• Journal of the Korea Concrete Institute
• /
• v.18 no.3 s.93
• /
• pp.425-429
• /
• 2006

Horizontal cracks at the mid-depth of concrete slabs were observed at a section of the continuously reinforced concrete pavement(CRCP) structures on the Korea Highway Corporation's Test Road. To investigate the existence and the extent of horizontal cracks in the concrete slab, a number of cores were taken from the section of CRCP. To identify the causes of horizontal cracks, numerical analyses were conducted. Several variables relative to design, material, and environment were considered in the studies to evaluate possible causes of horizontal cracking. A numerical model of CRCP was developed using the finite element discretization, and the shear and normal tensile stress distributions in CRCP were investigated with the model. Numerical analysis results show that the maximum shear and normal tensile stresses develop near the depth of steel bars at transverse cracks. If those maximum stresses reach the strength of concrete, horizontal cracks occur. The maximum stresses become higher as the environmental loads, coefficient of thermal expansion of concrete, and elastic modulus of concrete increase.

• Journal of Ocean Engineering and Technology
• /
• v.21 no.5
• /
• pp.55-60
• /
• 2007

Bellows are mechanical components which prevent the damage of system by absorption of the vibration and the displacement of axle and radial direction. Thermal piping system is expanded by the fluid of the high temperature from the heat engine inside. At this time, bellows prevent the damage of the piping due to the thermal expansion. Recently, design of bellows is required to fit some other operational environments which are not suggested in the E.J.M.A code book. And it is difficult to produce and to maintain bellows of high temperature and high pressure bemuse of its complicated shape and this causes the manufacturing cost to rise. The objective of this study is to determine optimum shape of bellows which can endure in the high temperature and high pressure. The maximum stress has an effect on the fatigue life of bellows, therefore it needs to be minimized. This study attempts to find a shape which minimizes the stress occurring in the bellows by the design of experiment. The model used in this study is not presented in the E.J.M.A code book, therefore, from the result of design of experiment we find the factors which give effects on the characteristic value and we presents the recession model using the RSM, which can predict the characteristic values depending on the change of factor values.

• International Journal of Highway Engineering
• /
• v.16 no.6
• /
• pp.95-104
• /
• 2014

PURPOSES : The purpose of this study is to evaluate the degree of restraint (DOR) of longitudinal steel at continuously reinforced concrete pavement (CRCP) against environmental loadings. METHODS : To measure the longitudinal steel strain, 3-electrical resistance and self-temperature compensation gauges were installed to CRCP test section (thickness = 250mm, steel ratio = 0.7%) and continuously measured 10 min. intervals during 259 days. In order to properly analyze the steel strains first, temperature compensation process has been conducted. Secondly, measured steel strains were divided into 12 phases with different events such as before paving, during concrete hardening, and after first cracking, etc. RESULTS : Thermal strain rate (TSR) concept is defined as the linear strain variations with temperature changes and restraints rate of longitudinal steel against environmental loadings (especially thermal loading) with different cases is defined as degree of restraint(DOR). New concept of DOR could be indirect indicator of crack width behaviors of CRCP. CONCLUSIONS : Before paving, DOR of longitudinal steel is almost same at the coefficient of thermal expansion of steel ($12.44m/m/^{\circ}C$) because of no restraint boundary condition. After concrete pouring, DOR is gradually changed into -1 due to concrete stiffness developing with hydration. After first cracking at crack induced area, values of DOR are around -3~-5. The negative DOR stands for the crack width behavior instead of steel strain behavior. During winter season, DOR reached to -5.77 as the highest, but spring this values gradually reduced as -1.7 as the lowest. Based on this observation, we can presume crack width decreased over time within the time frame of this study. This finding is not consistent with the current theory on crack width variations over time, so further study is necessary to identify the causes of crack width reducing. One of the reasons could be related to concrete stress re-distribution and stress relaxation.

• Journal of Korean Society of Steel Construction
• /
• v.18 no.6
• /
• pp.717-726
• /
• 2006

Traditional external post-tensioning method using either steel bars or tendons is commonly used as a retrofitting method for steel composite bridges. However, the method has some disadvantages such as stress concentration at anchorages and inefficient load-carrying capability of live loads. Multi-stepwise prestressing method using thermal expanded coverplate is a newly proposed prestressing method, which was originally developed for prestressing steel structures. A new retrofitting method for steel girder bridges founded on a simple concept of thermal expansion and contraction of cover plate, the method is a hybrid of and combines the advantages of external post-tensioning and thermal prestressing. In this paper, basic concepts of the method are presented and an illustrative experiment is introduced. From actual experimental data, the thermal prestressing effect was substantiated and the FEM approach for its analysis was verified. The retrofitting effects ofa single-span bridge were analyzed and the feasibility of the developed method was examined.

• Journal of the Microelectronics and Packaging Society
• /
• v.11 no.4 s.33
• /
• pp.13-22
• /
• 2004

In MEMS devices, packaging induced stress or stress induced structure deformation become increasing concerns since it directly affects the performance of the device. In this paper, deformation behavior of MEMS gyroscope package subjected to temperature change is investigated using high-sensitivity moire interferometry. Using the real-time moire setup, fringe patterns are recorded and analyzed at several temperatures. Temperature dependent analyses of warpages and extensions/contractions of the package are presented. Linear elastic behavior is documented in the temperature region of room temperature to $125^{\circ}C$. Analysis of the package reveals that global bending occurs due to the mismatch of thermal expansion coefficient between the chip, the molding compound and the PCB. Detailed global and local deformations of the package by temperature change are investigated, concerning the variation of natural frequency of MEMS gyro chip.

