• Structural Engineering and Mechanics
• /
• v.90 no.4
• /
• pp.357-370
• /
• 2024

Due to the fact that the mechanism of the effects of temperature and initial geometric imperfection on low-velocity impact problem of axially moving plates is not yet clear, the present paper is to fill the gap. In the present paper, the nonlinear dynamic behavior of axially moving imperfect graphene platelet reinforced metal foams (GPLRMF) plates subjected to lowvelocity impact in thermal environment is analyzed. The equivalent physical parameters of GPLRMF plates are estimated based on the Halpin-Tsai equation and the mixing rule. Combining Kirchhoff plate theory and the modified nonlinear Hertz contact theory, the nonlinear governing equations of GPLRMF plates are derived. Under the condition of simply supported boundary, the nonlinear control equation is discretized with the help of Gallekin method. The correctness of the proposed model is verified by comparison with the existing results. Finally, the time history curves of contact force and transverse center displacement are obtained by using the fourth order Runge-Kutta method. Through detailed parameter research, the effects of graphene platelet (GPL) distribution mode, foam distribution mode, GPL weight fraction, foam coefficient, axial moving speed, prestressing force, temperature changes, damping coefficient, initial geometric defect, radius and initial velocity of the impactor on the nonlinear impact problem are explored. The results indicate that temperature changes and initial geometric imperfections have significant impacts.

• Journal of Korean Ophthalmic Optics Society
• /
• v.20 no.2
• /
• pp.125-131
• /
• 2015

Purpose: The effect of pigmentation in circle contact lens on lens wearers' tear film stability was investigated in this study. Methods: Non-invasive tear film break-up times of 54 normal eyes before and after lens wearing in twenties and the portion of first tear breakup were measured. The frontal and back surface roughness of circle contact lens was further observed with a scanning electron microscope. The clear lens which was made of hilafilcon B and circle contact lenses which were made of hilafilcon B, polymacon and copolymer of HEMA NVP were tested. Results: Non-invasive tear film breakup time was significantly reduced compared with it before lens wearing. In some case, statistically significant difference in tear film breakup time was also observed between circle contact lenses. The tear file break-up was usually early started in lower than upper portion, and in temporal than nasal portion although there was variation in the lens material. The tear film break-up of circle contact lenses made of hilafilcon B was more commonly observed in peripheral portion(pigmented area) compared with clear contact lens made of same material. Conclusions: The present study revealed that the stability of tear film was affected by wearing circle contact lens and the result seems to be used as the basis for the problem solving that can occur due to the pigmentation when wearing circle contact lens.

• Korean Journal of Soil Science and Fertilizer
• /
• v.36 no.5
• /
• pp.263-271
• /
• 2003

Effects of the existence of $CO_2$ gas and pH on the silica surface characteristics at silica/Pb(II) and sodium dodecyl sulfonate (SDS, $C_{12}H_{25}SO_3Na$) solution interface were studied. The hydrophobic characteristics of silica surface was delineated by contact angle measurement and surface force measurement using atomic force microscopy (AFM). In $CO_2$ free condition provided by purging $N_2$ gas, the contact angle of fused silica surface in $10^{-4}M$ Pb(II) and SDS solution increased greatly up to $90^{\circ}$ compared with $40^{\circ}$ in atmospheric condition. It was due to the precipitation of $PbCO_3$ in atmospheric condition. In $CO_2$ free condition the change of contact angle and adhesion force ($F_{ad}$) in AFM, affected by pH change, was similar to the distribution of $PbOH^+$ ion in speciation diagram corresponding to $10^{-4}M$ total Pb(II). Therefore, it was convinced that the $PbOH^+$ ion among Pb(II) species would be the main adsorbing type on silica surface. Both of contact angle measurement and surface force measurement using AFM showed that the Pb only treatment made the silica surface hydrophobic. However, it could not be explained theoretically by current knowledge, and required further study in atomic level to solve the problem.

• Transactions of the Korean Society of Mechanical Engineers A
• /
• v.34 no.2
• /
• pp.161-166
• /
• 2010

Elastomeric seals are widely used in dynamic seal applications, and it is well known that the sealing surfaces can be gradually worn out. Abrasive wear is known to be the most dominant factor; however, little research has been carried out on this problem until now. In this study, a new contact problem related to elastomeric seals-a small spherical particle and steel surface-was modeled and analyzed using MARC. Variations of von-Mises and residual stress distributions as well as deformed seal and steel surface shapes with seal materials and interferences are presented. The stress distribution and surface deformation are highly affected by the elastic properties of seal. For PTFE, the maximum von-Mises stress exceeds the yield strength, and plastic deformation occurs on the steel surface. Therefore, the sealing surface can also be worn down by sub-surface fatigue due to intervening hard particles in the sealing surfaces together with the well-known abrasion.

