The observed maximum removal force was limited by the strength of the copper foam, not by friction, so it is reasonable that the observed removal force was less than the theoretical maximum friction force. 5 Discussion and conclusions Foamed material is considerably less stiff and strong than the same material in solid form. Therefore, the strength of a fastener based on negative Poisson's ratio cellular solids will be less than that of a corresponding solid screw of the same diameter. The difference in strength could be compensated for by designing a fastener of larger diameter than would otherwise be used. The advantage of a negative Poisson's ratio fastener is the simplicity of the press-fit insertion.
Fasteners based on stiff polymer foam are possible as well.
We conclude that press-fit type fasteners may be designed using negative Poisson's ratio materials, and that such fasteners perform in a manner consistent with a simple analysis. 6 References 1. Poisson's ratio vs longitudinal strain for re-entrant foam. 2 Load vs displacement for experimental insertion and removal of fastener core.