• Journal of the korean academy of Pediatric Dentistry
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• v.35 no.1
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• pp.18-29
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• 2008

The purpose of this study was to compare the polymerization shrinkage of several filling methods using strain gauges. In this study, a light-emitting diode(LED) curing unit(Elipar Freeligh2, 3M EPSE, USA) and plasma arc lamp(PAL) curing unit(Flipo, LOKKI, France) were used for curing, Filtek $Z350^{TM}$(3M EPSE, USA) composite resin was used for the cavity filling. Sixty permanent bicuspid teeth, that were extracted for orthodontic treatment, were studied. The cavities were prepared on the occlusal surface and were filled using the following methods : 1) bulk filling, 2) parallel filling, 3) oblique filling The strain was recorded on the buccal, lingual, mesial and distal surfaces and the strain values were computed into stress values. The shear bond strength of each filling method was tested using a Micro Universal Testing machine. The results can be summarized as follows: 1. In the strain changes, all LED and PAL curing groups showed an increase on the buccal surface and a slow decrease as time elapsed. 2. In the strain changes of the mesial and distal surfaces, the decreases and increases were shown repeatedly and reduced as time elapsed. 3. There were no significant statistical strain changes among filling methods in the LED or PAL curing groups. 4. There were significant statistical strain changes between the LED and PAL curing groups on the buccal surface(p<0.05). 5. From the shear bond strength results, in the LED curing group, filling method 3 showed lower surface stress than filling method 1 and 2(p<0.05). In the PAL curing group, there were no significant statistical strain changes between each filling method. 6. The surface stress of each group was lower than the shear bond strength.

• Korean Journal of Materials Research
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• v.33 no.2
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• pp.54-62
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• 2023

The most comprehensive and particularly reliable method for non-destructively measuring the residual stress of the surface layer of metals is the sin²ψ method. When X-rays were used the relationship of ε_φψ-sin²ψ measured on the surface layer of the processing metal did not show linearity when the sin²ψ method was used. In this case, since the effective penetration depth changes according to the changing direction of the incident X-ray, σ_φ becomes a sin²ψ function. Since σ_φ cannot be used as a constant, the relationship in ε_φψ-sin²ψ cannot be linear. Therefore, in this paper, the orthogonal function method according to Warren's diffraction theory and the basic profile of normal distribution were synthesized, and the X-ray diffraction profile was calculated and reviewed when there was a linear strain (stress) gradient on the surface. When there is a strain gradient, the X-ray diffraction profile becomes asymmetric, and as a result, the peak position, the position of half-maximum, and the centroid position show different values. The difference between the peak position and the centroid position appeared more clearly as the strain (stress) gradient became larger, and the basic profile width was smaller. The weighted average strain enables stress analysis when there is a strain (stress) gradient, based on the strain value corresponding to the centroid position of the diffracted X-rays. At the 1/5 I_max max height of X-ray diffraction, the position where the diffracted X-ray is divided into two by drawing a straight line parallel to the background, corresponds approximately to the centroid position.

• Transactions of Materials Processing
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• v.9 no.4
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• pp.404-412
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• 2000

It is desirable to use the square grid analysis with the aid of the stereo vision and image processing techniques in order to automatically measure the surface-strain distribution over a stamped part. But this method has some inherent problems such as the difficulty in enhancement of bad images, the measurement error due to the digital image resolution and the limit of the area that can be measured at a time. Therefore, it is still hard to measure the strain distribution over the entire surface of a medium-or large-sized stamped part even by using an automated strain measurement system. In this study, several methods which enable to solve these problems considerably without losing accuracy and precision In measurement are suggested. The superposition of images that have different high-lightened or damaged part from each other gives much enhanced image. A new algorithm for constructing of the element connectivity from the line-thinned image helps recognize up to 1,000 elements. And the geometry assembling algorithm including the global error minimization makes it possible to measure a large specimen with reliability and efficiency.

• Transactions of Materials Processing
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• v.20 no.3
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• pp.229-235
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• 2011

The formability of magnesium alloy sheets at room temperature is generally low because of the inherently limited number of slip systems, but higher at temperatures over $150^{\circ}C$. Therefore, prior to the practical application of these materials, the forming limits should be evaluated as a function of the temperature and strain rate. This can be achieved experimentally by performing a series of tests or analytically by deriving the corresponding modeling approaches. However, before the formability analysis can be conducted, a model of flow stress, which includes the effects of strain, strain rate and temperature, should be carefully identified. In this paper, such procedure is carried out for Mg alloy AZ31 and the concept of flow stress surface is proposed. Experimental flow stresses at four temperature levels ($150^{\circ}C$, $200^{\circ}C$, $250^{\circ}C$, $300^{\circ}C$) each with the pre-assigned strain rate levels of $0.01s^{-1}$, $0.1s^{-1}$ and $1.0s^{-1}$ are collected in order to establish the relationships between these variables. The temperature-compensated strain rate parameter which combines, in a single variable, the effects of temperature and strain rate, is introduced to capture these relationships in a compact manner. This study shows that the proposed concept of flow stress surface is practically relevant for the evaluation of temperature and strain dependent formability.

