• Proceedings of the Korea Concrete Institute Conference
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• 1998.10b
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• pp.839-844
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• 1998

The purpose of this study is to analyze the interface debonding failure on RC beams strengthened with carbon fiber sheet(CFS). The behavior of damaged RC beams strengthened with CFS is analytically investigated using both linear elastic fracture mechanics (LEFM) approach and the finite element method. This study includes the investigation of the separation mode by interface fracture of the strengthening materials due to the interfacial shear and normal stresses.

• Magazine of the Korea Concrete Institute
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• v.9 no.4
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• pp.187-196
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• 1997

시멘트를 기초로하는 복합재료의 파괴거동은 주균열이 진행하기 이전에 파괴진행영역이라고 하는 미세균열대가 콘크리트 내부에 형성고기 때문에 선형파괴역하게 입각하여 해석하게 되면 실험치와 상당한 차이를 나타낸다. 이러한 문제점을 해결하기 위해 가상균열모델이나 균열띠 모델, 두 파라메터 파괴모델 등 비선형해석에 따른 여러 파괴역학모델들이 제안되었으나 이들 모델들은 2차원 해석에 근거를 두고 있기 때문에 구조체의 두께 방향으로 동일한 균열이 형성되며, 특히 콘크리트 실험에서 관찰되는 비연속적 균열발생에 대해서 설며이 어려웠다. 이에 본 연구는 콘크리트를하나의 다종복합체로 가정하고 연립변형모드 및 진행파괴모드 방향으로 구성재료를 배열한 상태에서 가상균열 이론에 근거한 비선형해석방법으로 모델링하였다. 진행파괴모드로 구성재료를 배열하면 강성이 높은 구성재료를 통과하여 균열이 진행될 때 균열선단으로부터 분포된 응력이 상층의 허용인장강도를 초과하게 되어 균열이 발생되며 이러한 균열은점진적인 균열진행과는 달리 비연속 동시 발생 균열ㄹ로 나타났다. 본 연구는 진행파괴모드에서의 파괴 해석 방법과연립변형모드에서의 해석 방법을 제시하였으며, 해석결과를 실험결과와 비교함으로써 검증하였다.

• Transactions of the Korean Society of Mechanical Engineers
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• v.13 no.2
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• pp.252-256
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• 1989

In the low cycle fatigue fracture testing with KS(or JIS) SS41, crack growth rate, AE count rate and J-integral range are measured to get empirical relations between crack growth rate and J-integral range, AE count rate and J-integral range as well as AE count rate and crack growth rate. All the relations are shown to be linear on the log-log graphs. It is also shown that the linear relations can be formulated by using Dunegan's assumption and elastic-plastic fracture mechanics along with the well-known relation of crack growth rate and J-integral range. It is concluded that the differences between experimental and theoretical values are due to Dunegan's assumption.

• Structural Engineering and Mechanics
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• v.72 no.1
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• pp.31-41
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• 2019

Structural integrity assessment of piping components is of paramount important for remaining life prediction, residual strength evaluation and for in-service inspection planning. For accurate prediction of these, a reliable fracture parameter is essential. One of the fracture parameters is stress intensity factor (SIF), which is generally preferred for high strength materials, can be evaluated by using linear elastic fracture mechanics principles. To employ available analytical and numerical procedures for fracture analysis of piping components, it takes considerable amount of time and effort. In view of this, an alternative approach to analytical and finite element analysis, a model based on relevance vector machine (RVM) is developed to predict SIF of part through crack of a piping component under fatigue loading. RVM is based on probabilistic approach and regression and it is established based on Bayesian formulation of a linear model with an appropriate prior that results in a sparse representation. Model for SIF prediction is developed by using MATLAB software wherein 70% of the data has been used for the development of RVM model and rest of the data is used for validation. The predicted SIF is found to be in good agreement with the corresponding analytical solution, and can be used for damage tolerant analysis of structural components.