• Journal of the Society of Naval Architects of Korea
• /
• v.28 no.1
• /
• pp.60-68
• /
• 1991

The updated Lagrangian Finite Element Method is introduced to analyse rigid body-fluid impact problem which is characterized by incompressible Navier-Stokes equations and impact-contact conditions between free surface and rigid body. For the convenience of numerical computation, velocity fields are splinted into vicous and pressure parts, and then the governing equations and boundary conditions are decomposed in accordance with the decomposition. However, Viscous stresses acting an the solid boundaries are neglected on the assumption that very small velocity gradients may occur during extremely small time interval of the impact. Four coded quadrilateral elements are used to discretize the space domain and the fully explicit time-marching algorithm is employed with a reasonably small time step. At the beginning of each time step, contact velocity of the rigid body is computed from the momentum balance between the body and the fluid. The velocity field is then computed to satisfy the discretized equations of motions and incompressibility and contact constraints as well as an exact free surface boundary condition. At the end of each time step, the fluid domain is updated from the velocity field. In the present time stepping numerical analysis, behaviour of the free surface near the body can be observed without any difficulty which is very important in the water impact problem. The applicability of the algorithm is illustrated by a wedge type falling body problem. The numerical solutions for time-varying pressure distributions and impact loadings acting ion the surface are obtained.

• Structural Engineering and Mechanics
• /
• v.90 no.6
• /
• pp.551-568
• /
• 2024

The present study deals with static and dynamic behaviors including forced vibrations of an elastic rectangular nano plate on the two-parameter foundation. Firstly, the rectangular plate is assumed to be subjected to uniformly distributed and eccentrically applied concentrated loads. The governing equations of the problem are derived by considering the dynamic response of the plate, employing a series of the Chebyshev polynomials for the displacement function and applying the Galerkin method. Then, effects of the non-essential boundary conditions of the plate, i.e., the boundary conditions related to the shearing forces, the bending moments and the corner forces, are included in the governing equation of motion to compensate for the non-satisfied boundary conditions and increase the accuracy of the Galerkin method. The approximate numerical solution is accomplished using an iterative process due to the non-linearity of the unilateral property of the two-parameter foundation. The plate under static concentrated load is investigated in detail numerically by considering a wide range of parameters of the plate and the foundation stiffnesses. Numerical treatment of the problem in the time domain is carried out by assuming a stepwise variation of the concentrated load and the linear acceleration procedure is employed in the solution of the system of governing differential equations derived from the equation of motion. Time variations of the contact region and those of the displacements of the plate are presented in the figures for various numbers of the two-parameter of the foundation, as well as the classical and nano parameters of the plate particularly focusing on the non-linearity of the problem due to the plate lift-off from the unilateral foundation. The effects of classical and nonlocal parameters and loading are investigated in detail. Definition of the separation between the plate and the two-parameter foundation is presented and applied to the given problem. The effect of the lift-off on the static and dynamic behavior of the rectangular plate is studied in detail by considering various loading conditions. The numerical study shows that the effect of nonlocal parameters on the behavior of the plate becomes significant, when nonlinearity becomes more profound, due to the lift-off of the plate. It is seen that the size effects are significant in static and dynamic analysis of nano-scaled rectangular plates and need to be included in the mechanical analyses. Furthermore, the corner displacement of the plate is affected more significantly from the lift-off, whereas it is less marked in the time variation of the middle displacement of the plate. Several numerical examples are presented to examine the sensibility of various parameters associated with nonlocal parameters of the plate and foundation. Both stiffening and softening nonlocal parameters behavior of the plate are identified in the numerical solutions which show that increasing the foundation stiffness decreases the extent of the contact region, whereas the stiffness of the shear layer increases the contact region and reduces the foundation settlement considerably.

