1,721,295 research outputs found
Convergence analysis for a smeared crack approach in brittle fracture
No full textOur analysis focuses on the mechanical energies involved in the propagation of fractures: the elastic energy, stored in the bulk, and the fracture energy, concentrated in the crack. We consider a finite element model based on a smeared crack approach: the fracture is approximated geometrically by a stripe of elements and mechanically by a softening constitutive law. We define in this way a discrete free energy Gh (h being the element size) which accounts for both elastic displacements and fractures. Our main interest is the behaviour of Gh as h tends to 0. We prove that, for a suitable choice of the (mesh dependent) constitutive law, Gh converges to a limit functional Gphi with a positive (anisotropic) term concentrated on the crack. We discuss the mesh bias and compute it explicitly in the case of a structured triangulation
A unilateral gradient flow and its quasi-static limit in phase-field fracture by alternate minimization
No full textWe consider an evolution in phase field fracture which combines, in a system of {sc pde}s, an irreversible gradient-flow for the phase-field variable with the equilibrium equation for the displacement field. We introduce a discretization in time and define a discrete solution by means of a 1-step alternate minimization scheme, with a quadratic -penalty in the phase-field variable (i.e.~an alternate minimizing movement). First, we prove that discrete solutions converge to a solution of our system of {sc pde}s. Then, we show that the vanishing viscosity limit is a quasi-static (parametrized) -evolution. All these solutions are described both in terms of energy balance and, equivalently, by {sc pde}s within the natural framework of
A discontinuous finite element approach for the approximation of free discontinuity problems
No full textThe anisotropy introduced by the mesh in the finite element approximation of the Mumford-Shah functional
No full textDealing with Italian Temporal Expressions: the ITA-Chronos System.
No full textThis paper presents ITA-Chronos, the system developed
at FBK-irst to participate in the “Temporal Expressions
(TE) Recognition and Normalization Task” at EVALITA
2007. ITA-Chronos adopts a rule-based approach, with
different sets of hand-crafted rules specialized to deal
with different aspects of the problem. The system
(FBKirst_Negri_TIME) achieved good results both in
recognition (TERN-Value: 85,7%) and normalization
(TERN-Value: 61,9%), ranking 1st in both the sub-tasks
Γ-convergence for high order phase field fracture: Continuum and isogeometric formulations
No full textWe consider high order phase field functionals introduced in Borden et al. (2014) and provide a rigorous proof that these functionals converge to a sharp crack brittle fracture energy. We take into account three dimensional problems in linear elastic fracture mechanics and functionals defined both in Sobolev spaces and in spaces of tensor product B-splines. In the latter convergence holds when the mesh size vanishes faster than the internal length of the phase-field model. On the theoretical level, this condition is natural since the size of the phase field layer, around the crack, itself scales like the internal length; on the numerical level, it should be satisfied by local h-refinement. Technically, convergence holds in the sense of Γ-convergence, with respect to the strong topology of L1, while the sharp crack energy is defined in GSBD2. The constraint on the phase field to take values in [0,1] is taken into account both in the Sobolev setting and in the iso-geometric setting; in the latter, it requires a special treatment since the projection operator on the space of tensor product B-splines is not Lagrangian (i.e., interpolatory)
Sense-based Blind Relevance Feedback for Question Answering
No full textThis paper addresses the problem of enhancing document retrieval under the specific restrictions posed by the Question Answering scenario. In particular, given an input question, we aim at defining a reliable method for expand ing its keywords with semantic information extracted from WordNet (e.g. synonyms or hypernyms). This is a challenging task, since it is intrinsically dependent on high quality disambiguation of natural language questions which so far has been out of the reach of state-of-the-art Word Sense Disambiguation too ls. The proposed solution relies on a two-step access to the target document collection, and can be seen as a ``sense-based` relevance feedback. According to this technique, once the top d_1,d_2,...,d_n documents have been retrieved using the question keywords, the most frequent senses of the question terms are considered instead of drawing for expansion the most relevant words that appear within d_1,d_2,...,d_n. Query enrichment is then carried out adding terms semantically related to these senses. Our experiments, carried out using part of the TREC-2003 factoid questions set and th e AQUAINT corpus for document retrieval, demonstrate the viability of this approach. Preliminary results show that the application of Sense-based Relevance Feed back to the QA scenario can improve retrieval up to 7% in terms of answer-bearing documents obtained with the best performing expansion strategy
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