• Journal of the Computational Structural Engineering Institute of Korea
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• v.29 no.5
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• pp.437-445
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• 2016

Recently, polyurethane foam has been used in various industry fields to preserve temperature environment of structures, and a wide range of loads from the static to the dynamic are imposed on the material during a life period. The biggest characteristic of polyurethane foam is porosity as being polymeric material, and it is generally known that insulation performance of the material strongly depends on internal void size. In addition, polyurethane foam's mechanical behavior has high dependence on strain rate and temperature as well as being highly non-linear ductile for compression. In the non-linear compressive behavior, volume fraction of voids and elastic modulus decrease as strain increases. Therefore, in this study, temperature-dependent viscoplastic-damage constitutive model was developed to describe the non-linear compressive behavior with the aforementioned features of polyurethane foam.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.27 no.9
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• pp.1562-1570
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• 2003

The prediction of the inelastic behavior of the structure is an essential part of reliability assessment procedure, because most of the failures are induced by the inelastic deformation, such as creep and plastic deformation. During decades, there has been much progress in understanding of the inelastic behavior of the materials and a lot of inelastic constitutive equations have been developed. The complexity of these constitutive equations generally requires a stable and accurate numerical method. The radial return mapping is one of the most robust integration scheme currently used. Nonlinear kinematic hardening model of Armstrong-Fredrick type has recovery term and the direction of kinematic hardening increment is not parallel to that of plastic strain increment. In this case, The conventional radial return mapping method cannot be applied directly. In this investigation, we expanded the radial return mapping method to consider the nonlinear kinematic hardening model and implemented this integration scheme into ABAQUS by means of UMAT subroutine. The solution of the non-linear system of algebraic equations arising from time discretization with the generalized midpoint rule is determined using Newton method and bisection method. Using dynamic yield condition derived from linearization of flow rule, the integration scheme for elastoplastic and viscoplastic constitutive model was unified. Several numerical examples are considered to demonstrate the efficiency and applicability of the present method.

• Journal of the Society of Naval Architects of Korea
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• v.34 no.1
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• pp.77-86
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• 1997

The advanced development in many fields of engineering and science has caused much interests and demands for crashworthiness and non-linear dynamic transient analysis of structure response. Crash and impact problems have a dominant characteristic of large deformation with material plasticity for short time scales. The structural material shows strain rate-dependent behaviors in those cases. Conventional rate-independent constitutive equations used in the general purposed finite analysis programs are inadequate for dynamic finite strain problems. In this paper, a rate-dependent constitutive equation for elastic-plastic material is developed. The plastic stretch rate is modeled based on slip model with dislocation velocity and its density so that there is neither yielding condition, nor loading conditions. Non-linear hardening rule is also introduced for finite strain. Material constants of present constitutive equation are determined by experimental data of mild steel, and the constitutive equation is applied to uniaxile tension loading.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.35 no.4
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• pp.769-777
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• 2015

In this study, the rheological models were introduced and developed to reflect rate dependent tensile behaviour of concrete. In general, mechanical properties(e.g. strength, elasticity, and fracture energy) of concrete are increased under high loading rates. The strength of concrete shows high rate dependency among its mechanical properties, and the tensile strength has higher rate dependency than the compressional strength. To simulate the rate dependency of concrete, original spring set of RBSN(Rigid-Body- Spring-Network) model was adjusted with viscous and friction units(e.g. dashpot and Coulomb friction component). Three types of models( 1) visco-elastic, 2) visco-plastic, and 3) visco-elasto- plastic damage models) are considered, and the constitutive relationships for the models are derived. For validation purpose, direct tensile test were simulated, and characteristics of the three different rheological models were compared with experimental stress-strain responses. Simulation result of the developed visco-elasto-plastic damage(VEPD) model demonstrated well describing and fitting with experimental results.

• Geotechnical Engineering
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• v.6 no.4
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• pp.19-32
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• 1990

Generally foundation is composed of complicated multi - layers. Primary objective of this study is to perform numerical analysis on the distribution of stresses on the subgrade with the variation of constitutive equations, the structures and the depth of layer, rigidity, loading condition, etc. Multi - layered soils has been treated as Burmister's elastic model. However, in this research it was intended to analyzed the distribution of stresses on the subgrade with all of the multi - layered soils by using the EVP(elasto - viscoplastic) model, one of the numerical program based on the Biot's equation as governing equation. The numerical results are compared with those by the Burmister's and the Fox'method, which in turn proves to be satisfactory. This research is aiming at investigating the mechanism of stress transfer within a foundation by using computer program for multilayers foundation.

