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Re: QUERY: Fatigue

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Sharpshooter! (in jest!)

My response was specific to the stated situation and hence not complete.
Further elaborations follow:

A)It is true that the fatigue mechanism is the same in welds as in base metal
at a microscopic level. However, there are two differences: 1. The weld
material and the adjacent heat affected zone (HAZ) generally have a reduced
ductility compared to base metal and 2. The weld (even the best quality)
contains microcracks not detectable in non destructive examinations like
ultrasonic, radiographic, MPI or other tests. The ends of these microcracks
experience high stress concentrations. This is not the case in base metal.
Hence the distinction between material failure and weld failure.

B) AISC implicitly includes stress concentration factors in categorising the
type of weld and type of load application. The number of load cycles is also
approximately (and conservatively) taken into consideration in the loading
condition numbers 1-4. The situation is more complex in cases of wind or wave
induced fatigue, wherein the element experiences different numbers of cycles
of different stress ranges over its life time. Codes for offshore structures
deal with such situations. In many cases, explicit stress concentration
factors are computed and used, with S-N curves defining allowable number of
cycles for each stress range, and a cumulative damage computation procedure is
followed.

C. Threshold limit is the endurance limit. If the stress range is less than
the endurance limit, there is no possibility of fatigue failure. In this
specific instance, there is no stress concentration at these points.

D. I said Weld failure is more critical because of the microcracks. Offshore
structures have every welded connection examined non destructively for
identifiable faults, and rectified. This is generally not so in onshore
structures.

E. "You have taken care of fatigue" was intended to mean that the fatigue
requirements have been met. The stress limitation of 16 ksi for the type of
weld as per AISC for the number of cycles, and 0.5 Fy (endurance limit) in the
base metal at other points. The other aspects mentioned should already have
been considered in the design and specification, irrespective of fatigue
requirements.

Thanks for the comments. I am sure this discussion would have helped clarify a
number of issues not common in onshore structures. As might have been evident,
I have been designing offshore structures for long, and fatigue of welded
joints is one of our primary design considerations. There is a lot more to
Fatigue than what has been discussed, but that may not be of immediate
interest to the forum.

M. Hariharan
Engineers India Limited,
New Delhi, India

Christopher Wright wrote:

> >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
>