Grietas De Solidificacion

Fatigue Analysis of a Welded Assembly Using ANSYS Workbench Environment
Klaus-Dieter Schoenborn ANSYS Service @ CADFEM GmbH, Germany
Abstract Fatigue analysis of welded joints often requires the handling of large structures, since typical weld seams are small details in large assemblies. FE models deduced from the global structure require idealization and are not well suited for fatigueconcepts that are based on the local stress state. The model of the global structure is used to locate hot spots and to transfer the applied loads to a local deformation. The analysis concepts proposed by the IIW and other organizations require a very detailed and specific representation of the welded joints. ANSYS Workbench preprocessing capabilities combined with ANSYS submodeling technology allowdemonstrating a fast and reliable workflow. The ANSYS Workbench Fatigue tool performs the high cycle fatigue life calculation.

Introduction
Over the last decades, numerous concepts have evolved for Fatigue analysis of welded assemblies. The traditional approach is to analyze the fatigue life based on the nominal stress concept. This concept derives the stress state at the weld seam according to abeam theory approach. The Forces and Moments acting on a certain cross section are divided by the section properties to yield the nominal membrane and bending stress. This approach however is limited. The stress state is defined according to a nominal cross section and the fatigue properties are taken from tabulated geometric constellations, the FAT classes. Complicated geometries often do notallow defining a nominal cross section and the stress state is often not just a pure combination of membrane and bending stress. Also, the geometric constellation of the structure might not fit into one of the FAT classes. In the recent years, other concepts have been developed to address those limitations. The extrapolation concept proposed by the IIW and other institutes derives the stress statefrom a numerical FE stress result and thus eliminates the need to define a nominal cross section. The stress is evaluated at certain distances away from the weld and then extrapolated towards the critical spot. Thus there is no need to model the weld seam itself. The evaluated stress state is not limited to membrane and bending stress. The IIW approach is well proven and supported by a large amountof test data, thus being widely accepted in the industry and by the certification authorities. However, the preprocessing effort imposed on the analyst to follow those concepts may be quite substantial, since they require a very distinct type of Shell or Solid modeling. The effective notch stress concept offers a different approach to analyze the stress state at the weld seam. This concept isbased on a volumetric representation of the weld seam geometry. The representation differs among concepts, the most prominent being the R1MS concept. The process starting from CAD Geometry to the determination of fatigue life requires extensive preprocessing and computing resources. The required number of DOF may largely exceed those for the nominal stress or the extrapolation concept, but it offers aquick and comprehensible workflow to the analyst.

Submodeling – The analysis concept to cope with large structures
Structural analysts that are doing fatigue calculations on large structures have always faced the problem of FE models that grow beyond any reasonable DOF limit. This is due to the fact that the global model stiffness has to be reflected to a certain level of accuracy. Theaccuracy of the global deformation state affects the local stress state and thus largely influences the accuracy of fatigue life prediction. The accuracy of the local stress state itself crucially depends on the local mesh density, which is imposing tough

constraints on the maximum element edge length and in consequence to the model DOF number. To cope with this problem, ANSYS has developed the...