• Transactions of the Korean Society of Mechanical Engineers
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• v.11 no.2
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• pp.191-197
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• 1987

It has been well known that the fracture mechanics can be applied to large through crack growth. But the growth rate of small surface cracks initiated from a small defect under rotary bending fatigue tests can not be treated as a function of stress intensity factor range. In this paper, to investigate the growth behavior of surface small fatigue cracks in the view-point of both fracture mechanics and strength of materials, the fatigue test has been carried out on two kinds of plain carbon steels with a small surface defect. Applying the concept of the cyclic strain intensity factor range .DELTA. $K_{\epsilon}$/$_{t}$ to the analysis of small surface fatigue crack growth, it is found that the relationship between cyclic strain intensity factor range and crack growth rate shows linear relation on logarithmic coordinates regardless of defect sizes and two kinds of carbon steels.s.s.

• Korean Journal of Computational Design and Engineering
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• v.7 no.3
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• pp.190-201
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• 2002

Surfaces of many engineering structures, specially, those of ships and airplanes are commonly fabricated as doubly curved shapes as well as singly curved surfaces to fulfill functional requirements. Given a three dimensional design surface, the first step in the fabrication process is unfolding or planar development of this surfaces into a planar shape so that the manufacturer can determine the initial shape of the flat plate. Also a good planar development enables the manufacturer to estimate the strain distribution required to form the design shape. In this paper, an algorithm for optimal approximated development of a general curved surface, including both singly and doubly curved surface is developed in the sense that the strain energy from its planar development to the design surface is minimized, subjected to some constraints. The development process is formulated into a constrained nonlinear programming problem, which is on basis of deformation theory and finite element. Constraints are subjected to characteristics of the fabrication method. Some examples on typical surfaces and the practical ship surfaces show the effectiveness of this algorithm.

• Korean Journal of Optics and Photonics
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• v.22 no.1
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• pp.1-4
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• 2011

A new fabrication technique of long-period fiber gratings with periodically surface-etched structures is proposed. Transmission characteristics of the periodically surface-etched long-period fiber gratings are improved by changing strain because of variation of coupling strength between the core and the cladding modes. The sensitivities of the periodically surface-etched long-period fiber gratings to strain, torsion, and ambient refractive index were measured. The strain and the torsion sensitivities were measured to be $-0.033\;dB/\mu\varepsilon$ and -1.30 nm m/rad, respectively. The ambient index sensitivity was measured to be -31.33 nm/RIU from a range from 1.33 to 1.42.

• Journal of Power System Engineering
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• v.12 no.3
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• pp.55-59
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• 2008

In this paper, we have investigated on collection efficiency of fine particle of glass fiber-filter bag according to the surface treatment. The solution consisted of polytetrafluoroethylene(teflon), graphite powder, silicon resin and water was used as a basic surface treatment agent. Tensile strength of glass filter-bag increased with up to 3hrs and then decreased with surface treatment time. Tensile strength and initial modulus of the glass fiber-filter bag treated by iodine after basic surface treatment for 3hrs were lower than those of basic surface treatment for 3hrs, however collection efficiency and fracture strain were higher than those of basic surface treatment for 3hrs. Glass fiber-filter bag with lower initial modulus and more strain will be extend the durable period and the one treated by iodine after basic surface treatment 3or 3hrs is expected high collection efficiency of fine particle. This method makes it possible to manufacture glass fiber-filter bag of the optimum condition.

• Journal of Ocean Engineering and Technology
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• v.10 no.1
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• pp.53-64
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• 1996

In this paper, rotating bending fatigue tests have been carried out to investigate the growth behabiors of surface fatigue crack initiated from a small artificial surface defect, that might exist in real structures, on 2024-T3 and 6:4 brass. The test results are analysed in the viewpoints of both strength of materials and fracture mechanics, it can be concluded as follows. The effect of a small artificial surface defect upon the fatigue strength is very large. The sensitivity of 2024-T3 on the defect is higher than that of 6:4 brass. The growth behavior of the surface fatigue crack of 2024-T3 is different from that of 6:4 brass. The growth rate of the surface fatigue crack of 2024-T3 is considerably rapid in the early stage of the fatigue life and apt to decrease in the later stage. It was impossible to establish a unifying approach in the analysis of crack growth begabior of 2024-T3 and 6:4 brass using the maximum stress intensity factor because of their dependence on stress level. But if the elastic strain and cyclic total strain intensity factor range were applied to obtain the growth rate of surface fatigue cracks of the materials, the data were found to be nearly coincided.

• Journal of The Korean Society of Agricultural Engineers
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• v.55 no.2
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• pp.1-8
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• 2013

This study was performed for the purpose of analyzing the effect of tide on the stress-strain behavior in the boundary surface of sea dike embankment. Tide is a dynamic condition, but there are not suitable numerical models to solve the dynamic embankment condition caused by tide. So the analysis was simplified to quasi dynamic as follow. First, seepage by tide was analyzed according to elapsed time, and the results of the analysis at every hour during one periodic cycle time of 12 hours were applied to the pore water pressure conditions of stress-strain analysis with hyperbolic model by Duncan and Chang. The place at which maximum shear strain took place in the analysis result moved up and down repeatedly along the boundary of the dredged sand fill section and the crashed stone filter section. The value of maximum shear strain was large at high water level of tide. This result means that contraction and relaxation occur in turn repeatedly at every specific position along the boundary, and the repeated action compact loose position with sand moved down from the upper position by gravity. The experiment with the small sea dike model showed the result consistent with the numerical analysis. The surface of sea side on the dike collapsed at high water level after a couple of repetition of the rising and falling of water.