High Temperature Properties of Fiber Reinforced Composites under the Different Loading Conditions

( Hu Weiguang ),( Soo-jeong Park ),( Yun-hae Kim ) 한국복합재료학회 2017 Composites research Vol.30 No.3

The mechanical properties of composites are significantly affected by external environment. It is essential to understand the degradation of material performance and judge the material`s lifetime in advance. In the current research, changes in mechanical properties of glass fiber and unsaturated polyester composite materials (GFRP, Glass fiber reinforced plastic) were investigated under different bending stress and submerged in hot water at a temperature of 80°C. Loading time of 100 H (hours), 200 H, 400 H, 600 H, 800 H for testing under stresses equal to 0% (stressfree state), 30%, 50% and 70% of the ultimate strength was applied on the GFRP specimens. From the values of bending stress, obtained from three-point bending test, fracture energy, failure time, and life curve were analysed. Moreover, a normalized strength degradation model for this condition was also developed. It was observed that within 100 H, the decline rate of the bending strength was proportional to the pressure.

Comparative Study on Mechanical Properties of CR340/CFRP Composites through Three Point Bending Test by Using Theoretical and Experimental Methods

이민식,강충길,서형윤 한국정밀공학회 2016 International Journal of Precision Engineering and Vol.3 No.4

In this study, Cold Rolled 340 (CR340)/Carbon Fiber Reinforced Plastic (CFRP) composites were fabricated, and experiments and simulation were performed to compare their flexural properties from three-point bending tests. The mechanical properties of CR340 and CFRP were input by dividing the material regime into elastic, plastic, and fracture regions in order to improve the simulation reliability. A forming limit diagram was determined through a stretch test for the CR340 steel plate and used as data input in the simulation. For CFRP, simulation was carried out using the Hashin damage theory and damage evolution obtained from references. Results showed that the maximum bending stress, fracture displacement, and gradient in the experiments closely matched those obtained from the simulation.

A virtual experimental approach to evaluate transverse damage behavior of a unidirectional composite considering noncircular fiber cross-sections

Jeong, Gyu,Lim, Jae Hyuk,Choi, Chunghyeon,Kim, Sun-Won Elsevier 2019 COMPOSITE STRUCTURES -BARKING THEN OXFORD- Vol.228 No.-

<P><B>Abstract</B></P> <P>This study investigated the transverse damage behavior of a unidirectional composite containing a complex microstructure having noncircular fiber cross-sections. For this purpose, a finite-element (FE) model based real microscopic images of the M55J/M18 composite was generated with the signed distance function (SDF) by the level-set method and the trimming mesh technique. In addition, the interphase zone was constructed along the interface between the fiber and the matrix. Subsequently, a virtual experiment simulating a three-point bending test was conducted with the generated FE model. Crack propagation analysis was carried out with the cohesive zone model (CZM) by applying transverse tension under plane strain condition. The crack length and propagation directions were compared to those of the three-point bending test. To make a nice correlation between the virtual experiment and actual experiment, the effects of the fiber shape, thermal residual stress, and various fracture toughnesses were investigated on the length and propagation direction of the cracks. It was found that, the fiber shape is vital to the inter-fiber distance and fiber volume fraction (<I>V</I> <SUB>f</SUB>), which strengthen or weaken the stress concentration, thus making the propagation direction of the cracks warped and the length of cracks varied.</P> <P><B>Highlights</B></P> <P> <UL> <LI> A complex microstructure is modelled by the trimming mesh and the level set method. </LI> <LI> The proposed scheme can handle convex and concave shape of fibers accurately. </LI> <LI> A virtual three-point bending simulation propagates cracks in a complex microstructure. </LI> <LI> The crack propagation simulation show excellent agreement with the test results according to the applied displacement. </LI> <LI> The effect of the fiber shape on the length and propagation angle of cracks is investigated. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

Non-destructive assessment of the three-point-bending strength of mortar beams using radial basis function neural networks

