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Hi, while you have increased the mass of the impactor the initial kinetic energy is still not enough to plastically deform the crash box. You can load the model with imposed displacement and obtain the energy required to deform the crashbox by plotting the total energy in Hypergraph and observing the corresponding deformation in Hyperview. Then you can use this energy as the initial kinetic energy to calculate the required velocity and mass of the impactor. There are modeling errors: the base plate of crashbox is not attached to the fixed supporting structure so it will slide sideways when loaded. the impactor has a relatively small contact area and it can lose contact during deformation. Provide some friction or use larger impactor plate the mesh quality is bad. Use F10 Check elements>3D to check the solid element quality and remesh accordingly. the impactor and some other components are meshed inappropriately by double walls, effectively halving the stresses. You have to mesh a single mid surface instead. imposed_displacement_0000.rad

Please refer attached MFLUID example in OptiStruct. MFLUID.pdf MFLUID_Example.zip