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2019 Vol. 40, No. 4
Published: 2019-08-28
287
Plastic deformation mechanisms in magnesium alloys: A review
Pure magnesium has various microscopic plastic modes, especially twinning, which lead to a complex plastic deformation. The mechanical properties also show significant difference from those of the common FCC and BCC metals. However, due to the lack of sufficient understanding on the plastic mechanisms in dislocation slip and twinning deformation, the mechanical properties of magnesium alloys were not improved well. Compared with aluminum alloys, the mechanical properties of magnesium alloys still have much room for improvement. Based on this background, I first reviewed the development history and application status of magnesium alloys. Then I introduced the research progress in plastic mechanisms of dislocation slip and twinning deformation as well as computer simulation methods. Especially <c+a> dislocations, twinning, grain boundaries, precipitates, and solute atoms were highlighted. Finally, some research topics worthy of attention were proposed.
2019 Vol. 40 (4): 287-325 [
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326
An Experimental Investigation on the Temperature-Dependent Inelastic Behaviors of Filled Rubber
Filled rubbers have been widely used in various industrial applications, including tires, sealing materials, shock absorbers, artificial muscles and soft robotics. They exhibit complex inelastic behaviors, including a rate-dependent viscoelastic effect and a deformation-history-dependent Mullins effect. In the past, various works have been done to characterize the mechanical behaviors of filled rubber. However, most of these works were either performed at room temperature or only focused on one type of the inelastic behaviors. To overcome this limitation, in this work we performed a series of experiments to investigate the influence of temperature and strain rate on the Mullins effects and viscoelastic behaviors of a filled rubber Viton. To separate the coupled response between viscoelastic and the Mullins effects, the repeated loading-unloading tests were used to obtain the preconditioned specimens without the Mullins effect. Thus, in the same loading condition, the difference between the stress response of the virgin specimens and the preconditioned specimens is only contributed by the Mullins effect. The results show that the Mullins effect is independent on temperature and deformation rate. Uniaxial compression tests were also performed on the preconditioned specimens at different temperatures and strain rates. The results show that the viscoelastic response is significantly influenced by temperature and strain rate. At lower temperatures, an obvious strain hardening response can also be observed. The results of the stress relaxation tests demonstrate that the applied strain also affects the viscoelastic behaviors. More specifically, a large applied strain suppresses the stress relaxation of Viton. These results can advance our understanding of the inelastic behaviors in filled rubber.
2019 Vol. 40 (4): 326-333 [
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334
A model reduction method based on proper orthogonal decomposition for stiffened shells
Stiffened shells have been widely utilized in the fuel tank and in launch vehicles due to high specific stiffness and strength. Since these structures are subjected to complex dynamic loads, reliable prediction of natural vibration characteristics is essential in preventing excessive vibration levels, which may result in failure or very high noise levels. The natural frequency of a system is calculated via high fidelity model, such as using the finite element (FE) method, however, the detailed FE model has a very high computational cost. In order to obtain frequency of these complex structures accurately and efficiently, reduced order models (ROMs) are used as substitution to decrease the computational expense. This paper presents a model reduction method based on proper orthogonal decomposition technique (POD). The basis idea is to extract the principal component as the transformation matrix from the correlation matrix assembled by nodal displacement field of full order models (FOMs) subjected to different cross-section loads. The relationship between the ability of ROMs to predict mode shapes of interest and cross-section loads is investigated by a clamped-clamped stiffened shell, and the accuracy and efficiency of the proposed model reduction method are also validated by this example. Numerical results show that the maximum frequency error is about 1.01% and the computational time is only 0.03% of the associated FOM. Finally, a free-clamp stiffened shell subjected to harmonic external force is studied and the frequency response function is calculated through FOM and associated ROM. The number of degrees of freedom reduces from 48960 to 480 and average calculation time at each frequency point is only 0.04s for ROM and 4.65s for the associated FOM. And the displacement response in frequency range shows good agreement between the FOM and ROM.
