Mechanical metamaterials (or meta-structures) exhibit extraordinary physical and mechanical properties due to their unique microstructural designs. By combining the design ideas of mechanical metamaterials with intelligent and flexible materials (IFMs), it is possible to create intelligent flexible mechanical metamaterials (IFMMs) with self-sensing and self-actuating capabilities. This paper takes conventional mechanical metamaterials as a starting point and analyzes the fundamental design ideas, deformation mechanisms, and mechanical properties of IFMMs. According to recent research progress on IFMMs, this novel metamaterial is categorized as mechanical metamaterial based on shape memory polymers (SMPs), hydrogel, magnetoactive soft materials, and dielectric elastomers, with a particular focus on the first two types. On the basis of our previous work, we present a general approach that utilizes analytical methods and numerical simulations to analyze the mechanical properties of negative Poisson's ratio, negative expansion, and multi-stable metamaterials under the assumptions of small deformation and large deformation with multi-field coupling, respectively. In the case of small deformation, the use of beam theory and energy methods proves to be essential for obtaining fundamental mechanical parameters of the materials. Moreover, accurate constitutive models and numerical implementation under large deformation and multi-field coupling offer the possibility to analyze more complex deformations and structures. In addition, the preparation and performance testing of IFMMs remain crucial. Advanced manufacturing techniques have introduced new opportunities for the preparation of IFMMs, and currently, various methods are available to effectively prepare these materials. The performance testing of IFMMs includes both experiments applicable to traditional materials and specialized experiments only for IFMs. Finally, this paper concludes by highlighting some key issues and potential trends of IFMMs. These challenges primarily revolve around material properties, fabrication methods, mechanical models, and structural designs. This review may bring beneficial inspiration for the future development of IFMMs.

Shell structures are widely used in engineering, especially in aerospace and civil engineering. As a result, the study on dynamic behaviors of shell structures is crucial for engineering applications. Over the years, shell theory has undergone continuous improvement and development, leading to analytical solutions for specific shell structures. However, solving analytical solutions for complex shell structures with intricate shapes becomes highly challenging and even unattainable. Therefore, numerical methods like the finite element method (FEM) and the meshless method are employed for further solutions. The meshless method is a powerful complement to FEM, relying solely on nodal information for the formation of shape functions and enabling easy construction of higher-order smooth approximations. Consequently, it naturally holds an advantage in the numerical analysis of plate shell structures. Based on the 3D continuous shell theory and the moving least-squares (MLS) approximation, a meshless model for arbitrary shells is established in this paper. The MLS approximation is used not only for geometric surface interpolation, but also for displacement field approximation. The meshless equation governing the forced vibration of arbitrary shells is derived under Hamilton's principle and solved using the time-domain implicit Newmark method. The full transformation method is used to impose the essential boundary conditions. The code for the proposed method is developed in the MATLAB platform and used to compute several representative shell examples, obtaining the first ten natural frequencies of each shell type and the time history deflection at the center point under different pulse loads, considering both with and without damping. The calculated results are compared with ABAQUS solutions to verify the effectiveness and accuracy of the presented method. When using the meshless method to solve the forced vibration of arbitrary shell structures, it does not rely on grid partitioning. This enables effective analysis of various shell structures with different shapes, showcasing its strong adaptability and vast potential for applications.

