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2022 Vol. 43, No. 1
Published: 2022-02-28
1
Compressive strength of a short fiber reinforced composite
The internal stresses in a composite obtained by a micromechanics theory are homogenized quantities. They must be converted into true values before a failure assessment and strength prediction can be made. While the theories for obtaining all of the other directional true stresses have already been established, only the true stress of the matrix in the short fiber reinforced composite (SFRC) under a longitudinal compression is not known. The matrix true stress is obtained by multiplying its homogenized counterpart with the stress concentration factor (SCF). The stress field in the matrix of the SFRC subjected to a longitudinal compression is determined elastically, and is used to derive the longitudinal SCF of the matrix in the composite. Different from definition of the other directional SCFs, a longitudinal SCF of the SFRC must be obtained by subdividing its RVE (representative volume element) into three segments, i.e., two end plus a central ones, so that we can obtain a SCF applicable to any fiber aspect ratio. The homogenized stresses of the fiber and matrix are calculated by Bridging Model. And the compressive strength of a SFRC with any aspect ratio can be predicted easily. Only the geometrical and mechanical properties of the constituent materials are required. The predicted results are in good agreement with the experimental values, the efficiency of the theory established in this paper is finally verified.
2022 Vol. 43 (1): 1-15 [
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16
FINITE ELEMENT SIMULATION OF THE EVOLUTION PROCESS OF INCLUSIONS DUE TO STRESS-INDUCED INTERFACE DIFFUSION
2022 Vol. 43 (1): 16-27 [
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28
Experimental and GTN Model Numerical Study on Hydrogen Induced Brittle Fracture of 45 Steel
The hydrogen induced brittle fracture of 45 steel was studied by uniaxial tension after hydrogen charging with electrochemical method. It is observed that with the increase of hydrogen content, the elongation and average fracture strain decrease, the number of dimples decreases and the quasi-cleavage area increases. However, the change with hydrogen charging time is gradually slowed down, and the ductility and fracture strain of the sample do not change after 144 h of hydrogen charging. According to the experimental observation, the HEDE mechanism and the evolution mechanism of material microvoids affected by hydrogen are introduced into the GTN constitutive model, and the phenomenon that the volume fraction of microvoids nucleation increases with the increase of hydrogen concentration, and the initiation of nucleation and polymerization will be advanced is considered. The results show that with the increase of hydrogen concentration, the elongation and contraction at rupture decrease, and the position of the crack initiation point changes with the change of hydrogen concentration distribution. These results can reasonably explain and reproduce the process and phenomena observed in the experiment.
2022 Vol. 43 (1): 28-39 [
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167
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40
Investigation on Effective Elastic Properties of Honeycomb Materials Using Three-Dimensional Model
Effective elastic properties of conventional hexagonal honeycomb and the hexagonal honeycomb with circular joints are investigated by use of three-dimensional models. Average stresses in the representative volume element of the honeycomb materials are obtained by employing the finite element analysis combined with periodical boundary condition, and then all the effective elastic constants are derived from the average-field theory. Comparisons of the present conventional hexagonal honeycomb results with the existing results derived from beam model and test data show that three-dimensional model have higher accuracy compared with beam model, causes of errors of beam model are discussed. In-plane stiffness of the hexagonal honeycomb with circular joints can be improved dramatically by adjusting the ratio of the wall thickness of the circular joint to the honeycomb wall thickness under fixed relative density.
2022 Vol. 43 (1): 40-49 [
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153
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50
Thermoelastic Analysis of Periodic Cracks Originating from A Circular Hole in A Thermoelectric Material
In this paper，the problem of a circular hole with periodic radial cracks in thermoelectric material subjected to the uniform electric current and energy flux is studied. Considering the electrical and thermal impermeable boundary conditions，the exact solutions of electric current density，energy density and stress field in thermoelectric material are obtained by using complex function theory and conformal mapping method. According to the theory of fracture mechanics， the electric current，energy and stress intensity factors of periodic cracks are obtained by using Cauchy integral formula. The numerical results show that the intensity factors are proportional to the applied far-field loads，and these field intensity factors are closely related to the radius of the circular hole and the length of the crack. The results of this paper can promote the establishment of fracture mechanics theory system of thermoelectric material， and provide technical support for the reliability design and optimization of thermoelectric components.
2022 Vol. 43 (1): 50-59 [
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122
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60
Researches on boundary effects of film wrinkles sputtered on viscous substrates
Complex surface microstructures are ubiquitous in nature and endow biological tissues with superior physical and chemical performances. Inspired by nature, various bionic structures have been well designed and extensively applied in engineering fields such as micro-electro-mechanical system, optical devices, energy materials, biological templates and sensors. Self-organization of surface wrinkles in film-substrate systems driven by stresses is a facile and effective method to achieve various complex surface patterns. The wrinkle phenomena are typically observed in stiff films on elastic substrates when the imposed compression is beyond a critical value and they have been extensively investigated in the past decades. Recently, the surface instabilities of homogeneous stiff films on viscous substrates via stress relief have also received a great deal of attention. However, the instability modes and mechanical mechanisms of heterogeneous or structured stiff films on viscous substrates are still unclear up to now. In this work, metal iron films are deposited on viscous gel substrates by magnetron sputtering and spontaneous wrinkles of the films are investigated. Especially, the effects of constrained boundary and thickness-gradient boundary on wrinkle morphologies are elaborated. It is found that as the film thickness increases, the film surface evolves from networked folds to labyrinth-like wrinkles gradually, and the wrinkle wavelength decreases slightly first and then keeps constant. Near the constrained boundary, the wrinkle wavelength successively changes, exhibiting a hierarchical characteristic. Due to the boundary effect, the stress is anisotropic near the boundary and the wrinkles are always perpendicular to the boundary. In the thickness-gradient region, the wrinkles exhibit herringbone or wavy shapes along the gradient direction while the wavelength is almost unchanged. The morphological characteristics, evolutional behaviors and physical mechanisms of the wrinkles on viscous substrates are analyzed deeply based on the stress theory. This study could promote better understanding of the sputtering effect of metal films on viscous substrates and implement controllable surface microstructures by design of patterned structures.
