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2020 Vol. 41, No. 4
Published: 2020-08-28
293
Progress and challenge of research on Crashworthiness of civil airplane fuselage structures
The crashworthiness of civil aircraft in emergency landing is directly related to the safety of passengers, which is an important reflection of the safety of civil aircraft. Crashworthiness of airframe structure is a complex nonlinear impact dynamics problem, which involves large deformation, fracture failure and dynamic contact. The airworthiness standards for transport aircraft have clearly stipulated the crashworthiness of airframe structures, requiring reasonable design to avoid the crew from bearing excessively severe impact loads and to maintain the effective restraint of the large mass body in the cabin, to maintain the occupant viable space and the availability of the occupant evacuation channel, the relevant design needs to be evaluated by experimental or validated analytical methods. In this paper, several key problems in the research of crashworthiness of civil aircraft fuselage structure, such as the impact dynamics behavior of fasteners and mechanical connecting structures, the design method of high efficient energy absorbing structures, and the experimental and numerical methods of crashworthiness assessment in recent years are analyzed, the main research progress is summarized, and the main challenges are analyzed.
2020 Vol. 41 (4): 293-323 [
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324
Free Vibration of Imitated Super Carbon Nanotubes Based on Coarse-grained Method
Free vibration of the imitated super carbon nanotube (ISCNT) is studied based on the coarse-grained method together with finite element method of the original structure of the ISCNT. The carbon nanotubes have many excellent mechanical and electrical properties. Based on the structure of the carbon nanotubes, the super carbon nanotubes (SCNT) is a new type of structure, replacing each carbon-carbon bond with carbon nanotubes. The ISCNT is the corresponding macro structure of SCNT which keeps the exterior structure of the SCNT and enlarges the scale to the macro scale. The vibration behavior of ISCNT is important to its application and thoroughly understanding its mechanical properties. The imitated carbon nanotubes which constructs the ISCNT are equivalent to Euler beams to build the coarse-grained model of the ISCNT. The coarse-grained method based on the equivalent Euler beam element is presented to investigate the free vibration of the ISCNT. The effective bending Young’s modulus of the equivalent Euler beam can be obtained by the first-order natural frequency obtained by the finite element calculation of the original structure of imitated carbon nanotubes. The Poisson’s ratio of the equivalent Euler beam can be obtained by the stretching of the original structure of imitated carbon nanotubes via finite element method. The coarse-grained method is validated in the vibration behavior analysis of the ISCNT with different sizes. The results show that the increasing of radius of imitated bond and imitated carbon nanotubes leads to the increase of natural frequency of the ISCNT. The increasing of length of imitated carbon nanotubes and the ISCNT leads to the decrease of natural frequency of the ISCNT. The results show that the coarse-grained method can be effectively used to study the free vibration of ISCNT, so as to save time in building ISCNT models and calculating the vibration behavior of the ISCNT.
2020 Vol. 41 (4): 324-333 [
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334
A coupling model for hydrostatic pressure induced critical temperature degradation of Nb3Sn polycrystalline superconductors
Nb3Sn superconducting material has promising application prospect in high-field magnet. In the material application in the high-field magnet design and manufacture, strain induced superconducting performance degradation of Nb3Sn is of great importance and significance. In order to reveal the different degradation responses of Nb3Sn single crystal and Nb3Sn polycrystal under hydrostatic pressure, we establish a constitutive relation characterizing the electro-mechanical coupling behavior in Nb3Sn single crystal. The constitutive relation is based on maki de Gennes (MDG) relation. By using the relation, we predict the hydrostatic pressure induced critical temperature degradation response of Nb3Sn polycrystalline superconductor, whose deformation is modeled by polycrystalline finite element method. The predicted results are qualitatively consistent with the experimental observations. The proposed relation can consistently describe the hydrostatic pressure induced degradation responses of both Nb3Sn single crystal and Nb3Sn polycrystal. The results are helpful to improve the understanding of the electro-mechanical coupling behavior of Nb3Sn superconducting material and to lay a foundation for modeling and simulation methods for multi-field and multi-scale characterization of Nb3Sn in extreme serving environment. At the same time, the results can be used to evaluate high field superconducting magnet design and to give insight on how to fabricate superconducting materials with high strain resistance.
