Bolt strength comparison

If you wanted to compare the strength of two bolts of differing composition.
16mm of uts 860Mpa with 19mm of uts 585Mpa, would multiplying
the xsectonal area of each bolt by its uts be a valid comparison?

16mm...............201*860=1.73*10^5 N
19mm...............284*585=1.66*10^5 N
So 16mm bolt is stronger? (marginally)

 

If you're looking at the basic maximum tensile load capacity, yes.

Since Force (max) = stress (yield not UTS) x XSA.

So if you use say the16mm dia as your basis, you can use this as a ratio.

(Yield stress x XSA)basis/(Yield stress x XSA) new.

If this exceeds 1.0 the basis is best, if less than 1.0, the new is the better.

You must use yeild stress NOT UTS.

 

Maybe,

It could be if you are comparing identical installations, loading conditions, and with the bolt material as the limiting strength factor. The two materials would have to be compared for the properties in the direction of loading. For example, if the bolt is loaded in shear or in tension, you would have to compare the sheer or tension properties of the two materials (they are not the same for different materials). Or if for some reason the bolt was loaded in bending, you would have to compare the bending properties of each material (which is not common, but loading geometry is also considered if bending is the primary loading condition). Typically to size bolts you have to look at shear, tensile loading, and bending condition (if any) and size the bolts to the most severe condition.

The base material in which the bolts are installed may also have an influence in fastener selection. If a bolt is installed in wood, composite, or something soft like aluminum or bronze, the base material may be the limiting strength, and bolt material may be irrelevant. If you are installing a lag type of bolt into wood or composite, and it is in tension, than the withdrawal strength of the base material may be the limiting factor, not the bolt material strength.

You can also have a situation where the base material configuration (like the thickness of the flange, edge margin, etc.) with a high strength bolt is what determines the number of fasteners, but with a lower strength bolt it is the bolt material that determines the number and size of fastener that is necessary.

And than you can also have a condition where you have to use many smaller fasteners that just to spread the loads out in a larger area to prevent the supporting structure from buckling or failing.

And than there are also incompatible materials, or corrosive environments that sometimes has to be considered. For example, if you do not use galvanized bolts in treated lumber, the treatment will corrode right through smaller fasteners pretty fast. You also would never want to use cadmium plated fasteners in titanium parts, especially in areas where it get hot like around an engine or exhaust.

Also, some materials have different fatigue properties, and can be important if it is in a cyclic loading installation. This would be like an engine mount, or a pressure vessel that is pressurized and than unpressurized regularly.

So if you can give more details about what your intended use, installation details, load conditions, etc. that I can tell you if that is a valid assumption about bolt strength.

 

Ad Hoc,

For most bolting, you are correct that yield strength is the generally the better property.

Soft stainless bolting can be an exception. A193 B8 bolting has a low yield (30 ksi) but the material work hardens very quickly. The tensile capability of this bolting is much higher than yield and this is sometimes usefull.

 

Thanks!
The reason I used breaking strength was because it came from David Gerr's
"Elements of boat strength". Table three P346.
His methodology is what I have based my scantlings on.
The calculation of bolt diam. was based on load per bolt & referred to the table
of bolt breaking strength which I assumed to be UTS?
Will 585Mpa for stainless 316L be uts?

 

It depends, there are different grades of bolt

A4 50 500 UTS 210Yeild
A4 70 700 UTS 450Yeild
A4 80 800 UTS 600Yeild

A4 is 316 and you can also get A2 which is 304 with the same strength markings

 

Roly,

Are you looking for shear strength or tensile strength?

-Tom

 

They are keel bolts. LOA 34' Displ. 5000kg 14 bolts 1500kg balast@1.2m from hull.
David Gerr does not give figures for 2205 and I need the extra strength of this material to comply with the table.(Along with its slightly more corrosion resistance.) Using ms I would have to go up to approx. 22mm which is not possible as the b/ holes get too close to the edge..
The keel is bolted through the keelson to the floors via these 250mm bolts .
.
The keel is mounted to the hull via a steel plate 3m x 140mmx 10mm with a steel structure below supporting the lead.The steel plate will have a SS nut tacked to the underside. All bolts will be fully encapsulated including a removeable cap on the floors so a test bolt can be removed periodically. All bolts through an epoxy annulli. Keel/hull junction will have a high elongation epoxy/fabric laminate over,to seal joint & hopefully not load hull skin.

 

Predominately tension, I guess.

 

Never us UTS, always use yield strength.

P Flados:
If you have selected a work hardened alloy then again, you use the yield for that work hardened condition, it is still "yield stress". Whatever the material, you use the yield strength, for whatever temper of THAT alloy you have selected.

 

Gidday Roly

Don't forget that for bolts in tension you need to consider the root diameter of the thread instead of the shank diameter for your CSA.

Your original post is as good as any to do an initial comparison, but always do the complete calculation for the final iteration.

 

Another odd question.......
I guess fine thread is more likely to cross-thread than coarse?
Just thinking ahead to extracting bolts & re-inserting them.
(maintenance). The joys of SS & galling.
Guess lanocoat would be the go.

 

There are very large variances in the strengths of metals depending on the prior processing so I would not use and values that are published in a boat building book, use the manufacturers/suppliers numbers. From a Sandvik document SAF 2205 (quench annealed) has a 0.2% proof strength (not yield but treated in the same manner) of 400MPa and a tensile strength of 680-880MPa so it is not as strong as and A4 80 bolt which is 316.....

 

Cheers!
I was actually using the manufacturers specs, but using Davids loading formula. I hope his formula has an extra margin for the difference between UTS & YS!
I don't know why he used UTS in his tables but he used it with bronze as well. His safety factor of 8x may have something to do with it.
The batch of SAF2205 that I could purchase was batch tested at 875mPa.
The alternative, as you suggested would be to use A4 800, but means I am
down 75mPa. The 800 I suspect, is nominal & could be +/- 100mPa.
Whether to use studs threaded both ends or bolts, will limit my options
bolting up too. SAF2205 also has cost & corrosion benefits. Its here & I can custom machine. Anzor would have to indent A4 800 or Bumax stud$.

DG's loading formula:
Load/bolt = 8(S.F.) X ballast depth X ballast weight / 2 X Bolt- moment to edge of mount plate X # of bolts

 

If they have tested it they should have the 0.2% proof strength as well. Without working out that formula I will assume it gives a force in Newtons per bolt with a safety factor of 8, if this is the case I would use the force from this equation, the bolt cross sectional area at the thread root and the 0.2% proof load to calculate the bolt size.

That said Im not sure I trust that equation especially when you consider the safety factor has to account for all of your fatigue loads as well