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I agree the energy absorption is going to have negligible effect in the context of saving the chassis. Just regular aluminum should do what you want.

OTH if you want to make them weak enough that they cant transfer too much force to the bumpers, which could be useful for saving bumpers in parking lot incidences, you could do alternating milling pattern on either side of the tube. It would sorta turn the tube in an S shape when viewed from the top. This will ensure the tube bucles right away and will use the whole length for deformation.

I are an engineer, but this isn't really my area, so grain of salt and all that