• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2008.05a
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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.

• Advances in aircraft and spacecraft science
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• v.8 no.1
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• pp.31-51
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• 2021

Selective laser melting (SLM), one of the most widely used powder bed fusion (PBF) additive manufacturing (AM) technology, enables the fabrication of customized metallic parts with complex geometry by layer-by-layer fashion. However, SLM inherently poses several problems such as the discontinuities in the molten track and the steep temperature gradient resulting in a high degree of residual stress. To avoid such defects, thisstudy proposes a temperature thread multiscale model of SLM for the evaluation of the process at different scales. In microscale melt pool analysis, the laser beam parameters were evaluated based on the predicted melt pool morphology to check for lack-of-fusion or keyhole defects. The analysis results at microscale were then used to build an equivalent body heat flux model to obtain the residual stress distribution and the part distortions at the macroscale (part level). To identify the source of uneven heat dissipation, a liquid lifetime contour at macroscale was investigated. The predicted distortion was also experimentally validated showing a good agreement with the experimental measurement.

• Journal of the Korean Society of Safety
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• v.14 no.4
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• pp.43-52
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• 1999

In this study, theoretical stress field analysis near the crack tip under tension-compression loading was performed. The results of the theoretical stress analysis were compared to the results of Finite Element Method(FEM). From this study, generation of tensile residual stress at crack tip was proved after 1-cycle of tension-compression loading, and the fracture toughness and the fracture load of a structure can be decreased by the residual stress.

• Korea-Australia Rheology Journal
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• v.19 no.4
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• pp.191-199
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• 2007

Precision injection molding process is of great importance since precision optical products such as CD, DVD and various lens are manufactured by those process. In such products, birefringence affects the optical performance while residual stress that determines the geometric precision level. Therefore, it is needed to study residual stress and birefringence that affect deformation and optical quality, respectively in precision optical product. In the present study, we tried to predict residual stress, final shrinkage and birefringence in injection molded parts in a systematic way, and compared numerical results with the corresponding experimental data. Residual stress and birefringence can be divided into two parts, namely flow induced and thermally induced portions. Flow induced birefringence is dominant during the flow, whereas thermally induced stress is much higher than flow induced one when amorphous polymer undergoes rapid cooling across the glass transition region. A numerical system that is able to predict birefringence, residual stress and final shrinkage in injection molding process has been developed using hybrid finite element-difference method for a general three dimensional thin part geometry. The present modeling attempts to integrate the analysis of the entire process consistently by assuming polymeric materials as nonlinear viscoelastic fluids above a no-flow temperature and as linear viscoelastic solids below the no-flow temperature, while calculating residual stress, shrinkage and birefringence accordingly. Thus, for flow induced ones, the Leonov model and stress-optical law are adopted, while the linear viscoelastic model, photoviscoelastic model and free volume theory taking into account the density relaxation phenomena are employed to predict thermally induced ones. Special cares are taken of the modeling of the lateral boundary condition which can consider product geometry, histories of pressure and residual stress. Deformations at and after ejection have been considered using thin shell viscoelastic finite element method. There were good correspondences between numerical results and experimental data if final shrinkage, residual stress and birefringence were compared.

• Journal of Welding and Joining
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• v.27 no.6
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• pp.31-35
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• 2009

When conducting CTOD test, especially in thick welded steel plate, fatigue pre-cracking occasionally failed to satisfy the requirements of standards thus making the test result invalid. Internally accumulated residual stress of test piece has been thought as one of the main reasons. The propagation of fatigue crack, started from the tip of machined notch, which might have propagated irregularly due to residual stress field. To overcome this kind of difficulty three methods to modify the residual stress are suggested in standard i.e. local compression, reverse bending and stepwise high-R ratio method. In this paper not only multi pass welding but also local pre-compressing process of thick steel plate has been simulated using finite element method for clarifying variation of internal welding residual stress. The simulated results show that welding residual stress is compressive in the middle section of the model and it is predominantly increased after machining the specimen. Comparing as-welded state all component of the welding residual stress changing to compressive in the tip of machine notch whereas residual stress of the outer area remain as tensile condition relatively. Analysis results also show that this irregular residual stress distribution is improved to be more uniformly by applying local compression.

• Proceedings of the Korea Committee for Ocean Resources and Engineering Conference
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• 2002.10a
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• pp.329-334
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• 2002

Preflex beam is a method of construction to hold the Pre-compressive stresses over the concrete pier by the Preflexion load. During the fabrication of the girder, welding causes the welding residual stresses. The welding residual stresses must be relieved to generate the accurate compressive pre-stresses that is designed. On this study, to find out the thermal distribution characteristics on the girder by welding, both the three-dimensional finite element analysis and the two-dimensional finite element analysis in a quasi-steady state is carried out. After comparing with each result between the three-dimensional analysis and the two-dimensional analysis, finite element analysis is carried out against the actual girder and analyze welding thermal distribution characteristic over the preflex beam and obtain the input data for the analysis of the welding residual stresses.

• International Journal of Automotive Technology
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• v.8 no.2
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• pp.243-248
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• 2007

The durability of a cylinder head is influenced by the thermal and mechanical history during the manufacturing process, as well as engine operation. In order to improve the durability of cylinder head, both load from engine operation and the preload conditions from the manufacturing process must be considered. The aluminum cylinder head used for a HSDI diesel engine is investigated to reduce the possibility of high cycle fatigue crack in this study. FE analysis is performed to elucidate the mechanism of high cycle fatigue crack in the HSDI diesel cylinder head. Two separate approaches to increase the durability of the cylinder head are discussed: reducing load from engine operation and re-arranging preload conditions from the manufacturing process at the critical location of the cylinder head. Local design changes of the cylinder head and modification of pretension load in the cylinder head bolt were investigated using FE analysis to relieve load at the critical location during engine operation. Residual stress formed at the critical location during the manufacturing process is measured and heat treatment parameters are changed to re-arrange the distribution of residual stress. Results of FE analysis and experiments showed that thorough consideration of the manufacturing process is necessary to enhance the durability of the cylinder head.

• Journal of the Korean Ceramic Society
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• v.31 no.8
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• pp.902-910
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• 1994

Residual stress measurements were made for cylindrical glass rods to compare experimental results with the calculated values obtained by Instant Freezing Model. According to the photoelastic measurements, the stress ratio of surface compression and center tension was increased from 1.4 to 2.0 as the heat-treatment temperature was lowered, the fictitious forzen temperature was found to be closer to the heat-treatment temperature and the fictitious coefficient was increased.

• Proceedings of the KWS Conference
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• 2006.10a
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• pp.18-19
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• 2006

The purpose of this study is to evaluate the behavior of residual stress at the weldment of pressure vessel by local Post Weld Heat Treatment(PWHT). In order to do it, residual stress were measured before and after local PWHT by XRD on the test piece first. And then, the results of finite element(FE) analysis based on thermal-elasto-plastic-creep theories were verified by comparing with the measured results.

• Journal of the Korean Data and Information Science Society
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• v.8 no.1
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• pp.99-109
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• 1997

The estimation procedure of mean residual life function has been placed an important role in the study of survival analysis. In this paper, the product limit estimator on left truncated and right censoring model is proposed with asymptotic properties. Also, the small sample properties are investigated through the Monte Carlo study and the proposed product limit type estimator is compared with ordinary Kaplan-Meier type estimator.