"Christopher Tidy" wrote in message
...
Ed Huntress wrote:

snip

In need of something to argue about, Huntress suggests, look at what Nick
has said. "Again, a bolt that is plastically deformed by design is simply
an error." The point is that the bolts Richard is referring to are not
intended to deform in normal use, but are designed to deform when design
limits are exceeded. Depending on the design of the joint, bending may
prevent other modes of failure, and a weaker bolt that will bend often
has sufficient strength to prevent failure of the joint even when its
plastic limit *in bending* has been exceeded. The *ultimate tensile
strength* of the bolt will be quit a bit greater than its yield strength
in bending.

I agree. Some designs need to consider what might happen under abnormal
circumstances. Are you talking about bolts loaded in bending, or being
elongated? Loading bolts in bending is often a bad idea because of the
high stresses it creates. But I guess you can see a double-shear joint as
bending on a small scale, if it isn't a joint in which the shear force is
carried by friction, or if the limiting friction is exceeded.

That's the idea. You'll see joints of flattened tube, or shear plates, in
old race car designs, and similar things in some home-built aircraft. It can
be single-shear as well as double-shear. And it can be rivets or bolts.

As for loading in bending versus elongation, keep in mind that bending is
the result of tension on the outside of the bend, and compression on the
inside (and shear in between). Steel and most structural metals have similar
values for yield in tension and compression, so bending results in
elongation of the outside.


Complex structures, particularly those that have some bend and/or flex
intrinsic to their design, may not lend themselves to theoretically ideal
joint designs. A light aircraft frame may also incorporate a material
compromise, say in tube materials and their joints, that will yield and
break if a bolt doesn't yield first. The specific load imposed by a hard
and strong bolt may exceed the strength of the material being bolted
together by so much that the material being bolted fails, whereas it
wouldn't fail if the bolt deformed and thus redistributed the load on the
joint itself.

I can see what you're saying here, Ed. I'm not sure how many structures it
would apply to, though. It would need to be a (probably statically
indeterminate) structure in which the elongation of some bolts imposes a
safer distribution of stresses within the structure.

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.


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.

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.

The whole subject is treated in structural engineering texts, but I haven't
read one for years, so I can't give any references. Richard has experience
with airframe design so he can probably point to references better than I
can.

Keep in mind also that for complex structures, especially things like
airframes and other tetrahedral or geodesic structures, ductility of
individual joints is important for preventing failure of the overall
structure, because it allows a local overload to be distributed to other
joints in the structure without breaking the individual joint. A ductile,
but weaker joining element will "give," to put it in ordinary terms, without
breaking; before the ultimate strength of that individual joint is reached,
the load in a geodesic or tetrahedral structure will then be distributed to
other joints in the structure. Thus, weaker but more ductile joints can
result in greater overall strength and integrity of the structure.

--
Ed Huntress