Applied Mechanics and Materials Vols. 764-765

Abstract: The Interfacial Transition Zone (ITZ) between two basic materials having differentiations in their mechanical properties has always been intriguing. The stiffness disparities between the two will result in a very distinctive area, the interface. Cement based components such as mortar and concrete consist of the cement paste and aggregates, with the ITZ at the perimeter. When compared to the cement paste, this ITZ has a higher porosity with a dissimilar crystal formation. The resulting area therefore becomes the weak link in concrete. A Finite Element Model (FEM) was developed to construct the load-displacement behavior of a single inclusion specimen and to study the crack propagation within the ITZ. The ITZ was modeled as a linkage element having a double spring, perpendicular and parallel to the ITZ surface. The individual stiffness behavior of these springs was obtained from laboratory-tested specimens. Non-linearity was generated by evaluating the principal stresses and strains at Gauss points, while the CEB-FIB 2010 code was used for the constitutive material behavior of the mortar. Iteration is conducted by the arc-length method developed by Riks-Wempners. The load-displacement curves resulting from the FEM were validated with laboratory tested specimens to compare its effectiveness and assess the correctness of the model.

Abstract: Carbon nanotube(CNT) possesses excellent electrical, mechanical and thermal properties. Therefore, it has been applied in a variety of fields. In this study, characteristics of composite were investigated after adding CNT into copper, and validity of the extrusion method was reviewed. The CNT and Cu powder are deconcentrated by planetary ball mill. And ratio of CNT is 1 wt%, 5 wt% and 10 wt%, respectively. The ball mill is wet condition with ethanol(20ml) and ZrO₂ ball(90g). The composite powder was put in a billet (∅= 50mm, length= 100mm) by uniaxial press. Then, after billet heat for 1hr under 880°C, Specimen of round bar was made through extrusion process by universal tester in 200ton capacities.

Abstract: Concrete with blast furnace slag (BFS) shows varied strength development properties under general temperature conditions. Therefore, a precise prediction of compressive strength using a full maturity model is desired. The purpose of this study is to predict the compressive strength of concrete with BFS by calculating the apparent activation energy (E_a) for each BFS replacement ratio, applying this activation energy to the equivalent age model, and then using the Carino model. For BFS replacement ratios of 0%, 10%, 30%, and 50%, E_a is calculated as 33.475 kJ/mol, 37.325 kJ/mol, 41.958 kJ/mol and 45.541 kJ/mol respectively. Finally, the compressive strength of concrete with BFS is predicted.

Abstract: The wear behaviors of micrographite particles impregnating a cubic boron nitride (CBN) abrasive matrix using resin bonding was presented. CBN composite specimens containing 0.1, 1, and 5 wt% graphite and 50, 75, and 100 CBN concentrations were prepared by compression molding. Three dressing materials—copper, SiC, and Al₂O₃—were prepared to dress the CBN composite specimens. A carrier was designed and manufactured have a shape similar to that of a grinding wheel in order to hold the CBN composite specimens, which was then attached to the spindle of the grinder to carry out the grinding processes. The worn surfaces of the CBN composites were examined using scanning electron microscopy. Also, the surface characteristics and wear rate of the CBN composite specimens, the material removal rate, and the surface roughness of the SKD11 steel were investigated under dry grinding conditions. The experimental results indicated that there was considerable improvement in the process performance of the graphite-impregnated CBN structure.

Abstract: In the present work a comparative study was carried out on consolidation of Al-5083 alloy and 5wt. % nanoyttrium oxide powders by Equal channel angular pressing (ECAP). The powders were milled for 10, 15 and 20 hrs using planetary ball mill under optimized process parameters. The milled powders were characterised by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Milled powders exhibit nanocrystalline single phase after 10hrs. The crystallite size after 20 hr of milling, alloy and composite powders were found to be 23nm and 57nm respectively. The 20hr milled alloy and composite powder was consolidated by equal channel angular pressing (ECAP) through 90^o die channel angle using route-A for two passes with and without back pressure. Density of ECAPed samples were measured using Archimedes principle. The highest density was found as 96% for the alloy after 2 passes without backpressure and sintering and 94% for the composite after 2 passes with back pressure and sintering.

Abstract: Detailed observations of nanoindentation static creep effects exhibited in polymethylmethacrylate (PMMA) is presented and discussed in this study. One-indent nanoindentation technique, the partial unloading measurement (PUM) method, is compared with the basic measurement BM method (i.e., an array of multiple single indents with various magnitudes). The experimental results for PMMA reveal a strong dependence on the loading conditions. It was found that the PUM results for PMMA are not consistent with the corresponding BM results due to significant static. The creep depths accumulated and increased the indentation depth, and thus the contact area. The hardness and elastic modulus evaluated from these experimental results decreased with the indentation depth (or load).The effect of static creep exhibited in polymers for the PUM method during the determination of mechanical properties was studied. A significant static creep effect was found for polymers when the indentation load was held in order to attenuate the rapid direction change between the loading and unloading segments.

Abstract: In this study, the strains of artificial rock were analyzed in order to estimate the deformation behavior of rock-like materials in the SHPB test. The axial strain measured directly from the strain gage and obtained from the process analysis of three-wave method were compared. The analysis results show that: (1) the measurements of dynamic strains of material obtained from the strain gage method are feasible; (2) the axial strains and strain rates analyzed from the strain gage records are less than those obtained from the three-wave method; (3) the dynamic/static deformation modulus ratio E_d/E_s has the increasing tendency with the raising of stress rate, and the modulus ratio obtained from the strain gage method is more than that obtained from the three-wave method.

Abstract: This study is aimed to evaluate the tensile strength and impact resistance of cementitious materials which comprise steel fibers and silica fume in the mixes. Material variables include water-binder ratio, dosage of silica fume, steel fiber length and dosage. A designed tensile strength was used to perform the direct tensile in this study. Test results indicate that the compressive strength, splitting tensile strength and direct tensile strength of specimens for fiber length of 60 mm are higher than that of 35 mm. The inclusion of fibers in specimens containing silica fume has higher compressive and tensile strength; and lower impact resistance than the specimens made with silica fume. Incorporation of steel fiber and silica fume in composites achieves significantly higher increase in compressive strength, splitting tensile strength, and direct tensile strength than only individual use of steel fiber or silica fume and decrease in impact resistance than only individual use of steel fiber. Finally, the proposed direct tensile testing method is suitable for determining the tensile strength of fiber reinforce cementitious materials and generating the tensile stress-strain curves easily.

Abstract: Fiber cementitious materials are composed of fibers, pozzolan and cementitious. Addition of fibers in cementitious materials may enhance its mechanical properties, particularly tensile strength, and ductility. This project is aimed to evaluate the mechanical properties of fiber cementitious materials which comprise fibers and silica fume in the mixes. Test variables include dosage of silica fume, mix proportions, steel fiber dosage and type. Compressive strength, direct tensile strength and splitting tensile strength of the specimen were obtained through tests. Test results indicate that the splitting tensile strength, direct tensile strength, strain capacity and ability of crack-arresting increase with increasing steel fiber and silica fume dosages. The optimum composite is the mixture with 5 % replacement silica fume and 2 % fiber volume. In addition, the nonlinear regression analysis was used to determine the best-fit relationship between mechanical properties and test parameters.