Luiz Fernando Martha

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.

An algorithm for two-dimensional mesh generation for arbitrary regions with cracks

An algorithm for generating unstructured tetrahedral meshes of arbitrarily shaped three-dimensional regions is described. The algorithm works for regions without cracks, as well as for regions with one or multiple cracks. The algorithm incorporates aspects of well known meshing procedures, but includes some original steps. It uses an advancing front technique, along with an octree to develop local guidelines for the size of generated elements. The advancing front technique is based on a standard procedure found in the literature, with two additional steps to ensure valid volume mesh generation for virtually any domain. The first additional step is related to the generation of elements only considering the topology of the current front, and the second additional step is a back-tracking procedure with face deletion, to ensure that a mesh can be generated even when problems happen during the advance of the front. To improve mesh quality (as far as element shape is concerned), ana posteriori local mesh improvement procedure is used. The performance of the algorithm is evaluated by application to a number of realistically complex, cracked geometries.

Arbitrary crack representation using solid modeling

This paper describes the fundamental modeling approaches adopted for crack nucleation and propagation in a software system that is specifically designed to simulate problems with evolutionary geometry. Only the topological and geometrical aspects of crack modeling, and how these aspects affect the database representation in the system, are addressed in the present discussion. The following are the innovative features of the present crack modeling approach: (a) crack simulation is done with a true geometric representation of the structure, via solid modeling; (b) crack modeling relies on the sophisticated, topology-based data structure of this system to support linkage to the solid model, fast interaction and accurate representation of evolving flaw shapes; (c) the system provides the ability to specify flaws of arbitrary shape (including non-planar flaws), size and orientation at arbitrary locations in the geometric model; (d) the flaw is specified at the desired location in the actual structure geometry, rather than at a location in the mesh; (e) the system uses all its automatic and local remeshing capabilities for the simulation of flaw initiation and growth.

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.

A modeling methodology for finite element mesh generation of multi-region models with parametric surfaces☆

Abstract This paper presents a description of the reorganization of a geometric modeler, MG, designed to support new capabilities of a topological module (CGC) that allows the detection of closed-off solid regions described by surface patches in non-manifold geometric models defined by NURBS. These patches are interactively created by the user by means of the modelers graphics interface, and may result from parametric–surface intersection in which existing surface meshes are used as a support for a discrete definition of intersection curves. The geometry of realistic engineering objects is intrinsically complex, usually composed by several materials and regions. Therefore, automatic and adaptive meshing algorithms have become quite useful to increase the reliability of the procedures of a FEM numerical analysis. The present approach is concerned with two aspects of 3D FEM simulation: geometric modeling, with automatic multi-region detection, and support to automatic finite-element mesh generation.

Adaptive simulation of fracture processes based on spatial enumeration techniques

Abstract An interactive graphics computational system with self-adaptive, integrated, two-dimensional finite element analysis capabilities is described in this work. This system is able to handle both standard structural and fracture mechanics problems. The self-adaptive strategy is based on recursive spatial enumeration techniques: a binary tree partition for the boundary and the crack-line curves definition, and a quadtree partition for domain mesh generation. The ‘ a priori ’ refinement of the curves has the advantage of generating good transition meshes at the boundary regions. The system integrates different tools: a geometric modeler to create the model geometry, a pre-processor for mesh generation and attribute assignment, a numerical analysis module to evaluate the finite element response, and a post-processor for result visualization. The system is capable of deciding where to refine an initial mesh, of redoing the analysis, and of repeating this procedure until a pre-defined convergence criterion is achieved. Cracks can be introduced arbitrarily by the user at any position in the model. The system regenerates the meshes automatically taking into account the new created crack surfaces. For linear elastic analysis, quarter-point elements are inserted around the crack tips. The self-adaptive procedure is also considered in the crack propagation process. This procedure takes into account the arbitrarily generated crack geometry and the finite element error estimation analysis.

CRACK PROPAGATION MODELING

In this paper, we review recent advances in mathematical and computer science techniques for modeling crack propagation in solids. The fracture mechanics aspect of this problem is attacked by boundary integral equation methods, in particular the use of hypersingular integral equations for analyzing crack geometries. Key issues in the development of a software system capable of efficient crack propagation studies are also discussed. As an illustration of these techniques, calculations analyzing crack growth in a fuel door hinge on the Space Shuttle are presented.

A distributed-memory parallel technique for two-dimensional mesh generation for arbitrary domains

This work describes a technique for generating two-dimensional triangular meshes using distributed memory parallel computers, based on a master/slaves model. This technique uses a coarse quadtree to decompose the domain and a serial advancing front technique to generate the mesh in each subdomain concurrently. In order to advance the front to a neighboring subdomain, each subdomain suffers a shift to a Cartesian direction, and the same advancing front approach is performed on the shifted subdomain. This shift-and-remesh procedure is repeatedly applied until no more mesh can be generated, shifting the subdomains to different directions each turn. A finer quadtree is also employed in this work to help estimate the processing load associated with each subdomain. This load estimation technique produces results that accurately represent the number of elements to be generated in each subdomain, leading to proper runtime prediction and to a well-balanced algorithm. The meshes generated with the parallel technique have the same quality as those generated serially, within acceptable limits. Although the presented approach is two-dimensional, the idea can be easily extended to three dimensions.

GEOMECHANICS APPLIED TO THE WELL DESIGN THROUGH SALT LAYERS IN BRAZIL: A HISTORY OF SUCCESS

The lessons learned on the geomechanical salt behavior and its application in subsalt wells design are described in this article. In addition, the developed methodology validation, through comparison between computing modeling results with measurements carried out in experimental panels, in the potash mine, and with measurements obtained in an experimental well drilled for the purpose of calibrating and optimizing directional drilling in salt layers are presented. These parameters and methodology have been used for supporting the design of the wells drilled in the Pre-Salt giant oil fields in Brazil with very successful results.