Global and Local Design Method for Fatigue Resistant Structures

Results concerning Finite Element stress analysis and fatigue life predictions of various thin-walled structures designed according to the Statically Admissible Discontinuous Stress Fields (SADSF) method are presented and discussed in the paper. The SADSF method is one of a very few approaches enabling effective, from the load limit analysis viewpoint, design of geometrical shapes and spatial distribution of the material for prescribed geometrical and load boundary conditions. The resultant geometry of the structure is obtained by satisfying equilibrium conditions and maintaining the same equivalent stress in each of the discrete regions of the object. Such requirements can be satisfied if a relatively simple material model is used and as a consequence the ideal rigid-plastic material model is used when the system of discrete statically admissible stress field is looked for. Although the method does not rely on the actual stress fields in the case of linear elastic material the results obtained up to date indicated that all structures designed with the help of the SADSF method had optimum static load capacity. Basic principles of the method are briefly described illustrating also the use of the method in designing global geometry of a thin-walled structure. The same method is used in the subsequent application for designing the optimum geometry of a local welded connection. The Finite Element stress analysis of thin-walled structures designed according to the SADSF method revealed very uniform membrane-type distribution of stresses over the entire structure with negligible local bending moments. All weldments designed according to the SADSF method showed significant extension of fatigue durability in comparison with the standard geometry.