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Fibre-reinforced composites are rapidly increasing their market share in structural applications. Nevertheless, this increase would be much stronger if reliable failure predictions were available. These predictions are not only insufficiently reliable for complex loading of multidirectional composites, but even for longitudinal tensile failure of unidirectional (UD) composites. Since composite failure usually coincides with longitudinal failure of a 0° ply, the reliability often hinges on longitudinal failure predictions of UD composites. Despite great progress in the state-of-the-art models, significant obstacles remain in collecting the necessary input data and understanding the influence of the modelling assumptions. This review therefore surveys the mechanics, chemistry and physics involved in tensile failure of UD composites and highlights potential areas for improvement. Specific proposals are made to advance the state-of-the-art strength models, which could catalyse the use of composites in structural applications. publisher: Elsevier articletitle: A review of input data and modelling assumptions in longitudinal strength models for unidirectional fibre-reinforced composites journaltitle: Composite Structures articlelink: http://dx.doi.org/10.1016/j.compstruct.2016.05.002 content_type: article copyright: © 2016 Elsevier Ltd. All rights reserved. ispartof: Composite Structures vol:150 pages:153-172 status: published

The development of realistic numerical tools to efficiently model the response of concrete structures subjected to close-in detonations and high velocity impact has been one of the major quests in defense research. Under these loading conditions, quasi-brittle materials undergo a multitude of failure (damage) mechanisms. Dynamic tensile failure (e.g. spalling), characterized by a significant strength increase associated with loading rate, has revealed to be particularly challenging to represent. In this contribution, a rate-dependent stress-based nonlocal damage model has been introduced for the simulation of dynamic tensile failure of quasi-brittle materials. The recently proposed stress-based nonlocal criterion has been updated in order to be consistently combined with a rate-dependent version of the well-known Mazars damage model. The model was implemented in LS-DYNA using a fully explicit computational scheme. Two sets of numerical examples have been presented. First, one-dimensional numerical analyses were conducted to evaluate the model capabilities, applicability and limitations. Second, the model has been validated against experimental results. It has been shown that the proposed model, in addition to correcting spurious mesh sensitivity, also provides a more realistic representation of damage initiation and growth, in particular around discontinuities (notches and free boundaries) and damaged areas.

The development of realistic numerical tools to efficiently model the response of concrete structures subjected to close-in detonations and high velocity impacts has been one of the major quests in defense research. Under these loading conditions, quasi-brittle materials undergo a multitude of failure (damage) mechanisms. Dynamic tensile failure (e.g. spalling), characterized by a significant strength increase associated with loading rate, has revealed to be particularly challenging to represent. This phenomenon has been modeled by means of continuous damage mechanics in the last decades. To minimize pathological mesh sensitivity, a nonlocal formulation is generally considered. Nevertheless, these models fail to properly represent damage initiation and growth around discontinuities, such as notches, damage areas and free boundaries. These inconsistencies are the consequence of using a fixed interaction domain (characteristic length) in the nonlocal formulation. In spite of limited experimental knowledge about the definition of the characteristic length parameter, there is now consensus that this quantity is not constant. In this contribution, an enhanced nonlocal model, where the interaction domain of any gauss point contracts or expands according to the stress-state of its neighbors, is used. This formulation was coupled to the well-known Mazars damage model and implemented within the framework of LS-DYNA using a fully explicit computation scheme. Two sets of numerical studies are presented in this paper. One shows the applicability and limitations of the implemented explicit algorithm to compute nonlocal quantities. The other shows that with this stress-enhanced regularization model it is possible to represent damage initiation and growth more realistically and correct the inconsistencies emerging from the traditional nonlocal formulations.

