Are higher grade bolts more brittle?

In double shear a loose grade 8 will snap uner impact. A grade 5 will flex. It MAY permanently deform, A loose grade 8 bolt in shear WILL brake on impact.

You forget to consider the forces. A grade 5 bends under a certain load, where the grade 8 doesn't even bend with the same load. Wen the grade 5 breaks, the grade 8 just bends. Again, a bolt that is plastically deformed by design is simply an error.

Facts (metric grades): grade 5.6 : 500N/mm^2 and 300N/mm^2 grade 8.8 : 800N/mm^2 and 640N/mm^2

A bit simplified: The grade 5.6 bends at 300N/mm^2 and breaks at 500N/mm^2 The grade 8.8 bends at 640N/mm^2 and breaks at 800N/mm^2 So the grade 8.8 doesn't even bend when the grade 5.6 already failed completely.

Nick

That's the best argument I've ever seen to NOT use 8's in aircraft structures.

Exactly. I know why they are using lead. It bends at once and fails soon after.

Nick

Is it just me, or does that argument make no sense?

Chris

On Tue, 15 Jan 2008 07:48:04 -0800, Gunner penned the following well considered thoughts to the readers of rec.crafts.metalworking:

Hmmmm...... wonder if that will be made illegal, too?

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.

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.

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.

I anticipate argument on this point from Nick, but that's no problem, because he's wrong.

-- Ed Huntress

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.

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.

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. 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.

Best wishes,

Chris

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.

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.

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.

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

Well, grade 8 bolts are NOT used in aircraft, for what it's worth. AN bolts are closer to grade 5. They DO bend. They don't(theoretically, and hopefully) snap.

In a way, yes.

We design to a give load limit. Anything beyond that is excess weight. So we don't ever expect an unbreakable structure.

It will survive up to the yield point - beyond which the structure is damaged - but not broken.

At teh ultimate load point the structure breaks.

And yes, we know the elongation of both the joint material and the bolts (AN bolts are typically grade 5).

But the real deal is simply this...

WHEN the structure is over stressed - i.e.: it has been loaded beyond the yield point - how do you know? And - what gets damaged?

Do you want the bolt to bend visibly?

Or a few hundred rivets (or welds?) to be invisibly damaged?

?

Richard

I kind of see your argument. The grade 8.8 bolt has a stretch of 12% before he breaks (no numbers for the 5.8, should be something around 17%). But lets get back to the "I'll never use a grade 8 bolt": If the joint is *designed* the bolts are calculated to take the load. If that structure is overloaded, the two parts that are held together will lose contact and now we do have to distinguish two cases:

• The bolt was stretched, and the joint will rattle and no longer work (you'll see that later).

• The bolt was elastically stressed and the joint is back in contact after the overload and is still working (albeit maybe in a bit different position). Now we have a closer look at the second case: All joints with bolts relay on the elasticity modulus of the bolt. The modulus is the same for any grade. The bolt keeps two parts together with the preload given by the bolt (by the torque it was tightened with) and they will lift/move/shift as soon as the outer force is bigger than the inner force. Now there are two cases: A joint that is stressed with shearing forces: Design flaw: The *bolt* is designed to take the shearing force. You are fired! That is wrong by design! It always are the two parts and the friction between the two and the preload given by the bolt. There is no difference between different grades of bolts (-> modulus). OK, there are

