Antonio Miranda

Fatigue life and crack path predictions in generic 2D structural components

This paper proposes a reliable and cost-effective two-phase methodology to predict crack propagation life in generic two-dimensional (2D) structural components. First, the usually curved fatigue crack path and its stress-intensity factors are calculated at small crack increments in a specialized finite-element software, using automatic remeshing algorithms, special crack tip elements and appropriate crack increment criteria. Then, the computed stress-intensity factors are transferred to a powerful general-purpose fatigue-design program, which has been designed to predict both initiation and propagation fatigue lives by means of classical design methods. Particularly, its crack propagation module accepts any KI expression and any crack growth rate model, considering sequence effects such as overload-induced crack retardation to deal with 1D and 2D crack propagation under variable amplitude loading. Non-trivial application examples compare the numerical simulation results with those measured in physical experiments. � 2002 Elsevier Science Ltd. All rights reserved.

The magnitude and persistence of soil NO, N2O, CH4, and CO2 fluxes from burned tropical savanna in Brazil

Among all global ecosystems, tropical savannas are the most severely and extensively affected by anthropogenic burning. Frequency of fire in cerrado, a type of tropical savanna covering 25% of Brazil, is 2 to 4 years. In 1992 we measured soil fluxes of NO, N2O, CH4, and CO2 from cerrado sites that had been burned within the previous 2 days, 30 days, 1 year, and from a control site last burned in 1976. NO and N2O fluxes responded dramatically to fire with the highest fluxes observed from newly burned soils after addition of water. Emissions of N-trace gases after burning were of similar magnitude to estimated emissions during combustion. NO fluxes immediately after burning are among the highest observed for any ecosystem studied to date. These rates declined with time after burning and had returned to control levels 1 year after the burn. An assessment of our data suggested that tropical savanna, burned or unburned, is a major source of NO to the troposphere. Cerrado appeared to be a minor source of N2O and a sink for atmospheric CH4. Burning also elevated CO2 fluxes, which remained detectably elevated 1 year later.

SEASONAL AND DEPTH VARIATION OF SOIL MOISTURE IN A BURNED OPEN SAVANNA (CAMPO SUJO) IN CENTRAL BRAZIL

The soil water regimes of two areas of open savanna (campo sujo) near Brasilia, Brazil, were monitored between August 1999 and November 2000. Each area was subjected to a different fire regime. Soil water content was measured to a depth of 3.6 m, using a neutron probe. The profile storage at the end of the 1999 and 2000 dry seasons was very similar despite a difference in dry season duration and large differences in rainfall in the preceding wet seasons, indicating that the vegetation is conservative in its water use. In the last two months of the dry season, the water content of the upper 0.6 m of the soil profile did not decrease further, suggesting that the vegetation had used all of the available water in this layer. The seasonal variation in soil water storage to a depth of 3.6 m was 403 mm, 65% of which occurred below 1 m. The wet and dry season evaporation rates were estimated to be 2.4 mm/d and 1.6 mm/d, respectively, but for a month after fire, before regrowth started, the evaporation rate was less than 0.5 mm/d.

Photographic method to measure the vertical distribution of leaf area density in forests

Many current vegetation–atmosphere models require structural information describing the canopy in order to calculate rates of mass and energy exchange. One of the most important pieces of information is the variation with height in leaf area density, ρ (m2 leaf area per m3 canopy volume), but it is notoriously difficult to measure this in forest canopies. Consequently, very few data on ρ exist for tall tropical forests. We describe a relatively rapid photographic method to make this measurement and we demonstrate its use for a rain forest in Cameroon, and two rain forests in Brazil. The method requires an assumption or knowledge of leaf angle distribution, but is relatively rapid and has the benefit of sampling a relatively large volume of canopy. The technique works adequately, especially if the leaf area index of the site is already known; the results agree quantitatively and qualitatively with previous more laborious determinations.

Fatigue life prediction of complex 2D components under mixed-mode variable amplitude loading

Accurate residual fatigue life predictions under variable amplitude (VA) loading are essential to maximize the time between the required inspections in defect-tolerant structures. However, this is not a trivial task for real structural components, in which cracks may change direction as they grow due to mixed-mode loading. Such curved crack paths can be predicted using finite element (FE) techniques, but this approach is not computationally efficient to predict the residual life, because it would require timeconsuming remeshing of the entire structure after each rain-flow counted load event under VA loading. In this work, a two-phase methodology that is both precise and cost-effective is applied to solve this problem. First, the fatigue crack path and stress intensity factors KI and KII are calculated in a specialized (global) FE program using fixed crack increments, requiring only relatively few remeshing steps. Then, an analytical expression is fitted to the calculated KI(a) values, where a is the length along the crack path, and exported to a companion fatigue design program to predict the crack propagation life by the local approach, considering load interaction effects such as crack retardation or arrest after overloads. This two-phase methodology is experimentally validated by fatigue tests on compact tension specimens, modified with holes positioned to attract or to deflect the cracks.  2003 Elsevier Ltd. All rights reserved.

Surface mesh regeneration considering curvatures

This work describes an automatic algorithm for unstructured mesh regeneration on arbitrarily shaped three-dimensional surfaces. The arbitrary surface may be: a triangulated mesh, a set of points, or an analytical surface (such as a collection of NURBS patches). To be generic, the algorithm works directly in Cartesian coordinates, as opposed to generating the mesh in parametric space, which might not be available in all the cases. In addition, the algorithm requires the implementation of three generic functions that abstractly represent the supporting surface. The first, given a point location, returns the desired characteristic size of a triangular element at this position. The second method, given the current edge in the boundary-contraction algorithm, locates the ideal apex point that forms a triangle with this edge. And the third method, given a point in space and a projection direction, returns the closest point on the geometrical supporting surface. This work also describes the implementation of these three methods to re-mesh an existing triangulated mesh that might present regions of high curvature. In this implementation, the only information about the surface geometry is a set of triangles. In order to test the efficiency of the proposed algorithm of surface mesh generation and implementation of the three abstract methods, results of performance and quality of generated triangular element examples are presented.