2019 Vol. 40 (4): 334-341 [
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342
Liner Buckling and Collapse of Bi-material Metal Pipes under Combined Loadings
In this paper, the liner buckling and collapse of bi-material metal pipes subjected to combined loadings are studied systematically by using the finite element method. A 3-D finite element model is developed by considering the residual stress produced in the quasi-static inflation process. The effects of various parameters, such as the outer pipe diameter, the wall thickness of the liner pipe, the initial gap between the outer pipe and the liner pipe, the yield strength of the liner pipe, the internal pressure, and the friction between the outer pipe and the liner pipe on the liner buckling and collapse of bi-material metal pipes are analyzed in detail. It is found that the loading path, the geometric dimension and the yield strength of the liner pipe have significant influence on the buckling of bi-material metal pipes under combined loadings. Reducing the diameter of the outer steel pipe, the annular gap between the liner pipe and the outer steel pipe and the yield stress of the liner pipe can increase the curvature of liner buckling of the bi-material metal pipes. Increasing the wall thickness of the liner pipe and the internal pressure can delay the liner collapse. Thus the liner buckling and collapse of bi-material metal pipes subjected to combined loadings have become one of the important parts in the pipeline system design and the conclusions in this paper could be used to support the technical design and the safety assessment of the bi-material metal pipes.
2019 Vol. 40 (4): 342-353 [
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354
The recent progress in peridynamics and its numerical simulation in rock
The complex problem concerning the initiation and propagation of the crack in brittle materials is one of the most significant challenges in solid mechanics. As a common phenomenon, material failure may occur in the form of generation and growth of discontinuities such as cracks in engineering structures, resulting in great economic losses or even loss of life. At present, various numerical methods based on classical continuum mechanics are employed to model crack growth and material failure. However, the crack nucleation and singular crack-tip stress can hardly be solved due to the incapability of currently available continuum mechanics-based numerical methods to handle the discontinuous problems. Peridynmics (PD) is a nonlocal formulation based on integro-differential governing operators instead of the spatial derivative terms. The peridynamic theory is suited for modeling the non-continuity structural domains without any special treatment and can well describe the mechanical behaviors from continuity to non-continuity. In this paper, the current situation and the developing trend of peridynamics in material model and failure problems of rock and other materials are reviewed through introducing the theoretical basis of peridynamics. Numerical simulations of rock materials subjected to uniaxial tension, uniaxial compression, conventional triaxial compression and true triaxial compression are performed, respectively. The irregularity fracture of the rock slab with a circular hole under uniaxial tension loading condition is then analyzed by using the band-based peridynamics. Moreover, the uniaxial, conventional triaxial and true triaxial numerical tests under three-dimensional conditions are investigated based on state-based peridynamics. Subsequently, the fracture process of rock mass at the Mine-by tunnel in Canada is simulated. The present numerical results indicate that the peridynamic theory can be commendably applied in the non-continuity mechanical behaviors of rock. Finally, some current problems of peridyanmics in the rock fracture simulation and some worthy issues in further are discussed.
2019 Vol. 40 (4): 354-371 [
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372
FEM simulation of Dynamic Fragmentation of Liquid-driving Expanding Ring
The experimental platform of liquid-driving expanding ring can effectively realize the dynamic fracture of the ductile metal ring under the high strain rate tensile load. In this work, the finite element simulation of fluid–structure interaction method was used to simulate the metal ring’s movement, deformation and fracture under the action of high-pressure liquid. This paper analyzed the influence of the contact stress between the device and the specimen in the fragmentation process, and discussed how to realize the effective loading by hydraulic pressure to the specimen. The simulation results show that the initial contact stress and the longer liquid load have little effect on the fracture process of the specimen. By giving the proper loading condition, the liquid-driving expanding ring device can absolutely satisfy the one dimension stress condition and the free expanding of the ring.
2019 Vol. 40 (4): 372-380 [
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