scholarly journals New perspective of fracture mechanics inspired by gap test with crack-parallel compression

2020 ◽  
Vol 117 (25) ◽  
pp. 14015-14020 ◽  
Author(s):  
Hoang Nguyen ◽  
Madura Pathirage ◽  
Masoud Rezaei ◽  
Mohsen Issa ◽  
Gianluca Cusatis ◽  
...  
Keyword(s):  
Fracture Mechanics ◽  
Fracture Energy ◽  
Constitutive Law ◽  
Cohesive Crack ◽  
Coarse Grained ◽  
Particle Model ◽  
Band Model ◽  
Simple Modification ◽  
Gap Test ◽  
Line Crack

The line crack models, including linear elastic fracture mechanics (LEFM), cohesive crack model (CCM), and extended finite element method (XFEM), rest on the century-old hypothesis of constancy of materials’ fracture energy. However, the type of fracture test presented here, named the gap test, reveals that, in concrete and probably all quasibrittle materials, including coarse-grained ceramics, rocks, stiff foams, fiber composites, wood, and sea ice, the effective mode I fracture energy depends strongly on the crack-parallel normal stress, in-plane or out-of-plane. This stress can double the fracture energy or reduce it to zero. Why hasn’t this been detected earlier? Because the crack-parallel stress in all standard fracture specimens is negligible, and is, anyway, unaccountable by line crack models. To simulate this phenomenon by finite elements (FE), the fracture process zone must have a finite width, and must be characterized by a realistic tensorial softening damage model whose vectorial constitutive law captures oriented mesoscale frictional slip, microcrack opening, and splitting with microbuckling. This is best accomplished by the FE crack band model which, when coupled with microplane model M7, fits the test results satisfactorily. The lattice discrete particle model also works. However, the scalar stress–displacement softening law of CCM and tensorial models with a single-parameter damage law are inadequate. The experiment is proposed as a standard. It represents a simple modification of the three-point-bend test in which both the bending and crack-parallel compression are statically determinate. Finally, a perspective of various far-reaching consequences and limitations of CCM, LEFM, and XFEM is discussed.

scholarly journals Branching of hydraulic cracks enabling permeability of gas or oil shale with closed natural fractures

2019 ◽  
Vol 116 (5) ◽  
pp. 1532-1537 ◽  
Author(s):  
Saeed Rahimi-Aghdam ◽  
Viet-Tuan Chau ◽  
Hyunjin Lee ◽  
Hoang Nguyen ◽  
Weixin Li ◽  
...  
Keyword(s):  
Fracture Mechanics ◽  
Gas Permeability ◽  
Gas Production ◽  
Constitutive Law ◽  
Recent Analysis ◽  
Band Model ◽  
Secondary Creep ◽  
Tectonic Events ◽  
Crack Band ◽  
Permeability Increase

While hydraulic fracturing technology, aka fracking (or fraccing, frac), has become highly developed and astonishingly successful, a consistent formulation of the associated fracture mechanics that would not conflict with some observations is still unavailable. It is attempted here. Classical fracture mechanics, as well as current commercial software, predict vertical cracks to propagate without branching from the perforations of the horizontal well casing, which are typically spaced at 10 m or more. However, to explain the gas production rate at the wellhead, the crack spacing would have to be only about 0.1 m, which would increase the overall gas permeability of shale mass about 10,000×. This permeability increase has generally been attributed to a preexisting system of orthogonal natural cracks, whose spacing is about 0.1 m. However, their average age is about 100 million years, and a recent analysis indicated that these cracks must have been completely closed by secondary creep of shale in less than a million years. Here it is considered that the tectonic events that produced the natural cracks in shale must have also created weak layers with nanocracking or microcracking damage. It is numerically demonstrated that seepage forces and a greatly enhanced permeability along the weak layers, with a greatly increased transverse Biot coefficient, must cause the fracking to engender lateral branching and the opening of hydraulic cracks along the weak layers, even if these cracks are initially almost closed. A finite element crack band model, based on a recently developed anisotropic spherocylindrical microplane constitutive law, demonstrates these findings [Rahimi-Aghdam S, et al. (2018) arXiv:1212.11023].

Wedge-Splitting Test – Determination of Minimal Starting Notch Length for Various Cement Based Composites Part I: Cohesive Crack Modelling

2010 ◽  
Vol 452-453 ◽  
pp. 77-80 ◽  
Author(s):  
Václav Veselý ◽  
Ladislav Řoutil ◽  
Stanislav Seitl
Keyword(s):  
Fracture Mechanics ◽  
Cohesive Crack ◽  
Crack Model ◽  
Singular Stress ◽  
Initiation Point ◽  
Linear Elastic ◽  
Wedge Splitting Test ◽  
Splitting Test ◽  
Notch Length ◽  
Wedge Splitting

The geometric proportions of cube-shaped specimens subjected to wedge-splitting tests are numerically studied in the paper. The minimal notch length for specimens made of cement based composites varying in characteristic length of the material (a measure of material brittle-ness/heterogeneity) is verified using finite element method code with an implemented cohesive crack model (ATENA). The problem of assigning the crack initiation point (the notch tip vs. the groove corner in the load-imposing area of the specimen) is solved numerically also using both the theory of linear elastic fracture mechanics and the theory of the fracture mechanics of generalized singular stress concentrators in the second part of the two-part paper. Results ob-tained by the different approaches are compared. The minimal notch length is recommended.