During the manufacturing process of a rough workpiece, the non-uniformity of material mechanical properties can result in the generation of residual stresses within the workpiece, which can lead to structural failure. During the cutting removal process of the workpiece, the residual stresses gradually release, causing deformation. In this study, the "birth-death element" technique of finite element analysis was used to simulate the cutting removal process of the material. This process was then transformed into the release of residual stresses. A novel analytical method combining radial basis function interpolation with geometric and physical equations was proposed based on plate shell theory, small-deformation theory, theory of elasticity, and the superposition principle. The method aimed to invert the residual stress field and calculate the resulting deformation. This study was divided into two parts. In the first part, the influence of stress equilibrium equations was neglected, and the radial basis function interpolation method was used to invert the release of residual stresses in thin plates according to the initial residual stress field and the residual stress field after material removal. Next, the stresses were substituted into the physical equations to calculate the strain. The strain was then substituted into the geometric equations, and the plane displacement was calculated by strain integration from geometric equations. Based on the plate shell theory equations, the buckling deformation was inverted according to the plane displacement. In the second part, the reverse process of the first part was performed. Firstly, the buckling deformation caused by material removal in the thin plate was inverted. Then, the buckling deformation was substituted into the plate shell theory equations to invert the release of residual stress and the reconstructed residual stress field. The results demonstrated the reversibility of these two processes. Furthermore, the analytical solutions showed high agreement with the finite element solutions. This suggested that the analytical method proposed in this paper is applicable to thin-plate structures under elastic conditions and can be used to estimate residual stress distribution and predict deformation in the thin-plate cutting removal process.

The minimal surface structure is a continuous and smooth porous structure. It has the advantages of low density, high intensity, and excellent energy absorption capability. This paper has studied the mechanical properties and energy absorption characteristics of the minimal surface prepared by additive manufacturing process using nylon PA12. First, using the parametric modelling method, three kinds of minimal surface porous structures (G-surface, P-surface, and D-surface) with the same volume fraction of 20% are designed. The corresponding minimal surface structures are manufactured with Multi Jet Fusion (MJF) additive manufacturing technology. The mechanical response and energy absorption characteristics of different minimal surface structures are then analyzed by combining quasi-static compression tests and numerical simulations. For the mechanical response, it is found that the three kinds of minimal surface structures show better load-bearing capacities compared with the traditional BCC lattice structure. In detail, the nominal stresses of the three minimal surface structures (G-surface, P-surface, and D-surface) are 4.0 MPa, 2.1 MPa, and 4.75 MPa, respectively. The nominal stress value of the BCC lattice structure under the same volume is 2.0 MPa. It is clear that all values of the three minimal surface structures are significantly higher than that of the BCC lattice structure. For the study of energy absorption, the energy absorption per unit volume is used as one of key parameter to evaluate the energy absorption characteristic of the porous structure. The results indicate that the values of the energy absorption per unit volume for the three minimal surface structures (G-surface, P-surface, and D-surface) are all higher than that of the BCC lattice structure. The energy absorption per unit volume for the three minimal surface structures can approximately reach 7, 4, and 8 times that of the BCC lattice structure. In conclusion, the minimal surface structure can show excellent characteristics of mechanical property and energy absorption and has extensive application prospects in the fields of aerospace, automotive industry, and machinery.

In this study, the angle-preserving transformation method is employed to establish a propagation model for I/II composite lip-shaped cracks under tensile loading conditions. Based on Irwin's small-scale yielding equivalent hypothesis, a plastic propagation zone model is formulated for Ⅰ-Ⅱ composite lip-shaped cracks under tensile loading. This model provides expressions for the stress intensity factors (SIFs) of mode I and mode II at the tip of lip-shaped cracks within the plastic zone. Additionally, the stress distribution along the extension line of the lip-shaped crack tip is characterized. A tensile simulation model is developed, and the theoretical solution for stress distribution at the lip-shaped crack tip is compared with the elastoplastic and linear elastic simulation results. It is found that, based on Irwin's small-scale yielding equivalent hypothesis, the modified dimensions of lip-shaped cracks lead to increased crack sizes and greater equivalent SIFs. Geometric alterations in lip-shaped crack parameters also influence the plastic zone, with larger semi-lengths resulting in larger plastic zones under equivalent width-to-length ratios. Conversely, greater width-to-length ratios lead to smaller plastic zones under equivalent semi-lengths. Moreover, an increase in the inclination angle of the lip-shaped crack corresponds to a proportional increase in the plastic zone size. The plastic correction theory at the lip-shaped crack tip, founded on the basis of Irwin's small-scale yielding equivalent hypothesis, aligns well with plastic finite element simulations. As the inclination angle of the lip-shaped crack rises, stress levels at the crack tip diminish. On the one hand, this phenomenon arises from the transition from mode I crack extension to Ⅰ-Ⅱ composite crack extension, coupled with stress yielding at the concave region of the lip-shaped crack for larger inclination angles. On the other hand, this stress yielding serves to mitigate stress concentration at the crack tip, ultimately resulting in reduced stress levels at the crack tip.

