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2017 Vol. 38, No. 3
Published: 2017-06-28
189
Friction of Two-dimensional Materials at the Nanoscale: Behavior and Mechanisms
Two-dimensional (2D) materials are materials consisting of a single or a few layers of atoms. Owing to their outstanding physical, mechanical and chemical properties, 2D materials bring new horizons to the development of atomically thin solid lubricants with exceptional performances. In addition, the unique and simple topological structure of 2D materials also render them ideal objects for exploring the origins/mechanisms of friction. In this paper, the friction and wear properties of 2D materials at the nanoscale are reviewed. According to the mode of relative motions, frictional behavior of both inter-layer sliding and surface sliding are presented and their underlying physical mechanisms are elaborated based on current understandings. We also discuss the factors that influence the frictional behavior of 2D materials and present a few typical strategies of friction tuning. In addition to friction, different mechanisms associated with nanoscale wear processes of 2D materials are examined and illustrated. Finally, we briefly summarize the progress and give an outlook in tribological study of 2D materials.
2017 Vol. 38 (3): 189-214 [
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215
High Mechanical Performance and Multi-functionalities of Plant-fiber-reinforced Composites
During the past decade, plant fibers have been thrust into the global spotlight as environment-friendly materials with attractive advantages of low cost, renewability and biodegradability, and have become promising alternatives to traditional synthetic fibers in making fiber-reinforced composites owing to their interesting mechanical and physical properties. However, the limited benefits of the mechanical properties of plant-fiber-reinforced composites (PFRCs) become the bottleneck for their large-scale industrial applications. As we all know, the mechanical performances of composite materials are largely dependent on their interfacial properties owing to the decisive role of the interface in composite structure design. The poor interfacial bonding between hydrophilic plant fibers and hydrophobic polymer matrices is one of the main reasons for the unsatisfactory mechanical properties of PFRCs. In this paper, the unique microstructure, chemical composition and mechanical properties of plant fibers were introduced, together with a review of the latest research progress in improving the interfacial mechanical properties of PFRCs by fiber surface modifications. The limitative effects of the reported improvements by ignoring the hierarchical structure of plant fibers were then analyzed and discussed. Furthermore, the distinct multi-layer and multi-scale microstructure characteristics of plant fibers were considered from the point of views of structural design and manufacturing of the composites. Hybrid technology and nano-modification techniques were employed to design and optimize the interfacial properties of PFRCs. The improved interfacial properties and high mechanical performances of PFRCs were achieved by fully taking advantages of their multi-layer and multi-scale interfacial failure behaviors and damage mechanisms. Based on this, the structural design principles focusing on the mechanical properties, flame-retardant properties and acoustic properties of PFRCs were proposed. In addition, the demonstration applications of the above fundamental research findings on the high mechanical performances and multi-functionality of PFRCs in aviation, railway transportation and automotive industries were introduced. Finally, some suggestions on future research were put forward for achieving the structural and functional integrated green eco-composite materials, so that the large-scale real applications of PFRCs in the fields of aerospace, railway transportation, automotive engineering, civil infrastructures, and so on could be fulfilled. At the same time, expansions of the theories on multi-scale mechanics of composite materials could be expected.
2017 Vol. 38 (3): 215-243 [
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440
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244
Cyclic Constitutive Model of Saturated Sand Based on the Bounding Surface Theory
Significant changes in fabric and accumulated plastic strain of saturated sand occur during the unloading-reloading process. Test results have shown that the plastic modulus in the first cycle is related to but different from that in the following loading cycles. To describe the deformation during the unloading-reloading process, a dilatancy internal variable accounting for the effect of fabric changes was introduced, and an of cyclic plastic modulus was proposed. A cyclic elasto-plastic boundary surface constitutive model of saturated sand was then established based on a previous monotonic model. The model is capable of simulating the cyclic stress-strain behavior of sand with different densities and confining pressures as well as under both drained and undrained conditions. The model has been validated against the cyclic test results of Ottaw sand, Fuji River sand and Toyoura sand with good agreement.
2017 Vol. 38 (3): 244-252 [
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421
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253
Dynamic Stability of a Stepped Drillstring System Subjected to Fluctuating Weight on Bit
The dynamic response of a drillstring that is limited in a wellbore is influenced by the bottom hole assembly (BHA), the drilling fluid inside and outside the drillstring, the drilling parameters, and so on. One of the major causes that lead to hole collapse and drilling tool damage is the severe drillstring vibrations in a state of dynamic instability. It is necessary to study the influences of various parameters, such as the structural parameters and drilling parameters, on the lateral vibrations of drillstring, and to take steps to improve its stability. In this paper, taking into account the big difference between drill pipe (DP) and drill collar (DC), the drillstring is modeled as a stepped pipe conveying drilling fluid that is pumped down to the bottom inside the drillstring and returns to the ground through the annulus. Meanwhile, the stabilizer is simplified as a linear spring. For the lateral vibrations of drillstring in a vertical well, an analytical model that considers the drillstring gravity, weight on bit (WOB) varying harmonically with time, arrangements of stabilizer, and hydrodynamic force and damping force of drilling fluid is established and discretized into the four order ordinary differential equations using the finite element method. The instable regions are determined by solving the critical frequency equation obtained using Bolotin’s method. The effects of WOB, length of drillstring, stabilizer location, and velocity and density of drilling fluid on the stability of drillstring system are studied. The results show that both the average value and the fluctuation amplitude of WOB can be the driving factors behind the system instability, and the stability of the system is not sensitive to the length of the drill pipe in tension. Furthermore, reducing the flow rate and density of drilling fluid, and shifting the stabilizer downward are all helpful in maintaining the stability of the system within the parameter ranges discussed in this paper.
