Dae-Jin Kim

Analysis of Interacting Cracks Using the Generalized Finite Element Method With Global-Local Enrichment Functions

This paper presents an analysis of interacting cracks using a generalized finite element method (GFEM) enriched with so-called global-local functions. In this approach, solutions of local boundary value problems computed in a global-local analysis are used to enrich the global approximation space through the partition of unity framework used in the GFEM. This approach is related to the global-local procedure in the FEM, which is broadly used in industry to analyze fracture mechanics problems in complex three-dimensional geometries. In this paper, we compare the effectiveness of the global-local FEM with the GFEM with global-local enrichment functions. Numerical experiments demonstrate that the latter is much more robust than the former In particular, the GFEM is less sensitive to the quality of boundary conditions applied to local problems than the global-local FEM. Stress intensity factors computed with the conventional global-local approach showed errors of up to one order of magnitude larger than in the case of the GFEM. The numerical experiments also demonstrate that the GFEM can account for interactions among cracks with different scale sizes, even when not all cracks are modeled in the global domain.

A Global-Local Approach for the Construction of Enrichment Functions for the Generalized FEM and Its Application to Three-Dimensional Cracks

Existing generalized or extended finite element methods for modeling cracks in three-dimensions require the use of a sufficiently refined mesh around the crack front. This offsets some of the advantages of these methods specially in the case of propagating three-dimensional cracks. In this paper, a strategy to overcome this limitation is investigated. The approach involves the development of enrichment functions that are computed using a new global-local approach. This strategy allows the use of a fixed global mesh around the crack front and is specially appealing for non-linear or time dependent problems since it avoids mapping of solutions between meshes. The resulting technique enjoys the same flexibility of the so-called meshfree methods for this class of problem while being more computationally efficient.

Robust Elevator Button Recognition in the Presence of Partial Occlusion and Clutter by Specular Reflections

This paper deals with vision-based elevator button recognition for a robot arm manipulating elevator buttons. The major difficulties in elevator button recognition are the presence of partial occlusion of the target objects and image clutter caused by specular reflection from mirrorlike walls inside the elevator. As a remedy for the elevator button recognition problem in highly complicated settings, we propose a robust button recognition algorithm, which is modularized into feature extraction, initial recognition, and postrefinement modules. In consideration of the diverse button shapes in the form of convex quadrilaterals with rounded button corners, a set of features is specially designed to describe each button contour subject to perspective distortion. A homography-based image transform is also employed for the template matching to achieve a more reliable matching performance. Like a grammar in linguistics, geometric button alignment, plays a key role in the elimination of false positives and the estimation of the missing buttons. The proposed algorithm is extensively tested in a robotic platform to verify effectiveness, robustness, and real-time performance.

Bond strength of steel deformed rebars embedded in artificial lightweight aggregate concrete

The recycling of industrial waste such as bottom ash from furnaces is an important issue in construction industry, since it enables reduction in construction cost and has beneficial effect on the environment. In this study, we have investigated the bond characteristics of steel deformed bars embedded in artificial lightweight aggregate concrete which is manufactured from bottom ash. A pullout test was performed on 144 lightweight aggregate concrete specimens to measure the bond strengths. In this test, the parameters included the compressive strength of the concrete and embedment length of rebar. The pullout load vs. slip responses and modes of failure of the specimens were identified during the test. A bond strength equation for lightweight concrete is formulated by performing a regression analysis on the test results and compared with the predictions by the existing equations such as ACI 408, Orangun’s, and Darwin’s. The comparison shows that the existing bond strength equations cannot be directly applied to the design of lightweight concrete structures and the proposed equation is able to provide a more accurate estimation of the bond strength of lightweight concrete than the existing equations.

Experimental study on bond strength of fiber reinforced polymer rebars in normal strength concrete

The bond behavior of reinforcing bars is an important issue in the design of reinforced concrete structures and the use of fiber reinforced polymer (FRP) rebars is a promising solution to handle the problems of steel reinforcement corrosion. This study investigates the bond characteristics of carbon and aramid FRP (CFRP and AFRP) bars embedded in normal strength concrete. A pullout test was performed on 63 normal strength concrete specimens reinforced with FRP and steel rebars with different embedment lengths and bar diameters. The average bond stress versus slip curve is plotted for all specimens and their failure modes are identified. The effects of the embedment length and diameter of an FRP rebar on its bond strength are examined in this work. The bond strengths obtained from the test results are compared with the predictions by the bond strength equation proposed by Okelo and Yuan (2005), and its validity is evaluated.