2020 Vol. 41 (4): 334-342 [
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343
Electroelastic Fracture Analysis of Electrically Permeable Multiple Nano Cracks Emanating from A Circular Hole with Surface Effects
The fracture characteristics of radial electrically permeable multi-cracks originating from a nanoscale circular hole in piezoelectric materials subjected to inplane electric loads and far-field antiplane mechanical loads were analytically investigated. Based on the Gurtin-Murdoch surface elasticity theory, the analytical solution of the electroelastic field, the electroelastic intensity factor and the energy release rate at the crack tip were obtained by using the conformal mapping technique and the complex elastic theory. The size effects of the dimensionless electroelastic intensity factor and the dimensionless energy release rate were revealed and the influences of the number of cracks and defect parameters (hole size, crack length, hole/crack ratio) on dimensionless electroelastic intensity factor and dimensionless energy release rate were discussed. The results indicate that the dimensionless electroelastic field intensity factor and the dimensionless energy release rate have a significant size-dependent effect. As the hole size increases, the size effect of the dimensionless field intensity factor and dimensionless energy release rate gradually disappear, and the dimensionless stress intensity factor is different from the dimensionless electric displacement intensity factor. The electroelastic field intensity factor can reach its maximum only when the crack length is close to the hole size, and overlarge or oversmall crack/hole ratio shields the dimensionless field intensity factor. When the number of cracks is 3, the dimensionless field intensity factor reaches its maximum value, and then gradually decreases with the increase of the number of cracks. The influence of the crack/hole ratio on the variation of the electroelastic field intensity factor with the number of cracks gradually weakens when the number of cracks increases. Overlarge or oversmall crack/hole ratio greatly weakens the effect of crack length on the energy release rate. When the ratio of hole size and crack length is in the range of (0.1, 10), the effect of the crack length on the dimensionless energy release rate is greater.
2020 Vol. 41 (4): 343-351 [
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352
A Finite Element-Temporally Adaptive Algorithm for Asymptotic Coupling Analysis of Rock Linings
A Finite Element-temporally adaptive algorithm is presented for the coupling analysis of rock linings. A coupled spatial and temporal problem is decoupled into a series of spatial problems which are recursively solved by FEM, and a steady computing accuracy can be maintained via an adaptive process with different step sizes. Insufficient release of stress/displacement of rock/linings, and impact of excavation are analogized as a kind of asymptotic load. The whole process is divided into three stages by excavation time, implement time of primary and secondary linings, and compatible conditions at the joints of different stages are proposed. Parameters related with asymptotic load can be determined by solving an inverse problem. Numerical examples are provided to verify the proposed approach.
2020 Vol. 41 (4): 352-365 [
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366
Accurately modeling thermomechancial behavior of shape memory polymer by explicit method
A constitutive model for shape memory polymer is established by constructing a multi-axial compressible strain energy with thermal-coupled property to obtain relationships between stress-strain、stress-temperature and strain-temperature. Three invariants based on Hencky strain are introduced to (i) simulate compressible condition; (ii) applicable to at least two deformation mode, i.e. uniaxial tension and compression and bi-axial tension; (iii) effective for multi-axial case. By using explicit method to (i) give out the explicit formulations of free energy and entropy to prove that the constitutive equations satisfy the thermodynamic conditions; (ii) give out the shape functions of stress-strain、stress-temperature、and strain-temperature. The multi-axial model can automatically reduce to their uniaxial case, respectively. The model result can accurately match the experiment data by adjust parameters in the shape functions. The new constitutive model can accurately guide engineer designing for the shape memory polymer.
2020 Vol. 41 (4): 366-378 [
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379
Structural Reliability Calculation With Multiple Failure Modes Using LCF-Kriging Model
In order to balance the construction cost and the prediction accuracy of the surrogate model used to estimate structural system reliability with multi-failure modes, the Largest Contribution Function (LCF) is proposed. The LCF is used to identify the best samples from candidate samples in the construction process of surrogate model. Since the best samples have great influence on the variance of system failure probability, the prediction accuracy of system failure probability can be improved. Mathematical formulas of LCFs for the series system, parallel system and series-parallel system are derived based on the structural reliability theory. The learning stopping condition of LCF is established by using confidence level and allowable relative error, it can ensure that the information about existing samples is not wasted. A multi-output Kriging model is selected as surrogate model, it can approximate multiple performance functions at the same time. The structural system reliability is calculated by using the LCF-Kriging model and MCS, the correlation between performance functions can be considered by the logical relationship of failure modes. Some examples are introduced to demonstrate the efficiency and the accuracy of the proposed method. It is found that the proposed method can achieve the satisfactory balance between the construction cost and the accuracy for structural reliability estimations of series system, parallel system and series-parallel system under the appropriate learning stopping condition.
2020 Vol. 41 (4): 379-390 [
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