Failure Surface Variation Obtained with the Truss-Like Discrete Element Method

2015 ◽  
Vol 784 ◽  
pp. 225-232
Author(s):  
Guilherme Schumacher da Silva ◽  
Fabrício Goulart Fernandes ◽  
Angélica Bordin Colpo ◽  
Vicente B. Puglia ◽  
Luis E. Kosteski
Keyword(s):  
Spatial Distribution ◽  
Discrete Element Method ◽  
Fracture Energy ◽  
Failure Criterion ◽  
Failure Surface ◽  
Discrete Element ◽  
Constitutive Law ◽  
Surface Variation ◽  
Spatial Lattice ◽  
Element Method

This paper presents the study of failure surface obtained in the truss-like Discrete Element Method (DEM). The element constitutive law considers the fracture energy of the material and its spatial variation is used to take into account the heterogeneity of the simulated materials. It is studied the influence of spatial distribution of fracture energy and the spatial lattice perturbation on the DEM failure surface. A DEM failure criterion is compared with concrete and rock failure.

Gap Test of Crack-Parallel Stress Effect on Quasibrittle Fracture and Its Consequences

2020 ◽  
Vol 87 (7) ◽  
Author(s):  
Hoang Thai Nguyen ◽  
Madura Pathirage ◽  
Gianluca Cusatis ◽  
Zdeněk P. Bažant
Keyword(s):  
Compressive Stress ◽  
Process Zone ◽  
Main Idea ◽  
Stress Effect ◽  
Test Beam ◽  
Simple Modification ◽  
Out Of Plane ◽  
Three Point Bend Test ◽  
Strength Models ◽  
Gap Test

Abstract In the standard fracture test specimens, the crack-parallel normal stress is negligible. However, its effect can be strong, as revealed by a new type of experiment, briefly named the gap test. It consists of a simple modification of the standard three-point-bend test whose main idea is to use plastic pads with a near-perfect yield plateau to generate a constant crack-parallel compression and install the end supports with a gap that closes only when the pads yield. This way, the test beam transits from one statically determinate loading configuration to another, making evaluation unambiguous. For concrete, the gap test showed that moderate crack-parallel compressive stress can increase up to 1.8 times the Mode I (opening) fracture energy of concrete, and reduce it to almost zero on approach to the compressive stress limit. To model it, the fracture process zone must be characterized tensorially. We use computer simulations with crack-band microplane model, considering both in-plane and out-of-plane crack-parallel stresses for plain and fiber-reinforced concretes, and anisotropic shale. The results have broad implications for all quasibrittle materials, including shale, fiber composites, coarse ceramics, sea ice, foams, and fone. Except for negligible crack-parallel stress, the line crack models are shown to be inapplicable. Nevertheless, as an approximation ignoring stress tensor history, the crack-parallel stress effect may be introduced parametrically, by a formula. Finally we show that the standard tensorial strength models such as Drucker–Prager cannot reproduce these effects realistically.

State-of-the-Art Review on Concrete Softening Curve

2010 ◽  
Vol 168-170 ◽  
pp. 669-673
Author(s):  
Zhi Fang Zhao ◽  
Zhi Gang Zhao ◽  
Xiao Jie Feng ◽  
Ming Li
Keyword(s):  
Constitutive Law ◽  
Cohesive Crack ◽  
Crack Model ◽  
Cohesive Crack Model ◽  
Direct Tension ◽  
Tension Softening ◽  
Nonlinear Form ◽  
Determination Methods ◽  
Relationship Of ◽  
Inverse Analysis Method

The cohesive crack model is widely employed to the fracture analysis of concrete for mode I crack. The tension softening relationship is a very important constitutive law in the cohesive crack model. The determination methods of tension softening relationship of concrete are introduced in this paper which are direct tension methods, J-integral method and inverse analysis method. Meanwhile, those simplified softening curves including linear form and nonlinear form are summarized.

Finite Element Simulation of Ceramics Machining

2016 ◽  
Vol 693 ◽  
pp. 775-779
Author(s):  
J.X. Xue ◽  
H.B. Wu ◽  
Q.P. Sun
Keyword(s):  
Finite Element ◽  
Fracture Mechanics ◽  
Brittle Fracture ◽  
Fracture Energy ◽  
Finite Element Simulation ◽  
Material Removal ◽  
Machining Process ◽  
Removal Mechanism ◽  
Material Removal Mechanism ◽  
Element Simulation

The evolution of crack models based on fracture mechanics is reviewed. The brittle cracking model in Abaqucs is used to simulate the machining process of Al2O3. The result shows that it’s appropriate to simulate the machining process of ceramics with fracture energy cracking criterion and post-failure constitutive relation in a smeared cracking representation. Although more works are needed in the future to resolve the mesh sensitivity. The material removal mechanism of ceramics is confirmed to be the brittle fracture regime.

Fracture Mechanical Properties of Rocks and Mortar/Rock Interfaces

1994 ◽  
Vol 370 ◽  
Author(s):  
Manouchehr Hassanzadeh
Keyword(s):  
Mechanical Properties ◽  
Tensile Strength ◽  
Compressive Strength ◽  
Fracture Energy ◽  
Modulus Of Elasticity ◽  
Splitting Tensile Strength ◽  
Coarse Grained ◽  
Interfacial Zone ◽  
Test Results

AbstractThis study has determined the fracture mechanical properties of 9 types of rock, namely fine-, medium- and coarse-grained granites, gneiss, quartzite, diabase, gabbro, and fine- and coarse-grained limestones. Test results show among other things that quartzite has the highest compressive strength and fracture energy, while diabase has the highest splitting tensile strength and modulus of elasticity. Furthermore, the strength and fracture energy of the interfacial zone between the rocks and 6 different mortars have been determined. The results showed that, in this investigation, the mortar/rock interfaces are in most cases weaker than both mortars and rocks.

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