Fail-Safe Approach

[lLouie]

A fail safe strength investigation in which it is shown by analysis, tests, or both, that catastrophic failure of the structure is not probable after fatigue failure, or obvious partial failure, of a principal structural element, and that the remaining structure is able to withstand a static ultimate load factor of 75 percent of the critical limit load at Vc.

What should I understand exactly? The part is subject to fatigue, and it should not cause damage to other structures after it has fatigued. Am I right?
What should I do an analysis for this?

 

[rb1957]

oh, what a huge topic.

first, fail-safe static ... that the structure can sustain/support static loads (typically limit load, 2/3 of ultimate) without failure, including excessive deflection, with any part of the loadpath failed. this failed structure can be whatever you want ... an individual fastener in a bolt group, the connection of one element to another, a beam, multiple beams.

second, if you are using this as part of your DTA, then the failed structure needs to be inspectable (ie found by whatever inspection method you use) over some period of time (the repeat inspection interval). So you need to combine intact structure fatigue with failed structure fatigue. You have to analyze the critical sites of the failed structure, and account for the fatigue damage at thee locations from the intact structure.

Rarely,. in my experience, done as fatigue damage, but rather as crack growth.

But is this for your composite wing ?

An example of this (failed strucutre analysis) I've done is door stops. the door had multiple stops, intact, and each stop was failed individually and the model re-run (with 2/3 load) to see what was critical (statically) ... intact ultimate or failed limit.

Another example is rotor burst ... where the engine rotor damages the structure and the failed structure needs to support some static loads (here it's "get home" cases, not full limit load).

25.571e defines loads required to be analyzed for "discrete source damage".

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[lLouie]

Firstly, I really appreciate you. You always answer my questions.

This is for metal structure. You did an analysis using 2/3 of ultimate load. Then, how did you decide that there is no any problem or not. I tried to do an analysis in Ansys, I obtained a result that is the stress results are not higher than the metal structure's yield strength. Then, I decided that this structure will not affect the other structures. Did I do it right?

 

[rb1957]

"how did you decide that there is no any problem or not." ... the failed structure static loads are additional cases to be considered along with the intact ultimate cases. If you've already done the intact ultimate cases, then compare the failed structure limit loads to these ... if lower then no problem, if higher, revise MS calc.

"the stress results are not higher than the metal structure's yield strength" ... no, for failed structure you use the ultimate allowable. In the first place compare your failed structure internal loads with the intact ultimate internal loads ... if lower then failed (limit load) structure is covered by intact (ultimate load) ... yes. If not, revise the MS calc with the failed structure internal loads. Don't forget to consider deflections ... and overall stiffness (as this could impact flutter).

there are three static criteria ...
ultimate loads (compared to ultimate strength)
limit loads (compared to yield strength)
failed structure (limit loads at ultimate strength) ... the idea here is at failed structure is so unlikely to ever happen that limit load is sufficient strength requirement ... as different to limit (intact) loads.

And all this only considered the static component of failed structure ... there is a DT/Fatigue component as well.

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[lLouie]

I'm so sorry, I try to understand that I have a limit load of 100 N. According to the loading condition, I will apply a load of (100×0.75)×1.15, which is 86.25 N. How can I interpret the result based on the information you provided? Must I do static analysis or other analysis type? If it yes, I will obtain stress result, and what is the other road?

 

[lLouie]

"Partial Failure Scenario: To simulate the failure of one of the primary structural elements, remove or weaken this element in the model. Then, test whether the remaining structure can withstand a static ultimate load factor of 75 percent of the critical limit load."
I thought like this scenario. There are multiple parts that are connected to each other. When I remove one, I can see that it affects others or not. Am I right?

 

[rb1957]

limit is "limit" the defined load.
ultimate is 1.5*limit (you may have different factors, like 4/3 for ultimate load, rather than the typical 3/2)
"1.15" is a fitting factor, possibly some "uncertainty factor" (not usually required in civil cert). Fitting factors are applied in the detail calc as they apply to fittings, not "wings".

