Efecto de la zona de contacto entre fibra y matriz en las propiedades de fractura de materiales compuestos sometidos a carga transversal

El presente trabajo trata de dilucidar si en presencia de despegues entre fibra y matriz, se produce contacto circunferencial entre ambas fases cuando el conjunto está sometido a carga transversal a la fibra. Se utiliza el Método de los Elementos de Contorno para modelar el problema de fibra única embebida en matriz. El estado tensional se caracteriza colocando a ambos lados del fondo de grieta elementos de geometría circular y con variación discontinua singular en las tensiones. Con la consideración de la zona de contacto se pretende calcular valores de la energía unitaria liberada, usando la técnica del ciene de grieta, para compararlos con los obtenidos usando soluciones analíticas que conllevan interpenetraciones entre fibra y matriz. Ambos resultados serán comparados con medidas experimentales obtenidas en el ensayo transversal de fibra única, con una probeta obtenida del ensayo de fragmentación.This paper tries to ellucidate if in presence of debondings between fibre and matrix, circumferential·contact between them arises when the whole set is loaded transVersally to the fibre direction. The Boundary Element Method is used to model the case of unique fibre embedded in a matrix. The stress state is characterized placing at both si des of the crack tip elements with circular geometry and having discontinuous singular evolution of the stresses. Values of the energy release rate, considering the presence of a contact zone, are calculated, using the crnck closure technique, to be compared with those obtained using analytical solutions that involve interpenetrations between fibre and matrix. Both results are compared with the ex-perimental values obtained from the transversal test of unique fibre, the specimen having been obtained from a fragmentation test.Ministerio de Educación y Ciencia MAT 95-1672-EUnión Europea BRITE BRE2-0453/BE554

Evaluation of interfacial fracture toughness and friction coefficient in the single fiber fragmentation test

Single fiber fragmentation test is extensively employed to characterize the fiber-matrix interface in composites. A novel technique for evaluating fracture toughness and friction coefficient at the fiber-matrix interface in an epoxy sample containing a single glass fiber is proposed. Using experimental measurements of the average fragment and debond lengths, Boundary Element (BE) models of the portion of sample corresponding to the average fiber fragment are created for increasing values of the applied strain. From the solution of the BE models, energy release rate (ERR) during crack propagation is evaluated using a Fracture Mechanics based approach which accounts for fiber-matrix interfacial friction. The calculated evolution of the ERR has a reasonably linear dependency on the applied strain, with a decreasing slope for increasing values of the interfacial friction coefficient. Since debond growth is stable, ERR should equal the interfacial fracture toughness during debond propagation. Consequently, interfacial friction coefficient and fracture toughness can be simultaneously determined by parametrically varying the friction coefficient until a null slope is obtained in the linear fit of the numerical solution of the ERR as a function of the applied strain. The applicability of the proposed technique is demonstrated with experimental results taken from the literature.Junta de Andalucía P08-TEP-0405

El método de los elementos de contorno axisimétrico aplicado al análisis de grietas de interfase en materiales compuestos reforzados con fibras

En el presente artículo se muestra un análisis de los diferentes estados elásticos que aparecen durante la realización del ensayo de fragmentación de fibra única, empleando el Método de los Elementos de Contorno. El objetivo fundamental del trabajo es el análisis de la aparición y propagación de una grieta de despegue entre la fibra y la matriz, y de su influencia en las tensiones axiales a lo largo de la fibra. Se han empleado dos enfoques diferentes en el análisis: el enfoque de grieta abierta (el cual, aunque no es realista en este caso, ofrece algunos resultados aceptables) y el enfoque de grieta cerrada (considerando o no el efecto de la fricción entre las caras de la grieta). Se analiza, además, la solución asintótica de las tensiones y los desplazamiento en el entorno de dos grietas diferentes: primero, una grieta que parte la fibra y, segundo, una grieta de despegue.An analysis of the micromechanical elastic fields arising in the single fibre fragmentation test, carried out employing the Boundary Element Method, is presented in this paper. The main objective of the work is to study the initiation and growth of a debond crack along the fibre-matrix interface, and its influence on the axial stresses along the fibre. Two different models have been considered: the open crack model (which, although is clearly unrealistic in this particular case, give some acceptable results) and the closed crack model (excluding and including the effect of friction between crack faces). The asymptotic behaviour of stresses and displacements in the vicinity of the crack tips are studied, first, for a crack splitting the fibre and, second, for a debond crack

Sleep loss drives acetylcholine- and somatostatin interneuron-mediated gating of hippocampal activity to inhibit memory consolidation

Sleep loss disrupts consolidation of hippocampus-dependent memory. To characterize effects of learning and sleep loss, we quantified activity-dependent phosphorylation of ribosomal protein S6 (pS6) across the dorsal hippocampus of mice. We find that pS6 is enhanced in dentate gyrus (DG) following single-trial contextual fear conditioning (CFC) but is reduced throughout the hippocampus after brief sleep deprivation (SD; which disrupts contextual fear memory [CFM] consolidation). To characterize neuronal populations affected by SD, we used translating ribosome affinity purification sequencing to identify cell type-specific transcripts on pS6 ribosomes (pS6-TRAP). Cell type-specific enrichment analysis revealed that SD selectively activated hippocampal somatostatin-expressing (Sst+) interneurons and cholinergic and orexinergic hippocampal inputs. To understand the functional consequences of SD-elevated Sst+ interneuron activity, we used pharmacogenetics to activate or inhibit hippocampal Sst+ interneurons or cholinergic input from the medial septum. The activation of either cell population was sufficient to disrupt sleep-dependent CFM consolidation by gating activity in granule cells. The inhibition of either cell population during sleep promoted CFM consolidation and increased S6 phosphorylation among DG granule cells, suggesting their disinhibition by these manipulations. The inhibition of either population across post-CFC SD was insufficient to fully rescue CFM deficits, suggesting that additional features of sleeping brain activity are required for consolidation. Together, our data suggest that state-dependent gating of DG activity may be mediated by cholinergic input and local Sst+ interneurons. This mechanism could act as a sleep loss-driven inhibitory gate on hippocampal information processing.</p

