Ed Huntress wrote:

snip

I think it shows up in a lot of places in high-performance structures. I
recall seeing it in the design of seat-belt anchors in race cars; fastener
ductility also factors into the safety margins in bridge and building
design. Note that a lack of ductility in a bolt can increase stress
concentrations and thus can precipitate a failure in the material being
bolted, even when the loads don't even approach the strength of the bolt.

In the case of a seat belt anchor, the amount of energy it can absorb
before finally breaking is going to be important. It depends on what you
consider to be strength. I would consider strength to be a joint's
ability to resist the forces applied to it. And so a stronger joint
would be one which fails under a greater force. But you might also
consider strength to be a joint's ability to absorb energy before
breaking. Both are important, though sometimes one is more important
than the other.

This is one key reason why the elongation properties of materials often
are critical to the safety of a design. Any joint that is likely to be
loaded to a high percentage of its ultimate strength has to be engineered
as a whole. Stronger bolts may, in some circumstances, result in a weaker
joint.

Do you mean a weaker structure as a whole? If you're talking about
strength in terms of forces, then according to Nick's figures a joint made
with grade 8.8 bolts would either have the same strength (if the other
parts of the structure were the limiting factor), or a greater strength
(if the bolts were the limiting factor), than a joint made with grade 5.6
bolts.


That's incorrect, because it's unknown. All you can say for sure there is
that the BOLT will be stronger, not that the joint will be stronger. The
joint may, as we've been discussing, turn out to be weaker with the stronger
bolt because it may increase stress concentrations.

I disagree. Take a simple example: a single-shear joint made between two
mild steel flat bars. There's a hole in each bar, and a bolt connecting
the two holes.

Either the bars are weaker (the bolt will tear through one of the bars
when the joint fails), or the bolt is weaker (the bolt will fail in
shear). If a grade 5.6 bolt is weaker than the bars, then substituting a
grade 8.8 bolt can only make the joint stronger. If a grade 5.6 bolt is
stronger than the bars, subsituting a grade 8.8 bolt will make no
difference.

Now if the joint is part of a large and complex structure, it's possible
than using a weaker but more ductile bolt might, in some circumstances,
impose a safer distribution of forces within an overloaded structure,
making the whole structure stronger.

But even though the whole structure might be stronger, the individual
joint is still weaker. I can't see how the individual joint can possibly
be made stronger by substituting a weaker but more ductile bolt. If you
disagree, please explain why. Perhaps we agree and it's just a
misunderstanding?

But things might be different if you're talking about strength in terms of
the energy a joint can absorb before it fails, because we don't know the
elongation at which the two types of bolt break.


It's not only the bolts themselves. It's the entire design of the joint that
determines joint strength. Stronger bolts can, and sometimes do, result in a
weaker joint.

A weaker structure, possibly, but the individual joints are still stronger.

Best wishes,

Chris