraints at each iteration. Kinematic shakedown load factors are directly calculated in a monotonically converging manner. Numerical examples validate the accuracy and convergence of the developed method and illustrate the influences of limited and unlimited strain-hardening models on the kinematic shakedown load factors of thin square and circular plates.

For fragile products, packaging requires cushioning protection to prevent irreversible damage from accidental falls, transportation impacts, and other causes. The new polyurethane foam (PUF) material demonstrates superior cushioning and vibration isolation performance in practical applications, effectively minimizing damage from vibrations. Drop and vibration experiments were conducted on packages comprising novel PUF, expandable polyethylene, ethylene–vinyl acetate copolymer foam, and bracelets. Results verify that the new PUF material outperforms in cushioning and vibration isolation, as observed from the acceleration response. Furthermore, a random vibration analysis of a packaging unit involving different thicknesses of PUF materials and bracelets reveals the enhanced vibration isolation effect within a specific thickness range. The vibration results of the bracelet’s outer packaging align closely with finite element simulation results, validating the effectiveness of designing and optimizing the outer packaging. Through finite element simulation, deeper understanding and prediction of the bracelet’s vibration response under various conditions is achieved, facilitating optimized packaging design for better protection and vibration damping.

In this study, analytical and numerical methods are applied to investigate the dynamic response of an axially moving plate subjected to parametric and forced excitation. Based on the classical thin plate theory, the governing equation of the plate coupled with fluid is established and further discretized through the Galerkin method. These equations are solved using the method of multiple scales to obtain amplitude-frequency curves and phase-frequency curves. The stability of steady-state response is examined using Lyapunov’s stability theory. In addition, numerical analysis is employed to validate the results of analytical solutions based on the Runge–Kutta method. The multi-value and stability of periodic solutions are verified through stable periodic orbits. Detailed parametric studies show that proper selection of system parameters enables the system to stay in primary resonance or simultaneous resonance, and the state of the system can switch among different periodic motions, contributing to the optimization of fluid–structure interaction system.

Structural damage detection (SDD) remains highly challenging, due to the difficulty in selecting the optimal damage features from a vast amount of information. In this study, a tree model-based method using decision tree and random forest was employed for feature selection of vibration response signals in SDD. Signal datasets were obtained by numerical experiments and vibration experiments, respectively. Dataset features extracted using this method were input into a convolutional neural network to determine the location of structural damage. Results indicated a 5% to 10% improvement in detection accuracy compared to using original datasets without feature selection, demonstrating the feasibility of this method. The proposed method, based on tree model and classification, addresses the issue of extracting effective information from numerous vibration response signals in structural health monitoring.

In analyzing the complex interaction between the wellbore and the reservoir formation, the hydromechanical properties of the region proximal to the wellbore, referred to as the “wellbore skin zone”, play a pivotal role in determining flow dynamics and the resulting formation deformation. Existing models of the wellbore skin zone generally assume a constant permeability throughout, resulting in a sharp permeability discontinuity at the skin-reservoir interface. This paper introduces a model for a wellbore with a continuously graded skin zone of finite thickness within a poroelastic medium. Analytical solutions are derived using the Laplace transform method, addressing both positive and negative skin zones. Numerical results are presented to illustrate the effects of graded permeability/skin zone thickness on pore pressures and stresses around a wellbore. The results highlight a distinct divergence in stress and pore pressure fields when comparing wellbores with negative skin zones to those with positive skin zones or no skin at all.

Pressure and tilt data are jointly inverted to simultaneously map the orientation and dimensions of a hydraulic fracture. The deformation induced by a fracture under internal pressure is modeled using the distributed dislocation technique. The planar fracture is represented by four quarter ellipses, joined at the center and sharing semi-axes. This configuration provides a straightforward model for characterizing asymmetric fracture geometry. The inverse problem of mapping the fracture geometry is formulated using the Bayesian probabilistic method, combining the a priori information on the fracture model with updated information from pressure and tilt data. Solving the nonlinear inverse problem is achieved by pseudo-randomly sampling the posterior probability distribution through the Markov chain Monte Carlo method. The resulting posterior probability distribution is then explored to assess uncertainty, resolution, and correlation between model parameters. Numerical experiments are conducted to verify the accuracy and validity of the proposed analysis method in mapping the fracture geometry using synthetic pressure and tilt data.

