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

snip

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.


I think you just disagreed, and then agreed. d8-) Yes, it applies mostly
to structures with multiple joints, or with multiple fasteners in one
joint.

I did. But I've thought this through very carefully, and I think I'm
prepared to mostly concede the argument. Suppose a joint has many bolts
arranged in a line parallel to the force on the joint, and that all the
bolt holes are equally spaced when the material is in the unstressed
condition. The joint is then overloaded to the point where the limiting
friction is exceeded. The first few bolts will carry most of the load,
because the material between the bolt holes stretches, causing the other
bolts to go slack. It's the same as the way in which the first five turns
of a long screw thread carry most of the load. So if relatively brittle
bolts are used, it's possible that the first bolts might break before they
transfer enough load to the bolts further down the line, then the bolts
further down the line will break, and so on until the joint fails
completely. The same would be true for rivets.

But I suspect this is only important for joints with many bolts or rivets.
Perhaps more than five in a line parallel to the force, at a guess. Of
course, if the load on the joint was constant and known (not an overload)
it might be possible to vary the spacing of the holes to ensure that all
the bolts carried an equal load. Now there's an interesting idea.

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?


It would have to be a very contrived case to make the point with a single
fastener, but the principle still applies. In a car or aircraft crash,
for example, you could have an extreme overload applied to a joint, bot
only through a distance of a fraction of an inch. If the bolt doesn't
give, something will break.

Using a bolt in single-shear, perhaps. But a pin in double-shear or
multiple-shear is pretty common. Think of a tractor's three-point linkage,
a towing hitch or an eyebar suspension bridge.

I wouldn't get into energy because it complicates things, although it's
an issue with impact. We're just talking about distribution of forces,
or, in the case of a single fastener, an excessive force applied through
a very short distance.

The selt belt anchor is certainly a case in which energy is important. I
just raised it because I wasn't absolutely certain that you weren't
talking about energy at first.

A joint can have more than one fastener, as they often do in bridges and
vehicle structures.

I don't believe this argument applies to a joint consisting of a single
bolt, considered in isolation.

Thanks for an interesting discussion. This is what I come to RCM for!

Best wishes,

Chris

Since you've put some thought into this, you may find that reading a more
sophisticated engineering treatment of it would be worth your while. It's
too far in the past for me to remember much of it but there is plenty of
material on ductility and brittle failure in the field of aerospace
engineering. It's a very big issue there.

You could try the SAE website, which has a bookstore and white papers in the
aerospace division.

--
Ed Huntress