Papers by Keyword: Size Effect

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Authors: Chong Yang Gao, W.R. Lu
Abstract: In this paper, a constitutive description of the true stress-strain behaviors of nano-twinned metals has been proposed. The size effects of nano-scale twin boundaries (TBs) and ultra-fine grain boundaries (GBs) are considered in the athermal stress. The evolution of the dislocation density with strain under the influence of strain rate and temperature is introduced in the thermal stress based on our previous meso-scale constitutive model. The new model can effectively describe the strength transition regime in nano-twinned metals. The proposed model’s predictions of true stress-strain relation curves for nano-twinned copper are compared with the experimental results of uniaxial tension tests for validation. The comparisons show that the previous models in literature for the dependence of initial yield strength on twin spacing cannot describe the experimental data correctly when the twin spacing tends to zero; however, the phenomenological model proposed in this paper for the twin spacing depending relation is theoretically rational and can well describe the experimental data in the whole range of twin spacing.
Authors: Arash Mir, Giang Dinh Nguyen, Abdul H. Sheikh
Abstract: Failure in quasi-brittle materials, such as concrete and rock, usually develops in a fracture process zone (FPZ), in which dissipative processes takes place. At the onset of bifurcation or upon formation of FPZ the homogeneity of kinematic fields is lost and the stress field is redistributed which gives rise to the so called deterministic size effect problem. The total strain energy stored within a specimen of quasi-brittle materials will scale with its size; however, the amount of dissipated energy does not depend on the specimen size but only on the width of the FPZ. This width is related to the microstructure of the material and is considered a characteristic of the material. In this paper, a cohesive frictional interface is used to model the dissipative behaviour of material inside FPZ. Fundamental micro-mechanisms of energy dissipation such as micro-crack opening in mode I and frictional sliding between micro-crack surfaces are formulated within the frame work of Thermodynamic with internal variables (TIV) to ensure the thermodynamics admissibility of the model. The link between the material behaviour inside and outside FPZ is given through the continuity of tractions along the boundaries of FPZ. It is shown that although the shape of the post-peak stress-strain varies, for specimens of different slenderness, the amount of dissipated energy remains the same in all cases.
Authors: D.M. Hunter, R.I. Grynszpan
Authors: Wei Zheng, Guang Chun Wang, Liang Chen, Bing Tao Tang
Abstract: In the paper, a region model considering grain shape, orientation and boundary is built based on Hall-Petch Equation. The constitute relationships of materials in grain region and grain-boundary region are deduced by introducing the scale parameter. The proposed model and constitutive relationships are verified by the experimental data. Using the region model, the micro-upsetting processes are simulated. The researches in the paper can be used to guide the other micro-forming processes which are more complex.
Authors: Zi Yang Cao, Ning He, Liang Li
Abstract: In order to investigate the effects of cutting edge radius on micro/meso-scale cutting process, the current paper is concerned with a fundamental investigation of the contribution of cutting edge radius to cutting temperature, stress field and size effect by means of two-dimensional finite-element simulation for orthogonal cutting process. The results indicated that cutting edge radius has remarkable effects on cutting temperature and stress field, and the existence of cutting edge radius is one of the main reasons generating size effect. The cutting edge radius affects the micro/meso scale cutting process at smaller uncut chip thickness by altering the effective rake angle and enhancing the plowing effect, affecting the material deformation process, expanding and widening the plastic deformation zone, and causing higher energy dissipation due to increased tool-chip contact length.
Authors: François Ducobu, Edouard Rivière-Lorphèvre, Enrico Filippi
Abstract: The foundations of micro-milling are similar to macro-milling but the phenomena it involves are not a simple scaling-down of macro-cutting. The importance of the minimum chip thickness is one of the significant differences between the two processes. The lagrangian FEM model presented in this paper aims to study the depth of cut influence on chip formation of Ti6Al4V in orthogonal cutting. It is firstly used to compare the modelled saw-toothed macro-chip morphology and cutting forces to experimental cutting results from literature. Then a minimum chip thickness prediction is performed by decreasing the depth of cut. Finally this study is the opportunity to highlight the specific features of micro-cutting reported in literature, such as the effective negative rake angle of the tool or the size effect. The model presented brings therefore a numerical contribution to the comprehension of these phenomena.
Authors: Jun Liang Yuan, Jin Gen Deng, Qiang Tan, Lian Bo Hu
Abstract: With the ever increasing energy demands, unconventional gas becomes the new force in energy field. However, shale gas horizontal drillings are prone to instability problems because of the brittleness of gas shale. The casing programs of shale gas wells in China and North America demonstrate that the more drilling problems are encountered with the larger borehole sizes. In the thrust faults, based on vutukuri's size effect theory and Mclamore's failure criteria, a new prediction model of collapse pressure in transversely isotropic rock mass is proposed to analyze the critical mud weight of micro-hole in gas shale formations. The trajectory sensitivity analysis is also carried out. The results show that micro-hole drilling technique in thrust faults can significantly reduce the collapse pressure of directional wells by 12% approximately.
Authors: Jian Sheng Zhang, Yue Wu, Di Wu
Abstract: A number of methods for assessing wind-induced vibration of structures are available ranging from simplified procedure using quasi-static methods to the detailed procedure using statistical methods. The appropriate procedure should be selected in accordance with wind sensitivity of structures. However, it still remains unresolved concerning how to provide a universal criterion with physically meaningful and convenient for the concept of the wind sensitivity until now. In order to solve the previous problem of how to distinguish between those structures for which the wind effects can be treated by simplified procedure, and those for which the wind effects must be treated by detailed procedure, a concept of sensitivity for wind-resistance is presented in this paper. The essential idea of this theory is to provide a general expression of wind sensitivity of structures by synthesizing three factors between wind load and the structure, including the size-effect factor, frequency-effect factor and mode-effect factor, which are based on the analytical derivation and take duly into account the influence of all the significant parameters for the response. Based on that, two case studies of cantilevered roof and single-layer reticulated shell structures under wind actions are demonstrated as illustrative examples.
Authors: Lu Ming Shen, Zhen Chen
Abstract: To better understand the responses of ultrananocrystalline diamond (UNCD) under extreme working conditions, a numerical study is performed to investigate the size, loading rate and thermal effects on the material properties of UNCD films. A combined kinetic Monte Carlo (KMC) and molecular dynamics (MD) method is first applied to simulate the growth of polycrystalline UNCD films. The responses of the resulting UNCD films with various grain sizes are then investigated by applying displacement–controlled tensile loading with different rates and temperatures in the MD simulations. The preliminary results presented in this paper provide a better understanding of the combined size, rate and thermal effects on the material properties of UNCD.
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