2017 Vol. 38 (3): 253-262 [
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230
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263
The Influence of Crack Location on Stress Intensity Factor at Crack Tip Near the Bi-material Interface of Finite Size
With the application and development of composite materials, the interface structure of different materials has been paid more and more attention. The dissimilarity of material properties on two sides of the interfacial layer can cause singularity of the interface. The interface and the cracks around it can lead to stress singularity at crack tips. The mechanical analysis near the bimaterial interface is thus complicated. In this paper, the rectangular interface model of bimaterial was established, and the initial crack was designed near the interface of the material. The influence of finite material size on stress field and stress intensity factor was calculated. The Goursat formula in elasticity was used to obtain the stress field and the stress intensity factor of the rectangular interface of finite size. The stress intensity factor at the crack tip near the rectangular interface was further obtained by the superposition principle and the Green function method. The results show that the stress intensity factor at the crack tip near the interface decreases with the increase of the distance between the crack and the interface, and gradually becomes stable. The results can provide a reference for predicting the failure position of bimaterial interface.
2017 Vol. 38 (3): 263-270 [
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522
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271
Modelling and Solution on Vibration Characteristics of Ring-stiffened Cylindrical Shell with Arbitrary Boundary Conditions
The vibration of a ring-stiffened cylindrical shell is an important technical issue in engineering applications, such as pressure vessels, rockets and submerged marine structures. The presence of structural discontinuities and arbitrary boundary conditions does not permit an analytical solution, so we have to resort to numerical approaches to address the problem. Two approaches have been developed to determine the dynamic behavior of ring-stiffened cylindrical shell. One approach, called the smeared approach, assumes that the stiffeners are close together with equal spacing and are evaluated by averaging their properties over the surface of the shell. For a more general model, the ring stiffeners have to be treated as discrete elements, and many methods have been proposed by researchers. The existing literature was restricted to the calculation of vibration characteristics of ring-stiffened cylindrical shell with only a few classical boundary conditions, such as the free, simply supported and clamped boundary conditions. With the changes of boundary conditions, the displacement functions and boundary parameters should be changed, which means more functions and programs should be built. The main objective of this paper was to develop an alternative and unified solution for the vibration analysis of ring-stiffened cylindrical shell with arbitrary elastic boundary conditions. In this paper, an improved Fourier series was introduced as an admissible displacement function. Based on the energy method, the dynamic model of ring-stiffened cylindrical shell with arbitrary boundary conditions was constructed while the stiffeners were treated as discrete elements. The Rayleigh-Ritz technique was used to solve the Lagrange’s function of the structure, and the vibration modes and frequency response characteristics were obtained. The accuracy of the present method was validated by comparing the results with those from the modal experiment and calculated using the finite element method (FEM). In addition, the effects of the parameters of stiffener eccentricity, cross section dimension, position distribution and spring stiffness on the vibration characteristics of ring-stiffened cylindrical shell were studied. The present method for the ring-stiffened cylindrical shell should not only be useful in solving the arbitrary boundary conditions, but also serve as a reference source for future researchers.
2017 Vol. 38 (3): 271-280 [
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287
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281
Numerical Simulation for Three-point Bending Fractures of Asphalt Mixture Based on Extended Finite Element Method
In this paper, regular octagon coarse aggregates with AC-16 gradation and sizes greater than 2.36 mm were generated by utilizing the random aggregate generation and packing algorithm, and fine aggregates were combined with asphalt in the matrix. As a result, some two-dimensional heterogeneous asphalt mixture beam models with two phases, namely coarse aggregates and matrix, were built. The enrichment functions were added into the standard finite element approximation in the extended finite element method so that fracture could step across the continuous finite elements. With the aid of ABAQUS software, the extended finite element simulations of three-point bending fracture were performed on the beam models with prenotches at the midspan, the 20 mm offset and the 40 mm offset, respectively. The bilinear damage evolution model and the maximum tensile stress criterion were employed to simulate micro-crack initiation and evolution as well as macro-fracture onset. Three groups of experiments were carried out, and some model parameters were obtained by the comparison between the simulations and the experimental results. The influences of prenotch location on the crack path and the load vs. displacement curve were analyzed. It was shown that the farther is the prenotch away from the midspan, the larger is the deflection angle between the crack propagation and the extension line of prenotch. It was also found that a specimen with a prenotch farther away from the midspan has a greater peak load, which means that a pernotch farther away from the midspan weakens the carrying capacity of asphalt mixture beam more gently. The qualitative consistency between experiments and simulations indicated that the extended finite element method is suitable for simulating the fracture behavior of asphalt mixture.
2017 Vol. 38 (3): 281-286 [
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371
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