• Journal of the Korean Society for Precision Engineering
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• v.10 no.3
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• pp.152-160
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• 1993

This paper presents experimental study of the static and dynamic fracture toughness behavior of a A1-6061 aluminum alloy reinforced alumina( .delta. -A1$_{2}$0$_{3}$) whiskers with 5%, 10%, 15% volume fraction. The static fracture tests using three-point bending specimen were performed by UTM25T. And drop weight impact tester performing dynamic fracture tests was used to measure dynamic locads applied to a fatigue-precracked specimes. The oneset of crack initiation was detected uwing a strain gage bonded near a crack tip. The value of static fracture toughness $K_{IC}$ and dynamic fracture toughness $K_{ID}$ were decided on the basis of linear elastic fracture mechanics. The effects of fiber volume fraction and loading on fracture toughness were investigated. The distribution of whiskers, bonding state and fracture interfaces involved in void, fiber pull-out state were investigated by optical microscopy(OM) and scanning electron microscopy(SEM)

• Journal of the Korea Concrete Institute
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• v.12 no.3
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• pp.31-37
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• 2000

When a crack is produced in a concrete structure, a micro crack zone of fracture process zone (FPZ) appears at the crack tip. To investigate the behaviour of this the micro crack zone, nonlinear fracture mechanics (NLFM) must be applied. However, when a massive concrete structure such as a concrete gravity dam is considered, the micro crack zone can be neglected and the structure can be assumed to have linear elastic fracture mechanics (LEFM) behaviour. This study is divided into two main topics : (1) Calculating stress intensity factor (SIF) at the crack tip by surface integral method and (2) Investigating the propagation of the initial crack. If the initial crack propagates, the angle of the propagation is calculated by using maximum circumferential tensile strength theory. This study, also, contains the effects of body forces and water pressures on the crack face.

• Proceedings of the Korean Society of Machine Tool Engineers Conference
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• 2001.04a
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• pp.40-45
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• 2001

The relationship between structural geometry and number of life cycles to failure is investigated to improve the fatigue life of structural components. The linear elastic fracture mechanics(LEFM) approach is integrated with shape optimal design methodology. The primary objective of this study is to decide an optimal shape for enhancing the life of the structure. The results from LEFM analyses are used in the fatigue model to predict the life of the structure before failure is occurred. The shape of the structure is optimized by using the growth strain method. Relevant issues such as problem formulation, finite element modeling are explained. Three design examples are solved, and the results show that, with proper shape changes, the life of structural systems subjected to fatigue loads can be enhanced significantly.

• Proceedings of the KSME Conference
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• 2004.11a
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• pp.43-48
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• 2004

When structures are loaded by a combination of primary and secondary stresses, plasticity effects occur which cannot be evaluated by a simple linear addition of the effects resulting from the two independent stress systems. Thermal stress due to temperature gradient is classified as secondary stress. It is known that secondary stress is released as increase of plastic zone. In this paper, two and three dimensional elastic-plastic finite element analyses are performed for the cracked plates and pipes under combined thermal and mechanical loading. And V-factor is introduced to account for plasticity effect. The present results provide that V-factor is function of thermal factor and loading and is consistent regardless of geometry. We developed the prediction method of elastic-plastic fracture mechanics parameter under combined primary and secondary loading from the present results.

• Journal of Ocean Engineering and Technology
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• v.31 no.4
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• pp.288-298
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• 2017

A study of the damage evolution laws for ductile materials was carried out to predict the ductile fracture behavior of a marine structural steel (EH36). We conducted proportional and non-proportional stress tests in the experiments. The existing 3-D fracture strain surface was newly calibrated using two fracture parameters: the average stress triaxiality and average normalized load angle taken from the proportional tests. Linear and non-linear damage evolution models were taken into account in this study. A damage exponent of 3.0 for the non-linear damage model was determined based on a simple optimization technique, for which proportional and non-proportional stress tests were simultaneously used. We verified the validity of the three fracture models: the newly calibrated fracture strain model, linear damage evolution model, and non-linear damage evolution model for the tensile tests of the asymmetric notch specimens. Because the stress evolution pattern for the verification tests remained at mode I in terms of the linear elastic fracture mechanics, the three models did not show significant differences in their fracture initiation predictions.

• International Journal of Concrete Structures and Materials
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• v.18 no.3E
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• pp.191-198
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• 2006

The objective of this study is to examine fracture characteristics of concrete at early ages, i.g. critical stress intensity factor, critical crack-tip opening displacement, fracture energy, and bilinear softening curve based on the concepts of effective-elastic crack model and cohesive crack model. A wedge splitting test for Mode I was performed on cubic wedge specimens with a notch at the edge. By experimenting with various strengths and ages, load-crack mouth opening curves were obtained, and the results were analyzed by linear elastic fracture mechanics and FEM(finite element method). The results from the test and analysis showed that critical stress intensity factor and facture energy increased while critical crack-tip opening displacement decreased with concrete aging from 1 day to 28 days. Four parameters of bilinear softening curve from 1 day to 28 days were obtained from a numerical analysis. The obtained fracture parameters and bilinear softening curves at early ages from this study are to be used as a fracture criterion and an input data for the finite element analysis of concrete at early ages.