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2024 Vol. 45, No. 1
Published: 2024-02-28
1
Research on Constant-amplitude Impact Fatigue Test Method Based on Hopkinson Bar
Structural components in the fields of aviation, aerospace, weapons, and energy are often subjected to repeated impacts of small loads (or small energies). This type of load is different from the single-pulse impact of large 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 the 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 the 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 could 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. The 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 of other loading modes (such as tension, compression, etc.) can also be realized.
2024 Vol. 45 (1): 1-15 [
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16
Modeling of temperature rise for projectile penetrating into concrete target and analysis of influencing factors
A large amount of heat is generated by friction in the process of projectile penetration, resulting in a large increase in the temperature of projectile body, which may change the mechanism of action in the process of projectile penetration and further affect the penetration ability. In order to study the temperature rise of the projectile body under the action of high-speed penetration, a temperature rise calculation model for the parallel calculation of the penetration and heat conduction of the projectile body was established. Firstly, the heat flow data set of different parts of the projectile body during the whole process of penetration was obtained according to the motion equation of the projectile body and the friction heat generation mechanism, and then the temperature distribution of the projectile body at different times and positions was calculated based on the heat conduction theory and the finite difference algorithm. Based on the proposed temperature rise calculation model, the heat flux and temperature distribution of the projectile body during the process of motion are studied, and the factors affecting the temperature rise of the projectile body are discussed and analyzed. The results show that the temperature rise is very obvious in the process of penetration, and the high temperature is mainly distributed near the surface of the projectile body. The position of the highest surface temperature of the projectile in the process of penetration is related to the shape of the warhead. During the penetration time, the ratio of the heat conduction distance to the radius of the projectile decreases with the increase of the projectile size.
2024 Vol. 45 (1): 16-28 [
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29
Numerical simulation of target damage zone under hypervelocity impact
In the problem of hypervelocity impact, it is difficult to accurately obtain the crater morphology in some conditions. Studying the damage zone can make up for this deficiency, and provide important basis for the study of impact mechanism and verification of numerical simulation. There is relatively little research on numerical simulation of damage zone, mainly due to the lack of experimentally validated damage zone criteria. In this paper, the results of the existing quantitative measurement of damage zone were summarized. It is found that, for the same target, the depth of the damage zone obtained by multiple microscopic testing methods is relatively consistent, which provides convenience for 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 damage zone criteria is analyzed. The results indicate that, TPS = 0.1 is suitable as damage criterion; D = 1 could be used as damage criterion with caution; peak pressure is not suitable as damage criterion. Through parameter analysis, it is found that with the increase of porosity and target strength, the damage zone gradually decreases.
2024 Vol. 45 (1): 29-37 [
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Laser Ablation Characteristics of Pre-stressed LY12-CZ: An Experimental and Simulation Study
As aircraft is the main object of laser weapon, it is necessary to research on response of aeronautical materials under laser irradiation. In this work, experiments and finite element simulation were applied to investigate ablation characters of 1080nm continuous laser on LY12-CZ aluminum alloy under pre-stressed loading. The experiment results show that three kinds of microstructure exist in the remaining target material after laser ablation, namely dendrite, equiaxed grain and primitive microstructure. Element segregation among grain boundaries present in dendrite and equiaxed grain structures. High internal stress exists in the dendrite structure. The finite element simulation results show that the temperature field is not sensitive to the applied pre-stress. One of the reasons for the formation of burn-through hole is the local yield caused by hot softening and high level thermal stress. Compared with thermal stress, pre-stress is not easy to reach the yield strength which is decreased by the increasing temperature.
2024 Vol. 45 (1): 38-51 [
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52
Insertion and Withdrawal Forces of Electrical Connectors Considering Wear
Electrical connectors widely used in electrical and electronic devices suffer from severe contact failure problems, which is determined to the reliable service of these devices or even cases their destruction. In this paper, the finite element method is used to simulate the insertion and withdrawal processes of an electrical connector and the wear profile due to the repeated processes in the application. In cooperation with the adaptive mesh of the finite element software ABAQUS, the wear model based on the frictional dissipation energy is applied to predict the wear morphology. Based on the development of the FEM model of the electrical connector, the equivalent insertion and withdrawal forces with and without the consideration of wear are calculated. The wear profiles of the contact surface are obtained at different numbers of the insertion and withdrawal processes, and the effect of the wear on the insertion and withdrawal forces of the electrical connector is discussed.
