Hi Greg,

Overloading can be beneficial or detrimental to the structure's useful
life depending on whether the overloading causes compressive (good, in
most case) residual stress or tensile (usually bad) residual stress.

If you have a few loading cycles that cause yielding, it might be a good
idea to use "strain-life" approach instead of stress-life approach. This
topic is found in most books on Fatige Analysis.

Please note that this is a view from a person who had taken a course on
Fatigue and hence may be limited or impractical. Other feusers members who
have hand-on experience may be able to give you a better insight.

Kamaiton Wongkaew

Ph.D. Candidate
Structures Area
School of Civil Engineering
Purdue University
West Lafayette, IN 47907
USA

On Wed, 20 Oct 1999, Greg Cruse wrote:

> Pedro's comments got me to thinking about a fatigue question that comes up in my company
> from time to time.  (My background has been static FEA analysis -- over the past few years
> in my present job I've had to add fatigue analysis to my bag of analysis tools, though I'm
> very much still learning.)
> 
> Normally our analysis is done with an S/N approach to fatigue with a Miner's rule
> summation for the various fatigue loads/cycles that are given.  The stresses are obtained
> from an FEA analysis (linear elastic) and are almost always below yield.  The question
> arises when we have a set of high-cycle, low-stress fatigue loads given (several hundred
> thousand to a few million cycles) combined with 1-2 cycles of a certain load that causes
> stresses to go above yield.
> 
> Does the fact that we have 1 cycle of stresses above yield negate the using of an S/N
> curve that was generated from testing in the elastic region?
> 
> Another place where this has come into play is when a small area of the component exceeds
> yield during factory acceptance hydrostatic testing (according to analysis).  Can we still
> use S/N curves on this part for calculations of fatigue life or should we use a crack
> growth method?
> 
> I know this only partially relates to FEA, but I'd love to get some insight from some of
> you who do/have done fatigue life/crack growth analysis more than I.  Thanks.
> 
> Greg Cruse
> 
> 
> ----- Original Message -----
> From: marcalpv <[log in to unmask]>
> To: Fea-l List Member <[log in to unmask]>
> Sent: Wednesday, October 20, 1999 10:00 AM
> Subject: [Fea-l] Failure Criterion {01}
> 
> 
> Hi Karl H.,
> The various failure criterion you describe and elaborated on by others here,
> describe various points on the stress strain curves of different types of
> materials. As an analyst I prefer to think of failure as systemic. That is
> to say governed by our overall structural conditions ie loadings,boundary
> conditions and etc. Your failure conditions are then conditions for failure
> initiation. (Sometimes these are used as failure criterion by the codes as
> part of acceptable practice.). In order to classify the various systemic
> failures ,  I divide the material behavior into three parts. The elastic
> (opening game), plastic(middle game) and material failure(end game). Then we
> can perform various analysis or tests to assess systemic failure according
> to the type of material behavior that we expect.
> 
> material       Systemic type of failure
> 
> elastic          large deflection, buckling, elastic fracture, resonance,
>                     high cycle fatigue
> plastic          limit loads and other unconstrained flow, low cycle
>                     fatigue, nonlinear versions of the above.
> mat. failure  propagation of material failure, release of stored energy,
>                     creep, corrosion.
>                     (Our analysis models do a poor job of this third phase)
> 
> It is important to note here that there are extensive approximate methods to
> help us assess systemic failure. If this is the case you might well ask "why
> are there failures?"  I am sure the experience of this group can help us
> understand this better. Perhaps a consistent application of all the above
> failure modes will lead us to sigma six land.
> regards,
> Pedro,
> [log in to unmask]
> 
> 
> 
> 



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