The Prandtl–Tomlinson (PT) model has been widely applied to interpret the atomic friction mechanism of a single asperity. In this study, we present an approximate explicit expression for the friction force in the one-dimensional PT model under quasi-static conditions. The ‘stick–slip’ friction curves are first approximated properly by sawtooth-like lines, where the critical points before and after the ‘slip’ motion are described analytically in terms of a dimensionless parameter η. Following this, the average friction force is expressed in a closed form that remains continuous and valid for η > 1. Finally, an analytical expression for the load dependence of atomic friction of a single asperity is derived by connecting the parameter η with the normal load. With the parameters reported in experiments, our prediction shows good agreement with relevant experimental results.

Stress-dependent permeability models are developed for the organic pores and inorganic cleats/ fractures in unconventional gas reservoirs, which are modeled as Biot’s porous media of dual-porosity. Further considering multiple flow mechanisms such as dynamic effects of gas flow and surface diffusion, apparent permeability models are obtained to investigate the characteristics of unconventional gas migration. Compared to the gas transfer in single-porosity reservoirs, the gas migration ability of cleats in dual-porosity stratums rarely changes while that of organic pores is greatly improved because cleats sustain major geomechanical shrinkage deformation when the pore pressure drops. Further, the mass flux of reservoirs is dominated by the mass flux of cleats, which has a lower peak value, but a much longer production term than those in single-porosity reservoirs due to the interaction between organic pores and cleats. Parametric analysis is conducted to identify key factors significantly impacting mass flux in unconventional reservoirs. Reasons for the mass flux variation are also explored in terms of gas migration ability and pore pressure distribution.

Flexoelectricity is a two-way coupling effect between the strain gradient and electric field that exists in all dielectrics, regardless of point group symmetry. However, the high-order derivatives of displacements involved in the strain gradient pose challenges in solving electromechanical coupling problems incorporating the flexoelectric effect. In this study, we formulate a phase-field model for ferroelectric materials considering the flexoelectric effect. A four-node quadrilateral element with 20 degrees of freedom is constructed without introducing high-order shape functions. The microstructure evolution of domains is described by an independent order parameter, namely the spontaneous polarization governed by the time-dependent Ginzburg–Landau theory. The model is developed based on a thermodynamic framework, in which a set of microforces is introduced to construct the constitutive relation and evolution equation. For the flexoelectric part of electric enthalpy, the strain gradient is determined by interpolating the mechanical strain at the node via the values of Gaussian integration points in the isoparametric space. The model is shown to be capable of reproducing the classic analytical solution of dielectric materials incorporating the flexoelectric contribution. The model is verified by duplicating some typical phenomena in flexoelectricity in cylindrical tubes and truncated pyramids. A comparison is made between the polarization distribution in dielectrics and ferroelectrics. The model can reproduce the solution to the boundary value problem of the cylindrical flexoelectric tube, and demonstrate domain twisting at domain walls in ferroelectrics considering the flexoelectric effect.

This paper intensively explores the critical issues related to the quantitative and accurate evaluations of FCG behavior in the early stage, macro fatigue fracture toughness, and the critical crack size for damage tolerance in nuclear graphite. To address these issues, scale-span FCG tests were carried out using two typical specimens, CT and SEM in-situ specimens. These results indicate that the FCG threshold and the effective FCG length have a significant correlation with the modified maximum loop stress theory for a mixed I/II mode. In particular, the effective FCG length (a_eq) and the applied stress threshold of polycrystalline graphite are important parameters for fatigue damage tolerance design in engineering application. The influencing factors of ΔK_th,eq and a_eq were discussed in detail using the mixed I/II mode, respectively. In addition, the scattered values of ΔK_IC for this graphite can be quantitatively estimated using the Weibull distribution equation. The predicated parameters and experimental results demonstrate a strong correlation.