2024 Vol. 45 (1): 52-60 [
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61
Analytical solution of an arbitrary location through-crack emanating from a nano-hole in magnetoelectroelastic materials
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 a more complex internal structure 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-defect (pores and cracks) is investigated in this paper. Based on the Gurtin-Murdoch surface theory and conformal mapping theory, the mode III fracture properties of the MEE materials containing an arbitrary location through-crack emanating from a nano-hole under anti-plane mechanical load, in-plane electrical load and in-plane magnetic load are studied. The accurate solution of the MEE field in the matrix was obtained by using the MEE theory and the far-field load conditions. The analytical expression of the MEE field intensity factors of the tips at both ends of the through-crack under the condition that the surface of the nano-defect are magnetoelectric impermeable are given. The comparison between the obtained results and existing research demonstrates the correctness of the proposed method. The effects of the crack location, crack interaction, and the application of multiple physical loads on the dimensionless MEE field strength factors were discussed. The results show that the dimensionless MEE field intensity factors exhibits a significant size effect. The surface effect of the 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 crack length of the through-crack and the applied MEE loads. The results obtained in this article 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.
2024 Vol. 45 (1): 61-73 [
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74
Analysis of Circumferential Free Vibration of Functionally Graded Conical-cylindrical Joined Shells
In order to promote the application of functionally graded materials (FGMs) in the aerospace field, this paper presents an analysis of the circumferential free vibration of a functionally graded conical-cylindrical joined shell. The properties of the FGMs are described by the Voigt model and the four-parameter power function volume fraction. Based on the Donnell thin shell theory, the displacement and strain relations of the conical shell and the cylindrical shell are derived, and the energy expressions of the conical shell and the cylindrical shell are obtained. Artificial springs are introduced to simulate arbitrary boundaries and joined conditions, and the displacement function is constructed based on Chebyshev polynomials. Then, the modal frequencies of the FGMs conical-cylindrical joined shell are calculated using the Rayleigh-Ritz method, and the effects of gradient exponent, boundary conditions, and geometric parameters on modal frequencies are analyzed. Moreover, the main results of this paper include: for the circumferential wave number is less than 6, stronger boundary constraint conditions lead to a higher overall modal frequency of the structure, increasing the ceramic volume fraction can also effectively increase the modal frequency of the structure, the axial spring stiffness has a more significant impact on the modal frequency of the structure compared to the circumferential and radial spring stiffness; for the circumferential wave number is greater than 3, the modal frequency of the structure increases linearly with increasing shell thickness, while increasing the conical-cylindrical shell length ratio results in a lower modal frequency of the structure.
2024 Vol. 45 (1): 74-87 [
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88
Mechanism Analysis of Adhesive Layer Effect on Scratch Damage of Polymethylmethacrylate Coating
To investigate the influence of adhesive layer thickness and adhesion strength on the scratch damage of polymethylmethacrylate (PMMA) coating, scratch experiments were systematically conducted to 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 by finite element method. The physical mechanisms of complex scratch damage modes were revealed. The results show that: Different from the coating structures with 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. Adhesive layer with strong adhesion strength restricts the deformation of the coating during scratching, preventing severe buckling of coating in front of the scratch tip and avoiding the formation of longitudinal crack 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 crack and longitudinal crack. These findings contribute to the understanding of the scratch mechanism and further improvement of the scratch resistance and functional integrity of PMMA coatings.
