Papers by Keyword: Grain Size Distribution

Abstract: Crystal plasticity finite element (CPFE) simulations of three-dimensional representative volume elements (RVEs) enable the prediction of polycrystalline material behavior under complex loading conditions. Plastic deformation is modeled through crystallographic slip on lattice slip systems, subject to the Schmid yield criterion based on the maximum resolved shear stress (CRSS). In this work, an efficient rate-independent crystal plasticity (CP) and gradient enhanced crystal plasticity (GECP) formulation is used to investigate the influence of microstructural characteristics on the mechanical performance of FCC materials. Three-dimensional periodic RVEs with irregular grain morphologies are simulated within Abaqus/Standard to study the effects of grain size, grain size distribution, and grain shape under uniaxial loading. Comparative analyses between the CP and GECP frameworks are performed to assess the predictive capabilities and applicability for increasingly heterogeneous microstructures. The results demonstrate that GECP accurately captures grain size dependent work hardening and grain size distribution effects through the intrinsic length scale introduced by strain gradient calculations. In contrast, grain shape variations result in only minor changes in the macroscopic response for both frameworks.

Abstract: Earth-fill dams are commonly constructed by composing different geomaterials to optimally utilize local natural resources. In Indonesia, random fill materials are frequently used as a major composition in dam construction. The term random fill material originated from its broad range of grain size. Grain size distribution influences shear strength characteristics of geomaterials. There are 2 shear strength equations to model the behavior of fill material, i.e., linear Mohr-Coulomb and non-linear power curve. Two series of large scale in situ direct shear tests were performed at Keureuto Dam, Indonesia. Sieve analysis tests were performed accordingly. The random fill material was composed of cobbles, gravel, and less than 25% of sand. The stress-displacement characteristics of random fill material indicated that plastic deformation occurred at shear strain of 1% to 4%. The shear failure was reached in shear displacements of 60 – 90 mm, equivalent to shear strain of 8% – 12 %. Stress-strain relationships showed a dilative behavior indicating the random fill was in a relatively dense form. The dilatancy tends to decrease as the normal stress increases. The linear Mohr-Coulomb failure criteria and non-linear power curve equation are suggested to characterize the shear strength of the random fill material. To obtain a realistic value of the non-cohesive strength parameter of granular material, the Mohr-Coulomb approach should be intercepted at zero. A relationship between secant friction angles for different normal stresses is presented. This angle tends to decrease at higher normal stresses.

Abstract: Fine powders of strontium hexaferrite are widely used in powder metallurgy for the production of permanent magnets resistant to atmospheric oxygen and high working temperatures. Obtaining powders with predefined technological characteristics in minimal time and with minimal energy consumption is an actual problem of powder metallurgy. The paper provides the results of experimental studies of technological characteristics of strontium hexaferrite powder (SrFe₁₂O₁₉) during milling in a beater mill. Mechanical milling of coarse strontium hexaferrite was carried out in the mill with the system of rotating beaters for 120 minutes without and with the creation of a pseudo fluidized bed. The fluidization was formed by a perpendicular constant and alternating magnetic field with induction gradients of 150 and 210 mT/m. Average particle size and powder bulk density dependencies from milling time were studied. Experimental data show that milling with the formation of a magneto fluidized bed allows intensifying the process. Beginning from 70 minutes, the dependencies of average particle size and bulk density come to almost asymptotic behavior making further milling rather ineffective. Carried out research allows choosing optimal milling duration for obtaining the required average particle size.

Abstract: The evolution of microstructural features such as local grain size and local grain size distribution are essential in view of the final physical and mechanical properties of the nickel base alloy 718 for aircraft parts forged in a multi-step production route. Due to increasing standards and the need of the prediction of fracture mechanical properties, a multi-class grain size model for a more detailed microstructure prediction is necessary. Therefore, a multi-class model considers the real initial non-uniform grain size distribution and structure of the pre-material at the beginning of the forging process, which affects the evolution of grain sizes during thermo-mechanical treatment and leads to different results than commonly used uniform grain structures. The initial distribution is defined by grain classes according the ASTM standard. It is shown that the presence of different classes and distributions of grains are as import as the applied strain, strain rate and temperature on dynamic, meta-dynamic and static recrystallization. Additionally, dissolution processes of delta phase and grain growth kinetics are included in the model to properly indicate the recrystallized fractions and represent the resulting multi-class microstructure. A series of simulations with different initial distributions is discussed and compared with examined forged samples in terms of the resulting microstructure for typical forging parameters. Based on these results the microstructure model can be used in combination with collected process data to predict the resulting properties and for the design of new aircraft parts.