*rare* exceptions where a shearing pin (which needs to have tolerances in the diameter together with the hole it goes into) and a bolt are merged into one piece. But that is *not* a normal bolt. A joint that is stressed by pull: Lets take two hypothetical bolts. One with 500 N/mm^2 and one with 1000 N/mm^2. I call them grade 5 and grade 10 for now. For a given designed load, the grade 10 bolt can have half the cross-section of the grade 5. Now if we overload the joint, the grade 10 acts *softer* than the grade 5 (half the cross-section, same modulus. acts like a spring, half the spring rate) that looks to be an advantage, because parts can move easier with the grade 10, before we do have a plastic deformation. Now if we come into the pastic region of stress, the grade 5 bolt was elongated *half* the distance of the grade 10. The fact that it has about 30% (17% vs. 12%) more plastic elongation than the grade 10 bolt doesn't help, because it has double the spring rate of the grade 10. So by design -assuming propper design- a grade 10 is more forgiving than a grade 5. And now finally to the case a grade 5 is replaced by a grade 10 without changing the diameter: Well, something will bend/stretch. It makes a difference what will bend. But it is already a failure. We will find enough examples where the slight bending of the structure is better (and keeps the whole structure in a still perfect working condition) contrary to where a rattling joint will have its advantages. Uh? Read above, a joint that is loaded by shearing forces no longer works *at* *all*. A joint that is stressed by pulling forces is just rattling and not keeping things together.

Nick

Actually, in aircraft work it's the exact opposite. Bolts are never (?) loaded in tension. Shear only.

For what it's worth...

So you are suggesting that the bolts aren't tightened? Maybe you have to re-read what I wrote. Friction & clamping forces are the keywords to look for. If it is shear only, you are at the *second* failure of the joint.

Nick

This whole discussion makes no sense to me. Stronger is stronger. Would you rather have a joint fail (by bending) at some stress or have it fail at a LOT higher stress by breaking. It's a no- brainer to me. ...lew...

If you ever design an airplane, I want to make sure I'm never in it. d8-)

-- Ed Huntress

Most joints work as you have described. Most structures are designed for ridigity, not for strength, and the stronger the fasteners, the more rigid is the structure. But ductility/elongation come into play in structures that require high ratios of strength to weight, as is the case with some bridges and many aircraft, as well as some other things.

Where that's the case, designing for ductility can be an important issue for the overall strength of the structure in at least a couple of different ways (not counting the visible near-failure that Richard is talking about, which also is of practical importance). The primary way relates to distribution of loads by avoiding point loads that will cause progressive failure. Some riveted sheet structures and shear panels are examples. It can also be an issue where there are multiple bolts in a heavier plate structure, with holes that have some reasonable production tolerance and where the bolts are going to be loaded in shear as the limits of strength are approached. This is what I had in mind with the trailer-hitch example. As with the sheet structures, the value of some ductility here can be distribution of the load among multiple fasteners, where harder/stronger ones would localize the load until the first hole tore out, and then then second, and so on.

As I said, I was just looking for an argument. Most of the time, what you're talking about is the important design parameter. But not always. And aircraft designers, as well as designers of many other types of highly loaded structures, must design around ductility to avoid excessive point loads. One well-known example is in the welding of tubular space frames. A pure space frame does not load joints in bending but a *real* space frame always does. If the weld is too strong it will not allow plastic deformation of the joint. That will cause a failure where the tube joins the weld, because of the high point loads. The situation with bolts and rivets is similar.

Sometimes, weaker is stronger. That's why we have structural engineers.

-- Ed Huntress

Can't be done that way, Lew.

A solid steel wing would be a WHOLE lot stronger - but won't get off the ground...

We design to load limits. Say (typical light plane) 4 Gs + 50% safety margin.

The structure should take the load up to 4 G's, flex under load, and return to it's original shape (exactly) when the load is removed.

Then we enter the plastic region.

Above the yield limit (4 Gs in this case) the structure does not return to original shape when the load is removed. It has deformed - and is now "damaged" by over stress. But it should not break (catastrophic failure) below the 6 G ultimate limit.

Then comes the scary part...

Above the ultimate limit, you WANT the structure to break. If it doesn't, it simply weighs too much.

Weight verses strength.

Not absolute strength.

Try not to think about it next time you board an airliner...

Richard

Not quite true. The bolt is in tension to hold parts together so the friction takes the shear. Clamping load is still tension.