A coupled hydro-mechanical analysis for prediction of hydraulic fracture propagation in saturated porous media using EFG mesh-less method

The details of the Element Free Galerkin (EFG) method are presented with the method being applied to a study on hydraulic fracturing initiation and propagation process in a saturated porous medium using coupled hydro-mechanical numerical modelling. In this EFG method, interpolation (approximation) is based on nodes without using elements and hence an arbitrary discrete fracture path can be modelled.The numerical approach is based upon solving two governing partial differential equations of equilibrium and continuity of pore water simultaneously. Displacement increment and pore water pressure increment are discretized using the same EFG shape functions. An incremental constrained Galerkin weak form is used to create the discrete system of equations and a fully implicit scheme is used for discretization in the time domain. Implementation of essential boundary conditions is based on the penalty method. In order to model discrete fractures, the so-called diffraction method is used.Examples are presented and the results are compared to some closed-form solutions and FEM approximations in order to demonstrate the validity of the developed model and its capabilities. The model is able to take the anisotropy and inhomogeneity of the material into account. The applicability of the model is examined by simulating hydraulic fracture initiation and propagation process from a borehole by injection of fluid. The maximum tensile strength criterion and Mohr-Coulomb shear criterion are used for modelling tensile and shear fracture, respectively. The model successfully simulates the leak-off of fluid from the fracture into the surrounding material. The results indicate the importance of pore fluid pressure in the initiation and propagation pattern of fracture in saturated soils. © 2013 Elsevier Ltd.

Fracture of brittle lattice materials: A review

The mechanics of failure for elastic-brittle lattice materials is reviewed. Closed-form expressions are summarized for fracture toughness as a function of relative density for a wide range of periodic lattices. A variety of theoretical and numerical approaches has been developed in the literature and in the main the predictions coincide for any given topology. However, there are discrepancies and the underlying reasons for these are highlighted. The role of imperfections at the cell wall level can be accounted for by Weibull analysis. Nevertheless, defects can also arise on the meso-scale in the form of misplaced joints, wavy cell walls and randomly distributed missing cell walls. These degrade the macroscopic fracture toughness of the lattice. © 2010 Springer Science+Business Media B.V.

Mechanical and fracture properties of cement-based bi-materials after thermal cycling

This study investigates the effect of thermal cycles on the fracture properties of the cement-based bi-materials. Sixty eight cubes were exposed to a varied number of 24-hour thermal cycles ranging from 0 to 90 and subsequently were tested in a wedge splitting configuration. The mechanical and fracture properties of normal strength and high strength concretes are substantially improved after 30 thermal cycles, but less so after 90 thermal cycles both in isolation and when bonded to an ultra high-performance fibre-reinforced cement-based composite. © 2009 Elsevier Ltd. All rights reserved.

Dipole assisted photogated switch in spiropyran grafted polyaniline nanowires

In this work we show dipole-assisted photogated switching by covalent grafting of photoactive molecules to conducting polymers. Photochromic spiropyran molecules were covalently attached to polyaniline (PANI) nanowires via N-alkylation reaction to the quinoic part of PANI. Upon irradiation with ultraviolet light spiropyran transformed to a large dipole containing molecule, merocyanine form. We show that this transformation leads to a substantial (ca. 2 orders of magnitude) increase in conductance of the photochromic PANI nanowires, which were evident by an increase in field-effect mobility and calculated band gap narrowing of the system. Finally, this transformation was found to be fully reversible with no significant photofatigue. © 2011 American Chemical Society.

A fracture criterion for the notch strength of high strength steels in the presence of hydrogen

High strength steels can suffer from a loss of ductility when exposed to hydrogen, and this may lead to sudden failure. The hydrogen is either accommodated in the lattice or is trapped at defects, such as dislocations, grain boundaries and carbides. The challenge is to identify the effect of hydrogen located at different sites upon the drop in tensile strength of a high strength steel. For this purpose, literature data on the failure stress of notched and un-notched steel bars are re-analysed; the bars were tested over a wide range of strain rates and hydrogen concentrations. The local stress state at failure has been determined by the finite element (FE) method, and the concentration of both lattice and trapped hydrogen is predicted using Oriani's theory along with the stress-driven diffusion equation. The experimental data are rationalised in terms of a postulated failure locus of peak maximum principal stress versus lattice hydrogen concentration. This failure locus is treated as a unique material property for the given steel and heat treatment condition. We conclude that the presence of lattice hydrogen increases the susceptibility to hydrogen embrittlement whereas trapped hydrogen has only a negligible effect. It is also found that the observed failure strength of hydrogen charged un-notched bars is less than the peak local stress within the notched geometries. Weakest link statistics are used to account for this stressed volume effect. © 2013 Elsevier Ltd.

Time-dependent fracture toughness of cornea.

The fracture and time-dependent properties of cornea are very important for the development of corneal scaffolds and prostheses. However, there has been no systematic study of cornea fracture; time-dependent behavior of cornea has never been investigated in a fracture context. In this work, fracture toughness of cornea was characterized by trouser tear tests, and time-dependent properties of cornea were examined by stress-relaxation and uniaxial tensile tests. Control experiments were performed on a photoelastic rubber sheet. Corneal fracture resistance was found to be strain-rate dependent, with values ranging from 3.39±0.57 to 5.40±0.48kJm(-2) over strain rates from 3 to 300mmmin(-1). Results from stress-relaxation tests confirmed that cornea is a nonlinear viscoelastic material. The cornea behaved closer to a viscous fluid at small strain but became relatively more elastic at larger strain. Although cornea properties are greatly dependent on time, the stress-strain responses of cornea were found to be insensitive to the strain rate when subjected to tensile loading.

Defect-assisted sub-bandgap avalanche photodetection in interleaved carrier-depletion silicon waveguide for telecom band

This paper demonstrates on chip sub bandgap detection of light at 1550 nm wavelength using the configuration of interleaved PN junctions along a silicon waveguide. The device operates under reverse bias in a nearly fully depleted mode, thus minimizing the free carrier plasma losses and significantly increases the detection volume at the same time. Furthermore, substantial enhancement in responsivity is observed by the transition from reverse bias to avalanche breakdown regime. The observed high responsivity of up to 7.2 mA/W at 3 V is attributed to defect assisted photogeneration, where the defects are related to the surface and the bulk of the waveguide. © 2014 AIP Publishing LLC.

Fracture mechanics of idealised bituminous mixes

© 2014 Taylor & Francis. The durability of asphalt pavements is strongly impaired by cracks, caused primarily by traffic loads and environmental effects. In this work, fracture behaviour of idealised asphalt mixes is investigated. Experiments on idealised asphalt mixes under pure-tension mode (mode I cracking) were performed and fracture parameters were evaluated. In these three-point bend fracture tests, the test variables were temperature and load rate. The test data were stored in an asphalt materials database and special-purpose tools were implemented to analyse and handle the laboratory data automatically. Fracture mechanism maps were constructed, showing the conditions associated with ductile, brittle and ductile-brittle transition regimes of behaviour. The mechanism maps show the failure response of the material in terms of the stress intensity factor, strain energy release rate and J-integral as a function of the temperature-compensated crack mouth opening strain rate. Fracture behaviour of asphalt mix specimens was simulated by cohesive zone model in conjunction with a novel material constitutive model for asphalt mixes. The finite element model agrees well with the experimental results and provides insights into fracture response of the notched asphalt mix beam specimens.