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.

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.

In hypervelocity impact problems, it is difficult to accurately obtain the crater morphology under certain conditions. Studying the damage zone can compensate for this limitation and serve as an important basis for understanding the impact mechanism and validating numerical simulations. However, there has been limited research on numerical simulation of the damage zone due to the lack of experimentally validated criteria. This paper presents a summary of the existing quantitative measurement results for the damage zone. The findings reveal that, for the same target, the depths of the damage zone obtained through multiple microscopic testing methods are relatively consistent, which facilitates the analysis of the damage zone. Based on the iSALE code, the applicability of total plastic strain (TPS), damage factor (D), and peak pressure as criteria for assessing the damage zone is analyzed. The results indicate that, TPS=0.1 is appropriate as a damage criterion, D=1 could be used as a damage criterion with caution, while peak pressure is not a suitable damage criterion. Through parametric analysis, it is found that the damage zone gradually decreases with increasing porosity and target strength.

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.

Electrical connectors widely used in electrical and electronic devices suffer from severe contact failure problems which determine the reliable service of these devices or even cause their destruction. The insertion and withdrawal forces are significant factors to evaluate the performance and quality of electrical connectors. The wear behavior due to the repeated insertion and withdrawal processes has a non-negligible influence on the forces. Therefore, the forces and wear behavior are investigated in this paper. A simulation model is developed to study and evaluate the performance of electrical connectors. The finite element method (FEM) is used to simulate the insertion and withdrawal processes and the wear profile of an electrical connector in the application. According to the development of the FEM model of the electrical connector, the equivalent insertion and withdrawal forces with and without the consideration of wear behavior are calculated. Frictional behavior is used to study the insertion and withdrawal forces without considering wear. In cooperation with the adaptive mesh technique of the commercial FEM software ABAQUS, the wear model based on frictional dissipation energy is applied to predict the wear morphology. The characteristics of the insertion and withdrawal procedure are analyzed using the forces vs. time curves. Furthermore, the wear profiles of the contact surface are obtained at different times of the insertion and withdrawal processes, and the effect of wear on the insertion and withdrawal forces of the electrical connector is discussed. The critical number of insertions and withdrawals leading to a larger wear depth is found. The size parameter of the contact component is modified to validate the conclusion of wear influence. It is worth emphasizing that this study is significant to the failure problem of electrical connectors and provides a guideline for the production and application of electrical connectors.

With the development of engineering technology and materials science, pure elastic materials can no longer meet the application needs of materials in industrial manufacturing. Magneto-electro-elastic (MEE) materials have more complex internal structures compared to classical elastic materials, and the methods for solving mechanical and physical performance are more difficult compared to classical elastic materials. Therefore, the mode III fracture behavior of MEE materials with nano-defects (pores and cracks) is investigated in this study. Based on the Gurtin-Murdoch surface theory and conformal mapping theory, the mode III fracture properties of MEE materials containing an arbitrary-location through crack emanating from a nano-hole under anti-plane mechanical loading, in-plane electrical loading, and in-plane magnetic loading are studied. The accurate solution of the MEE field in the matrix is obtained using the MEE theory and the far-field loading conditions. Analytical expressions for the MEE field intensity factors of the tips at both ends of the through crack, assuming that the surface of nano-defects is magneto-electric impermeable, are given. The proposed method is validated through a comparison with existing research. The effects of crack location, crack interaction, and the application of multiple physical loads on the dimensionless MEE field strength factors are discussed. The results show that the dimensionless MEE field intensity factors exhibit a significant size effect. The surface effect of nano-defects on the MEE tip fields of the cracks is constrained by the crack location. The dimensionless MEE field intensity factors are significantly affected by the ratio of the through crack length to the applied MEE loads. The results obtained in this study provide a theoretical basis for the experiments and numerical simulations of the mode III fracture behavior of an arbitrary-location through crack emanating from a nano-hole in MEE materials.

This paper focuses on analyzing the circumferential free vibration of the functionally graded joined conical-cylindrical shell to enhance the vibration performance and stability of the structure, particularly in the aerospace field. First, the properties of the functionally graded materials (FGMs) are described using the Voigt model and the four-parameter power function volume fraction. The energy expressions for the conical shell and cylindrical shell are derived based on the previously obtained displacement-strain relationships formulated utilizing the Donnell thin shell theory. Then, artificial springs are introduced to simulate the continuity conditions and boundary conditions. The displacement function is constructed using Chebyshev polynomials to enable a more accurate analysis of the structural response and performance. The modal frequencies of the functionally graded joined conical-cylindrical shell are calculated employing the Rayleigh-Ritz method with this displacement function. Hence, the influence of gradient exponent, boundary conditions, and geometric parameters on the modal frequencies is analyzed to reveal the vibration characteristics of the structure. The main results indicate that increasing the volume fraction of ceramics effectively enhances the modal frequencies of the structure, while higher gradient exponents lead to a decrease in the modal frequencies. Stronger boundary constraints result in higher modal frequencies for the functionally graded joined conical-cylindrical shell. With an increase in the circumferential wave number, the influence of boundary conditions on the structural modal frequencies diminishes. The effect of boundary constraints is more pronounced on the cylindrical shell compared to the conical shell. Additionally, the axial spring stiffness has a more significant impact on the modal frequencies of the structure compared to the circumferential and radial spring stiffnesses. When the circumferential wave number is greater than 3, the modal frequency of the structure exhibits a linear increase with increasing shell thickness, whereas increasing the conical and cylindrical shell length ratio leads to a decrease in modal frequency. Finally, when the length ratio of the conical and cylindrical shell is fixed, increasing the cone angle initially results in an increase in the modal frequencies of the structure until it reaches a peak value, after which it starts to decrease.

