• Journal of Institute of Control, Robotics and Systems
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• v.20 no.1
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• pp.58-69
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• 2014

This paper proposes a method for predicting maximum friction coefficients and optimal slip ratios as optimal control parameters for traction control or slip control of autonomous mobile robots on rough terrain. This paper focuses on strength of ground surface which indicates different characteristics depending on material types on surface. Strength of various material types can be estimated by Willoughby sinkage model and by a developed testbed which can measure forces, velocities, and displacements generated by wheel-terrain interaction. Estimated strength is collaborated on building improved Brixius model with friction-slip data from experiments with the testbed over sand and grass material. Improved Brixius model covers widespread material types in outdoor environments on predicting friction-slip characteristics depending on strength of ground surface. Thus, a prediction model for obtaining optimal control parameters is derived by partial differentiation of the improved Brixius model with respect to slip. This prediction model can be applied to autonomous mobile robots and finally gives secure maneuverability on rough terrain. Proposed method is verified by various experiments under similar conditions with the ones for real outdoor robots.

• Journal of Korean Society of Steel Construction
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• v.18 no.6
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• pp.737-746
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• 2006

The elastic buckling strength of a Glass Fiber Reinforced Plastic (GFRP) pipe reinforced with ribs was evaluated. The height and thickness of a rib and the spacing between two adjacent ribs were considered as factors affecting the buckling strength of the pipe. And also, the ratio of the longitudinal stiffness and transverse stiffness was considered as the parameter affecting the buckling strength as the GFRP is orthotropic material. Buckling strengths of various GFRP pipe models with different shapes and stiffness ratios were evaluated by FE analyses and a formula to estimate the elastic buckling strength of a rib-reinforced pipe made of orthotropic material was suggested from the regression with the results from the FE analysis. Analytical results show that a rib-reinforced pipe has a buckling strength superior to a general flat pipe and the suggested formula estimates accurate buckling strength of the rib-reinforced pipe.

• Journal of the Korean Wood Science and Technology
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• v.38 no.3
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• pp.178-184
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• 2010

This study was carried out to investigate the properties of woodceramics made from woody part of Broussonetia Kazinoki at different impregnation ratios of phenolic resin of 40, 50, 60, 70%. The physical and mechanical properties increased with increasing impregnation ratio. The highest mean values of density, bending strength, Brinell hardness and compressive strength were 0.66 g/$cm^3$, 53 kgf/$cm^2$, 187 kgf/$cm^2$, 126 kgf/$cm^2$, respectively. There were close correlations between density and bending strength, Brinell hardness and compressive strength, and between MOE and MOR.

• International Journal of Concrete Structures and Materials
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• v.3 no.1
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• pp.25-32
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• 2009

A non-destructive testing (NDT) method for evaluating physical properties of concrete including the compressive strength is highly desirable. This paper presents such an NDT method based on measurement of electromagnetic (EM) properties of the material. Experiments are carried out on cement mortar with different water/cement (W/C) ratios. Their EM properties including the conductivity and the dielectric constant are measured at different exposure conditions and curing periods over a wide frequency range of the EM wave. The compressive strength of these specimens is also tested. It is found that both the conductivity and the dielectric constant increase as the W/C ratio decreases and the curing period increases, which lead strength development in the specimens. A linear correlation is observed between the averaged EM properties over the 5 to 20 GHz frequency range and the measured compressive strength, demonstrating the effectiveness of the EM property-based NDT method in evaluating strength of OPC mortar.

• Proceedings of the Korean Institute of Building Construction Conference
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• 2009.11a
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• pp.55-59
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• 2009

The epoxy resin without hardener can harden by a ring-opening reaction in the presence of the alkalies produced by the hydration of cement in epoxy-modified mortars and concretes. This paper investigates the effect of curing conditions on the strength improvement of polymer-modified mortars using bisphenol A-type epoxy resin without hardener. The polymer-modified mortars using epoxy resin are prepared with various polymer-cement ratios, and subjected to ideal, water, dry and heat cures. In the heat cure, the epoxy-modified mortars are sealed or unsealed with a PVDC (polyvinylidene chloride) film. The epoxy-modified mortars are tested for flexural and compressive strengths at desired curing methods. The microstructures of the epoxy-modified mortars are also observed by scanning electron microscope. The effects of curing conditions on the strength development of the epoxy-modified mortars are examined. From the test results, the marked effectiveness of the heat cure under the PVDC film sealing against the development of the strength of the epoxy-modified mortar without the hardener is recognized. The flexural and compressive strengths of 7-day-90℃ heat-cured, PVDC film-sealed epoxy-modified mortars without hardener reach 7 to 17MPa and 24 to 44MPa respectively, and are two to three times of Unmodified mortar. Such high strength development of the epoxy-modified mortars may be achieved by the dense microstructure formation by cement hydrates and the hardening of the epoxy resin in the mortars.