Alex Alexandridis,Ilias Stavrakas,Charalampos Stergiopoulos,George Hloupis,Konstantinos Ninos,Dimos Triantis 사단법인 한국계산역학회 2015 Computers and Concrete, An International Journal Vol.16 No.6

This paper presents a new method for assessing the three-point-bending (3PB) strength of mortar beams in a non-destructive manner, based on neural network (NN) models. The models are based on the radial basis function (RBF) architecture and the fuzzy means algorithm is employed for training, in order to boost the prediction accuracy. Data for training the models were collected based on a series of experiments, where the cement mortar beams were subjected to various bending mechanical loads and the resulting pressure stimulated currents (PSCs) were recorded. The input variables to the NN models were then calculated by describing the PSC relaxation process through a generalization of Boltzmannn-Gibbs statistical physics, known as non-extensive statistical physics (NESP). The NN predictions were evaluated using k-fold cross-validation and new data that were kept independent from training; it can be seen that the proposed method can successfully form the basis of a non-destructive tool for assessing the bending strength. A comparison with a different NN architecture confirms the superiority of the proposed approach.

적층한 콘크리트 복합구조체의 파괴역학적 거동

김상철,김연태,Kim, Sang-Chul,Kim, Yeon-Tae 한국콘크리트학회 1999 콘크리트학회지 Vol.11 No.1

본 연구는 시멘트를 기초로 하는 준 취성재료를 2개 결합시킨 복합구조체의 파괴거동을 시뮬레이션 하는 데 그 목적이 있다. 이를 위해 작용하중방향에 직각으로 콘크리트와 시멘트를 적층하여 시편을 제작하였고, 비선형파괴해석에서 사용되는 각 구성재료별 파괴계수들을 구하기 위해 콘크리트와 시멘트 각각에 대해 3점휨시험 및 간접인장강도시험을 실시하였다. 시험을 통해 산정한 계수들을 도출한 이론식에 적용하여 복합체로 제작한 시편의 실험 결과치와 비교한 결과, 가상균열이론에 입각한 이론치와 실험결과치는 거의 유사한 경향을 나타내었다. 또한, 파괴에너지와 강도의 향상은 복합체를 구성하는 구성재료의 적층순서에 크게 의존하는 것으로 나타났다. 따라서 구조물의 목적에 맞게 복합체를 적절히 배열하므로써 구조물의 취성과 연성을 상승시킬 수 있음을 본 연구를 통해 알 수 있었다. The objective of this study is to simulate the fracture behavior of composite structure bonded with more than 2 different cementitious materials. For this, concrete and cement were stacked and bonded in a direction perpendicular to loading and specimens were tested. Each constituent material of concrete and cement was fabricated independently also, and three point bending and indirect tensile tests were carried out for the acquisition of measured values applicable to the proposed model. As a result of comparing theoretical results and experimental ones, it was found that the proposed model derived from fictitious crack theory can be used to predict the fracture behavior of composite structures on the vases of well agreement with experimental results. It was also noted that the degree of improvement of fracture energies and strengths is greatly dependent on the stacking sequence of layers composing of a composite structure. Thus, it can be concluded that brittleness or ductility of a composite structure can be accomplished by a proper arrangement of layers on one's purpose throughout the proposed analysis.

Crack propagation in flexural fatigue of concrete using rheological-dynamical theory

Aleksandar Pančić,Dragan D. Milašinović,Danica Goleš 사단법인 한국계산역학회 2021 Computers and Concrete, An International Journal Vol.27 No.1

The concrete fatigue analysis can be performed with the use of fracture mechanics. The fracture mechanics defines the fatigue crack propagation as the relationship of crack growth rate and stress intensity factor. In contrast to metal, the application of fracture mechanics to concrete is more complicated and therefore many authors have introduced empirical expressions using Paris law. The topic of this paper is development of a new prediction of fatigue crack propagation for concrete using rheological-dynamical analogy (RDA) and finite element method (FEM) in the frame of linear elastic fracture mechanics (LEFM). The static and cyclic fatigue three-point bending tests on notched beams are considered. Verification of the proposed approach was performed on the test results taken from the literature. The comparison between the theoretical model and experimental results indicates that the model proposed in this paper is valid to predict the crack propagation in flexural fatigue of concrete.