KDM6A Loss Triggers an Epigenetic Switch That Disrupts Urothelial Differentiation and Drives Cell Proliferation in Bladder Cancer

Disruption of KDM6A, a histone lysine demethylase, is one of the most common somatic alternations in bladder cancer. Insights into how KDM6A mutations affect the epigenetic landscape to promote carcinogenesis could help reveal potential new treatment approaches. Here, we demonstrated that KDM6A loss triggers an epigenetic switch that disrupts urothelial differentiation and induces a neoplastic state characterized by increased cell proliferation. In bladder cancer cells with intact KDM6A, FOXA1 interacted with KDM6A to activate genes instructing urothelial differentiation. KDM6A-deficient cells displayed simultaneous loss of FOXA1 target binding and genome-wide redistribution of the bZIP transcription factor ATF3, which in turn repressed FOXA1-target genes and activated cell-cycle progression genes. Importantly, ATF3 depletion reversed the cell proliferation phenotype induced by KDM6A deficiency. These data establish that KDM6A loss engenders an epigenetic state that drives tumor growth in an ATF3-dependent manner, creating a potentially targetable molecular vulnerability

Interface Cohesive Elements to Model Matrix Crack Evolution in Composite Laminates

In this paper, the transverse matrix (resin) cracking developed in multidirectional composite laminates loaded in tension was numerically investigated by a finite element (FE) model implemented in the commercially available software Abaqus/Explicit 6.10. A theoretical solution using the equivalent constraint model (ECM) of the damaged laminate developed by Soutis et al. was employed to describe matrix cracking evolution and compared to the proposed numerical approach. In the numerical model, interface cohesive elements were inserted between neighbouring finite elements that run parallel to fibre orientation in each lamina to simulate matrix cracking with the assumption of equally spaced cracks (based on experimental measurements and observations). The stress based traction-separation law was introduced to simulate initiation of matrix cracking and propagation under mixed-mode loading. The numerically predicted crack density was found to depend on the mesh size of the model and the material fracture parameters defined for the cohesive elements. Numerical predictions of matrix crack density as a function of applied stress are in a good agreement to experimentally measured and theoretically (ECM) obtained values, but some further refinement will be required in near future work

A stochastic multiple mapping conditioning computational model in OpenFOAM for turbulent combustion

Computational models for combustion must account for complex and inherently interconnected physical processes including dispersion, mixing, chemical reactions, particulate nucleation and growth and, critically, the interactions of these with turbulence. The development of affordable and accurate models that are widely applicable is a work in progress. Stochastic multiple mapping conditioning (MMC) is a fast-emerging approach that has been successfully applied to non-premixed, premixed and partially premixed flames as well to the modelling of liquid and solid particulate synthesis. The method solves the conventional PDF transport equation but incorporates an additional constraint in that the mixing is localised in a reference space. This paper describes the numerical implementation of stochastic MMC in an OpenFOAM compatible code called mmcFoam. The model concepts and equations along with alternative submodels, code structure and numerical schemes are explained. A focus is placed on validation of the computational methods in particular demonstrating numerical convergence and mass consistency of the hybrid Eulerian/Lagrangian schemes. Four validation cases are selected including a combustion direct numerical simulation (DNS) case, two combustion experimental jet flame cases and a non-combusting particulate synthesis case. The results show that the total mass and mass distribution of Eulerian and Lagrangian schemes are consistent and confirm that the solutions numerically converge with increasing number of stochastic computational particles and sections for describing particulate size distribution

Computational Implementation of a Thermodynamically Based Work Potential Model For Progressive Microdamage and Transverse Cracking in Fiber-Reinforced Laminates

A continuum-level, dual internal state variable, thermodynamically based, work potential model, Schapery Theory, is used capture the effects of two matrix damage mechanisms in a fiber-reinforced laminated composite: microdamage and transverse cracking. Matrix microdamage accrues primarily in the form of shear microcracks between the fibers of the composite. Whereas, larger transverse matrix cracks typically span the thickness of a lamina and run parallel to the fibers. Schapery Theory uses the energy potential required to advance structural changes, associated with the damage mechanisms, to govern damage growth through a set of internal state variables. These state variables are used to quantify the stiffness degradation resulting from damage growth. The transverse and shear stiffness of the lamina are related to the internal state variables through a set of measurable damage functions. Additionally, the damage variables for a given strain state can be calculated from a set of evolution equations. These evolution equations and damage functions are implemented into the finite element method and used to govern the constitutive response of the material points in the model. Additionally, an axial failure criterion is included in the model. The response of a center-notched, buffer strip-stiffened panel subjected to uniaxial tension is investigated and results are compared to experiment