• Korean Journal of Child Studies
• /
• v.31 no.3
• /
• pp.51-66
• /
• 2010

This study examined the effects of parental attachment and peer attachment in elementary school children upon early adolescent delinquency. The participants were 2,844 taken from data from the Korean Youth Panel Survey (KYPS). Data from secondary questionnaires data on parent and peer attachment, problem behavior, delinquent peers, and delinquency were used for this study. Data were analyzed by means of a Structural Equation Model using AMOS 5.0. Our results indicated that, firstly, parental attachment was negatively related to problem behavior for both male and female adolescents, whereas peer attachment was positively related to problem behavior only for males, but not for females. Secondly, problem behavior in childhood has a direct effect on adolescent delinquency, which also can be indirectly mediated by contact with delinquent peers. In conclusion, the path model from parental and peer attachment in early childhood to adolescent delinquency was supported only in the case of males.

• Journal of Korea Technical Association of The Pulp and Paper Industry
• /
• v.44 no.2
• /
• pp.18-28
• /
• 2012

This study was to develop a novel automatic system for measuring St$\ddot{o}$ckigt sizing degree and contact angle at a time. The conventional methods to measure sizing degree had serious problems in obtaining significant differences according to different dosages of a sizing agent, and moreover they disclosed unique limitation due to liquid types used and tester's subjectivity. However, the newly developed system could get reproducible results through total automation of all procedures including liquid dropping, image acquisition and measurement of both St$\ddot{o}$ckigt sizing degree and contact angle. For the St$\ddot{o}$ckigt sizing test, the automatic system could measure sizing degree with more definite differences according to different dosage of AKD, compared to the conventional method. For the contact angle test, the automatic system showed a similar trend to the conventional method but had smaller contact angles due to distortion of an image focus by a sheet curl than the conventional testing machine. The problem from the image out of focus due to specimen curl will be overcome with adopting a new specimen holder for the future system.

• The Journal of Korean Academy of Prosthodontics
• /
• v.45 no.4
• /
• pp.534-545
• /
• 2007

Statement of problem: In spite of increasing sports injury, there was no collected data on the rate and type of dental injuries for athletes in Korea. Purpose: The purpose of this study was to investigate the occurrence of maxillofacial injuries and attitude of college sports player in Korea towards mouthguard. Material and methods: Total 617 athletes answered a series of questionnaire concerning their sports injury and using mouthguard. Results: 1. 81%(502) of athletes had, playing or training a sports, suffered an injury. Female athletes suffered an injury more than male athletes(p<0.05). Contact sports athletes were injured more than non contact sports athletes(p<0.05). 2. 335 athletes(54.8%) had suffered maxillofacial injuries while playing or training. 81.8% of athletes suffered an maxillofacial injury in contact sports(p<0.05). 3. Laceration of oral area, wrick in neck, fracture or avulsion on upper incisors, concussion, TMJ injury, fracture or avulsion on lower incisors, fracture or avulsion on lower molars were frequently injured area. 4. 67.2% of athletes answered that mouthguard could prevent sports injury especially high in contact sports(p<0.05). But only 39.1% of athletes required mouthguard while playing. 44.6% of athletes showed their intention of using mouthguard. Conclusion: This study shows that the incidence of maxillofacial injuries is very high while the actual use of mouthguard is very low. To prevent sports injury, a dentist must inform sports players and coaches of accurate information about mouthguard so that they can use it well. A dentist also has to provide them with better mouthguard on the basis of user's complaint.

• Journal of Communications and Networks
• /
• v.15 no.5
• /
• pp.504-513
• /
• 2013

Communication in delay tolerant networks (DTNs) relies on message transport by mobile nodes, and a correct understanding of the node mobility characteristics is therefore crucial to the design of an efficient DTN routing protocol. However, previous work has mainly focused on uncovering all behaviors of node movement, which is not conducive to accurately detecting the specific movement characteristics of a different node. In this paper, we seek to address this problem based on a consideration of social relationships. We first consider social ties from both static and dynamic perspectives. For a static perspective, in addition to certain accidental events, social relations are considered for a long time granularity and tend to be stable over time. For a dynamic perspective, social relations are analyzed in a relatively short time granularity and are likely to change over time. Based on these perspectives, we adopted different efficient approaches to dividing node pairs into two classes, i.e., familiar and unfamiliar pairs. A threshold approach is used for static social ties whereas a density-based aggregation method is used for dynamic social relationships. Extensive experimental results show that both familiar and unfamiliar node pairs have the same inter-contact time distribution, which closely follows a power-law decay up to a certain point, beyond which it begins to exponentially decay. The results also demonstrate that the inter-contact time distribution of familiar pairs decays faster than that of unfamiliar pairs, whether from a static or dynamic perspective. In addition, we also analyze the reason for the difference between the inter-contact time distributions of both unfamiliar and familiar pairs.