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The optimization The optimisation calculates an approximated von Mises stress and shows these values in the postprocessingPost Processing.
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The elected stress goal selected Stress Goal is the overall optimization goal for the optimization optimisation and has a big influence on the resulting design. The target is that the material of the part is evenly loaded and the stress uniformly distributed. Thus, the stress goal Stress Goal isn’t the maximum stress which can occur at design space Design Space limits or due to FE singularities, but the value which is a good middle value for a long part life (green areas in the example above). Not in every case the yield strength of the material is the best value for this parameter, e.g. when the applied loads are too small to result in such stresses. By changing the stress goalStress Goal, the stiffness of the structure can be influenced as well. Due to uniformly distributed stresses, lower stress goals Stress Goals result in stiffer parts at the most lightweight design. A safety factor can be introduced by considering this factor in the Stress Goal value. The user needs to take into account if a safety factor is needed and adjust the Stress Goal accordingly.
The used stress goal Stress Goal is dependent on the resolution level during the optimizationoptimisation. The optimization optimisation runs on different resolution levels and switches between them to achieve the best results. The first iterations are calculated on a coarse resolution level. With rising number of iterations, the resolution gets finer. This means that the first iterations calculate much faster than the last iterations but aren’t as detailed as the last. With the increasing resolution, the size of the output data, the calculation time as well as the surface quality increase.
For each resolution level a different stress percent goal is defined which is dependent on the stress goal Stress Goal set for the optimizationoptimisation. On the lower resolution levels the stress goal varies on purpose from the requested stress goal Stress Goal to realize a fast material reduction and therefore design generation. On the highest resolution level, the requested stress goal reaches 100% and thus satisfies the defined stress goal Stress Goal as the geometry allows it. 
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In one optimization optimisation several Events (load cases) can be considered. Each Event is calculated by itself and the stresses of all Events are enveloped and the highest stress for each area is considered. For information regarding event specific stress constraints Event Specific Stress Goals have a look here.
The stress calculation is from an FE point of view an estimation. That’s why we recommend a FE reanalysis using Apex Structures/Nastran to verify the results regarding all boundary conditions. For the reanalysis the intersected model should be used which will be manufactured and usedused after manufacturing and post manufacturing processes. The reanalysis can also take dynamic loading & other boundary conditions into consideration that are not considered in the design process.
For more further information regarding intersection the stresses in the Post Processing, have a look here.