Hee Sun Kim

Finite element modeling technique for predicting mechanical behaviors on mandible bone during mastication

PURPOSE The purpose of this study was to propose finite element (FE) modeling methods for predicting stress distributions on teeth and mandible under chewing action. MATERIALS AND METHODS For FE model generation, CT images of skull were translated into 3D FE models, and static analysis was performed considering linear material behaviors and nonlinear geometrical effect. To find out proper boundary and loading conditions, parametric studies were performed with various areas and directions of restraints and loading. The loading directions are prescribed to be same as direction of masseter muscle, which was referred from anatomy chart and CT image. From the analysis, strain and stress distributions of teeth and mandible were obtained and compared with experimental data for model validation. RESULTS As a result of FE analysis, the optimized boundary condition was chosen such that 8 teeth were fixed in all directions and condyloid process was fixed in all directions except for forward and backward directions. Also, fixing a part of mandible in a lateral direction, where medial pterygoid muscle was attached, gave the more proper analytical results. Loading was prescribed in a same direction as masseter muscle. The tendency of strain distributions between the teeth predicted from the proposed model were compared with experimental results and showed good agreements. CONCLUSION This study proposes cost efficient FE modeling method for predicting stress distributions on teeth and mandible under chewing action. The proposed modeling method is validated with experimental data and can further be used to evaluate structural safety of dental prosthesis.

Crack-closing of cement mortar beams using NiTi cold-drawn SMA short fibers

In this study, crack-closing tests of mortar beams reinforced by shape memory alloy (SMA) short fibers were performed. For this purpose, NiTi SMA fibers with a diameter of 0.965 mm and a length of 30 mm were made from SMA wires of 1.0 mm diameter by cold drawing. Four types of SMA fibers were prepared, namely, straight and dog-bone-shaped fiber and the two types of fibers with paper wrapping in the middle of the fibers. The paper provides an unbonded length of 15 mm. For bending tests, six types of mortar beams with the dimensions of 40 mm × 40 mm × 160 mm (B×H×L) were prepared. The SMA fibers were placed at the bottom center of the beams along with an artificial crack of 10 mm depth and 1 mm thickness. This study investigated the influence of SMA fibers on the flexural strength of the beams from the measured force- deflection curves. After cracking, the beams were heated at the bottom by fire to activate the SMA fibers. Then, the beams recovered the deflection, and the cracks were closed. This study evaluated crack-closing capacity using the degree of crack recovery and deflection-recovery factor. The first factor is estimated from the crack-width before and after crack-closing, and the second one is obtained from the downward deflection due to loading and the upward deflection due to the closing force of the SMA fibers.

Structural damage evaluation of reinforced concrete beams exposed to high temperatures

The objective of this study is to investigate the effect of temperature distribution, concrete strength, cover thickness, and heating time on the structural behavior of reinforced concrete beams. Toward this goal, reinforced concrete beams with different concrete compressive strength and cover thickness are fabricated and subjected to furnace heating for 60, 90, and 120 min under a loaded state. In order to analyze structural behavior based on the thermal behavior of the beams, transient temperature distribution is measured during the furnace heating. After furnace heating, spalling is observed. From loading tests performed on the damaged reinforced concrete beams, residual strength, maximum loads, and beam deflections are measured and examined. The experimental results show that significant damage occurs in the reinforced concrete beams under high temperatures. In addition, it is found that thermal and structural behavior of damaged reinforced concrete beams is dependent on cover thickness and concrete s...

Experimental Studies on the Effect of Various Design Parameters on Thermal Behaviors of High Strength Concrete Columns under High Temperatures

Although concrete is considered as fire proof materials, high strength concrete shows severe material and structural damages when exposed to fire. To understand such damages in high strength concrete structures, the effects of various design parameters and fire condition on the thermal behaviors of high strength concrete structures are investigated in this study. In order to achieve this goal, fire tests are performed on high strength concrete columns with different fire conditions and design parameters including cross sectional area, cover thickness, and reinforcement alignment. To investigate thermal behaviors, temperature distributions and amount of spalling are measured. In overall, the columns show rapidly increasing inner temperatures between 30~60 mins of the fire tests due to spalling. In detail, the higher temperature distributions are observed from the columns with the larger cross section and less cover thickness. Moreover, among the columns with same reinforcing ratio, larger number of reinforcements with the smaller diameter causes the higher temperature distribution. The findings from the experimental study allow not only understanding of thermal behaviors of high strength concrete columns under fire, but also guidance in revising fire safety design.