I understand you've already done your intact ultimate loadcase. And have the internal loads, good.
Now fail some element and apply limit load. Are the internal loads less than the above intact case ? yes, then done; No, then revise the calcs where the loads are higher.
Repeat for other failure cases. Typically we'd combined the failure cases together as a "max/min", and then max/min with the intact, and look for where the intact is not critical.

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[lLouie]

Thank you so much. I will start to do analysis

 

[drawoh]

lLouie,

Can you define catastrophic? I would assume that massive property damage, and/or people seriously injured or killed is catastrophic. Expensive hardware and/or people should not plummet from high elevations. Expensive hardware and/or people on the ground should not be hit by falling objects. Expensive hardware should not explode, unless that was part of the design purpose. Expensive hardware and/or people should not be impacted by explosions. I don't think there is good theory about this. You need to look at what you are doing and do basic reasoning.

If the failure is catastrophic, maybe your safety factors should increase.

As an engineer or designer, the time will come when you must do very through calculations, or just walk away.

--
JHG

 

[rb1957]

we have to be careful with that word ... "catastrophic".

there is always some level of damage that is catastrophic (= loss of airplane) for the finite strength of the plane.
the question becomes how do we "prevent" that damage from happening. DTA is a tool that allows us to say "we can find the damage in the skin before it becomes critical".
An important element to this is "is the failure detectable ?". In commercial planes, if some structure is uninspectable, then you have to assume it has failed.

And remember there are a bunch of other issues with failed structure, beyond simple stress work.

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[lLouie]

drawoh,
This study is related to UAV, so expensive hardware should not be damaged or when it drops, people must not be damaged. and,

rb1957,
you are right, the failure must be detected.

 

[rb1957]

you do realise that you don't have to design your wing to be fail-safe ? Fail-safe requires a multi-loadpath structure. You can have a single loadpath structure, only then you need to ensure that you won't have failure of a loadpath in service.

This is for your UAV wing ? Your composite wing ?

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[lLouie]

This is for UAV wing which is composite. Thank you so much for reply. I will work fail-safe approach

 

[lLouie]

I was going to open a new topic, but I want to ask here. I think I've made the form too busy. I also want to perform a fatigue analysis of the landing gear, but what should be the loads I need to determine for the static analysis? After determining a critical load, should I comment that it can withstand this load for X amount of time? Or should I install a strain gauge on the landing gear and use the loads during and after landing?

 

[rb1957]

ok, but how to apply fail-safe to a composite wing ?? (which is pretty much your original question !!)

to the fuselage attachments ? the easiest way here is to make these connections duplicate fittings with each fitting able to support limit load. Though that isn't the end of that story ... you need DTA (presumably metal fittings) for a typical flaw growth for a factored inspection interval, and limit load residual strength (to allow you to inspect for the failed loadpath).

waht does "fail-safe" mean for a composite panel ? that you can fail one ply (and have limit load capability ... I should think so, given you're no longer applying the ultimate load FoS) ?? That you can fail an inspectable piece of structure and still have limit load capability ?

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[lLouie]

I am thinking of doing it this way. After doing the structural analysis, I will do an IRF check, remove the critically damaged part and run it again without that part. I need to see if the others will withstand the load without that part.

 

[rb1957]

"IRF" ?

yes, revise the model, also revise the loads (maybe with a simple factor rather than creating a whole new set of loads).

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[lLouie]

Inverse Reserve Factor
Thanks to it, composite damage can be find, if there is. Like hoffmann, tsai-wu etc.

 

[rb1957]

well that makes more sense to me than "failure index" which is how I've heard it in the past.

"Thanks to it, composite damage can be find" ... not as I understand it.
finding damage is a function of the inspection method.
can the structure support some level of damage ? ... ok, that is a FEM issue
does the detectable level of damage grow under in-service loadings ? ... again, a different question.

it's my understanding that people assume a level of damage that is both detectable and static (not growing) under fatigue loads and supports limit load.

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[lLouie]

Yes, the structure can support some level of damage. Actually my aim is this, after analyzing at ultimate load I will detect the damaged structures. Then I will remove those damaged areas and apply 75% less load to determine the strength of other structures.