7
$\begingroup$

The specifications for standard (ASTM A 325) bolts do not list minimum service temperatures. I am thinking of outdoor applications like Alaska or Canada (lows of -60F) and wondering if the cold temperatures would have an effect on the bolts.

There is a type of bolt (ASTM A 320) that has a grade that specifies testing at low temperatures (-150F). Those bolts seem to be for cryogenic facilities. It seems that this is over-kill for most outdoor locations where the coldest ever recorded temperature on earth is about -128F in Antarctica.

Is the cold a concern for standard structural bolts? Are there European standards that might apply?

$\endgroup$
  • $\begingroup$ The RCSC Guide (boltcouncil.org/files/2ndEditionGuide.pdf) touches on this in section 2.7. They don't have an specific guidance though, just "fracture mechanics are complicated." It does reference a couple of other sources for more information. I'm not aware of any more specific guidance in the RCSC or AISC specs. $\endgroup$ – Ethan48 Feb 7 '15 at 21:23
  • 1
    $\begingroup$ @Ethan48: It is exactly sections in codes like that scare me. Saying that something could be an issue without giving any guidance does not help. I want to get projects done, not use my client's money to fund research. $\endgroup$ – hazzey Feb 7 '15 at 22:21
  • $\begingroup$ Agreed! Not very informative, and not at all prescriptive, just ominous. Sorry I don't have any more knowledge on the subject. $\endgroup$ – Ethan48 Feb 7 '15 at 22:24
  • $\begingroup$ How does the pricing of the 'cryogenic' bolt compare to 'standard' (and to the whole project value)? It's overkill only if it significantly impacts the budget, otherwise it's just 'generous safety margin' ;) (like in our case, we order 'industrial quality' boards where 'consumer' would suffice, doubling the board cost but increasing the final product cost by 0.3% and getting it far above required specs instead of 'just only', and having the product just perform in adverse conditions instead of explaining to the customer that they aren't supposed to use it in these conditions.) $\endgroup$ – SF. Feb 8 '15 at 9:45
4
$\begingroup$

To answer your question bluntly, yes, the cold can be a concern for standard structural bolts, because the cold can be a concern for just about any metal or plastic. I can give some insight into why the cold is a factor, but I want to be clear that I can't make a recommendation on acceptable temperature ranges for the standard bolts, so if you can't find some data to ensure the proper operation of them, it may be best to use the low-temperature bolts to be safe.

In metals, this phenomenon is known as the ductile-brittle transition, which occurs at a specific temperature based on the material and the strain rate. Note that this means it's dependent on how quickly you deform the material, not how much force you apply.

There are two types of deformation, elastic (where the material returns to its original shape) and plastic (where the material permanently deforms.) On a molecular level, elastic deformation happens when the bonds between molecules in the material are stretched. Because no permanent change in the structure occurs, the material can retain its original shape. At a certain point though, the structure begins to change, often in the form of dislocation movement. Existing defects in the material begin to move, and this movement cannot be spontaneously reversed, causing the permanent change.

The ease and rate of dislocation movement is in part based on temperature. Temperature is a measure of energy, and if there's more energy in a material, some of that energy goes towards dislocation movement. This is important because dislocation motion can help prevent fracture. Cracks exist in all parts, they are impossible to avoid, and parts will fracture at those cracks because stress is concentrated there and the material is naturally weaker.

At these cracks, part of the energy goes into forming new surfaces (propagating the crack further into the part.) Most of the rest of the energy goes into moving the dislocations by plastically deforming the material. If the material is too resistant to deformation (too brittle), no energy will be dissipated by deformation, leaving more energy available for crack propagation. This is what causes metals to be more likely to fracture in lower temperatures. At a certain critical temperature, the material becomes more brittle than ductile, and its fracture toughness is significantly reduced.

Determining this temperature is not simple from a theoretical point, and it's usually done experimentally I believe.

This paper is what I used to refresh my memory from my molecular material behavior class, it's good some good info, but I wouldn't worry about getting bogged down in the math because it's incredibly complex for non-simplified cases. The explanations aren't as bad though.

As I said at the top, the simple answer is yes, it will affect the bolt strength, and I think the standards are reluctant to give specific answers if the bolt isn't designed for a temperature range because bolt size, defect size, and type of loading can all affect this.

| improve this answer | |
$\endgroup$
  • $\begingroup$ You have done a good job of adding background for why I am concerned. This is the reason that structural steels undergo Charpy testing. $\endgroup$ – hazzey Feb 8 '15 at 21:21

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.