DSpace Collection:
http://hdl.handle.net/10174/995
2019-10-23T19:17:41ZSpeaking about heat flow, ice fusion and temperature alterations
http://hdl.handle.net/10174/25856
Title: Speaking about heat flow, ice fusion and temperature alterations
Authors: Duque, Maria Rosa
Abstract: Heat flow density measurements are difficult to obtain in Antarctica and Greenland due to climate and ice thickness in those regions. The works published about that subject shows that spatial distribution of the heat flux values is highly heterogeneous. Maximum and minimum values of heat flux vary with different authors and method used, but all of them say that East Antarctic is characterized by low heat flux values (45-85 mW m-2) with the lowest values found especially in the central part. The values obtained in West Antarctic (65-180 mW m-2) and in the Antarctic Peninsula (maximum of 170 mW m-2). In the work whose data is used high values were obtained in the West Antarctic Rift system (maximum value of 130 mW m-2). Some local geothermal anomalies have been reported such as Lake Wilhams (285 mW m-2) or Siple Dome (69 mW m-2). Elevated heat flux values are obtained in volcanic regions and in regions with relatively recent tectonic activity, in West Antarctic. The east Antarctic is characterized by low values, however the Coastal part of Queen Mary Land, the Lambert Rift and Victoria Land, show higher values, suggesting Cenozoic processes in the region, including volcanism or extension. Ice fusion occurs near the base of the ice sheet. In order to study these fact relations between heat flux, bed, thickness and ice altitude were studied. In regions with bottom of the ice below sea level ice melting of the ice occurs. The amount of ice obtained is under the measured value. The effect of sea water interacting with ice must be considered. Temperature values alterations at the surface may be found in the ice, including alterations due to last glaciations must they are not the cause of ice melting.2019-01-01T00:00:00ZGeneral constitutive updating for finite strain formulations based on assumed strains and the Jacobian
http://hdl.handle.net/10174/25509
Title: General constitutive updating for finite strain formulations based on assumed strains and the Jacobian
Authors: P, Areias
Abstract: Compatibility between element technology featuring assumed (finite)-strains based on least-squares and current constitutive formulations employed in elastic and inelastic contexts is a demanding task. Local frames are required for anisotropic and cohesive laws, some assumed-strain element technologies do not explicitly provide the deformation gradient, and total Lagrangian approaches are often inadequate for advanced plasticity models. Kirchhoff stress-based Fe Fp decompositions are also not convenient for ductile damage models. In addition, if rotational degrees-of-freedom are used, as is the case in beams and shells, the adoption of a fixed undeformed configuration causes implementation brittleness. An additional aspect to consider is remeshing by element partitioning, which precludes the storage of constitutive tensors in local frames, invalidating the stored quantities. Based on seven algorithmic requirements and the corresponding design solutions, we introduce a general constitutive updating algorithm based on the strain and the Jacobian provided by the element. This allows the use of virtually any constitutive law with any finite-strain element formulation while satisfying the seven requirements. In addition, Newton-Raphson convergence properties are extraordinary, at the cost of precision in the strain rate estimation. As a prototype element implementation, we present a stable hexahedron based on least-squares strains. A BFGS secant estimation is employed for the weight in the least-squares so that softening constitutive laws can be adopted without stability issues at the element level.2018-01-01T00:00:00ZEffective 2D and 3D crack propagation with local mesh refinement and the screened Poisson equation
http://hdl.handle.net/10174/25506
Title: Effective 2D and 3D crack propagation with local mesh refinement and the screened Poisson equation
Authors: Areias, P; Reinoso, J.; Camanho, P; César de Sá, J; Rabczuk, T
Abstract: In this paper, we propose a simple 2D and 3D crack evolution algorithm which avoids the variable/DOF mapping within mesh adaptation algorithms. To this end, a new area/volume minimization algorithm for damaged elements is introduced with the goal of improving the crack path representation. In addition, the new algorithm consists of: (i) mesh-creation stage where a damage model is employed to drive the remeshing procedure (ii) a subsequent analysis stage with a localization limiter in the form of a modified screened Poisson equation. This is exempt of crack path calculations. In the second stage, the crack naturally occurs within the refined region. A staggered algorithm for equilibrium and screened Poisson equations is used in this second stage. Element subdivision is based on edge split operations in 2D and 3D using the damage variable. Both 2D and 3D operations are described in detail. With the objective of assessing the robustness and accuracy of the algorithm, we test its capabilities by means of four quasi-brittle benchmark applications.2018-01-01T00:00:00ZFully-coupled piezoelectric assumed-strain least-squares nonlinear shell
http://hdl.handle.net/10174/25498
Title: Fully-coupled piezoelectric assumed-strain least-squares nonlinear shell
Authors: Areias, P; Rabczuk, T; César de Sá, J; Mota Soares, C
Abstract: Relevance of finite strain shell piezoelectric analysis is significant due to the general use of polyvinylidene fluoride (PVDF). A finite-strain geometrically exact shell model for the analysis of piezoelectric laminated structures is introduced. An assumed-strain formulation is employed, with least-squares fitting of contravariant linear stress fields. This allows the condensation of internal degrees-of-freedom corresponding to the assumed strains. The resulting piezoelectric shell has 8 degrees-of-freedom in each node, with 3 position/displacement degrees-of-freedom, 3 rotation parameters and the upper and lower electrostatic potential at the nodes. This contrasts with available formulations where only one electric degree-of-freedom is considered. A total of 32 degrees-of-freedom in each 4-node element are used. In term of implementation, we use a generalized strain and generalized stress formulation to reproduce the conventional finite element organization. Six examples are presented, with transversely isotropic and orthotropic cases, including finite strains and asymmetric plies. Results show a remarkably good agreement with the sources and we achieve higher values of actuation.2018-01-01T00:00:00Z