Stress triaxiality is a parameter that expresses the stress state and can be used as a variable to characterize the plasticity and fracture damage model of materials. It plays an important role in structural strength and failure analysis. The round bar tensile test with a notch can be used to calibrate the parameters in the plastic and damage models. However, there are two different formulas in the literature to calculate the triaxiality of the minimum cross-sectional axis of a notched round bar under tensile loading, which were proposed by internationally renowned scholars Bridgman and Wierzbicki, respectively. Their differences often cause confusion in application. Through refined finite element numerical analysis, this article attempts to clarify the validity and applicability of the two formulas. The results show that the Bridgman formula is more accurate only in the elastic stage and in a specific a/R range. The Bao-Wierzbicki formula, on the other hand, is in good agreement with the experimental data and simulation results, which can be used to calculate the arithmetic mean value of triaxiality during the entire tensile process. Based on further analysis, a new revised stress triaxiality formula in the plastic stage under elastic-perfectly-plastic condition is proposed, and the notch geometry effect and strain-hardening effect are further discussed. It is pointed out that notch ratio can affect the neck stress field. The smaller is the notch ratio, the closer is the stress triaxiality value in the elastic stage to 1/3. When the notch ratio is too small, it can also affect the change of stress triaxiality throughout the entire tensile process. The strain-hardening effect can change the trend of stress triaxiality during the stretching process, and an increase in the strengthening modulus will lead to a decrease in the peak value of the plastic stage. The higher is the strengthening modulus, the faster is the decrease of stress triaxiality after entering the plastic stage.

In this paper, the filter function in the ICM method and the penalty function in the variable density method are both referred as the mapping functions. Different forms of mapping functions have a significant impact on the convergence efficiency of structural topology optimization. Therefore, it is necessary to study how to construct a suitable mapping function for the optimization model. Aimed at this problem, how to construct and select a mapping function in the establishment of the structural topology optimization model is studied, and the influence of different mapping functions on the convergence efficiency of structural topology optimization is discussed. An approach is proposed to construct a mapping function to achieve high-efficiency convergence in structural topology optimization. Five common forms of mapping functions are also given. An optimization model and a solution algorithm matching the mapping function with highly efficient convergence (MFHEC) are proposed. Firstly, the convergence rates of the filter function and the quasi-filter function of the same form of mapping functions are compared. Then the convergence rates of the fast filter function of different forms of mapping functions are compared. Taking the structural topology optimization problem of minimizing structural volume under displacement constraints as an example, the ICM method is adopted to establish the models and solve the problems. The higher convergence efficiency of MFHEC is verified by the results of numerical comparison. The results show that the fast filter function has a faster convergence rate than other functions in the same form of mapping functions. Compared with five different forms of mapping functions, the filter function of power function form has the fastest convergence efficiency. Finally, it should be emphasized that the conclusions of the mapping function studied in this paper are equally applicable to the filter function of the ICM method and the penalty function of the variable density method. The proposed method for constructing MFHEC is very useful for improving the efficiency of the ICM method and the variable density method.

