Study on the Hydromechanical Behavior of Single Fracture under Normal Stresses

Ni Xie,Jinbao Yang,Jianfu Shao 대한토목학회 2014 KSCE JOURNAL OF CIVIL ENGINEERING Vol.18 No.6

The coupling between hydraulic and mechanical behavior of the fractured rock mass is of great significance for various civil andenvironmental engineering projects. In order to study the hydro-mechanical behavior of single fracture, seepage tests under differentconfining pressures and fracture water pressures were conducted on single shear fractures produced by triaxial loading of diabaserock samples from Danjiangkou Water Reservoir, China. Test results show that fluid pressure acting on fracture surfaces has stronginfluences on the hydraulic behavior of the fracture. Based on the classic Biot poroelasticity theory and by taking the fracture asassembling of a set of voids in rock mass, a generalized Biot coefficient is introduced to describe the interaction effect between porefluid pressure and fracture deformation. Then, a nonlinear constitutive equation for single fracture under both normal stress and fluidpressure is developed. Later, the mechanical deformation of the fracture is related to the fracture hydraulic conductivity through“cubic law”, so that a coupled mechanical-hydraulic model is proposed. All the four parameters involved in this model have theirphysical significances and can be determined through mechanical compression tests and seepage tests. A first validity of the model ismade by predicting the variation of fracture flowrates versus normal stress under different fluid pressures.

Finite Element Analysis for Fracture Resistance of Fiber-reinforced Asphalt Concrete

백종은,유평준 한국도로학회 2015 한국도로학회논문집 Vol.17 No.3

PURPOSES : In this study, a fracture-based finite element (FE) model is proposed to evaluate the fracture behavior of fiber-reinforced asphalt (FRA) concrete under various interface conditions. METHODS: A fracture-based FE model was developed to simulate a double-edge notched tension (DENT) test. A cohesive zone model (CZM) and linear viscoelastic model were implemented to model the fracture behavior and viscous behavior of the FRA concrete, respectively. Three models were developed to characterize the behavior of interfacial bonding between the fiber reinforcement and surrounding materials. In the first model, the fracture property of the asphalt concrete was modified to study the effect of fiber reinforcement. In the second model, spring elements were used to simulated the fiber reinforcement. In the third method, bar and spring elements, based on a nonlinear bond-slip model, were used to simulate the fiber reinforcement and interfacial bonding conditions. The performance of the FRA in resisting crack development under various interfacial conditions was evaluated. RESULTS : The elastic modulus of the fibers was not sensitive to the behavior of the FRA in the DENT test before crack initiation. After crack development, the fracture resistance of the FRA was found to have enhanced considerably as the elastic modulus of the fibers increased from 450 MPa to 900 MPa. When the adhesion between the fibers and asphalt concrete was sufficiently high, the fiber reinforcement was effective. It means that the interfacial bonding conditions affect the fracture resistance of the FRA significantly. CONCLUSIONS: The bar/spring element models were more effective in representing the local behavior of the fibers and interfacial bonding than the fracture energy approach. The reinforcement effect is more significant after crack initiation, as the fibers can be pulled out sufficiently. Both the elastic modulus of the fiber reinforcement and the interfacial bonding were significant in controlling crack development in the FRA.

탄소섬유직물/에폭시 복합재의 혼합모우드 층간파괴 거동

윤성호(Sung-Ho Yoon),허광수(Kwang-Soo Heo),오진오(Jin-Oh Oh) 한국항공우주학회 2007 韓國航空宇宙學會誌 Vol.35 No.1

MMF 시험을 적용하여 혼합모우드 비율을 20%~90%의 범위 내에서 변화시키면서 탄소섬유직물/에폭시 복합재의 혼합모우드 층간파괴 거동을 조사하였다. 혼합모우드 층간파괴 거동을 예측하기 위해 NL점과 5% offset점에 근거한 혼합모우드 층간파괴 결정식을 제시하였다. 파단면 양상과 균열진전 거동은 이동식 현미경과 전자현미경을 통해 조사하였다. 연구결과에 따르면 혼합모우드 층간파괴 거동은 NL점에 근거한 경우 매개변수 m=1.5와 n=0.5, 5% offset점에 근거한 경우 매개변수 m=2와 n=3인 혼합모우드 충간파괴 결정식에 의해 잘 예측되어진다. 파단면 양상과 균열진전 거동은 혼합모우드 비율에 매우 민감하게 변하며 MMF 시험은 혼합모우드 층간파괴인성의 평가에 성공적으로 적용됨을 알 수 있었다. Mixed mode interlaminar fracture behaviors of carbon fabric/epoxy composites were investigated through MMF (Mixed Mode Flexural) test by varying mixed mode ratio ranging from 20% to 90%. Mixed mode interlaminar fracture criteria based on NL point and 5% offset point were also suggested in order to predict mixed mode interlaminar fracture behaviors. Fracture surfaces and crack propagating behaviors were examined through a travelling scope and a scanning electron microscope. According to the results, mixed mode interlaminar fracture behaviors can be predicted by mixed mode interiaminar fracture criterion with m=1.5 and n=0.5 on the basis of NL point or mixed mode interlaminar fracture criterion with m=2 and n=3 on the basis of 5% offset point. Fracture surfaces and crack propagating behaviors are sensitive to mixed mode ratios. MMF test can be successfully applicable in evaluating mixed mode interiaminar fracture toughness of carbon fabric/epoxy composites.