The development of realistic numerical tools to efficiently model the response of concrete structures subjected to close-in detonations and high velocity impacts has been one of the major quests in defense research. Under these loading conditions, quasi-brittle materials undergo a multitude of failure (damage) mechanisms. Dynamic tensile failure (e.g. spalling), characterized by a significant strength increase associated with loading rate, has revealed to be particularly challenging to represent. This phenomenon has been modeled by means of continuous damage mechanics in the last decades. To minimize pathological mesh sensitivity, a nonlocal formulation is generally considered. Nevertheless, these models fail to properly represent damage initiation and growth around discontinuities, such as notches, damage areas and free boundaries. These inconsistencies are the consequence of using a fixed interaction domain (characteristic length) in the nonlocal formulation. In spite of limited experimental knowledge about the definition of the characteristic length parameter, there is now consensus that this quantity is not constant. In this contribution, an enhanced nonlocal model, where the interaction domain of any gauss point contracts or expands according to the stress-state of its neighbors, is used. This formulation was coupled to the well-known Mazars damage model and implemented within the framework of LS-DYNA using a fully explicit computation scheme. Two sets of numerical studies are presented in this paper. One shows the applicability and limitations of the implemented explicit algorithm to compute nonlocal quantities. The other shows that with this stress-enhanced regularization model it is possible to represent damage initiation and growth more realistically and correct the inconsistencies emerging from the traditional nonlocal formulations.

The development of realistic numerical tools to efficiently model the response of concrete structures subjected to close-in detonations and high velocity impacts has been one of the major quests in defense research. Under these loading conditions, quasi-brittle materials undergo a multitude of failure (damage) mechanisms. Dynamic tensile failure (e.g. spalling), characterized by a significant strength increase associated with loading rate, has revealed to be particularly challenging to represent. This phenomenon has been modeled by means of continuous damage mechanics in the last decades. To minimize pathological mesh sensitivity, a nonlocal formulation is generally considered. Nevertheless, these models fail to properly represent damage initiation and growth around discontinuities, such as notches, damage areas and free boundaries. These inconsistencies are the consequence of using a fixed interaction domain (characteristic length) in the nonlocal formulation. In spite of limited experimental knowledge about the definition of the characteristic length parameter, there is now consensus that this quantity is not constant. In this contribution, an enhanced nonlocal model, where the interaction domain of any gauss point contracts or expands according to the stress-state of its neighbors, is used. This formulation was coupled to the well-known Mazars damage model and implemented within the framework of LS-DYNA using a fully explicit computation scheme. Two sets of numerical studies are presented in this paper. One shows the applicability and limitations of the implemented explicit algorithm to compute nonlocal quantities. The other shows that with this stress-enhanced regularization model it is possible to represent damage initiation and growth more realistically and correct the inconsistencies emerging from the traditional nonlocal formulations.

International audience; The evaluation of the location of slab tensile failure represents an important concern for the evaluation of the extent of avalanche release zones and hence hazard assessment. In this study, a mechanically-based statistical model of the slab-weak layer system accounting for weak-layer spatial variability, stress redistributions by elasticity of the slab and the slab possible tensile failure is simulated using a stochastic finite element method. Two types of avalanche releases are distinguished in the simulations: (1) full slope releases, for which the entire simulated slope is released and the heterogeneity is not sufficient to trigger a tensile failure within the slab; (2) partial slope releases, for which tensile failure occurs within the slab due to the heterogeneity so that only a part of the slope is released. We present the proportion of these two release types as a function of the different model parameters obtained from finite element simulations. One of the main outcomes is that, for slab tensile strength higher than the average cohesion of the weak layer, all the releases appear to be full-slope, justifying the major influence of topographical and morphological features such as rocks, trees, slope curvature, ridge and heterogeneous snow cover often claimed in the literature.

Master's thesis in Industrial economics Oil and gas production is moving to harsher geological conditions such as deep water drilling and high-pressure high-temperature reservoirs, so accurate knowledge of wellbore stability is crucial. The main causes of wellbore instability are high pore pressure in the formation, drilling induced disturbance of stable formations and the possible chemical reactions between the reservoir formation and the drilling and completion fluids. The thesis has studied the occurrence of drilling induced fractures, which can eventually cause fluid losses to the formation, hence become a costly issue during drilling. Borehole image tool are the only tool as of today that can detect drilling induced fractures, however one would like to prevent them from occurring. The thesis is interested in examine what can be the primary effect of their occurrence, and have chosen to focus on the effect of temperature. A new fracturing model was also introduced in this examination, where three scenarios were developed. These scenarios provided important information on the fracture gradient’s sensitivity towards temperature. Finally, the coefficient of thermal expansion was suggested to investigate further as it may have a bigger impact on the fracture gradient than initially presumed. Additionally, the importance of time-dependent downhole MWD data along with effective ECD management was accentuated.