Seismic fragility analysis of lap-spliced reinforced concrete columns retrofitted by SMA wire jackets

The aim of this study is to provide seismic fragility curves of reinforced concrete columns retrofitted by shape memory alloy wire jackets and thus assess the seismic performance of the columns against earthquakes, comparing them with reinforced concrete columns with lap-spliced and continuous reinforcement. For that purpose, this study first developed analytical models of the experimental results of the three types of columns, (1) lap-spliced reinforcement, (2) continuous reinforcement and (3) lap-spliced reinforcement and retrofitted by SMA wire jackets, using the OpenSEES program, which is oriented to nonlinear dynamic analysis. Then, a suite of ten recorded ground motions was used to conduct dynamic analyses of the analytical models with scaling of the peak ground acceleration from 0:1g to 1:0g in steps of 0:1g. From the static experimental tests, the column retrofitted with SMA wire jackets had a larger displacement ductility by a factor of 2.3 times that of the lap-spliced column, which was 6% larger compared with the ductility of the continuous reinforcement column. From the fragility analyses, the SMA wire jacketed column had median values of 0.162g and 0.567g for yield and collapse, respectively. For the yield damage state, the SMA wire jacketed column had a median value similar to the continuous reinforcement column. However, for the complete damage state, the SMA wire jacketed column showed a 1.33 times larger median value than the continuously reinforcement column. (Some figures may appear in colour only in the online journal)

Experimental Study for Evaluating Structural Behavior of RC Beams Strengthened by Different Width of FRP Layers

This paper reports experimental studies of reinforced concrete (RC) beams strengthened by carbon fiber reinforced polymer sheet (CFRPs) having different width. The objective of this study is to evaluate effect of different width of CFRPs on structural behaviors of RC beams and investigate most effective height of CFRPs on the side surfaces of RC beams for flexural strengthening. Toward this goal, five RC beams are fabricated and strengthened with different width of CFRPs, and then four-point bending tests with the simply supported beams are performed. Results of this study show that the width of strengthened CFRPs on RC beams has a significant influence on structural behaviors of RC beams. Index Term—CFRPs, CFRP width, RC beams, strengthening effect

Effect of asymmetry on hemodynamics in fluid-structure interaction model of congenital bicuspid aortic valves

A bicuspid aortic valve (BAV) is a congenital cardiac disorder where the valve consists of only two cusps instead of three in a normal tricuspid valve (TAV). Although 97% of BAVs include asymmetric cusps, little or no prior studies investigated the blood flow through physiological three-dimensional BAV and root. This study presents four fully coupled fluid-structure interaction (FSI) models, including native TAV, asymmetric BAV with or without a raphe and an almost symmetric BAV. The FSI simulations are based on coupled structural and fluid dynamics solvers that allow accurate modeling of the pressure load on both the root and the cusps. The partitioned solver has non-conformal meshes and the flow is modeled employing an Eulerian approach. The cusps tissue in the structural model is composed of hyperelastic finite elements with collagen fiber network embedded in the elastin matrix. The tissues behavior of the aortic sinuses is also hyperelastic. The coaptation is modeled with master-slave contact algorithm. A full cardiac cycle is simulated by imposing the same physiological blood pressure at the upstream and downstream boundaries, for all the TAV and BAV models. The latter have significantly smaller opening area compared to the TAV. Larger stress values were also found in the cusps of the BAV models with fused cusps, both at the systolic and diastolic phases. The asymmetric geometry cause asymmetric vortices and much larger wall shear stress on the cusps, which is a potential cause for early valvular calcification in BAVs.

Structural Capacity Evaluation of High Strength Concrete Short Columns with Various Design Parameters under High Temperatures

It is well known that high strength concrete with compressive strength higher than 50 MPa shows severe material and structural damages under fire due to spalling. To understand degradation of structural capacity of fire damaged high strength concrete structures, not only thermo-mechanical behavior needs to be defined, but also structural behavior of high strength concrete member under high temperature needs to be investigated. In this study, structural tests are performed by applying axial loads on high strength concrete columns exposed at elevated temperatures for assigned amount of time. The tested columns are prepared to have different concrete strength and polypropylene fiber percentage. The test results show that structural capacity of the columns decreased with increased compressive strength of concrete under same heating condition. Especially, it is interesting to note that high strength concrete columns with polypropylene fiber for spalling proof did not improve structural capacity compared to the columns without polypropylene fiber. The findings from the test are able to improve fire proof design of high strength concrete structural members and predicting structural performance of fire damaged structural members.

Analytical Study to Investigate Structural Behaviors of Skull During Mastication According to Occlusal Relationships

This study proposes an effective modeling method to simulate masticatory action. Based on computed tomography (CT) images, finite element (FE) models of human skull with two different occlusal relationships, such as class I and end-on class II are constructed. After applied loading and boundary conditions for masticatory simulation, FE analyses are performed considering linear material properties and geometrical effect. As a result, the relationships between the mandibular movement and occlusal force of both two models show similar tendency in range of human occlusal force. However, stress distributions are different due to changes of occlusal surface. In addition, an experiment is conducted using replica skull model for the validation. The experimental results show a good agreement with the analytical results.

Analytical Study for Predicting Structural Performance of Buildings According to Opening Conditions Under Fire

This paper aims at predicting structural performance of buildings according to opening conditions under fire. Towards that goal, fire simulations are performed to investigate effects of opening conditions on heat propagation. In addition, residual performance of the structural members is evaluated using finite element method to see if fire damaged structural members satisfy structural design criteria. As a result, in the model with opened windows, maximum temperature is observed from the fire initiated room, while maximum temperature is moved from fire initiated room to stairways in the model with closed windows. This influences on structural behaviors of the fire damaged building such that the area where structural design criteria is not satisfied increases in the case of model with opened windows.