Structural components in the fields of aviation, aerospace, weapons, and energy are often subjected to repeated impacts of small loads (or low energy). This type of load is different from the single-pulse impact of high energy and the conventional low-strain-rate fatigue, which is called impact fatigue. Because the energy-based impact fatigue test methods can only detect the relation between impact energy and fatigue life, the industrial application of impact fatigue test results in structural design and performance evaluation has been limited. Therefore, this paper focuses on exploring a new impact fatigue loading method. First, based on a brief review of the development of existing impact fatigue test methods, this paper affirms the superiority of the stress wave method based on the Hopkinson bar principle, and raises the problem of non-constant amplitude loading in impact fatigue tests. The waveform generated by the Hopkinson bar is controllable and measurable, which is beneficial to realizing constant-amplitude cyclic loading. Then, three impact fatigue loading techniques (namely the one-wave, two-wave, and three-wave techniques) based on the Hopkinson bar are proposed. The feasibility of these methods is verified through experiments, focusing on studying whether there is a non-constant amplitude loading problem caused by secondary loading. The three-wave technique is found to be the most effective impact fatigue loading method because it can achieve constant amplitude loading and obtain comprehensive test data. Finally, a constant-amplitude dynamic shear fatigue test method is developed using the three-wave technique. Impact fatigue performance tests are carried out on the TC4 titanium alloy. The test loading frequency is 0.1 Hz, and the test strain rate ranges from 6800/s to 8400/s. It is proved that this method can realize dynamic shear fatigue testing of metal materials at the strain rate level of 10³/s. This study provides a new idea for the constant-amplitude impact fatigue test. By changing the forms of the specimen and the loading bars, the impact fatigue loading in other loading modes (such as tension, compression, etc.) can also be realized.

Considering that aircrafts are typical targets for laser weapons, and aircraft materials are used under complex mechanical conditions, the response of these materials under laser and prestressing holds significant value in the aeronautic industry. However, current research on material response under combined laser and prestressing mainly focuses on mechanical failure affected by laser irradiation, with limited in-depth understanding of laser ablation characteristics and microscopic mechanisms. In this study, we investigate the laser ablation characteristics of LY12-CZ alloy by means of experiment and simulation. We analyze the ablative morphology under prestressing and characterize microstructure and element distribution in the remaining samples. The Vickers hardness test along the radial direction of the heat source is carried out. In addition, a finite element simulation of laser ablation under prestressing is performed based on the fully coupled thermal-stress analysis method. The experimental findings reveal three distinct structures along the radial direction of the spot: the dendrite structure near the spot, the equiaxed grain structure in the middle, and the original structure far away from the spot. Both the dendrite and equiaxed grain structures exhibit severe segregation of Cu element, while the equiaxed grain structure also experiences high internal stress. The hardness in the dendrite structure is close to 80 HV and increases sharply to around 120 HV in the equiaxed grain structure. In the transition zone from the equiaxed grain structure to the original structure, the hardness decreases first and then increases. Prestressing does not have an obvious effect on the distribution of microstructure, element segregation, and hardness after laser ablation. Simulation results indicate that the temperature field is not significantly influenced by the applied prestress. The thermal stress caused by the rise in irradiation temperature is much higher than the applied prestress, leading to the formation of burn-through cavities and local thermal softening of the material. It is found that prestressing does not significantly alter the laser ablation characteristics of LY12-CZ alloy.

During penetration, the temperature of the projectile will sharply rise due to the large amount of heat generated by the sliding friction between the projectile and the target. High temperature can soften the projectile, potentially change its shape, penetration mechanism, and further affect the penetration ability. In order to study the temperature rise of the projectile during high-speed penetration, a two-part temperature rise calculation model of the penetration is established. In the first part, the heat flux data set of the projectile at different positions in the process of penetration is obtained according to rigid body dynamics theory and the friction heat generation mechanism. The second part takes the heat flux data set as the boundary condition and calculates the temperature distribution of the projectile based on heat conduction theory and the finite difference algorithm. The stability of the model is discussed from the two aspects of time step and projectile mesh, and appropriate values are selected. The calculation model of temperature rise is used to study the heat flux and temperature distribution of the projectile, and the factors influencing the temperature rise of the projectile are discussed and analyzed. The results show that the temperature rise is obvious during high-speed penetration, but it only lasts for a very short time, and the high temperature is mainly distributed near the surface of the projectile head. The position of the highest surface temperature outside the projectile during penetration is related to the shape of the projectile. During the penetration time, the ratio of the heat conduction distance to the radius of the projectile decreases with the increase of the size of the projectile. The research results are useful for the design and material selection of high-speed penetration projectiles.