This is the post-print version of the final paper published in Engineering Structures. The published article is available from the link below. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. Copyright @ 2008 Elsevier B.V. It is evident from a series of tests on simply supported reinforced concrete slabs that the failure of the slabs at large deflections is due to the formation of individual large cracks. This failure mode was also observed in the Cardington full-scale fire tests. Previous research indicates that the global behaviour of concrete slabs subject to large deflections can be well predicted by the smeared cracking model; however, the model cannot quantitatively predict the openings of individual cracks within the slabs at large deflections. For the discrete approach it is usually assumed that the cracks are formed along element edges, therefore continuous re-meshing is required during the analysis. Consequently, the results are mesh-dependent and the computing cost is high. In recent years, mesh independent finite element procedures, such as embedded (EFEM) and extended (XFEM) approaches, were widely used for modelling of the crack initiation and growth in structural members. However, most of the meshless models developed are either based on in-plane loading conditions or confined to thin shells with assumed full-depth cracks, which form apparent displacement jumps within an element. For a reinforced concrete slab, an out-of-plane load causes coupled stretching and bending of the slab, cracks are usually initiated at discrete positions and then propagated, until at last some individual full-depth cracks are formed. Pure stretching or assumed full-depth cracking is inadequate for modelling this kind of failure. Therefore, in this research, a non-linear layered procedure with embedded weak discontinuity is developed to quantitatively model the progressive tensile failure of reinforced concrete slabs subjected to large deflections. The current model inherits the advantage of the smeared approach, and at the same time, introduces the opening width of crack explicitly by taking the advantage of the better description of the kinematic characteristics of the EFEM approach. A series of validations have been conducted against test data at both ambient and elevated temperatures, and the research shows that the model developed in this paper is not sensitive to the FE mesh size and the aspect ratio of the slab. The results predicted by the model developed agreed well with the test data in terms of deflection and crack open width, also agreeing well with those modelled by the smeared model. Hence, this new approach provides a numerical method to predict the load capacity as well as identifying the occurrence and severity of crack failure in reinforced concrete slabs subjected to extreme loading conditions, such as fire.

The geological disposal of nuclear waste, in underground openings and the long-term performance of these openings demand a detailed understanding of fundamental rock mechanics. A full scale field experiment: Äspö Pillar Stability Experiment was conducted at a depth of 450 m in sparsely fractured granitic rock to examine the rock mass response between two deposition holes. An oval shaped tunnel was excavated parallel to the σ3 direction to provide access to the experiment and also provide elevated stress magnitudes in the floor. In the tunnel floor two 1.75-m diameter 6-m deep boreholes were excavated so that a 1-m thick pillar was created between them. In one of the holes a confinement pressure of 700 kPa was applied and in the other displacement transducers were installed. The pillar volume was monitored by an Acoustic Emission System. Spatially distributed thermocouples were used to monitor the temperature development as the pillar was heated by electrical heaters. The excavation-induced stress together with the thermal-induced stress was sufficient to cause the wall of the open borehole to yield. The temperature-induced stress was increased slowly to enable detailed studies of the rock mass yielding process. Once the rock mass loading response was observed, the rock mass was unloaded using a de-stress slotting technique. This thesis focuses on the in-situ study of the rock mass response to coupled mechanical thermal loading and thermal-mechanical unloading. The experiment, its design, monitoring and observations are thoroughly described. An estimate of the yielding strength of the rock mass is presented and compared with laboratory test and results from other rock mass conditions reported elsewhere in the open literature. General conclusions about the effect of the confining pressure and the observations from the unloading of the pillar are also presented. Important findings are that the yielding strength of the rock mass has been successfully determined, low confinement pressures significantly affects the onset of yielding, the primary mode of fracture initiation and propagation is extensional, no significant time dependency of the yielding process was observed. The unloading studies also indicated that what appeared to be shear bands likely was a propagating zone of extensile failure that weakened the rock so that displacements in the shear direction could occur. QC 20100622

International audience; The study of the clay tensile behaviour is one of the topics which requires a specific lighting especially when we give a closely attention to the pathology of the constructions built with or on the clays submitted to significant tensilestrenght. Therefore, failure or damage of clay can be related especially to tensile limit and not to shear limit overtaking. It is the case of compacted clay liners in wastes landfill cover or for embankments built on high compressible soils. In order to study the tensile compacted clay behaviour, a series of laboratory bending tests were carried out. On the basis of the test results, different numerical simulations using Discrete Element Method were engaged. Two numerical bending tests were carried out: a three points bending and a four points bending. A comparison between the two numerical protocols is given. Four analytical models (classical elasticity, bimodular elasticity, differential model and struts-tie method) are used in order to interpret bending tests results and are compared with the numerical simulations. At the same time, the validity of the assumptions relative to the four models was discussed. To outcome to these results, a study and an adjustment of numerical and micromechanical parameters were carried out. It was demonstrated that the Discrete Element Method has a strong correlation with laboratory tests and can contribute somewhat to the understanding and discussion on the validity degree of the kind of indirect tests and of their interpretation.

