• 한국생산제조학회지
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• 제9권5호
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• pp.47-57
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• 2000

In this study, a fundamental approach to make clear the mechanism of the mutual interference and coalescence of stress fields in the vicinity of two crack tips on the process of their slow growth, using boundary element method. Automatic generation of quadratic discontinuous elements along both of the crack boundaries which can be defined by an arbitrary piece-wise straight geometry. The direction of the crack-extension increment is predicted by the maximum principal stress criterion, corrected to account for the discreteness of the crack extension. Along the computed direction, the crack is extended one increment. Automatic incremental crack-extension analysis with no remeshing, computation of the stress intensity factors by J-integral. Numerical stress intensity factors for two growing cracks in plane-homogeneous regions were determined.

• 한국지반공학회:학술대회논문집
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• 한국지반공학회 1994년도 연약지반처리위원회 봄 학술발표회 논문집 연약지반처리
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• pp.3-12
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• 1994

Based on the shear failure mechanism, hydraulic fracturing criteria are extended to three dimensional stress state. According to the situation of the directions of borehole and major principal stress axes, three equations can be derived for three dimensional hydraulic fracturing problems. By comparing these equations, a single criterion is selected for hydraulic fracturing pressure in cohesive soils. The criterion is a function of maximum principal stress, minimum principal stress and soil parameters in UU conditions. The equation indicates that with any increase in maximim principal stress, hydraulic fracturing pressure decreases. In order to prove the integrity of the criteria, laboratory tests are performed on compacted cubical specimens using true a triaxial apparatus. The shape and direction of fractures are determined by injecting colored water after fracture initiation. It is found that the direction of fractures are perpendicular to the o1 plane.

• 터널과지하공간
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• 제31권6호
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• pp.480-493
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• 2021

일반화된 Hoek-Brown(GHB) 파괴기준식은 GSI 값을 이용하여 현장 암반조건이 반영된 강도정수 값을 효과적으로 결정할 수 있기 때문에 암반공학 분야에서 표준 파괴기준식의 하나로 인식되고 있다. 그러나 GHB 파괴기준식의 비선형적 형태는 이 식의 수학적 취급을 어렵게 하고 이 식의 적용 범위를 제약하는 요인이 되고 있다. GHB 파괴기준식의 이러한 단점을 극복하기 위한 노력의 하나로 Taylor 다항함수 근사원리를 적용하여 파괴 최대주응력에 대응하는 최소주응력을 근사적으로 계산할 수 있는 명시적, 해석적 수식을 유도하였다. 근사식으로 구한 최소주응력과 수치해석적으로 계산한 정해를 비교하여 이 연구에서 유도한 최소주응력 근사식의 정확성을 검증하였다. 연구결과의 응용사례를 제시하기 위해 근사 최소주응력 계산식을 활용하여 GHB 암반에 굴착된 원형터널 주변에 예상되는 소성영역의 등가 마찰각과 등가 점착력을 계산하였다. 소성영역의 등가 Mohr-Coulomb 강도정수를 정밀하기 산정하기 위해서는 mi, GSI, 초기지압의 크기를 동시에 고려하는 것이 중요한 것으로 나타났다.

• 대한기계학회논문집A
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• 제28권3호
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• pp.304-310
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• 2004

In this paper, cracking threshold for nanoindentation is analyzed by using 3D finited-element method. The analysis by maximum principal stress criterion can obtain the reliable results for determining to crack initiation location and load. Because the ratio of maximum principal stress to indentation depth for Victors indentation is smaller than flat-plane-column indentation and cracking for Victors indentation occurs from the inner part of specimen difficult to measure crack length, the nanoindentation facture test for flat-plane-column indentation is more effective.

• 한국세라믹학회지
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• 제28권10호
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• pp.810-818
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• 1991

Fracture responses of Al2O3 tubes were investigated for various loading paths under combined tension/torsion. The fracture criterion did not depend on loading paths. Fracture angles agreed well with the maximum tensile stress criterion. As the loading condition approaches a shear dominant state, the tensile principal stress at fracture increases compared to the uniaxial fracture strength. By using the Weibull modulus obtained from tension and torsion tests, the Weibull statistical fracture strengths were compared with experimental data. This comparison suggests that fracture may occur at the surface of the specimen when tensile stress is dominant, but within the volume of the specimen when shear stress is dominant. The Weibull fracture strength increased as the loading conition approached a shear dominant state, but underestimated compared to experimental data. Finally, a new fracture criterion was proposed by including the effect of compressive principal stress. The proposed criterion agreed well with experimental data of Al2O3 tubes not only at combined tension/torsion but also at balanced biaxial tension.

• Journal of Welding and Joining
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• 제17권6호
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• pp.25-31
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• 1999

Stainless steel sheets are commonly used for vehicles such as the bus and the train. These are mainly fabricated by spot-welding. By the way, its fatigue strength is lower than base metal due to high stress concentration at the nugget. edge of the spot-welding. By the way, its fatigue strength is lower than base metal due to high stress concentration at the nugget edge of the spot-welding point. Especially, it is influenced by welding conditions as well as geometrical factors of spot welded joint. Therefore, it is not too much to say that structural rigidity and strength of spot-welded structures is decided by fatigue strength of spot welded lap joint. Thus, it is necessary to establish a reasonable and systematic long life design criterion for the spot-welded structure. In this study, numerical stress analysis was performed by using 3-dimensional finite element model on IB-type spot-welded lap joint of 304 stainless steel sheet under tension-shear load. Fatigue tests were also conducted on them having various thickness, joint angle, lapped length, and width of the plate. From the results, it was found that fatigue strength of IB-type spot-welded lap joints was influenced by its geometrical factors, however, could be systematically rearranged by maximum principal stress ({TEX}$σ_{1max}${/TEX}) at the nugget edge of the spot-welding point.