2022 Vol. 43 (1): 60-70 [
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218
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71
Analysis on Added Mass of Fluid-Coupled Coaxial Cylindrical shells
Abstract：Based on the structural features, boundary conditions and fluid flow velocity of the narrow gap coaxial equipment supporting cylinders in the reactor vessel, the structure is simplified to a coaxial cylindrical shell with one fixed end and free end, and the fluid inside and outside the cylindrical shell is simplified to irrotational, non-viscous, incompressible fluid. Under seismic conditions, the coaxial cylindrical shell is coupled through the fluid pressure field, and its radial displacement mode determines the pressure field in the narrow gap fluid domain. Therefore, the radial orthogonal displacement modes of the cylindrical shell in the form of series are constructed to satisfy the boundary condition with one fixed end and the other free end, and the pressure field basis function of the wave equation is satisfied. From this basis functions, the additional mass theoretical formula of the shell-typevibration modes of a coaxial cylindrical shell with a fixed end and a free end is derived considering fluid-solid interraction. With the increase of the basis function series terms, the additional mass results calculated by this formula converge quickly. When the number of basis function terms is greater than 50, the additional mass calculation accuracy meets the requirements. When n=1 in this formula, namely, only the beam mode of the coaxial cylindrical shell is considered, the formula deduced in this paper degenerates to the beam-type vibration mode additional mass formula of the coaxial cylindrical shell deduced by Au-Yang M K [3]. In order to facilitate engineering applications, the additional mass coefficient is defined with reference to the Russian standard "Nuclear Power Plant Equipments and Pipes Strength Calculation Specification", and the additional mass results of the coaxial cylindrical shell derived in this paper are given in the form of a design curve. In order to verify the above theoretical formulas, this paper establishes a finite element model of coaxial cylinders containing narrow gap fluid domains with different height-to-diameter ratios, different gaps and different wall thicknesses, and the comparable calculation results are obtained with the theoretical results. The modal results of the finite element model show that when the height-to-diameter ratio of the cylindrical shell is less than or equal to 2.0, the frequency error of the main vibration mode is within 5%; at the same time, the author designed a modal experiment of a double-layer coaxial cylinder taking fluid-structure interraction into account. The frequency and vibration mode results of the modal test further verify the correctness of the additional mass theoretical formula derived in this paper.
2022 Vol. 43 (1): 71-80 [
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81
Crack Propagation Analysis of Short Fiber Reinforced Rubber Composites Based on Extended Finite Element Method
In order to study the crack propagation evolution law of short fiber reinforced rubber composites under tensile loading. The crack propagation of short fiber reinforced rubber composite with prefabricated cracks was simulated by using the extended finite element method. The random sequence adsorption algorithm was used to generate model in ABAQUS. The effects of failure criteria parameters (maximum allowable principal stress and crack surface energy) and short fiber meso-parameters (volume content, length and orientation angle) on crack propagation behavior were analyzed. And investigated the toughening mechanism of short fibers. The numerical results were in good agreement with the experimental results. The results show that reducing the damage threshold of substrate material, can realize the propagation of the crack without changing the crack propagation characteristics. The existence of short fiber reinforced phase will change the extension direction of crack, crack extension path brings complexity to improve material tear resistance. The tortuosity of crack propagation path increases with the increase of short fiber content, length and orientation angle, and the work consumed in the specimen breaking is increased, and the tear resistance is improved.
2022 Vol. 43 (1): 81-94 [
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160
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95
Effect of T-stress on Cubic Quasicrystals with Unequal Arm Cracks
The T-stress near the tips of a cruciform crack with unequal arms in the cubic quasicrystals is analyzed. The associated boundary value problems can transform to a system of singular integral equations by using the integral transformation technique. According to the stress difference method, T-stresses of phonon field and phason field can be expressed as the sum of an integral involving crack opening displacements and applied loading at infinity. Crack opening displacements play a leading role in determining T-stresses of phonon field and phason field. The role of the T-stresses in brittle fracture for cubic quasicrystals is emphasized. Moreover, the influence of the ratio of two crack-arm lengths b/a and the crack opening displacements on the T-stresses of a cruciform crack is examined.
2022 Vol. 43 (1): 95-110 [
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144
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111
Explicit Topology Optimization Design of Stainless Steel Metro Carbody
Due to the manufacturing requirements and the large scale of calculation efforts, the topology optimization at the car-level is one of the difficulties in metro car design. Oriented to the structure design of the stainless steel metro carbody, moving morphable components (MMC)-based explicit topology optimization approach is studied. According to the EN 12663 standard, the MMC-based topology optimization model is established. Besides an integral design process, an optimization design platform under explicit topology optimization framework is also established. Resorting to MMC, the loading path of the whole carbody can be obtained directly. The effectiveness of the proposed approach is illustrated through some numerical examples.
2022 Vol. 43 (1): 111-120 [
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196
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