Abstract: Copper-Nickel alloy pipes in marine engineering have been suffering severe seawater corrosion and erosion-corrosion. In this work, six kinds of Cu-Ni alloy pipes with different service lives delivered by two manufacturers were used to clarify the relationship between corrosion resistance and microstructure. The corrosion behaviors of the samples in 3.5 wt.% NaCl solution were studied by electrochemical measurements. Chemical composition, grain size distribution, crystallographic orientation, and grain boundary characterization distribution (GBCD) were investigated by energy-dispersive spectrometry (EDS), metallography and electron backscattered diffraction (EBSD) technology. There were no obvious differences in chemical composition and GBCD in contrast with size and uniformity of grains. Pipes with large grains and a broader grain size distribution had better corrosion resistance. It was also found that the accuracy of experimental data greatly depended on the quality of the sample surface in EBSD analysis. The scratches and contamination during sample preparation have a strong impact on the imaging quality and the calculation of GBCD.

Abstract: The purpose of this work was to produce a mix design rarely reported in the literature on wood-cement composites, i.e., 1:1:0.5 (cement:wood:water). This design was achieved by using CP IV-32 RS cement and adding 4% of ferric chloride additive plus fibers of the species Simarouba amara (paparaúba) retained on ASTM 7 and ASTM 10 sieves. Before mixing the fibers into the cement paste, they were presoaked for 15 minutes to between 84% to 136% of their mass. In these conditions, all the heat of hydration tests presented low inhibition. Panels measuring 300 x 450 x 25 mm were prepared, and the values of compressive and flexural strength of the composite after 28 days showed no significant differences with respect to soak, but the effect of particle size was significant. These results indicate that more fiber can be added in the mix design of this cement-wood composite without changing its mechanical strength, controlling only the presoak conditions.

Abstract: A mean field model for discontinuous dynamic recrystallization (DDRX) has been developed and chained with a post-dynamic recrystallization (PDRX) model to predict transient and steady-state flow stresses and average grain sizes. Numerical results are compared with experimental data obtained on a 304L stainless steel yielding to a good agreement in terms of average grain size. However an unrealistic grain-size distribution is observed using DDRX, which affects results of the PDRX model. This result is discussed with respect to the fundamental equations of DDRX.

Abstract: This study presents the numerical simulation of a true triaxial test by means of the discrete element method (DEM). Experimental results performed on Toyoura sand are employed as reference and the calibration methodology is explained. Physical aspects of the real soil, such as the grain size distribution and the relative density, are considered during the generation of the virtual sample. It is shown that the main aspects of the macro-mechanical behaviour of granular soils during compression loading can be fairly represented by the idealised simulations with particles.

Abstract: Large landslides usually dam lakes that may easily fail and then result in catastrophic flood threatening the population downstream. The grain size composition within landslide dams is considered as a vital factor that impacts the timing of failure and the resulting magnitude of flood. Therefore, experiments were conducted to investigate the features of size distribution in accumulation bodies. The results indicate that the grain size distribution varies in different parts of the deposits. The inverse grading phenomenon can be observed. Grains in the front of the deposit are coarser, while grains in the rear are finer. For the front of the deposit, the grains in the middle are finer than the other two sides. The grain size segregation is believed to be the governing mechanism that contributes to this special distribution. The speculations of the grain size distribution inside the landslide dam are presented.

Abstract: The compressive strength of cement stabilized macadam base is an important index to evaluate the quality of road construction. In this paper, investigation and field sampling were conducted on roads in Shenyang city, and, indoor experiment study was then adopted to analyze effects of particle grade, cement dosage, molding method, compaction and curing age on compressive strength. The results show that rational particle grade greatly enhances the strength. Strength of vibration molding sample is closer to the field sample strength and is 0.6 times bigger than that of static pressure molding sample. 1% increment of construction compaction degree will induce 10% increment of compressive strength. Increment of cement dosage and curing age will induce increment of compressive strength, but the effect is limited. Studied in paper can provide a theoretical basis for site construction in Shenyang area.