• Transactions of the Korean Society of Mechanical Engineers A
• /
• v.34 no.9
• /
• pp.1227-1233
• /
• 2010

In the hot section of a gas turbine, the turbine blades were protected from high temperature by providing a thermal barrier coating (TBC) as well as by cooling air flowing through internal passages within the blades. The cooling air then passed through discrete holes on the blade surface, creating a film of cooling air that further protects the surface from the hot mainstream flow. The holes are subjected to stresses resulting from the lateral growth of thermally grown oxide, the thermal expansion misfit between the constituent layers, and the centrifugal force due to high-speed revolution; these stresses often result in cracking. In this study, the deformation and cracks occurring near a hole on a heat-resistant alloy subjected to thermo-mechanical cycling were investigated. The experiment showed that cracks formed around the hole depending on the applied stress level and the number of cycles. These results could be explained by our analytic solution.

• Korean Journal of Materials Research
• /
• v.8 no.11
• /
• pp.993-998
• /
• 1998

In this study TiNi/A16061 shape memory composite is introduced as one of new material using a shape memory alloy. High tensile strength of composite due to compressive residual stress in matrix by the shape memory effect of TiNi fiber can be produced. This composite can remove the tensile residual stress by the difference of coefficients of thermal expansion between fiber and matrix. one of the significant weak point of metal matrix composite. In this paper, shape memory composites are made by squeeze casting. And then, microstructure and fatigue properties of the composites by shape memory effect above inverse transformation temperature A, of TiNi alloy are discussed. The results of the fatigue crack control properties of TiNi/A16061 shape memory composite by a squeeze casting are summarized as follows the effect of fatigue crack propagation control at 363K increases according to the increase of volume fraction and prestrain in composites.

• Korean Journal of Plant Tissue Culture
• /
• v.28 no.5
• /
• pp.249-254
• /
• 2001

Effects of ultraviolet stress and elicitors, cellulase and jasmonic acid (JA), for the production of capsidiol, sesquiterpenoid phytoalexin, in seedlings and suspension cultures of pepper (Capsicum annum L. cv, Soobicho) were examined. Extracellular capsidiol in the medium of suspension cultures was absent from control cells, but accumulated in the elicitor treated cells with 0.05 $\mu\textrm{g}$/mL of cellulase or 0.1 $\mu\textrm{g}$/mL JA. Elicited cells gradually decreased their viability and eventually died within 48 hours of elicitor treatment by the toxicity of capsidiol accumulated in the culture medium. Capsidiol production in the leaves of pepper seedlings was markedly increased by the treatment of ultraviolet stress and reached maximum level at 48 hours of irradiation. Infiltration of elicitors, 0.05 $\mu\textrm{g}$/mL cellulase or 1.0 $\mu\textrm{g}$/mL JA, to the surface of leaf or fruit, stimulated the elicitation of the cells which resulted in the production of capsidiol and expansion of pathogene-like lesion around the elicitor treated region.

• Transactions of the Korean Society of Pressure Vessels and Piping
• /
• v.4 no.2
• /
• pp.40-45
• /
• 2008

There exist many types of pipe and component fatigue through vibrations, thermal fatigues or shifting. In some cases of thermal stratification/thermal fatigue, pipes & components are receiving thermal stress by means of material expansion and shrinkage by continuous thermal repetitive variation. Small cracks initially occur on the inside surface by thermal stress. These cracks grow in depth the pipe wall and finally come to a rupture. Pipe parts of susceptibility to thermal stratification and thermal fatigue are now being examined by conventional UT(ultrasonic test) as volumetric examination. It is difficult to fully satisfy the code & standards requirements because 3" weldolet weldments of RCS 16" pipe to 3" branch connection lines have complex structural shape. To solve the problems of conventional UT examination, we made a realistic mock-up and UT calibration block. We performed a simulation of phased array UT utilizing CIVA as NDE(Non-Destructive Examination) simulation software. Also we designed phased array UT transducer and wedge, optimal frequency by using simulation data. We performed phased array UT experiment through mock-up including artificial flaws(notch). The phased array UT technique is finally developed to improve the reliability of ultrasonic test at RCS 16" pipe to 3" branch connection weld.

• Journal of Surface Science and Engineering
• /
• v.30 no.4
• /
• pp.272-280
• /
• 1997

The Thermal Barrier Coation(TBC) to improve the that barrier and wear resistant propenrty in high temperature ofthe aircraftength between the accumlation of the aircraft engine and the automobile engine has usually the two layer structure. One is a creamic top layer for heat insulation and the other is a metal bond layer to facilitate the bond strength between the top ceramic layer and the substrate. But, the coated layers should be peeled off because of the accumulation of the thermal stress by the differance of the thermal expantion coefficient between metal and ceramics in a hrat cyclic environment. In this study, the intermediate layer by plasm spray process was introduced to reduce the thermal stress. The powders of plasm spray coating were the Yttria Stabilized Zirconia (YSZ), the Magnesia Stabillized Zirconia(MSZ) and NiCrAlY. the intermediate layer was sprayed with the powders of the bond cast for the purpose of test were executed. The high temperature wear resistance tends to decreasnceee wear and thermal shock test were exeucuted. The high temperature were resistance of the YSZ TBC is better that of the MSZ TBC. The wearrsistance tends to decrease accoring to incresing the temperature between $400^{\circ}C$to $600^{\circ}C$. The thermal shock life of the 3 layer TBC with YSZ top casting was the most outstanding thermal shock rsisstasnce. This means that the intermediate layer should play an importnat roll to alleviate the diffrerence of the thermal expansion coef frcients between metallic layer and cermics layer.