To investigate the influence of adhesive layer thickness and adhesion strength on the scratch damage of polymethylmethacrylate (PMMA) coating, scratch experiments were systematically conducted on PMMA coatings with different adhesive layer thicknesses and adhesion strengths. A constitutive model considering the competition between shear yielding and brittle fracture was employed to describe the mechanical behavior of PMMA coating. The scratch behavior of PMMA coating was simulated using the finite element method. The physical mechanisms behind the complex scratch damage modes were revealed. The results show that, different from the coating structures with a zero-thickness adhesive layer, the deformation of the finite-thickness adhesive layer leads to local bending of the PMMA coating, resulting in the formation of internal cracks in the bottom region of the coating beneath the scratch tip. An adhesive layer with strong adhesion strength restricts the deformation of the coating during scratching, preventing severe buckling of the coating in front of the scratch tip and avoiding the formation of longitudinal cracks that penetrate through the coating along the thickness direction. Increasing the coating thickness can enhance the resistance of the coating to bending and buckling during scratching, thereby delaying the formation of the internal and longitudinal cracks. These findings contribute to the understanding of the scratch mechanism and further improvement of the scratch resistance and functional integrity of PMMA coatings.

To study the problem of a semi-infinite plane with an embedded deflected crack and a microcrack at any position under tensile loading, the integral equation for the corresponding dislocation density is established based on the method of continuous distributions of dislocations, and its mechanical parameters are obtained using the Gauss-Chebyshev numerical integration method. The theoretical results are verified using the finite element method. Compared with the case without microcracks, microcracks in certain directions (i. e., the azimuth angle of the microcrack is between -90° and 45°) promote the growth of the main crack, while microcracks in other directions (i. e., the azimuth angle of the microcrack is between 45° and 90°) inhibit its growth. The stress intensity factor at the main crack tip decreases with the increase of the embedded depth of the crack and the distance from the center of the microcrack to the tip of the deflected main crack. When the azimuth angle of the microcrack is between -90° and -65°, or between -25° and 10°, the propagation direction of the main crack deflects clockwise from the original propagation direction. When the azimuth angle of the microcrack is between -65° and -25°, or between 10° and 90°, the propagation direction of the main crack deflects counterclockwise from the original propagation direction. Horizontal microcracks have a greater influence on the propagation direction of the main crack and the equivalent stress intensity factor compared with inclined microcracks.

Defects play a crucial role in understanding the physical and mechanical behavior of materials. In this study, the fracture problem of an infinite one-dimensional hexagonal piezoelectric quasicrystal material matrix containing secondary asymmetric straight cracks with lip-shaped pores is investigated. A defect mechanics model of secondary asymmetric cracks with lip-shaped pores is constructed for the first time. Utilizing conformal transformation technology, a conformal transformation formula from an infinite region containing secondary asymmetric cracks at the lip on the physical plane to the outer region of the unit circle is built. Using the complex variable method, analytical expressions for the field intensity factor and energy release rate at the crack tip are obtained. Under given conditions, these analytical results can be simplified into solutions for other defect models, such as secondary single cracks at the lip and secondary symmetric cracks at the lip. At the same time, they can also degenerate into the solutions of classical Griffith cracks and lip cracks without secondary cracks. Numerical examples reveal the effects of defect size, particularly the lip height and crack length, on the field intensity factor and energy release rate. The results show that increasing the length of both sides of the crack promotes crack propagation, while increasing the height of the lip inhibits crack propagation. These findings are consistent with the conclusions drawn from theoretical analysis. When the length of the secondary crack on one side of the lip is zero, as the height of the lip increases, the stress intensity factor and energy release rate at the crack tip on the other side first increase to a peak and then gradually decrease, eventually stabilizing at a constant level. The research results presented in this paper can contribute to the development of a theoretical framework for material fracture mechanics and provide technical assistance for nondestructive testing, reliability design, and optimization of piezoelectric quasicrystal material equipment and components.

Due to progress in micro and nano technologies, nanoscale piezoelectric bimorphs have gained extensive popularity in various fields such as nanosensors, nanoactuators, nanoscale energy recovery devices, and nanoresonators. With a decrease in size, the influence of scale effect becomes more prominent. The aim of this research was to investigate the scale effect on the frequency characteristics of nanoscale piezoelectric bimorphs according to scale-dependent theory. This work may broaden our understanding of the wave characteristics of piezoelectric nanostructures. On the basis of nonlocal strain gradient theory, the wave dispersion properties in nanoscale piezoelectric bimorphs were studied, taking into account surface elasticity and residual stress. The upper and lower piezoelectric layers of the bimorphs were subjected to an electric field and deposited on a viscoelastic substrate. The control equation was derived based on Hamilton's principle and sinusoidal shear theory. The equation of motion was derived according to the scale-dependent constitutive equation with nonlocal and length scale parameters, and the corresponding characteristic equation was solved by incorporating harmonic solutions. The obtained numerical results revealed the effects of surface elasticity, residual stress, scale parameters, wave number, and viscoelastic substrate on piezoelectric bimorphs. The research showed that the dispersion properties of piezoelectric bimorphs were influenced by a combination of surface residual stress and surface elastic coefficient. The existence of surface effects was found to be essential for the investigation of the frequency properties of piezoelectric bimorphs. Scale parameters and wave number also had a combined effect on dispersion characteristics, and the influences of elastic coefficient, damping coefficient, and piezoelectric layer thickness on frequency exhibited regional characteristics. Therefore, it is possible to use appropriate substrate materials to regulate the center frequency of piezoelectric bimorphs. This work contributes to the theoretical research on the dispersion mechanism of piezoelectric nanoresonators and provides useful reference for the design and manufacturing of piezoelectric nanofilters.