• Journal of the Korea institute for structural maintenance and inspection
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• v.6 no.4
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• pp.233-242
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• 2002

This study is to investigate experimentally the shear capacity of high-strength lightweight-aggregate reinforced concrete beams subjected to monotonic loading. Ten beams made of fly-ash artificial lightweight high-strength concrete were tested to determine their diagonal cracking and ultimate shear capacities. The variables in the test program were longitudinal reinforcement ratio; which variabled (between 0.83 and 1.66 percent), shear span-to-depth ratio (a/d=1.5, 2.5 and 3.5), and web reinforcement(0, 0.137, 0.275 and 0.554 percent). Six of the test beams had no web reinforcement and the other six had web reinforcement along the entire length of the beam. Most of beams failed brittly by distinct diagonal shear crack, and have reserved shear strength due to the lack of additional resisting effect by aggregate interlocking action after diagonal cracking. Test results indicate that the ACI Building Code predictions of Eq. (11-3) and (11-5) for lightweight concretes are unconservative for beams with tensile steel ratio of 1.66, a/d ratios greater than 2.5 without web reinforcement. Through a more rational approach to compute the contribution of concrete to the shear capacity, a postcracking shear strength in concrete is observed.

• Advances in concrete construction
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• v.10 no.3
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• pp.185-193
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• 2020

An analytical model is developed in this paper to predict the shear capacity of reinforced concrete (RC) columns with corroded transverse reinforcements. The shear strength model for corroded RC columns is proposed based on modifying the existing strut-and-tie model, which considers the deformational compatibility between truss and arch mechanisms. The contributions to the shear strength from both truss and arch mechanisms are incorporated in the proposed model. The effects of corrosion level of transverse reinforcements are considered in the proposed model through the minimum residual cross-sectional area of transverse reinforcements and the reduction of concrete compressive strength for the cover area. The shear strengths calculated from the developed model are compared with the experimental results from Vu's study (2017), which consisted of RC columns with corroded transverse reinforcements showing shear failure under the cyclic loading. The comparison results indicate satisfactory correlations. Parametric studies are conducted based on the developed shear strength model to explore the effects of column axial loading, aspect ratios, transverse reinforcements and the corrosion levels in transverse reinforcements to the shear strength of RC columns with corroded transverse reinforcements.

• Interaction and multiscale mechanics
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• v.5 no.2
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• pp.91-104
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• 2012

There is relatively little numerical study on the behavior of eccentrically loaded high strength rectangular concrete-filled steel tubular (CFST) slender beam-columns with large depth-to-thickness ratios, which may undergo local and global buckling. This paper presents a multiscale numerical model for simulating the interaction local and global buckling behavior of high strength thin-walled rectangular CFST slender beam-columns under eccentric loading. The effects of progressive local buckling are taken into account in the mesoscale model based on fiber element formulations. Computational algorithms based on the M$\ddot{u}$ller's method are developed to obtain complete load-deflection responses of CFST slender beam-columns at the macroscale level. Performance indices are proposed to quantify the performance of CFST slender beam-columns. The accuracy of the multiscale numerical model is examined by comparisons of computer solutions with existing experimental results. The numerical model is utilized to investigate the effects of concrete compressive strength, depth-to-thickness ratio, loading eccentricity ratio and column slenderness ratio on the performance indices. The multiscale numerical model is shown to be accurate and efficient for predicting the interaction buckling behavior of high strength thin-walled CFST slender beam-columns.

• Computers and Concrete
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• v.23 no.6
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• pp.409-419
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• 2019

In the present study, effect of size and placement of cubic specimens on compressive strength of self-compacting lightweight concrete (SCLC) were considered. To do so, 81 specimens of different sizes (50 mm, 75 mm, 100 mm, and 150 mm) were prepared by using three different mixes of SCLC. Results of the cured specimens were then used in regression analyses to find predictive equations with regard to both the placement direction and the size. Test results showed that the strength ratio in cases in which the direction of loading and placement were parallel, were higher than those specimens, whose configurations were normal between loading and placement. In addition, strength ratios in SCLC mixes were slightly higher than those are for self-compacting normal weight concrete. In order to analyze the effect of size on compressive strength the conventional size effect law as well as the modified size effect law (MSEL) were used. Besides, the convergence criterion of nonlinear regression process of size effect study has been discussed. Analyses of the results showed that the unconstraint nonlinear regression in size effect study of SCLC mixes could lead to erroneous results.

• Korean Journal of Materials Research
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• v.31 no.10
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• pp.546-551
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• 2021

To improve ferroelectric properties of PZT, many studies have attempted to fabricate dense PZT films. The AD process has an advantage for forming dense ceramic films at room temperature without any additional heat treatment in low vacuum. Thick films coated by AD have a higher dielectric breakdown strength due to their higher density than those coated using conventional methods. To improve the breakdown strength, glass (SiO₂-Al₂O₃-Y₂O₃, SAY) is mixed with PZT powder at various volume ratios (PZT-xSAY, x = 0, 5, 10 vol%) and coating films are produced on silicon wafers by AD method. Depending on the ratio of PZT to glass, dielectric breakdown strength and energy storage efficiency characteristics change. Mechanical impact in the AD process makes the SAY glass more viscous and fills the film densely. Compared to pure PZT film, PZT-SAY film shows an 87.5 % increase in breakdown strength and a 35.3 % increase in energy storage efficiency.