2024 Vol. 45 (1): 88-110 [
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Theoretical Study on the Distribution Dislocation of Subsurface Microcracks on the Growth Behavior of Deflection Main Cracks
To study the semi-infinite plane problem including an embedded deflection crack and a micro-crack at any position under tensile load. Based on the continuous distribution dislocation method, the corresponding dislocation density integral equation was established, and its mechanical parameters were obtained by using the GAUESS-CHEBSHEV numerical integration method. The theoretical results are verified by finite element method. The buried depth and the distance from the microcrack center to the main crack tip will affect the stress intensity factor of the main crack tip; Compared with the case without microcracks, microcracks in some directions promote the growth of the main crack tip, while microcracks in other directions inhibit the growth of the main crack tip; The propagation direction of the main crack tip and the equivalent stress intensity factor are more affected by the horizontal microcrack than the inclined microcrack. The stress intensity factor at the tip of the main crack decreases with the increase of the crack embedding depth and the distance from the center of the microcrack to the tip of the deflected main crack; In time, microcracks will promote the growth of main cracks, while in time, they will inhibit the growth of main cracks; When the microcrack is in and, the propagation direction of the main crack will deflect clockwise from the original propagation direction, while when the microcrack is in and, the propagation direction of the main crack will deflect counterclockwise from the original propagation direction; Horizontal microcracks have more influence on the propagation direction of the main crack and the equivalent stress intensity factor than inclined microcracks.
2024 Vol. 45 (1): 111-122 [
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123
The anti-plane Shear problems of a lip-shaped orifice with two unsymmetrical edge rips in one-dimensional hexagonal quasicrystals piezoelectric materials
The fracture problem of nonlinear cracks in one-dimensional hexagonal quasicrystal piezoelectric materials under anti-plane load is studied by using the complex variable function method and Stroh algorithm. The defect mechanics model of the secondary asymmetric cracks at the lip is first constructed, and the conformal transformation formula from the infinite region containing the secondary asymmetric cracks at the lip to the outer region of the unit circle is derived, the analytical expressions of the field intensity factor and the energy release rate at the crack tip are obtained. The numerical examples reveal the influence of the defect size, especially the lip height and the crack length, on the field intensity factor and the energy release rate. The results show that increasing the length of both sides of the crack will promote the crack expansion, and increasing the lip height will inhibit the crack growth. Finally, under given conditions, these analytical results can be simplified to the solutions of other defect models, for example, the solution of lip secondary single crack and lip secondary double symmetric crack can also be degenerated into the solution of classical Griffith crack and lip secondary crack. The above results are consistent with the conclusion of theoretical analysis
2024 Vol. 45 (1): 123-134 [
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135
Dispersion Analysis of Waves in Nanoscale Piezoelectric Double Crystals Considering Surface Effects
By the progress of micro and nano technologies, nanoscale piezoelectric bimorphs have become extensively popular in various fields including nanosensors, nanoactuators, nanoscale energy recovery devices, and nanoresonators. With the decrease of size, the influence of scale effects becomes more prominent. The aim of this research was to investigate scale effect on the frequency characteristics of nano piezoelectric bimorphs according to scale dependence theory. This work could broaden our understanding of the wave characteristics of piezoelectric nanostructures. According to nonlocal strain gradient theory, wave dispersion properties in nanoscale piezoelectric bimorphs was studied taking into account surface elasticity and residual stress. The upper and lower piezoelectric layers of piezoelectric bimorphs were subjected to an electric field and were deposited on a viscoelastic substrate. Control equation was derived based on Hamilton's principle and sinusoidal shear theory. Motion equation was derived according to 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 numbers, and viscoelastic substrates on piezoelectric bimorphs. Research has revealed that surface residual stress and surface elastic coefficient had a combined effect on the dispersion properties of piezoelectric bimorphs. Research has also suggested that the existence of surface effects was essential for the investigation of the frequency properties of piezoelectric bimorphs. Scale parameters and wave numbers 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 as a means to regulate the center frequency of piezoelectric bimorphs. This work will expand theoretical research on the dispersion mechanism of piezoelectric nano resonators and provide useful reference for the design and manufacturing of piezoelectric nanofilters.
2024 Vol. 45 (1): 135-144 [
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