May I request some clarification please? Here's my understanding, and I would appreciate the education if I'm misinformed.
The clamping force applied by a bolt is fundamentally a function of its elastic modulus, and all steels have roughly the same modulus of elasticity, about 3.0^7 psi.
The mechanical difference between the 12.9 and 8.8 is yield strength (roughly 90% of 1200 MPa vs 80% of 800 MPa). That means the 12.9 can be stretched further (i.e. torqued to some higher value) before it yields, which of course increases the clamp force. If an 8.8 and a 12.9 have the same modulus, then stretching them equally (same torque, assuming same thread pitch, form, and friction) should result in the same clamp force.
There's no reason why a 12.9 mustbe torqued to a higher level. The required clamp force is usually dictated by the anticipated cyclical stress; we want the clamp force to be higher than the cyclical stress which tries to pull the clamped joint apart. If an 8.8 was adequate in this regard, a 12.9 should be fine.
Lower ductility for a 12.9 has no bearing on this discussion. It merely indicates a short plastic region on the stress-strain plot. The stress applied here, using the torque specified for the 8.8, would not put a 12.9 anywhere near that plastic region.
So what am I missing?
POSTSCRIPT....just posted to find Dave has already covered the question.
Last edited by DanH : 11-17-2015 at 09:24 AM.