유한요소해석을 통한 섬유보강 아스팔트의 파괴거동특성 분석

Baek, Jongeun,Yoo, Pyeong Jun 한국도로학회 2015 한국도로학회논문집 Vol.17 No.3

PURPOSES : In this study, a fracture-based finite element (FE) model is proposed to evaluate the fracture behavior of fiber-reinforced asphalt (FRA) concrete under various interface conditions. METHODS: A fracture-based FE model was developed to simulate a double-edge notched tension (DENT) test. A cohesive zone model (CZM) and linear viscoelastic model were implemented to model the fracture behavior and viscous behavior of the FRA concrete, respectively. Three models were developed to characterize the behavior of interfacial bonding between the fiber reinforcement and surrounding materials. In the first model, the fracture property of the asphalt concrete was modified to study the effect of fiber reinforcement. In the second model, spring elements were used to simulated the fiber reinforcement. In the third method, bar and spring elements, based on a nonlinear bond-slip model, were used to simulate the fiber reinforcement and interfacial bonding conditions. The performance of the FRA in resisting crack development under various interfacial conditions was evaluated. RESULTS : The elastic modulus of the fibers was not sensitive to the behavior of the FRA in the DENT test before crack initiation. After crack development, the fracture resistance of the FRA was found to have enhanced considerably as the elastic modulus of the fibers increased from 450 MPa to 900 MPa. When the adhesion between the fibers and asphalt concrete was sufficiently high, the fiber reinforcement was effective. It means that the interfacial bonding conditions affect the fracture resistance of the FRA significantly. CONCLUSIONS: The bar/spring element models were more effective in representing the local behavior of the fibers and interfacial bonding than the fracture energy approach. The reinforcement effect is more significant after crack initiation, as the fibers can be pulled out sufficiently. Both the elastic modulus of the fiber reinforcement and the interfacial bonding were significant in controlling crack development in the FRA.

Fracture of nanoscale Cu/Ag bimaterials with an interface crack

Cui, C.B.,Beom, H.G. Elsevier 2016 Computational materials science Vol.118 No.-

The fracture of nanoscale metallic bimaterials with an interface crack is investigated using atomistic simulations under uniaxial tensile loading conditions. Two Cu/Ag bimaterials with different crystal orientations are considered to examine the direction-dependent interface fracture behaviors. The effects of crack tip states on the fracture toughness and fracture behavior are also studied. The overall features for the selected models of both bimaterials are brittle; however, the fracture patterns are slightly different. Three methods were adopted to calculate the fracture toughness including atomistic simulations, linear elastic fracture mechanics, and Griffith theory. The results from different methods show good consistency because the models of the two bimaterials exhibit a linear elastic response under the applied tensile loading conditions. The scatter of the fracture toughness for the same bimaterials obtained from atomistic simulations is attributed to the different initial states of the models induced by the discrete nature of the materials on the atomic scale.

A Study on Fracture Behavior and Impact Stability of Sintered Rare-earth Permanent Magnets

Li Wei,Li Anhua,Wang Huijie,Dong Shengzhi,Guo Yongquan 한국분말야금학회 2006 한국분말야금학회 학술대회논문집 Vol.2006 No.1

The fracture behavior and mechanical characteristics of sintered rare-earth magnets were investigated. It shows that the fracture behavior and bending strength of the magnets obviously exhibit anisotropy. Sm-Co magnets tend to cleavage fracture in the close-packed (0001) plane or in the (10 11 ) plane. The fracture mechanism of Nd2Fe14B magnet mainly appears to be intergranular fracture. The anisotropy of fracture behavior and mechanical strength of sintered rare-earth magnets is caused mainly by the strong crystal-structure anisotropy and the grain alignment texture. The effects of Nd content, and Pr, Dy substitution on the impact stability of Nd2Fe14B magnets were also reported.

콘크리트 보강용 고연성 하이브리드 FRP 보강근의 인장 및 파괴 특성

박찬기,원종필,Park, Chan-Gi,Won, Jong-Pil 한국농공학회 2004 한국농공학회논문집 Vol.46 No.1

FRP re-bar in concrete structures could be used as a substitute of steel re-bars for that cases in which aggressive environment produce high steel corrosion, or lightweight is an important design factor, or transportation cost increase significantly with the weight of the materials. But FRP fibers have only linearly elastic stress-strain behavior; whereas, steel re-bar has linear elastic behavior up to the yield point followed by large plastic deformation and strain hardening. Thus, the current FRP re-bars are not suitable concrete reinforcement where a large amount of plastic deformation prior to collapse is required. The main objectives of this study in to evaluate the tensile behavior and the fracture mode of hybrid FRP re-bar. Fracture mode of hybrid FRP re-bar is unique. The only feature common to the failure of the hybrid FRP re-bars and the composite is the random fiber fracture and multilevel fracture of sleeve fibers, and the resin laceration behavior in both the sleeve and the core areas. Also, the result of the tensile and interlaminar shear stress test results of hybrid FRP re-bar can provide its excellent tensile strength-strain and interlaminar stress-strain behavior.

