Yielded structures

[cyt4]

Hi all

I started working on some post yielded structures problem(overloaded during test) and this is new to me. Are there any ways to detect yielding on a surface besides the "visual inspection"? Are there any non destructive testing methods that have been proven to work? I think hardness is one of the tests from what I have read.What is the industry standard to detect yielding in a non stress concentration,radius, fillet,screw or hole threads? What is typical for this atypical problem? How can we ensure that we have not gone past the Ftu or close to that point with Ndt?

 

[SWComposites]

Well, there should have been strain gages in the areas of predicted high stress / yielding.

What do you plan to do with the test article now?

 

[rb1957]

is this a static test ? If so, why do you care if you have inconsequential local yielding ?

Fatigue loading is a very different question, if your peak load is very infrequent, then it'll set up a beneficial yield zone (research crack retardation).

A paint called "stresscote" might help, I remember undercarriage people using it in tests.

"Wir hoffen, dass dieses Mal alles gut gehen wird!"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[LiftDivergence]

The most common way, in association with instrumentation, would be dimensional tolerance checks. You know what the design parts are supposed to measure / look like. If you have permanent deformation, things would have shrunk or stretched in a way that is measurable. Preload/alignment at joints, fillet radii, flushness between parts, surface waviness, flange perpendicularity, etc. Where you check should be informed by analysis.

is this a static test ? If so, why do you care if you have inconsequential local yielding ?

I think that would depend on the load at which the yielding occurred... if it was only the result of the overload, or if it started at a lower load, and whether or not the nominal "static" envelope is set up to only allow yielding during a true limit load case (i.e. a load case that will only occur ONCE in the life of the vehicle).

if your peak load is very infrequent, then it'll set up a beneficial yield zone (research crack retardation).

That is only true of potential overload scenarios. Compressive yielding can have the opposite effect, accelerating fatigue crack growth.


Keep em' Flying
//Fight Corrosion!

 

[cyt4]

I tend to agree that dimensional checks are ideal and I had hoped for some other aha answer. Some of the features ,however, are in relatively inaccessible areas due to assembly and they happen to be areas of interest.

This overload is unusual due to the failure of fixture in the test. Some parts may have gone past Ftu stress(analytically) and those areas were not instrumented as they were inaccessible and were never supposed to be critical. I do not see post yielding as a benefit in this case as it also would invalidate requirements and may affect bolted joints.

Stresscote is an interesting option...

 

[SWComposites]

Stresscote is for use during the test, not after.

Sounds like you will need to disassemble things; something I’m sure program management does not want to hear.

 

[rb1957]

I doubt dimensional checks post test will show anything. I suspect that the yielded material is compressed within the bulk material.

But CYT4 ... you're not telling us much to go by. I guess you'll say it proprietary, which is ok but doesn't allow us to help. Why do you think some of your test piece yielded ? 'cause FEA told you ? If so run a loading in the time domain ... load and unload the test piece. Or run the test with Stresscote (I thought this is what you were doing, not trying to interrogate a test piece post test). Or maybe photoelasticity test ?

"Wir hoffen, dass dieses Mal alles gut gehen wird!"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[mfgenggear]

OP
NDT heavy stress areas or joints.
Verify if there are areas that have plastic deformation. And verify for indications .
Which would indicate damage and failure.

I agree there should have been a procedure for stress strain sensors or gages.

Remove and replace sections and test with
Tensile test.

 

[bigred89]

There are several non-destructive testing (NDT) methods that can help detect yielding on a surface beyond visual inspection. Hardness testing, as you mentioned, is one common technique - yielding can cause a change in the material's hardness. Other NDT methods include eddy current testing, which can identify changes in the material's electrical conductivity due to plastic deformation, and ultrasonic testing, which can detect changes in the material's acoustic properties caused by yielding. Additionally, techniques like digital image correlation (DIC) can measure surface strains and identify areas that have undergone plastic deformation. The industry standard for detecting yielding in non-stress concentration areas varies depending on the specific application and material, but a combination of visual inspection, hardness testing, and other NDT methods is typically used.