Papers by Keyword: Size Effect

Abstract: The continuous enhancement of reliability and durability requirements for many machinery components is significantly pushing the experimental research on the Very-High-Cycle Fatigue (VHCF) response of metallic materials. In order to significantly reduce testing time, ultrasonic testing machines are widely adopted when carrying out VHCF tests. Several fundamental material properties can be estimated from the fracture surfaces of specimens failed during ultrasonic VHCF tests. In the VHCF literature the critical Stress Intensity Factor (SIF) is generally estimated by applying analytical SIF formulations to the typical semi-circular surface crack geometry revealed by fracture surfaces at final failure. However, when subjected to ultrasonic VHCF tests, analytical SIF formulations valid for static loading conditions could eventually lead to significant estimation errors. The present paper aims at comparing the critical SIF at failure estimated from conventional analytical formulations and from Finite Element Models (FEMs). The fracture surfaces of two specimens with different shapes (Hourglass and Gaussian) are taken into account for modeling crack geometry at final failure. Through an implicit solving procedure, the critical SIF in resonance condition at 20 kHz is computed from the 3D geometry of the cracked specimens and compared with the corresponding analytical prediction.

Abstract: The work presented is a part of the french ANR (Agence Nationale pour la Recherche) project MACENA (Maitrise du Confinement en Accident), its main objective is to better present the role of concrete heterogeneities in RC structures in the cracking process. This paper aims to develop and use the size effect method (WL2) applicable to RC structures proposed by Sellier and Millard 2014 [1]. The originality of the method lies on introducing a weighting function defined in the direction of the maximum principal stress using a scale length. In this work, an inverse analysis of the method allows to identify this scale length using experimental test series of concrete specimens under tensile load and 3 point bending beams. The approach is then applied to predict the sensitivity of the mechanical behavior of a reinforced concrete tie under tensile load. The method is applied in the elastic phase and allows providing the structural tensile strength corresponding to the first crack which is affected by size effect and plays a key role because cracked and uncracked structures behave in severe environment in a very different way. In FE model, correlated random fields on the tensile strength of the concrete can be generated using the identified scale length to characterize the autocorrelation length.

Abstract: This study aims at improving the analysis of drying tests and taking better advantage of the measurements, by studying the influence of the specimen size. One self-consolidating concrete (SCC) was studied during three years. Drying started 24 hours after casting. The tests were carried on three sizes of cylinders: Φ78mm, Φ113mm and Φ163mm, under the same experimental conditions (20±1°C, 50±5% relative humidity). Total and autogenous shrinkages and mass-loss evolutions were monitored according to RILEM recommendations [1]. Assuming that concrete is a homogeneous and non-aging material, a simple model can be written and used to highlight size effects observed in real concrete. By changing the time variable to the ratio of the square root of the time to the notional size of cross-section, the drying curves of different specimen sizes theoretically follow a master curve [2]. However, experimental curves showed upshifts, thus size effect can be highlighted. Phenomena that occur at early age are actually not taken into account by diffusion equation. This paper introduces a new concept called drying depth as a layer where water loss occurs during the first days without resulting in shrinkage at the specimen level. This depth did not depend of the specimen size. However the relative volume of this layer increases as the size of the specimen decreases, which explains the size effect experimentally observed. The hydric properties of this zone can be assumed to be different from the inner concrete, with a higher porosity and higher permeability. Several phenomena could explain this effect, like the initial particle size distribution with a higher paste proportion near the surface due to the wall effect during casting [3] [4] [5]. Because of early drying hydration stops and this creates a coarser pores in the external layer. Finally, the micro-cracking at the surface induced by the differential shrinkage can also influence the hydric parameters.

Abstract: There is stress and friction size effect in meso scale deep drawing. Lots of difference appeared in mould producing and parts holding, fixing and conveying against in the macro scale. Therefore, many researches of deep drawing processes are hard to carry out. In this paper, the meso scale deep drawing mould was developed and the experiments were carried out, the height, destroyed method and thickness of the drawing tube were observed and tested. According to the analysis, the methods and factors which influenced the height, thickness of deep drawing were described.

Abstract: Micro forming is a promising technology and has drawn global attentions due to the urgent requirements on miniaturised products. Micro hydro deep drawing (MHDD) is a typical micro forming method and its products are widely applied in various fields, such as micro electro-mechanical systems (MEMS), medical and aerospace areas. However, tiny sizes of both products and tools are obstacles to investigate this novel technology. Therefore, finite element method (FEM) is adopted and modified to study the influence of processing parameters on micro drawing process and produced products. In this study, a normal hydro deep drawing model was developed, and then a voronoi blank model was generated to consider the size effects of materials. Additionally, a surface layer model was created based on the voronoi blank model. Therefore, the open and closed lubricant theory can be fulfilled and friction behaviour in MHDD can be considered. Subsequently, different processing parameters, such as hydraulic pressure and material inhomogeneity, were investigated. Simulation results indicate that a proper hydraulic pressure improves drawability and shape accuracy of the drawn cups. Moreover, material inhomogeneity affects the quality of drawn cups.

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.

Abstract: At present, design values in codes and regulations are mainly based on test results of small size specimens, which are different from large-scale members used in practical engineering, therefore size adjustment coefficients are needed to be established. The four-point bending test method was adopted to investigate four groups of different sizes of Larch Glulam beams in their flexural behavior. Experiment data such as ultimate bearing capacity, deflection, strains and others are obtained, and the failure pattern and failure mechanism of bending members are analyzed. The research results indicate that the bending modulus of elasticity of Larch Glulam beam is not affected by the size. Bending strength of the Larch Glulam beam show a declining trend as the size of specimens increases, however, the ultimate bending moment increases. In addition, by means of a two-parameter Weibull model, a so-called size effect coefficient has been calculated by the slope method, thus providing a basis for the design and application of Larch Glulam beams.

Abstract: An expandable thermal insulation material was prepared which can expand the volume at high temperature. The paper is aimed to study the effect of the particle size of expandable graphite for this material, through comparing properties including expansion rate, compression strength and heat conductivity coefficient. The material with 200 mesh has the lowest expansion rate, but compression strength is the maximum 0.901MPa at 800°C while the lowest heat conductivity coefficients 0.118W/m·k at 1100°C which has a 25% reduction in the 50 mesh.

Abstract: In this study, we investigated the cytocompatibility of ceramic nanoparticles on different types of cells. All ceramics nanoparticles investigated in this study except Copper oxide (CuO) exhibited good cytocompatibility and cell viability (90% or more) even at 20 ppm concentration. In contrast, CuO nanoparticles caused cell inflammation, and their effect depended on their particle size. Confocal fluorescence microscopy measurements indicated that some particles had penetrated into the cells. These results indicate that except CuO nanoparticles, all other ceramic nanoparticles reported herein exhibited excellent cytocompatibility even for lung epithelial cells.

Abstract: This paper presents an experimental study of cutting forces during micro end-milling of brass. The influences of cutting speed and feed per tooth on cutting forces have been researched. The results show that the resultant force F_r and feed force F_x significantly increase with increasing the feed per tooth. The resultant force F_r, feed force F_x and normal force F_y increase with increasing cutting speed. The specific shear energy is also investigated. It is observed that the specific shear energy increases greatly with decreasing the feed per tooth when the feed per tooth is less than minimum chip thickness.