• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2008년도 춘계학술대회 논문집
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• pp.288-292
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• 2008

Plastics is commonly used in consumer electronics because of it is high strength per unit mass and good productivity. But plastic parts are usually distorted after injection molding due to the residual stress after filling, packing, cooling process, and etc. And plastic material is to be deteriorated according to various temperature conditions and operating time, which can be characterized by stress relaxation and creep. The viscoelastic behaviour of plastic materials in time domain can be expressed by the Prony series of the commercial code, ABAQUS. In the paper, the process to predict the post deformation under cyclic thermal loadings was suggested. The process was applied to the real panel, and the deformation predicted by the analysis was compared with that of real test, which showed the possibility of applying the suggested process to predict the post deformation of plastic product under thermal loadings.

• 소성∙가공
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• 제16권3호
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• pp.193-200
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• 2007

Simulations of the thermally-induced residual stresses and birefringence in freely quenched plates of polycarbonate were performed by using the linear viscoelastic and photoviscoelastic constitutive equations for the mechanical and optical properties, respectively, and the first order rate equation for volume relaxation. The predictions for the birefringence showed good agreement with experimental measurements. However, for initial temperature close to the glass transition temperature, some differences existed around the surface layer. Based on the simulation, the influences of various cooling conditions on the residual stress and birefringence in plates were investigated. The residual stress and birefringence increased with increasing initial temperature, decreasing coolant temperature and increasing heat transfer coefficient of coolants.

• Journal of Welding and Joining
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• 제22권2호
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• pp.51-58
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• 2004

Residual stresses, which can be produced during the welding process, play an important role in an industrial field. Welding residual stresses are exerting negative effect on the fatigue behavior and integrity of structure. In this study, as a result of the thermal elasto-plastic finite element analysis for the welds of a nuclear component, the residual stress distributions are estimated for as-welded condition. Also, finite element techniques are developed to simulate the relaxation of the residual stresses according to the various mechanical stress relieving(MSR) loads such as hydrostatic pressure loading, tensile pipe-end loading, and mechanical stress improvement process(MSIP) loading. Finally, the results of residual stress redistributions for various loading conditions are compared and reviewed qualitatively and quantitatively to find an optimum loading condition.

• 소성∙가공
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• 제33권3호
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• pp.200-207
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• 2024

In this study, we developed a semi-analytical approach for the numerical analysis of residual stress in oxide scales formed on hot-rolled steels. The oxide scale, formed during the hot rolling process, experiences complex interactions due to thermal and mechanical influences, significantly affecting the material's integrity and performance. Our research focuses on integrating various stress components such as thermal stress, growth stress, and creep behavior to predict the residual stress within the oxide layer. The semi-analytical method combines analytical expressions for each stress component with numerical integration to account for their cumulative effects. Validation through instrumented indentation tests confirms the reliability of our model, which considers thermal expansion coefficient (CTE) differences, scale growth, and creep-induced stress relaxation. Our findings indicate that thermal stress resulting from CTE differences significantly impacts the overall residual stress, with growth stress contributing a compressive component during cooling, and creep behavior playing a minor role in stress relaxation. This comprehensive approach enhances the accuracy of residual stress prediction, facilitating the optimization of material design and processing conditions for hot-rolled steel products.

• 폴리머
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• 제36권2호
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• pp.131-136
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• 2012

사출성형품은 성형 중 온도변화와 전단응력에 의해 잔류응력이 형성된다. 이 잔류응력으로 인해 짧게는 하루 이내에 길게는 수 년 이상 시간이 흐른 뒤 변형과 뒤틀림 등이 발생될 수 있다. 따라서 잔류응력 최소화를 위한 사출성형조건 및 사출성형품의 열처리가 요구된다. 본 논문에서는 투명한 사출성형품을 어닐링 조건 즉, 상대습도와 온도, 그리고 어닐링 시간을 변화시켜가며 각 어닐링 조건이 잔류응력 해소에 미치는 영향을 조사하였다. 상대습도가 작을 경우 잔류응력의 해소가 매우 적었으나 상대습도 50% 이상에서는 그 영향이 크게 작용하였다. 또한 유리전이온도 부근에서 어닐링 시 매우 빠른 잔류응력 해소를 보였으며 이는 광탄성 장치를 통하여 관찰할 수 있었다. 상대 습도보다는 어닐링 온도가 잔류응력 해소에 더 큰 영향을 미치고 있음을 확인할 수 있다. 시편에서 잔류응력이 해소되면서 시편에서 수지의 흐름방향으로는 수축이, 흐름의 직각방향으로는 팽창이 발생하여 변형이 일어나는 것이 관찰되었다. 어닐링 시 제품의 변형을 최소화 하기 위해서는 지그나 고정구로 시편을 구속한 환경에서 어닐링이 필요할 것으로 판단되었다.