Measuring high speed crack propagation in concrete fracture test using mechanoluminescent material

Kim, Wha-Jung,Lee, Jae-Min,Kim, Ji-Sik,Lee, Chang Joon Techno-Press 2012 Smart Structures and Systems, An International Jou Vol.10 No.6

Measuring crack length in concrete fracture test is not a trivial problem due to high speed crack propagation. In this study, mechanoluminascent (ML) material, which emits visible light under stress condition, was employed to visualize crack propagation during concrete fracture test. Three-point bending test was conducted with a notched concrete beam specimen. The cracking images due to ML phenomenon were recorded by using a high speed camera as a function of time and external loadings. The experimental results successfully demonstrated the capability of ML material as a promising visualization tool for concrete crack propagation. In addition, an interesting cracking behavior of concrete bending fracture was observed in which the crack propagated fast while the load decreased slowly at early fracture stage.

디지털 이미지 처리와 강형식 기반의 무요소법을 융합한 시험법의 모서리 점과 이미지 해상도의 영향 분석

박준원,정연석,윤영철 한국전산구조공학회 2024 한국전산구조공학회논문집 Vol.37 No.1

본 논문에서는 역학적 변수들을 측정하는 방안으로 디지털 이미지 프로세싱과 강형식 기반의 MLS 차분법을 융합한 DIP-MLS 시험법을 소개하고 추적점의 위치와 이미지 해상도에 대한 영향을 분석하였다. 이 방법은 디지털 이미지 프로세싱을 통해 시료에 부착된 표적의 변위 값을 측정하고 이를 절점만 사용하는 MLS 차분법 모델의 절점 변위로 분배하여 대상 물체의 응력, 변형률과 같은 역학적 변수를 계산한다. 디지털 이미지 프로세싱을 통해서 표적의 무게중심 점의 변위를 측정하기 위한 효과적인 방안을 제시하였다. 이미지 기반의 표적 변위를 이용한 MLS 차분법의 역학적 변수의 계산은 정확한 시험체의 변위 이력을 취득하고 정형성이 부족한 추적 점들의 변위를 이용해 mesh나 grid의 제약 없이 임의의 위치에서 역학적 변수를 쉽게 계산할 수 있다. 개발된 시험법은 고무 보의 3점 휨 실험을 대상으로 센서의 계측 결과와 DIP-MLS 시험법의 결과를 비교하고, 추가적으로 MLS 차분법만으로 시뮬레이션한 수치해석 결과와도 비교하여 검증하였다. 이를 통해 개발된 기법이 대변형 이전까지의 단계에서 실제 시험을 정확히 모사하고 수치해석 결과와도 잘 일치하는 것을 확인하였다. 또한, 모서리 점을 추가한 46개의 추적점을 DIP-MLS 시험법에 적용하고 표적의 내부 점만을 이용한 경우와 비교하여 경계 점의 영향을 분석하였고 이 시험법을 위한 최적의 이미지 해상도를 제시하였다. 이를 통해 직접 실험이나 기존의 요소망 기반 시뮬레이션의 부족한 점을 효율적으로 보완하는 한편, 실험-시뮬레이션 과정의 디지털화가 상당한 수준까지 가능하다는 것을 보여주었다. In this paper, we presen t a DIP-MLS testing method that combines digital image processing with a rigid body-based MLS differencing approach to measure mechanical variables and analyze the impact of target location and image resolution. This method assesses the displacement of the target attached to the sample through digital image processing and allocates this displacement to the node displacement of the MLS differencing method, which solely employs nodes to calculate mechanical variables such as stress and strain of the studied object. We propose an effective method to measure the displacement of the target's center of gravity using digital image processing. The calculation of mechanical variables through the MLS differencing method, incorporating image-based target displacement, facilitates easy computation of mechanical variables at arbitrary positions without constraints from meshes or grids. This is achieved by acquiring the accurate displacement history of the test specimen and utilizing the displacement of tracking points with low rigidity. The developed testing method was validated by comparing the measurement results of the sensor with those of the DIP-MLS testing method in a three-point bending test of a rubber beam. Additionally, numerical analysis results simulated only by the MLS differencing method were compared , confirming that the developed method accurately reproduces the actual test and shows good agreement with numerical analysis results before significant deformation. Furthermore, we analyzed the effects of boundary points by applying 46 tracking points, including corner points, to the DIP-MLS testing method. This was compared with using only the internal points of the target, determining the optimal image resolution for this testing method. Through this, we demonstrated that the developed method efficiently addresses the limitations of direct experiments or existing mesh-based simulations. It also suggests that digitalization of the experimental-simulation process is achievable to a considerable extent.

