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Fatigue simulation during the design process of vehicle structures
Published June 14, 1999 by ISATA - Dusseldorf Trade Fair in United Kingdom
Event: ISATA 1999
The lifetime of dynamically loaded vehicle components can be improved dramatically by finding the crack initiation point with suitable software tools and optimization of the critical areas. With increasing capacities of computers the prediction of the lifetime for components by numerical methods gets more and more important. Using the program FEMFAT the assessment of uniaxially and multiaxially loaded components as well as welding seams and spot joints is possible.
The theory applied in FEMFAT differs in some aspects from classical approaches like the nominal stress concept or the local one and can be characterized by the term "influence parameter method". The specimen S/N-curve is locally modified by different influence parameters as e.g. stress-gradient to take into account notch effects, mean-stress influence which is quantified by means of a Haigh- diagram, surface roughness and treatments, temperature, technological size, etc. It is possible to consider plastic deformations which results in mean-stress rearrangements. The user can choose between several calculation methods for the quantification of the influence parameters, e.g. methods which are fixed by German guidelines (FKM, TGL, .) and such one which have been developed in our company. Generally, the fatigue life of a component under a time-varying multiaxial stress state is determined by utilizing a strength hypothesis. The classical method is to compute an equivalent stress which is compared to the strength value of an uniaxial load (i.e. von Mises stress or maximum shear stress for ductile materials and maximum normal stress for brittle materials compared to tensile strength of cylindrical test specimens). This procedure is applicable only for proportional loads, i.e. loads which scale the magnitude of the multiaxial stress state, but which do not change the directions of the principal stress axes.
If one has to consider non-proportional loads, this conventional strength cannot be used. A convenient method to consider situations with changing principal stress directions is the "Critical Plane Approach". This hypothesis was used to develop a "multiaxial-damage-module" for FEMFAT. The basic idea of the Critical Plane approach is that cracking starts in the cutting plane with "maximum damage". This method can be well applied for each combination of external loads, however, the calculation effort is extensive. Therefore only nodes on surfaces of solid structures are considered and two cutting plane filter methods have been implemented to select planes where a high damage is to be expected.
An example for a dynamically loaded structure will be presented. The locations and the numbers of load cycles, where the components broke, were well predicted.
The software FEMFAT exists for about fifteen years, is widely applied for extensive problems and its development is permanently continued since that time. It is a very important contribution to the development process and helps to improve the lifetime of components and exploitation of materials.