• Journal of Advanced Marine Engineering and Technology
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• 제7권2호
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• pp.29-35
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• 1983

In general, for measuring residual stresses in plane stress state, two principal stresses {\sigma}_1 , {\sigma}_2$ and their directions .theta., should be determined. Naturally, for deciding three unknowns ${\sigma}_1, {\sigma}_2, ${\theta}$-three informations are necessary and therefore three strain gages are required for determining residual stresses at one point. In this paper, we tried to measure the residual stresses of one point with only two strain gages by drilling twice the hole of different diameters at the point and by detecting relaxation strains for each hole-drilling. We present also the formulas for determining the residual stresses from relaxation strains detected by strain gages in each hole-drilling. We carried out experiment, determining principal stresses and their directions of specimens applied specified uniform stress, and compared experimental results with the values calculated by formulas presented in this paper. The values calculated by formulas presented in this paper are always a little greater than the experimental results.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2003년도 춘계학술대회
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• pp.438-444
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• 2003

A new method to measure residual stress in micron and nano scale films is described. In the theory it is based on Linear Elastic Fracture Mechanics. And in the techniques it depends on the combined capability of the focused ion beam (FIB) imaging system and of high-resolution digital image correlation (DIC) software. The method can be used for any film material (whether amorphous or crystalline) without thinning the substrate. In the method, a region of the film surface is highlighted and scanning electron images of that region taken before and after a long slot, depth a, is introduced using the FIB. The DIC software evaluates the displacement of the surface normal to the slot due to the stress relaxation by using features on the film surface. To minimize the influence of signal noise and rigid body movement, not a few, but all of the measure displacements are used for determining the real residual stress. The accuracy of the method has been assessed by performing measurements on a nano film of diamond like carbon (DLC) on glass substrate and on micro film of aluminum oxide thermally grown on Fecrally substrate. It is shown that the new method determines the residual stress ${\sigma}_R=-1.73$ GPa for DLC and ${\sigma}_R=-5.45$ GPa for the aluminum oxide, which agree quite well with ones measured independently.

• Journal of Biosystems Engineering
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• 제16권1호
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• pp.37-48
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• 1991

Rice plants are subjected to various forces such as natural force of wind and mechanical force of cultivating machines. Rheological behavior of the rice stem can be expressed in terms of three variables : stress, relaxation and time. The objectives of this study are to examine stress relaxation, creep and recovery characteristics on the rice stem in case of axial and radial loading. Stress relaxation with time was studied on three levels of loading rate and on four levels of applied stress. The results were summarized as follows : 1. The hysterisis losses of the rice stem distinctly observed at the radial compression in comparison with axial compression. The hysterisis loss implied that the stem to absorbed energy without being deformed beyond the yield point. 2. Ageneralized Maxwell model consisting of three elements gave a good description of the relaxation behavior of the rice stem. Rate of loading was more significant on the observed relaxation behavior within the short relaxation time, but there were little influences of rate of loading on the relaxation time. 3. The stress relaxation intensity and the residual stress increased in magnitude as the applied stress increased, but the relaxation time was little affected by the applied stress. 4. The coefficients of the stress relaxation model showed much differences in the radial compression and the axial compression, especially the higher relaxation stress of the third element was observed in the radial compression. 5. The behaviors of rice stem in creep and recovery test also might be represented by a four element Burger's model. But the coefficients of the creep model were different from those of the recovery model. 6. The steady-state phenomena of creep appeared at the stress larger than 20 MPa in Samkang and 1.8 MPa in Whajin. 7. The elastic modulus of the stem showed the range from 40 to 60 MPa. It could be considered, as a result, the rice stems had viscoelastic properties.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2003년도 추계학술대회
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• pp.1086-1091
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• 2003

The problem of residual stresses and fatigue behavior in welded structures is the main concern of welding research fields. The residual stresses and distortions of structures by welding exert negative effect on the safety of mechanical structures. Postweld heat treatment is usually carried out to relieve this residual stresses of welded joints. In this paper the influence of postweld heat treatment on fatigue life of butt-welded joint was investigated. To predict the effect of PWHT, an analytical model is developed by finite element and local strain approach and the result of fatigue life analysis is compared to experimental results. It is demonstrated that fatigue life estimates closely approximate the experimental results and PWHT provides some increase of fatigue lives in long-life fatigue region and no increase in short-life fatigue region because of the residual stress relaxation under tensile loads.

• 한국세라믹학회지
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• 제32권1호
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• pp.120-130
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• 1995

To analyze the mechanical property and the residual stress in functionally gradient materials(FGMs), disctype TZP/Ni-and TZP/SUS304-FGM were hot pressed using powder metallurgy compared with directly bonded materials which were fabricated by the same method. The continuous interface and the microstructure of FGMs were characterized by EPMA, WDS, optical microscope and SEM. By fractography, the fracture behavior of FGMs was mainly influenced by the defects which originated from the fabrication process. And the defectlike cracks in the FGMs induced by the residual stress have been shown to cause failure. This fact has well corresponded to the analysis of the residual stress distribution by Finite Element Method (FEM). The residual stress generated on the interface (between each layer, and matrix and second phase, respectively) were dominantly influenced on the sintering temperature and the material constants. As a consequence, the interfacial stability and the relaxation of residual stress could be obtained through compositional gradient.