Bolt strength comparison

Understood.

 

ANZOR Fasteners stock bolts and nuts with a material called bumax 109. This appears to have properties similar to 2205. They are used for highly corrosive industrial applications (and marine applications as well if the advertising is anything to go by)

Not an NA here, so do your homework first.

 

Thanks, been there. Post #14
Must be 60 knts outside & lights are flickering! At sea you wouldn't want to be wondering 'bout
your keel bolts. Bad enough wondering whether the kauri with Phytophthora is gonna land on our roof.

 

Ah, you gotta love weather 'eh!!

-Tom

 

Cheers JRD,
They DO have stock!

 

Is there a fatigue problem associated with hi-tensile steel or higher tensile stainless that anyone knows of?

 

For what purpose you use the bolts? what part are you bolting together?

 

Keel bolts securing 3mx140mmx10mm plate at top of keel thru keelson, fastened to top of
floors. Epoxy encapsulated. 160mm x60mm plates for each two keelbolts.

 

UTS-Proof-"Endurance limit"

I was about to order Sandvik SAF2205 16mm stock & nuts when I spoke to a Special Steels Ltd engineer on fatigue resistance.

A term came up that I am not familar with, "Endurance Limit".
Take for example 4340 (10.9) hi tensile steel uts 1000mPa, Yield 900mPa
EL= 333mPa, approx 1/3 UTS, for infinite number of cycles. Over 333mPa fatigue starts, cycle life decreases.
This is less than 1/3 yield strength!
The endurance limit of ATI 2205 is given as 45% of UTS.This isn't far from its
Yield stregth? (Discounting stress corrosion etc) So stainless is less prone to fatigue than ht steel?
And yet it works hardens so readily! Are these SS figures arrived at with different criteria?

How accepted is this "endurance limit"? Shouldn't endurance limit be the factor
to consider instead of yield?
http://www.alleghenytechnologies.com/defense/docs/ATI2205.pdf

 

And then you'll not be familiar with stress range/ratio either.

You need to calculate the min and max stress that the bolt shall experience as that influences it fatigue life, not just the endurance limit.

Thus the amount of preload you put on has an effect on the fatigue life.

You should really consult with a NA or structural engineer for something as critical as these keel bolts. Show them your layout the boat what the SOR is etc..and they can work out a suitable loading scenario and go from there.

You wont do it just by cherry picking definitions.

If you replace exactly what you had before with no changes, one can assume it shall be as satisfactory as before, assuming it was. Don't meddle if you're unsure. Unless you're willing to pay for a proper review of the whole "unit" for an answer.

 

The endurance limit 'method' so to speak is based on empirical data and then there are other modifying (fudge) factors that used as well such as loading conditions, stress concentration factors, reliability requirements which all reduce the endurance limit. A quick example that I know off the top of my head would be a 40mm tube welded (in a T configuration) to a 60mm tube made from Q235 which is has a yield or 235MPa, the endurance limit near the weld is around 16MPa.

Further to your question stainless does not necessarily have a better fatigue life than any other particular steel. Also something to consider is hydrogen embrittlement of the HT bolts when they are plated or hot dipped, even after baking the only way I would trust them is to destructively test one from the same batch.

Another point on fatigue, I doubt that you will be able to torque a large HT bolt enough to reduce the stress ratio without crushing the floor of the boat

 

I seem to remember fatigue as a concern only when stresses were cyclical (tensile then compressive then tensile, etc.). For typical fastener applications, there may be changes in the amount of tensile loading, but stresses never go over to the compressive side. As such, fatigue is usually not an issue.

If it were not for this, the stress riser at the root of every thread would make fatigue one of the primary failure mechanisms for bolting on machinery that has any real vibration.

The bolted joint holding the keel of a boat in place is probably as low risk as you can get for fatigue concerns.

 

That is not correct.

Fatigue cracks propagate during the tensile phase of loading. They do not propagate during compressive phase.

Without tensile loading there is no stress!

To clarify:

If the stress range is -50MPa to 100MPa, the cracks propagate in the 0 - 100MPa region.

If the stress range is 20MPa to 170MPa, it propagates in the whole range.

 

P Flados,

Do you mean cycling?

-Tom

 

"If you replace exactly what you had before with no changes, one can assume it shall be as satisfactory as before, assuming it was."



I am replacing what I had with 50% more effective bolt XS area. (1/2" to 16mm, unc to mc) 1297mm2 to 2016mm2. Everything has been upsized,& load spreading improved.
The numbers exceed both Tom McNaughtons calculations for my boat (consulted) and my own, using Gerr's method.
The old bolt material was a mixture of galv ms & hi-tensile.
The new bolts will be either 2205 or HT4340.
I have yet to decide.

Max bolt torque will be 148Nm unlubed for 2205 & perhaps twice that for 4340.(Floors permitting) (T = (Sig(y)) [the yield or proof stress] x 0.7 x Area x diameter/ 5000) Courtesy AdHoc