• Computational Structural Engineering
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• v.2 no.1
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• pp.71-78
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• 1989

In this paper a computer program for displacement control method was developed, in which a certain displacement of the structure is increased and the applied loads and another displacements are obtained. To simplify the nonlinear structural analysis, the relationships of moment-curvature were linearized as elasto-softening model for over-reinforced concrete beam and as elasto-plastic-softening model for under-reinforced concrete beam. Since the result of the analysis of reinforced concrete beam depended on the element size beyond elastic zone, the relationship of moment-curvature was modified for each element by using the concept of fracture energy approach. Overall, analytical results accurately predicted the load-displacement behavior of reinforced concrete beams.

• Transactions of the Korean Society of Mechanical Engineers
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• v.19 no.6
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• pp.1411-1421
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• 1995

A rigid visco-plastic finite element method has been developed for modeling superplastic forming processes. The optimum pressure-time relationship for a target strain rate and thickness distributions was predicted using two-node line element based on membrane approximation for plane strain and axisymmetric condition. Analysis of superplastic forming was carried out using the developed program and the numerical results were compared to the values available in the literature for plane strain problems. For description of the contact between the dies and sheet, the direct projection method was applied to the complicated problem and the validity of the scheme was tested. Experiments for the various geometries such as hemisphere and cone were performed with the developed forming machine using the calculated optimum pressure-time curves. Comparison between analysis and experiments showed good agreement.

• Journal of the Korean Geotechnical Society
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• v.16 no.3
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• pp.39-46
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• 2000

본 논문에서는 배수재가 설치된 구역과 설치되지 않은 구역으로 구성된 Olga-C 시험성토지반의 변형률속도 의존적인 압밀거동을 서술하였다. 배수재가 설치된 지반이 압밀거동에 대한 변형률속도의 영향을 해석하기 위하여 응력-변형률-변형률 속도의 관계식(v-$\varepsilon$v- v)을 이용한 축대칭 비선형 점소성 모델을 제안하였다. 제안된 모델은 실험실과 현장의 변형률속도 차이뿐만 아니라 간극수압의 소산과 생성의 복합적인 압밀과정을 고려할 수 있다. 연직 및 반경방향의 배수효과에 의해 배수재가 설치된 지반(Zone B)에서 유발되는 변형률 속도는 배수재가 설치되지 않은 연약지반 (Zone A)의 변형률 속도보다 크다. 유발된 변형률 속도의 영향으로 Zone B의 선행압밀하중도 Zone A에서 유발되는 선행압밀하중보다 크다. Olga-C 지역의 Zone A 에서는 응력완화효과가 유발되지만, Zone B에서는 응력완화효과가 유발 되지 않았다.

• Proceedings of the Materials Research Society of Korea Conference
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• 2003.11a
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• pp.40-40
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• 2003

최근 나노기술의 발달과 더불어 나노재료에 대한 특성평가 요구가 높아지고 있고, 따라서 나노스케일에서 재료의 기계적 거동을 분석할 수 있는 나노인덴테이션 기법이 심도있게 연구되고 있다. 본 연구에서는 나노인덴테이션, 주사탐침현미경(SPM), 투과전자현미경(TEM) 기법을 이용하여 여러가지 재료의 탄성 소성 변형 거동과 팝인/괍아웃 현상을 조사하고 해석하였다. 나노인덴테이션 기법으로는 50 마이크로뉴턴 (5 mg) 이하의 매우 작은 하중 하에서는 접촉 응력조건이라도 인장시험에서 관찰되는 영구변형이 제로인 완전탄성 변형 거동을 관찰할 수 있었다. 또한, 50-250 마이크로 뉴턴의 하중 범위에서 재료는 탄성변형 이후에 갑작스런 항복거동과 더불어 수십-수백 나노미터를 미끌어지듯 변형하는 팝인(pop-in), 또는 탈선(excursion) 현상을 관찰할 수 있었다. 이 현상은 하중을 가하는 동안에 여러 번 발생하였으며 재료의 표면상태와 전위밀도와 밀접한 상관관계를 보였다. 반복 압입 시험에서는 전형적인 가공경화 현상으로 항복점이 높아지고 새로운 항복점 이후에야 다시 팝인 발생함을 보였다. 한편, 하중을 가할 때 발생하는 팝인과는 달리 하중을 제거할 때 급격히 회복하는 팝아웃 현상 또한 관찰되었다.

• Journal of the Korean Society for Precision Engineering
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• v.19 no.8
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• pp.126-133
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• 2002

The cutting thickness of ultra-precision machining is generally very small, only a few micrometer or even down to the order of a few nanometer. In such case, a basic understanding of the mechanism on the micro-machining process is is necessary to produce a high quality surface. When machining at very small depths of cut, metal flow near a rounded tool edge become important. In this paper a finite element analysis is presented to calculate the stagnation point on the tool edge or critical depth of cut below which no cutting occurs. From the simulation, the effects of the cutting speed on the critical depths of cut were calculated and discussed. Also the transition of the stagnation point according to the increase of the depths of cut was observed.