Boolean operations on multi-region solids for mesh generation

An algorithm for Boolean operations on non-manifold models is proposed to allow the treatment of solids with multiple regions (internal interfaces) and degenerate portions (shells and wires), in the context of mesh generation. In a solid modeler, one of the most powerful tools to create three-dimensional objects with any level of geometric complexity is the Boolean set operators. They are intuitive and popular ways to combine solids, based on the operations applied to point sets. To assure that the resulting objects have the same dimension as the original objects, without loose or dangling parts, a regularization process is usually applied after a Boolean operation. In practice, the regularization is performed classifying the topological elements and removing internal or lower-dimensional structures. However, in many engineering applications, the adopted geometric model may contain idealized internal parts, as in the case of multi-region models, or lower-dimensional parts, as in the case of solids that contain dangling slabs that are represented as zero-thickness surfaces or wireframes in the model. Therefore, the aim of this work is the development of a generic algorithm that allows the application of the Boolean set operations in a geometric modeling environment applied to finite and boundary element mesh generation. This environment adopts a non-manifold boundary representation that considers an undefined number of topological entities (group concept), and works with objects of different dimensions and with objects not necessarily plane or polyhedral (parametric curved surfaces). Numerical examples are presented to illustrate the proposed methodology.

A Three-Dimensional Adaptive Mesh Generation Approach Using Geometric Modeling With Multi-Regions and Parametric Surfaces

This work presents a methodology for adaptive generation of 3D finite element meshes using geometric modeling with multiregions and parametric surfaces, considering a geometric model described by curves, surfaces, and volumes. This methodology is applied in the simulation of stress analysis of solid structures using a displacement-based finite element method and may be extended to other types of 3D finite element simulation. The adaptive strategy is based on an independent and hierarchical refinement of curves, surfaces, and volumes. From an initial model, new sizes of elements obtained from a discretization error analysis and from geometric restrictions are stored in a global background structure, a recursive spatial composition represented by an octree. Based on this background structure, the model’s curves are initially refined using a binary partition algorithm. Curve discretization is then used as input for the refinement of adjacent surfaces. Surface discretization also employs the background octree-based refinement, which is coupled to an advancing front technique in the surface’s parametric space to generate an unstructured triangulated mesh. Surface meshes are finally used as input for the refinement of adjacent volumetric domains, which also uses an advancing front technique but in 3D space. In all stages of the adaptive strategy, the refinement of curves, surface meshes, and solid meshes is based on estimated discretization errors associated with the mesh of the previous step in the adaptive process. In addition, curve and surface refinement takes curvature information into account. Numerical examples of simulation of engineering problems are presented in order to validate the methodology proposed in this work. [DOI: 10.1115/1.4024106]

Finite element mesh generation for subsurface simulation models

This paper introduces a methodology for creating geometrically consistent subsurface simulation models, and subsequently tetrahedral finite element (FE) meshes, from geometric entities generated in gOcad software. Subsurface simulation models have an intrinsic heterogeneous characteristic due to the different geomechanics properties of each geological layer. This type of modeling should represent geometry of natural objects, such as geological horizons and faults, which have faceted representations. In addition, in subsurface simulation modeling, lower-dimension degenerated parts, such as dangling surfaces, should be represented. These requirements pose complex modeling problems, which, in general, are not treated by a generic geometric modeler. Therefore, this paper describes four important modeling capabilities that are implemented in a subsurface simulation modeler: surface re-triangulation, surface intersection, automatic volume recognition, and tetrahedral mesh generation. Surface re-triangulation is used for regenerating the underlying geometric support of surfaces imported from gOcad and of surface patches resulting from intersection. The same re-triangulation algorithm is used for generating FE surface meshes. The proposed modeling methodology combines, with some adaptation, meshing algorithms previously published by the authors. Two novel techniques are presented, the first for surface intersection and the second for automatic volume recognition. The main contribution of the present work is the integration of such techniques through a methodology for the solution of mesh generation problems in subsurface simulation modeling. An example illustrates the capabilities of the proposed methodology. Shape quality of generated triangular surface and tetrahedral meshes, as well as the efficiency of the 3D mesh generator, is demonstrated by means of this example.

Quadrilateral Mesh Generation Using Hierarchical Templates

This paper describes a quadrilateral mesh generation algorithm ideally suited for transition subdomain meshes in the context of any domain decomposition meshing strategy. The algorithm is based on an automatic hierarchical region decomposition in which, in the last level, it is possible to generate quadrilateral elements with a conventional mapping strategy. In two dimensions, a subdomain is usually a triangle or a rectangle. In this algorithm, a subdomain with two boundary curves may also be allowed. Templates impose restrictions on the number of boundary curve segments of a subdomain to be meshed. The proposed hierarchical template scheme eliminates these restrictions, requiring only an even number of boundary segments. Other algorithms in the literature present similar characteristics. However, the implementation of the hierarchical decomposition and its templates presented here is quite simple compared to other approaches. Six high-level templates are considered for a subdomain, depending on the number of boundary curves and the number of segments on each curve. Several examples demonstrate that this simple idea may result in structured meshes of surprisingly good quality. We also show the possibility of obtaining different meshes for a subdomain with fixed boundary discretization by changing the corners between curves.