• Computers and Concrete
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• 제22권6호
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• pp.563-572
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• 2018

Uniaxial compressive strength test and uniaxial compression creep one were produced on four groups of twelve concrete specimens with different hole number by RLW-2000 rock triaxial rheology test system. The relationships between horizontal holes and instantaneous failure stress, the strain, and creep failure stress, the strain, and the relationships between stress level and instantaneous strain, creep strain were studied, and the relationship between horizontal holes and failure mode was determined. The results showed that: with horizontal hole number increasing, compressive strength of the specimens decreased whereas its peak strain increased, while both creep failure strength and its peak strain decreased. The relationships between horizontal holes and compressive strength of the specimens, the peak strain, were represented in quadratic polynomial, the relationships between horizontal holes and creep failure strength, the peak strain were represented in both linear and quadratic polynomial, respectively. Instantaneous strain decreased with stress level increasing, and the more holes in the blocks the less the damping of instantaneous strain were recorded. In the failure stress level, instantaneous strain reversally increased, creep strain showed three stages: decreasing, increasing, and sharp increasing; in same stress level, the less holes the less creep strain rate was recorded. The compressive-shear failure was produced along specimen diagonal line where the master surface of creep failure occurred, the more holes in a block, the higher chances of specimen failure and the more obvious master surface were.

• Nuclear Engineering and Technology
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• 제55권11호
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• pp.4134-4145
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• 2023

This paper proposes strain-based failure model of A533B1 pressure vessel steel to simulate failure, followed by application to OECD lower head failure (OLHF) test simulation for experimental validation. The proposed strain-based failure model uses simple constant and linear functions based on physical failure modes with the critical strain value determined either using the lower bound of true fracture strain or using the average value of total elongation depending on the temperature. Application to OECD Lower Head Failure (OLHF) tests shows that progressive deformation, failure time and failure location can be well predicted.

• 한국해양공학회지
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• 제27권2호
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• pp.19-26
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• 2013

Stress triaxiality is recognized as one of the most important factors for predicting the failure strain of ductile metals. This study dealt with the effect of the average stress triaxiality on the failure strain of a typical low-temperature high-strength marine structural steel, EH36. Tensile tests were carried out on flat specimens with different notches, from relatively smooth to very sharp levels. Numerical simulations of each specimen were performed by using ABAQUS. The failure initiation points in numerical simulations were identified from a comparison of the engineering stress vs. strain curves obtained from experiments with simulated ones. The failure strain curves for various dimensionless critical energy levels were established in the average stress triaxiality domain and compared with the identified failure strain points. It was observed that most of the failure initiation points were approximated with a 100% dimensionless critical energy curve. It was concluded that the failure strains were well expressed as a function of the average stress triaxiality.

• 한국해양공학회지
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• 제25권2호
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• pp.92-100
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• 2011

The main goal of this paper is to provide the theoretical background for the fracture phenomena in marine structural steels. In this paper, various fracture criteria are theoretically investigated: shear failure criteria with constant failure strain and stress triaxiality-dependent failure strain (piecewise failure and Johnson-Cook criteria), forming limit curve failure criterion, micromechanical porosity failure criterion, and continuum damage mechanics failure criterion. It is obvious that stress triaxiality is a very important index to determine the failure phenomenon for ductile materials. Assuming a piecewise failure strain curve as a function of stress triaxiality, the numerical results coincide well with the test results for smooth and notched specimens, where low and high stress triaxialities are observed. Therefore, it is proved that a failure criterion with reliable material constants presents a plastic deformation process, as well as fracture initiation and evolution.

• 콘크리트학회논문집
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• 제21권2호
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• pp.121-127
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• 2009

연화재료의 변형률 국부화 거동에 따른 파괴영역과 파괴영역 내에서의 변형률 추이는 구조물의 극한거동에 큰 영향을 미치지만 파괴영역의 예측과 영역 내에서의 변형률을 정량화하는 것은 실험적으로 명확히 구명되어 있다고 보기 어렵다. 이 연구에서는 국부화된 파괴영역 및 그 영역 내에서의 변형률 분포 추이를 추정하기 위해 화상처리기법을 이용하였다. 화상처리기법의 신뢰성 및 타당성을 검증하기 위해, 일축압축공시체를 대상으로 화상처리기법을 이용하여 변형률을 추정하고, 이를 콘크리트 게이지로부터 취득한 변형률과 비교분석하였다. 분석 결과를 바탕으로 전단압축파괴형 보의 정적파괴실험에 화상처리기법을 적용하여 실험체의 거동에 따른 주인장변형률 및 주압축변형률의 분포도를 작성하였다. 마지막으로 작성된 분포도를 이용하여 국부화영역 및 국부변형률을 추정하였다. 실험 결과, 화상처리기법을 이용하여 취득한 파괴영역 내에서의 변형률 분포 추이는 비교적 정확히 실험체의 거동을 예측할 수 있었으며 손상 정도에 따라 비교적 합리적인 국부변형률값을 얻을 수 있었다.

