About the meaning of volume fraction in Altair OptiStruct Posted May 6, 2019 · Report reply On 4/28/2019 at 10:10 PM, Hyperman said: Hi, Volumefrac response is the material fraction of the designable material volume (material available for optimization). The volume fraction response can be a fraction of: -the total volume, which includes design and non-design volume -by entity, specifying either props (properties) or mats (materials) Thanks for answering! I know the 30% is the material available for optimization. But I wanna know how the remained 70% of the volume change? what state doest the 70% stay? Low-density or some other?