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Attention: The Automatic Non-Design Growth needs to be activated to use Retained Volumes.
These two options create different transitions between the Non-Design Spaces and the overall Design structure.
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The disabled Non-Design growth (default) creates an undercut and therefore a smooth transition between the Non-Design Space and design structure. Nevertheless, in this area stress risings can appear. The Post Processing with a machining process like milling is much easier because no unintentional cuts into the design structure can happen.
The enabled Non-Design growth creates a hard transition with a notch to the design structure. This transition leads to stress risings at the notch which influences the fatigue life of the part. Furthermore, the Post Processing after the manufacturing of the part with e.g. milling is quite difficult. The position of the notch should not be moved any further in the Design direction.
It is not quite clear which transition has a better impact on the fatigue life of a part and has to be evaluated by the engineer for each case separately.