• Geomechanics and Engineering
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• 제15권3호
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• pp.919-925
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• 2018

Stress-strain responses of weak-to-strong carbonate rocks used for tunnel construction were studied. The analysis of applicability of exponential stress-strain models based on Haldane's distribution function is presented. It is revealed that these exponential equations presented in transformed forms allow us to predict stress-strain relationships over the whole pre-failure strain range without mechanical testing of rock samples under compression using a press machine and to avoid measurements of axial failure strains for which relatively large values of compressive stress are required. In this study, only one point measurement (small strain at small stress) using indentation test and uniaxial compressive strength determined by a standard Schmidt hammer are considered as input parameters to predict stress-strain response from zero strain/zero stress up to failure. Observations show good predictive capabilities of transformed stress-stress models for weak-to-strong (${\sigma}_c$ <100 MPa) heterogeneous carbonate rocks exhibiting small (< 0.5 %), intermediate (< 1 %) and large (> 1 %) axial strains.

• Geomechanics and Engineering
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• 제16권2호
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• pp.205-215
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• 2018

Strain-based criteria are known as a direct method in determining the stability of the geomechanical structures. In spite of the widely use of Sakurai critical strain criterion, it is so conservative to make use of them in rocks with initial plastic deformation on account of the considerable difference between the failure and critical strains. In this study, a new criterion has been developed on the basis of the failure strain to attain more reasonable results in determining the stability status of the tunnels excavated in the rocks mostly characterized by plastic-elastic/plastic behavior. Firstly, the stress-strain curve was obtained having conducted uniaxial compression strength tests on 91 samples of eight rock types. Then, the initial plastic deformation was omitted making use of axis translation technique and the criterion was presented allowing for the modified secant modulus and by use of the failure strain. The results depicted that the use of failure strain criterion in such rocks not only decreases the conservativeness of the critical strain criterion up to 42%, but also it determines the stability status of the tunnel more accurately.

• Composites Research
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• 제14권5호
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• pp.12-19
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• 2001

급격한 과도하중이나 충격 등에 의해서 발생만 복합적층 내부의 손상은 항공기 구조물과 같이 안전성이 중요시되는 구조의 신뢰성을 저하시키며 또한 큰 위험 요인이 될 수 있다. 따라서 본 연구에서는 구조의 건전성을 모니터링하고 파손여부를 실시간으로 감지하기 위해 단파장 레이저와 광대역광원을 동시에 적용한 광섬유 센서를 이용하여 변형률 및 파손을 실시간으로 동시에 모니터링 할 수 있는 시스템을 구성하였다 이때 서로 다른 파장대의 두 장원은 파장분할다중 송신기를 이용하여 하나의 광섬유 센서에 적용되었다 파손신호의 특징을 정량적으로 구분하기 위해 STFT와 Wavelet Transform 과 같은 시간 주파수 분석법을 사용하였으며, 광섬유 센서로 취득 긴 파손신호 및 변형률 측정값을 각각 압전 세라믹 센서와 스트레인게이지의 값과 서로로 비교하였다. 장시간동안 파손과 동시에 측정된 변형률의 값은 스트레인게이지의 측정값과 잘 일치하였으며 파손감지 시스템 또만 미세한 파손신호까지 민감하게 감지해 낼 수 있음을 알 수 있었다.

• Advanced Composite Materials
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• 제16권2호
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• pp.167-180
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• 2007

The tensile strength of unidirectional carbon fiber reinforced plastics under a high strain rate was experimentally investigated. A high-strain-rate test was performed using the tension-type split Hopkinson bar technique. In order to obtain the tensile stress-strain relations, a special fixture was used for the impact tensile specimen. The experimental results demonstrated that the tensile modulus and strength in the longitudinal direction are independent of the strain rate. In contrast, the tensile properties in the transverse direction and the shear properties increase with the strain rate. Moreover, it was observed that the strain-rate dependence of the shear strength is much stronger than that of the transverse strength. The tensile strength of off-axis specimens was measured using an oblique tab, and the experimental results were compared with the tensile strength predicted based on the Tsai-Hill failure criterion. It was concluded that the tensile strength can be characterized quite well using the above failure criterion under dynamic loading conditions.

• 한국농공학회지
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• 제37권2호
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• pp.64-72
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• 1995

A series of cubical triaxial tests with three independent principal stresses was per- formed on Baekma river sand( # 40~100). It was found that the major principal strain at failure remained approximately constant for b values larger than about 0.3 for both the drained and undrained condition, and thereafter increased as b value decreased. The test results showed that the direction of the strain increment at failure form acute angles with the failure surfaces for both the drained and undrained condition. The results were thus not in agreement with the normality condition from classic plasticity theory. Howev- er, it was found that the projections of the plastic strain increment vectors on the octahe- dral plane were perpendicular to the failure surface in that plane. Failure strength in terms of effective stress anlaysis was greatly influenced by the variation of intermediate principal stress and so was failure criterion. The effective stress failure surfaces for both the drained and undrained condition were estimated quite well by use of Lade's failure criterion.