Stress when part is deformed to a predetermined shape

pberry

I want to have a certain variable prestress profile along a composite part when it is in the final (ready to take the working load) shape. The unloaded working shape will be maintained by stays to counter the prestress.
I can generate according to some basic theory the required starting (unstressed) shape of the part. Once that shape is imported into Mecway, is there a way to force it into the final prestressed shape so I can see if the prestress models correctly?

Thank you,
Paul

prop_design

hi paul,

I believe you are talking about hot to cold shape differences. Yes, you can use mecway to do this. If you modelled the unstressed shape, just apply your loads and it should move to the desired hot shape. You can use transfer displacements to mesh feature under mesh tools. A value of 1 will move the mesh the same amount from the analysis. You can work from the hot shape backwards to the cold shape as well. But it depends on your loads to how you go about it. I have a video for doing this with blade loads. But that may not apply to your situation. But the bottom line is you can do it and it's pretty easy with mecway. Use a value of -1 if going from hot shape to cold shape. You may have to apply the loads in a sequence and use multi runs. You can transfer the deformed model out of fea and into cad using the save as binary stl option.

I've found when going from hot to cold shape you can get almost all the error out. but it would be useful if you could only apply certain directions. right now you can only transfer the total displacement to the mesh, rather than x, y, or z. also being in cylindrical coordinates can be useful in certain situations. but you can't do that right now. however, as it stands, you can get most of the error out of the model.

pberry

Thank you for the reply,

I was hoping I could predefine the the hot shape, but I suppose that's an unusual approach. To apply the prestressing load to produce the final shape would require that I also model the cable stays. In the past I have had no luck modeling tension-only elements, but I suppose I could give it another shot. I don't recall what the snag was.
Possibly another approach would be to prestress it into a fully supportive cavity of the desired hot shape. More pondering to do...

Regards,
Paul

prop_design

you can define the hot shape. that's what I have to do as well. you still use the transfer displacement to mesh, but with a value of -1. then export the mesh to cad via stl binary. the binary makes smaller files than the ascii. however, how you get back to the cold shape is load dependent. so you will probably have to do trial and error. in my case it's a multistep process.

Victor

Not sure if I'm properly understanding this, but it seems like you can already calculate the cold shape and just need modify the hot shape to that without any stress, is that right?

For that, you can use non-zero displacement constraints at various locations (where the stays are?) sufficient to hold it in the desired shape, then solve and use Transfer displacements from solution as prop_design said. But it should work quite simply because you're just copying the deformed shape from the solution, not trying to actually find the cold shape.

Sergio

Have worked with parts that suffer a lot of deformation/stress due to thermal shrinkage, what we did for that parts was model the geometry in "hot state", and then add a thermal step to the simulation to decrease the temperature and shrink the part to the final shape/dimensions/pretensions. Having the part modeled in hot state is usefull also to send directly to the tool/mold manufacturer and have a fine control of the shrinkage.