Residual static strength after fatigue loading

[izax1]

Does anyone have any clue to the residual static strength of materials after it has been exposed to fatigue loadings? I have to verify a structure's static strength after it has been exposed to a certain number of load reversals.

Will the static strength (yield and/or tensile) be influenced, and to what degree??


Thanks for any hints

Regards
Bernt

 

[WKTaylor]

bernt..

It depends... every material is VERY different and aerospace materials/fatigue spectrums are VERY different from automotive fatigue spectrums... etc...

Strongly recommend You get the following book from the SAE Booktore: Multiaxial Fatigue ISBN 0-7680-0453-5
NOTE: there are other books on this subject in the SAE and the ASM bookstores.

http://www.sae.org

http://www.asm-intl.org

Regards, Wil Taylor

 

[Kenneth]

I've heard of residual strength testing perofrmed after fatigue loading as a rapid "screening" or "load bracketing" test in an attempt to determine if the fatigue loads/spectrum applied were sufficient to initiate defects (micro-cracks, fretting, etc.) that had not yet progressed/propagated to the point that would result in catastrophic fatigue failure for the number of cycles run/tested. In other words, had the part "begun to fail" at the loads/cycles tested?

When used in this way, I believe the attraction is a perceived ability to shorten the fatigue test (cycle) time required for a given stress level, while still giving a feel for the initiation of fatigue damage (and the related potential for failure at that stress level for a greater number of cycles). I've heard of it used in this way for materials with "low" crack growth rates (and therefore "long" fatigue lives).

Then again, maybe you just want to quantify any effect on strength at some future projected point in the products life-cycle, or want some "extra" or "free" information/data at the end of a load cycling test.

In any case, I would hazard to guess that any effect on the residual strength would be dependent on the degree to which the loads/cycles applied resulted in crack initiation, fretting, etc. Stresses/cycles that didn't result in significant damage/initiation shouldn't shown measurable reductions in residual strength. Stresses/cycles that resulted in significant damage/initiation could show measurable reductions in residual strength (providing the final “overload” failure is in a area where such damage was located).

In summary: It depends – On the stress levels and number of cycles applied (and, as wktaylor has said, spectrum and material tested).

 

[prex]

In my experience (pressure vessels) there is no relationship between fatigue and direct stress failure.
This happens because fatigue is due to peak stresses that do not enter into the structural calculations for direct (primary) stresses.
Moreover for a ductile material (the only one type used in welded construction) the effect of fatigue is to plasticize at the locations of peak stresses with no effect on resistance to primary stresses.
Of course things may be different with brittle materials or where the quality of welds is not carefully inspected.
I think that for a common steel structure no conclusion may be drawn before a weld inspection campaign.
prex
motori@xcalcsREMOVE.com

http://www.xcalcs.com

Online tools for structural design

 

[diamondjim]

I too would think that the static strength would still
be the same unless your fatigue tests took you near the
elastic or plastic ranges.

 

[WKTaylor]

Guys, in a nutshell...

Materials, manufacturing and anyticipated usage all make a huge difference in life/durability. Aerospace design goals for light weight and durability are much different from other industries... especially those reliant on low-strength steel and high margins of safety.

Aerospace materials are substantially stronger and stiffer proportionally than almost any other types; and as such cracking WILL reduce residual strength capability... severely in some cases. Oh... and manufacturing techniques/ variables make big differences too!

In aerospace, a static test article is "failed" to verify engineering stress calculations and manufacturing issues... early-on in a program.

Fatigue testing is used to verify life-durability calculations that are peculiar to the materials, stress concentrations, fastening, production/fabrication, etc... realities of life. The test article is randomy loaded to simulate "real-life" flying experiences in-service. This fatigue spectrum loading simulates typical/anticipated operation and is applied to a "production" article for at least 2X, 2.5X, 3X or 4X life-times, as required by the FAA, DOD, etc... The "spectrum" rarely approaches "Limit Load" and almost never exceeds 1.25LL [Ultimate Load = 1.5LL min].

Harsh experiences and testing discovered that an airframe could be flying within the anticipated "spectrum" at the end of it's "life", ... but could have fatigue cracks that would cripple or fail the structure if a "sudden over-load" occured.

Most manufacturers accomplish a follow-on limit/ultimate static load tests to failure. Most airframes [NOW] survive the limit-load tests nicely... but fail some-what less predictably [IE: different deformations and failure-sites] than original static tests... hopefully at/beyond ultimate loads. The airframe is then inspected in great detail to determine post-fatigue static-loading failure-modes which are "fed" back to refine/verify the engineering model.

The (3) types of tests tell the manufacturers a LOT about the airframe and structural details and their analysis. These tests often give the manufacturer:

1. Insight into design methods and techniques... for both analytical improvements and manufacturing cost reductions, etc...

2. Starting data for future design and for the inspection requirements of the airframe at various points in it's life.

3. Comparison between fatigue-test data, flight test and operation data insures that anticipated goals are met. If NOT similar... then analytical revisions to the structural models are made to "refine the durability predictions"... and to revise predictions for INDIVIDUAL airframes that may NOT be flying the anticipated spectrum.

4. Insight into residual structural capability allows the manufacturer to exploite this extra "capability" to design enlarged airframes variants with higher gross weights... with minimum redesign effort and reduced costs.

5. Insight into manufacturing materials and process issues.


Regards, Wil Taylor

 

[boo1]

Much of the work we perform involves designs subject to cyclic loading. Analytical fatigue prediction is analogous to navigation by compass - it is a good indicator of the direction to head but is less than ideal for the distance to travel. Fatigue testing complements the analytical techniques for accuracy in the characterization of the design behavior. Without testing, conservatism by overdesign or failure tolerance is required.

The fatigue loading reduces the allowable Alternating Stress associated (Sa) with a mean stress. Use the Solerberg's or Modified Goodman Law. As long as the loading does not exceed the Sa you are OK.