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Re: QUERY: Fatigue

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>There are two types of fatigue failure: (a) material failure and (b) Weld 
>failure.
At the risk of seeming like a sharpshooter, I'd like to quibble with this 
reply a little. There aren't two types of fatigue failure--the fatigue 
mechanism is the same in welds as in the base metal. For structural 
metals fatigue failures occure at stress concentration points when 
plastic strain accumulates over a number of load cycles. Typically stress 
concentrations are not calculated for AISC and other structural codes 
rather different limits are imposed for different construction details. 
The stress concentration effects are built into the fatigue allowables; 
fatigue assessment is based on primary stress calculated for a specific 
detail. 

Thare _are_ two stages in fatigue failure--plastic strain accumulation 
leading to crack formation and crack growth where a crack is growing to 
the critical length which causes failure. The principles of plasticity 
govern the strain accumulation stage; fracture mechanics governs the 
crack growth stage. It isn't exactly simple in practice, but the physics 
is pretty straightforward.

> Normally, as a designer, I would like to keep the extreme fibre
> stress (at the tip of the bar stiffener, and well as tensile
> stress in the plate) below the threshold limit, which is
> 0.5 Fy(at least for A-36 material). Then there is no need
> to worry about fatigue failure of the metal 
If 'threshold limit' is intended to mean 'endurance limit' (the textbook 
term in the US) the endurance limit is about 1/2 the ultimate strength, 
not the yield strength. That puts the endurance limit for A-36 at about 
30 ksi. This relationship is illustrated in Timoshenko's strength of 
materials books among others and is somewhat conservative. The endurance 
limit estimate applies to the total stress including the effects of 
stress concentration. The 1/2 Fy limit is commonly used over here on the 
dark side for primary tensile stress in machine elements to allow a 
margin for impact. Stress concentration effects are assessed separately. 

>Weld failure is more critical, and is addressed by the AISC table and 
>illustrative examples.
Weld failure isn't necessarily more critical, although it requires more 
care to avoid.  The AISC cyclic stress limits were developed from tests 
on welded construction made according to good practice. There's no wiggle 
room for welding defects or poor procedures. Poor welding practice can 
really end up spoiling someone's whole afternoon, especially if it 
results in unseen cracking during construction.

Welded details, particularly fillet welds and partial penetration welds, 
_are_ subject to higher stress concentration effects and the inspection 
is a good deal more critical. But a properly designed and fabbed weld 
isn't any more 'critical' than any other load carrying detail. That said, 
my experience is that mechanical failures seem to occur at connections, 
both welded and bolted, so maybe paying attention to the design of welds 
is the critical item.

> Thus, if you size the plate + rib as a T-section with top stress
> limited to 16 ksi and bottom stress to 0.5 Fy, you have taken care of 
fatigue.
I confess I didn't follow this explanation. If it means that the AISC 
Appendix on Fatigue should be followed, then it's correct. There's a lot 
more to it than is contained in the quoted statement, however. You 
haven't 'taken care of fatigue' until you've assessed the number of 
cycles you expect, determined allowables for all the components of the 
joint including shear carried by the weld attaching the flange to the 
web, detailed the weld to reflect both the calculated stress, the welding 
electrode and limits for length and throat, and taken steps to insure 
that the weld quality is maintained.

Christopher Wright P.E.    |"They couldn't hit an elephant from
chrisw(--nospam--at)skypoint.com        | this distance"   (last words of Gen.
___________________________| John Sedgwick, Spotsylvania 1864)
http://www.skypoint.com/~chrisw