Development of an Efficient Steel Beam Section for Modular Construction Based on Six-Sigma

This study presents a systematic approach for the development of an efficient steel beam section for modular construction based on Six-Sigma. Although the Six-Sigma is frequently implemented in manufacturing and other service industries, it is a relatively new concept in the area of building design and construction. As a first step in this approach, market studies and surveys are conducted to obtain the opinions of potential customers. Then the opinions of customers are converted into quality characteristics for the steel beam using the quality function deployment methodology. A steel hollow flanged channel is chosen as the main modular beam shape, and the design concept is derived and developed by applying the Pugh matrix methodology. A pilot test was performed to validate the effectiveness of the developed beam section. The results indicated that the developed channel beam section showed excellent performance and retained high accuracy in fabrication, thus resulting in a significant reduction of steel consumption.

TWO-SCALE 3D ANALYSIS OF REFLECTIVE CRACKS IN AIRFIELD PAVEMENTS

Prediction and simulation of load-related reflective cracking in airfield pavements require 3D models in order to accurately capture the effects of gear loads on crack initiation and propagation. Furthermore, the size relations between aircraft landing gear, pavement area and reflective cracks make this a multi-scale problem. This paper presents a generalized finite element method (GFEM) based on the solution of interdependent global and local problems. The macro-scale component of the solution is approximated by a finite element global model, while the fine-scale is addressed by a local model. This allows accurate modeling of cracks in airfield pavements that are orders of magnitude larger than the critical zones in which the initiation of reflective cracking occurs. Two 3D pavement models are created for this study. The first model is subjected to the gear loading of a Boeing 777 aircraft. This example demonstrates the accuracy of solving a multi-scale pavement problem with a GFEM enriched with local problem solutions. The second problem studies the effect of the size of the local domain on the accuracy of the GFEM solution. The numerical simulations show that 3D multi-scale problems such as reflective cracks in airfield pavements, can be efficiently solved by a GFEM based on interdependent global and local problems.

Pull-Out Resistance Capacity of a New Perfobond Shear Connector for Steel Pile Cap Strengthening

This study proposes a new type of the perfobond shear connector, which can be used to strengthen the steel pile cap embedded into the structure foundation, and evaluates its pull-out resistance capacity by performing a test on ten specimens. Test parameters include the embedment length of the shear connector, existence of transverse rebars passing through holes in the shear connector, and their shape, size, and number. The pull-out load versus slip curve is plotted for all specimens, and their failure modes are identified. The effects of the test parameters on the peak pull-out load are examined in this work. The test results show that the perfobond shear connector proposed in this study can retain the peak pull-out load up to 6 times higher than the one without any holes. This indicates that the existence of holes in the shear connector enables the dowel action of concrete inside the hole, resulting in the improvement of the shear resistance capacity of the connector.

Evaluation of Shear Strength Equation of Concrete Beams Embedded with GFRP Plates

In this paper, we will study the possibility of the design standard applied by modifying the shear strength equation so that we can propose the shear strength equation of reinforced concrete beams strengthened with the new type of GFRP plate. The new GFRP shear reinforcement is manufactured into a plate shape with several openings to ensure perfect integration with concrete. The test was performed on 6 specimens with shear span-to-depth ratio of 2.4. Test variables are the amount of the shear reinforcement and the shape of shear reinforcement. The two types of GFRP plate with different shapes are a rectangle and a parallelogram. A shear strength equation for concrete beams with the proposed GFRP plate was suggested through modifying the existing shear strength equation for concrete beams in CSA A23.3-04 and ACI 318-11. Comparing the test results with the estimations based on the proposed equation, CSA A23.3-04 modified equation predicted the shear strength better than ACI 318-11 modified equation.

Soft Sign Language Expression Method of 3D Avatar

This paper proposes a 3D avatar which expresses sign language in a very using lips, facial expression, complexion, pupil motion and body motion as well as hand shape, Hand posture and hand motion to overcome the limitation of conventional sign language avatars from a deaf`s viewpoint. To describe motion data of hand and other body components structurally and enhance the performance of databases, we introduce the concept of a hyper sign sentence. We show the superiority of the developed system by a usability test through a questionnaire survey.