Stress Field of Structures with Internal Cracks by 3D-ILC Technology: Experimental and Numerical Analysis

Haijun Wang,Haitang Qi,Xuhua Ren,Lei Tang,Qianqian Dong 대한토목학회 2022 KSCE JOURNAL OF CIVIL ENGINEERING Vol.26 No.1

Great achievements have been made regarding the stress field in the two-dimensional state, however the study of three-dimensional stress field visualization has still not been comprehensively examined. The model with internal cracks was prepared by the laser-medium interaction (3D-ILC), and with the aid of the photoelastic test technology, it visually showed the distribution characteristics of the stress field inside the rock with defects under the three-point bending test method. Primarily, based on the two-dimensional and three-dimensional stress optics law, the isometric fringes were converted into phase differences or optical path differences to visualize the stress field. Moreover, transparent glass, which had better transparency and brittleness closer to real rock was selected as the specimen material. Internal cracks changed the stress fringe distribution of the specimen, and the combination of 3D-ILC technology and photoelasticity provided a new way to visualize the three-dimensional stress field of brittle materials with internal cracks. Through the secondary development and utilization of ABAQUS finite element analysis software, the internal stress fields of brittle solid materials with horizontal internal cracks were visualized. Accurate characterization of the three-dimensional stress field of brittle solid materials has been a long-term goal pursued by researchers in the field of fracture, and it is also the basis and key to solving many practical engineering problems such as design, safety analysis and evaluation.

세립분 함유량이 동결 사질토의 파괴특성에 미치는 영향

황범식,조완제,Hwang, Bumsik,Cho, Wanjei 한국지반공학회 2020 한국지반공학회논문집 Vol.36 No.3

동토의 세립분 함유량에 따른 파괴특성을 파악하기 위해 -10℃의 온도에서 다양한 세립분 함유량과 초기 노치(notch)의 위치를 조정한 직사각형 공시체를 제작하여 Three-point bending 시험을 수행하였다. 시험결과를 바탕으로 동토의 mode I 파괴인성(fracture toughness)을 산정하였으며, 하중-변형 곡선의 최대점까지의 fracture energy를 산정하여 동토의 mixed-mode(mode I + II) 파괴특성을 파악하였다. 시험결과, 최대하중 및 mode I 파괴인성은 세립분 함유량 10%까지 증가하다가 15%에서 다시 감소하는 경향을 나타내었다. 또한, 노치의 위치가 공시체 중심에서 멀어질수록 mode II 하중의 증가로 인해 균열이 진행하는데 필요한 fracture enenrgy가 증가하는 것으로 나타났으며, 세립분 함유량이 증가할수록 mode II 하중의 증가비율 또한 증가하는 것으로 나타났다. In this research, three-point bending tests were performed using a rectangular frozen specimen with various fine contents and notch offset distance from the center of the specimen to investigate the fracture characteristic of the frozen sand. Based on the test results, mode I fracture toughness was calculated, and mixed-mode (mode I + II) fracture characteristics were investigated using the fracture energy which was calculated until the maximum point of the load-displacement curve. As the fine contents increase, the peak load and mode I fracture toughness increase until 10% fine contents. Furthermore, as the notch offset distance increases, the fracture energy required for crack start also increases due to the increase in mode II load at the crack tip.