Properties controlling the bend-assisted fracture of AHSS

Lee, J.,Kim, J.H.,Lee, M.G.,Barlat, F.,Zhou, C.,Chen, Z.,Wagoner, R.H. Pergamon Press ; Elsevier Science Ltd 2015 International journal of plasticity Vol.75 No.-

Bend-assisted fracture, also commonly called shear fracture, is the splitting of metal sheets during forming in tight-bending regions. It has been shown to be predominantly a result of plastic localization for most advanced high strength steels (AHSS). Such fractures are poorly predicted by typical industrial methods involving finite element modeling (FEM) and forming limit diagrams (FLDs). In order to understand the source of the problem, the sensitivity of simulated shear-fracture formability to material and process parameters was determined using FEM in conjunction with a realistic range of constitutive models, element sizes, and friction coefficients. Two types of shear fracture process were simulated. (1) Draw-bend fracture (DBF) tests are laboratory analogs of industrial forming conditions producing shear fracture; they offer the opportunity of experimental validation but introduce complexity because of varying strain state and unavoidable transitions between shear fracture and tensile fracture. (2) Plane-strain (PS) draw-bend fracture simulations correspond more closely to industrial forming conditions; they simplify the modeling (fixed strain state, no transitions) but no corresponding full-scale laboratory experiments currently exist. The DBF test was found to be sensitive to every material and process parameter tested, with the largest factors being the form of 1-D hardening law and the yield function. Varying these quantities in ranges representing what practical measurements would produce showed variations in predicted formability of up to 80%. The PS simulations, which represent industrial practice more closely, showed large variations in predicted formability only for two variables: 1-D hardening law and friction coefficient. All other parameters were insignificant, except for thermo-mechanical effects, which were important for high-rate tests only. These results show why it is difficult or impossible to predict shear fracture using standard industrial techniques designed for traditional steels. They suggest ways to modify such techniques to accommodate advanced high strength steels. The results also give guidance to alloy designers in terms of which constitutive parameters are most important in inhibiting shear fracture, and which are relatively insignificant.

Non-linear analysis of dealamination fracture in functionally graded beams

Rizov, Victor I. Techno-Press 2017 Coupled systems mechanics Vol.6 No.1

The present paper reports an analytical study of delamination fracture in the Mixed Mode Flexure (MMF) functionally graded beam with considering the material non-linearity. The mechanical behavior of MMF beam is modeled by using a non-linear stress-strain relation. It is assumed that the material is functionally graded along the beam height. Fracture behavior is analyzed by the J-integral approach. Non-linear analytical solution is derived of the J-integral for a delamination located arbitrary along the beam height. The J-integral solution derived is verified by analyzing the strain energy release rate with considering the non-linear material behavior. The effects of material gradient, crack location along the beam height and material non-linearity on the fracture are evaluated. It is found that the J-integral value decreases with increasing the upper crack arm thickness. Concerning the influence of material gradient on the non-linear fracture, the analysis reveals that the J-integral value decreases with increasing the ratio of modulus of elasticity in the lower and upper edge of the beam. It is found also that non-linear material behavior leads to increase of the J-integral value. The present study contributes for the understanding of fracture in functionally graded beams that exhibit material non-linearity.

Microstructural Degradation and Creep Fracture Behavior of Conventionally and Thermomechanically Treated 9% Chromium Heat Resistant Steel

Javier Vivas,Carlos Capdevila,Eberhard Altstadt,Mario Houska,Ilchat Sabirov,David San‑Martín 대한금속·재료학회 2019 METALS AND MATERIALS International Vol.25 No.2

The microstructural degradation and the creep fracture behavior of conventionally and thermomechanically treated Grade91 steel were investigated after performing small punch creep tests. A remarkable reduction in creep ductility was observedfor the samples thermomechanically treated in comparison to those conventionally treated under the tested conditions ofload (200 N) and temperature (700 °C). A change in the fracture mechanism from a ductile transgranular fracture to a brittleintergranular fracture was observed when changing from the conventionally treated to the thermomechanically treated processingcondition, leading to this drop in creep ductility. The change in the fracture mechanism was associated to the localizedconcentration of creep deformation, close to coarse M23C6carbides, at the vicinity of prior austenite grain boundaries(PAGB) in the thermomechanically treated samples. The preferential recovery experienced at the vicinity of PAGB producedthe loss of the lath structure and the coarsening of the M23C6precipitates. The electron microscopy images provided suggestthat the creep cavities nucleate in these weak recovered areas, associated to the presence of coarse M23C6. After the coalescenceof the cavities the propagation of the cracks was facilitated by the large prior austenite grain size produced during theaustenitization which favors the propagation of the cracks along grain boundaries triggering the intergranular brittle fracture. This fracture mechanism limits the potential use of the proposed thermomechanical processing routes.