Load-deformation curves for reinforced concrete beams subjected to bending show size effects due to tensile failure of the concrete at early stages in the failure process and due to compression failure of the concrete when the final failure takes place. In this paper these effects are modelled using fracture mechanical concepts, and size effects of the models are studied and compared with experimental results. Load-deformation curves for reinforced concrete beams subjected to bending show size effects due to tensile failure of the concrete at early stages in the failure process and due to compression failure of the concrete when the final failure takes place. In this paper these effects are modelled using fracture mechanical concepts, and size effects of the models are studied and compared with experimental results.

In this work we investigate the effects of yarn diameter and gauge length on the statistical strength of yarns spun from carbon nanotubes (CNTs). Tensile tests are conducted on a large sample set of nanostructured CNT yarns. The data show that strength varies substantially and both strength and statistical dispersion in strength decreases as yarn diameter increases. To explain these phenomena and forecast their effects on larger-scale structures, a hierarchical set of Monte Carlo simulation models is developed: the lower-scale model aims to predict the relationship between yarn nanostructure and tensile strength and the higher-scale model aims to relate the strength of CNT yarns to the strength of composites reinforced with unidirectionally aligned CNT yarns. Predictions indicate that, for both structures, the mean and statistical variation in strength will decrease as the surface twist angle, number of CNTs in cross section and gauge length of the yarn increases. The predicted reductions in variability due to yarn nanostructure will be important for determining ways to minimize the detrimental effects of increasing length scale on strength.

TENSOR74 is a major revision of TENSOR, a computer code designed to solve stress wave propagation problems in two dimensions. The major physics modificatims in TENSOR74 are in the area of constitutive modeling of solid materials. The new models, which are described in detail, take into account pore collapse, ductile and strain softening brittle failure, as well as tensile failure with void opening and closure. In addition, a modified form of linear artificial viscosity is described.

To assess the effects of geological activities, models that take into account the short and long-term hydro-mechanical coupling including fracture failure are required. A new method to solve shear and tensile failure of fractures is proposed. It focuses in fragile rocks where it is possible to assume an elastic matrix and to impose irreversible displacements in fractures when fracture failure occurs. The formulation is based on an analytical solution combined with numerical methods solved using an iterative algorithm.

In recent years, some geological applications like the CO2 storage or the hydraulic fracturing have attracted the attention not only of the scientific community but also of the general public. One of the reasons of the controversy generated by these topics is the ignorance about some processes that can take place during the realization of this kind of operations. In fact, several questions are still unresolved: how long will the fractures propagate?, Will the contaminated fluid or the released gas leak to aquifers or to the atmosphere? Will the injection process induce seismicity? What magnitude? Hence, the aim of this work (developed as a part of the EU - FracRisk project) is to answer some of these questions identifying the parameters which are key in the hydraulic fracturing operation. In order to do that, a new thermo-hydro-mechanical Finite Element code has been developed in the Kratos framework using C++. As part of this code, a different method to solve the shear and the tensile failure has been proposed. In this method, fractures are represented as a zero thickness element, this means that both sides of the element lie together in the initial configuration (it seems a 1D element in a 2D domain, and a 2D element in a 3D domain) and separate as the adjacent matrix elements deform (Figure 1). Since the hydraulic fracturing operations take place in hard, fragile rocks, an elastic matrix can be assumed and irreversible displacements in fractures when rock failure occurs can be imposed. The formulation used to simulate shear and tensile failure is based on the analytical solution proposed by Okada (1992) and it is part of an iterative process. Thus, the fracture failure is calculated in a straightforward manner, avoiding numerical issues and capturing the domino effect that may occur when the failure process is triggered in a zone. In conclusion, the goal of this work is to analyze the main uncertainties related with the thermo-hydro-mechanical behavior of fractures derived from the hydraulic fracturing operations by means of the new code and method developed.