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The optimization calculates an approximated von Mises stress and shows these values in the postprocessing.
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For each resolution level a different stress percent goal is defined which is dependent on the stress goal set for the optimization. On the lower resolution levels the stress goal varies on purpose from the requested 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 as the geometry allows it. 
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In one optimization 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 have a look here.
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