• Geomechanics and Engineering
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• 제36권4호
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• pp.355-366
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• 2024

The original elastoplastic Hardening Soil model is formulated actually partly under hexagonal pyramidal Mohr-Coulomb failure criterion, and can be only used in specific stress paths. It must be completely generalized under Mohr-Coulomb criterion before its usage in engineering practice. A set of generalized constitutive equations under this criterion, including shear and volumetric yield surfaces and hardening laws, is proposed for Hardening Soil model in principal stress space. On the other hand, a Mohr-Coulumb type yield surface in principal stress space comprises six corners and an apex that make singularity for the normal integration approach of constitutive equations. With respect to the isotropic nature of the material, a technique for processing these singularities by means of Koiter's rule, along with a transforming approach between both stress spaces for both stress tensor and consistent stiffness matrix based on spectral decomposition method, is introduced to provide such an approach for developing generalized Hardening Soil model in finite element analysis code ABAQUS. The implemented model is verified in comparison with the results after the original simulations of oedometer and triaxial tests by means of this model, for volumetric and shear hardenings respectively. Results from the simulation of oedometer test show similar shape of primary loading curve to the original one, while maximum vertical strain is a little overestimated for about 0.5% probably due to the selection of relationships for cap parameters. In simulation of triaxial test, the stress-strain and dilation curves are both in very good agreement with the original curves as well as test data.

• 소성∙가공
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• 제25권2호
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• pp.91-95
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• 2016

This paper is concerned with modeling of a ductile fracture criterion for sheet metal considering anisotropy to predict the sudden fracture of advanced high strength steel (AHSS) sheets during complicated forming processes. The Lou−Huh ductile fracture criterion is modified using the Hill’s 48 anisotropic plastic potential instead of the von Mises isotropic plastic potential to take account of the influence of anisotropy on the equivalent plastic strain at the onset of fracture. To determine the coefficients of the model proposed, a two dimensional digital image correlation (2D-DIC) method is utilized to measure the strain histories on the surface of three different types of specimens during deformation. For the derivation of an anisotropic ductile fracture model, principal stresses (𝜎₁,𝜎₂, 𝜎₃) are expressed in terms of the stress triaxiality, the Lode parameter, and the equivalent stress (𝜂_𝐻, 𝐿,) based on the Hill’s 48 anisotropic plastic potential. The proposed anisotropic ductile fracture criterion was quantitatively evaluated according to various directions of the maximum principal stress. Fracture forming limit diagrams were also constructed to evaluate the forming limit in sheet metal forming of AHSS sheets over a wide range of loading conditions.

• 터널과지하공간
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• 제27권1호
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• pp.39-49
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• 2017

다양한 암반구조물의 안전도를 정량적으로 표현하는 지수로 안전율이 흔히 사용된다. 그러므로 동일한 응력조건에서 산정된 안전율이라 할지라도 안전율의 정의에 따라 안전율 계산 값의 차이가 있을 수 있으므로 적용하는 안전율의 정의와 특성을 이해하는 것이 중요하다. 이 연구에서는 Mohr-Coulomb 파괴조건식과 Hoek-Brown 파괴조건식을 활용하여 최대전단응력, 최대전단강도, 응력불변량, 최대주응력을 기반으로 하는 4종의 국부안전율을 정의하였다. 이어서 5가지 응력경로에 따른 각 안전율의 변화특성을 고찰하였다. 암반구조물 해석시 전통적으로 많이 이용되고 있는 전단강도 기반의 국부안전율은 최대주응력 및 응력불변량 기반의 안전율에 비해 높은 안전율 값을 계산함을 보였다. 이러한 결과는 보수적 암반공학적 설계가 필요한 경우 전단강도 기반의 국부안전율보다 최대전단응력이나 응력불변량 기반의 국부안전율 도입이 필요하다는 것을 말해준다. 또한 응력경로에 따라 파괴 시 최대주응력 값도 큰 차이가 있을 수 있음을 보였다.

• Structural Engineering and Mechanics
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• 제34권3호
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• pp.377-390
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• 2010

This paper studies fracture initiation direction of two parallel non-coplanar cracks of equal length. Using the dislocation pile-up modelling, singular integral equations for two parallel cracks subjected to mixed-mode loading are derived and the crack-tip field including singular and non-singular terms is obtained. The kinking angle is determined by using the maximum hoop stress criterion, or the ${\sigma}_{\theta}$-criterion. Results are presented for simple uniaxial tension and biaxial loading. The biaxiality ratio has a noticeable influence on crack growth direction. For the case of biaxial tension, when neglecting the T-stress the crack branching angle is overestimated for small crack inclination angles relative to the largest applied principal stress direction, and underestimated for large crack inclination angles.