Papers by Keyword: Size Effect

Abstract: This paper highlights the importance of strength correction factors for a correct assessment of concrete compressive strength as a destructive test is performed. Full-scale reinforced concrete columns were prepared and the experiment consists of the extraction of cores (core diameter of 64 mm, 79 mm, and 103 mm) to carry out destructive test. The results show that the assessment of the compressive strength of concrete depends strongly on the aspect ratio and the diameter. Furthermore, the comparison between the core compressive strengths of samples with an aspect ratio of one and two could be considered as non-conventional. In addition, a likely presence of a size effect contributes to complicate the assessment of the actual compressive strength of concrete. A need of codes unification account for correction factors to interpret the equivalent compressive strength of concrete has been discussed.

Abstract: : In this paper, activation procedures under size effects of some gold nanoparticles (Au₁₀₁, Au_naked and Au_citrate) and nanoclusters (Au₈ and Au₉) immobilized on powder Norit^® activated carbon (abbreviated to AC) and/or Vulcan carbon (abbreviated to VC) on the catalytic activity of gold nanocatalysts were studied. The gold nanostructures were activated through the washing procedure with a base in MilliQ water or hot toluene and then followed by heating in static air (abbreviated to W+S) or under vacuum (abbreviated to W+V) at 100 °C for 3 h. The highest activity of gold nanocatalysts for benzyl alcohol oxidation was obtained for activated (W+V) ‘naked’ gold nanoparticles immobilized on Norit^® activated carbon when the gold nanoparticle diameters was ~4.4 nm.

Abstract: Pairwise effective potentials in first seventeen shells of the Ni-22.5at.%Fe alloy are calculated using model potential method with account of the linear size effect. Using obtained values of pairwise effective potentials, the short range order parameters on the first seventeen shells of alloy are calculated by Krivoglaz-Clapp-Moss method. The calculated values ​​of the short-range order parameters were fitted to the experimental values ​​by varying the parameters of static atomic displacements. Reliable value of critical temperature of order-disorder phase transition in Ni-22.5at.%Fe alloy was calculated using obtained meanings of pairwise effective potentials.

Abstract: The properties of individual grains affect the mechanical behaviors and response of materials in micro-scaled deformation, viz., microforming, and there are unknown phenomena and deformation behaviors existing and limiting the wide application of microforming due to size effect. In this paper, a composite model combining crystal plasticity and grain boundary strengthening theories was developed for numerical investigation into the effect of grain boundaries on the plastic deformation of copper micro-upsetting. By comparing the results with and without grain-boundary structure, it is revealed that grain boundaries, which act as the barriers of crystal slip, result in the enhanced flow stress and the discontinuous distribution of stress and strain. The grain size effect is also considered in this research, and the results show the coarse-grained material reduces the flow stress and enhances the inhomogeneous deformation.

Abstract: In order to obtain the critical temperatures of energetic materials thermal reaction in different scales, a multi-scale thermal reaction test system has been developed. In this experiment, the measurement method of thermal diffusivity of energetic materials under test conditions is added. Based on the measured thermal diffusivity in thermal reaction test, activation energy and pre exponential factor measured in laboratory. The critical temperatures of thermal reaction in different scales of DINA are calculated, the calculation results are verified by two different scale thermal reaction tests, and the test results are in good agreement with the calculation results. According to the above calculation method, the critical temperature of thermal reaction of DINA under actual process conditions is obtained; it provides the basis of safety data for enterprise safety production.

Abstract: In this paper, a micromechanical finite element (FE) model has been proposed to investigate the effect of the nanoscale precipitates on the development of microplasticity for Inconel 718 (IN718) superalloy. A strain gradient crystal plasticity formulation has been developed with the considerations of the evolution of statistically stored dislocation density and geometrically necessary dislocation density. The mesh convergence has been examined, showing that sufficiently fine mesh is required in the FE model. The results show that the model with strain gradient effect incorporated shows less peak plastic strain and higher value of dislocation density than the model with no strain gradient effect. The present study indicates that the strain hardening process at the scale of strengthening precipitate is mainly governed by the evolution of geometrically necessary dislocation densities.

Abstract: Pure titanium TA1 foil with a thickness of 0.05mm under different grain sizes were carried out by the DT-C539 micro-stamping machine in the laboratory. The size effect of the pure titanium TA1 foil with grain sizes of 3, 7, 9 and 23 microns respectively on surface morphology of the microstamping sample were studied. It is found that the stamping samples with good surface quality can be obtained on the condition that the grain size is 23 microns and the stamping speed is 1mm/s. VORONOI model was established by using ABAQUS, NEPER and MATLAB software. Heterogeneous finite element simulation was carried out for the micro-stamping process under the same conditions. The results showed that the simulation results were more consistent with the experimental results.

Abstract: The size effect of metallic materials is one of the important factors for understanding characteristics of material. The higher-order gradient crystal plasticity is a powerful model for describing the size effect. However, it is known that the finite element method sometimes provides an improper solution. In this study, we analyze the higher-order gradient crystal plasticity model using a meshfree method, and a nodal integration scheme is introduced to improve the analysis accuracy. The effectiveness and stability of the meshfree method for the higher-order gradient crystal plasticity model are quantitatively discussed.

Abstract: Based on robust numerical formulations and various material models, finite element (FE) analysis becomes a powerful tool in conventional sheet metal forming process. Unfortunately, the present constitutive equations irrelevant to thickness that describe well conventional sheet deformation modes have difficulties being applied directly to ultra-thin sheet deformation modes. In the present study, a constitutive equation considering size effect is established by introducing a scale factor that represents size effects through thickness and width directions. Uniaxial tensile tests were used to evaluate the scale factor of different thicknesses together with the parameter identification. The developed constitutive equation reveals that thickness is the most important factor effecting on the constitutive relation of ultra-thin sheet. 2D draw forming process of C7035 ultra-thin sheet is analyzed using JSTAMP/NV introducing the developed constitutive equation. The analysis results show that there are obvious differences in the punch forces and loading geometries according to the size effect through thickness direction. Specimen width has slight effect on the flow stress although specimen thickness has strong effect on the flow stress. It is expected that the proposed constitutive equation gives good applicability to FE analysis of micro-scale forming.

Abstract: In this study, target size effects in the low energy impact response of plain CFRP plateswere investigated. It was found that increase the target size leads to a reduction in the maximumimpact force recorded during the test. This is due to the reduction on flexural rigidity of the largerpanels. The experimental results indicated that at energies above the first failure threshold, themaximum impact force does not coincidence with the predicting value. Two mathematical modelswere used to predict the maximum impact force including single degree of freedom (SDOF)spring-mass model and Energy-Balance (E-B) model. The predicting results were then comparedwith the experimental results, and both of the two models show good agreement with theexperimental results in elastic deformation region. In addition, the level of agreement between thepredictions and the experimental results indicate that both models are capable of modelling theimpact response of these CFRP panels at elastic regime.