1 HR and 10 HR Endurance Limits

[Kristina Sornikova]

Dear experts,

I looking for method to calculate 1 HR and 10 HR endurance limits from the working S-N curve of a life limited component, safe life part.
The limits will used to monitor flight load exceedances. Any help please?
Much thanks.
Kristina.

 

[Sparweb]

I'm sure I would need more information to help you with a specific answer, but what I CAN say right now is that you would gain a lot of insight by looking at this book:

Schijve, Jaap; Fatigue of Structures and Materials

https://www.springer.com/gp/book/9781402068072

I do believe an important piece of information needed to solve your problem is a stress spectrum. Also probably a specific definition of the flight operations and mission being flown, because the spectrum will change depending on long-cruise VS. hover-lift types of operations.

http://www.sparweb.ca

 

[WKTaylor]

Kristina... Please advise...

EXACTLY what spec/reg and circumstances are You referring to?

Helo mechanisms and/or rotors?

Mechanisms are all lubricated so FOD/contamination, loss of lubrication and/or battle damage are factors.

Rotors have their own unique problems/criteria.

Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[Kristina Sornikova]

Thank you Sirs SparWeb and WKTaylor for you kind answers.

SparWeb:
1HR and 10HR life being so close to left side of S-N curve. I say in most cases, it will be GAG damage due to alternating load or moment or stress close to limit load or moment or stress, maybe some exceptions.
Spectrum is many different kind of mission configuration and operation including training, sorties, live fire, turns, pull ups, pushovers, hovers, landings, autorotation and all kind of things. So for now, say I have working S-N curve lower than GAG S-N curve. Life is 1HR and 10HR. How to determine endurance monitoring limits for exceedances for these lives for a safe life part?

WKTaylor:
This is for a helicopter Flight Strain Survey, many regulations in 14 CFR 27 apply for safety of flight test. Components are nose to tail, many different life limited components.

 

[WKTaylor]

For the Airframe... Huhhh.?

I thought this would be related to flight critical mechanical systems... and would be more tribological phenomena related.

I thought most helos were being designed with significant/enhanced structure margins for crashworthiness/survivability... OK maybe only applies to MIL helos… or only the PAX related structure.

Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[Kristina Sornikova]

WKTaylor,

I not sure if I follow what you trying to say me?

Kristina.

 

[Sparweb]

By "1 HR endurance" do you actually mean "one hour of endurance before failure"? I have a sense that I haven't understood you correctly.
If anything in the helicopter could fail within 1 hour of operation there's no way you could consider certification.
Component life needs to be in the 1000 hour range. Or more commonly 1000 cycles if defined by GAG cycle.

Every SN curve is used with a scatter factor. Say it's 5 for your data. A "1 hour endurance" average with the scatter factor would be 12 minutes.
There would be a 5 to 10% chance of failure already existing at that time!

http://www.sparweb.ca

 

[Kristina Sornikova]

Yes, only is use to monitor test loads.

GAG is usually 4 to 6 cycles per hour depend on application.

But usually 6000 cycles per 1000 HR.

S-N curve already include all knockdowns scatter etc.

All I need is how to calculate 1Hr and 10HR monitoring limits....

 

[Sparweb]

Well, maybe (just maybe) all you want is Miner's Rule

More detail about it here: Saunders Samuel, Boeing Scientific Research Lab, D1-32-1019, Dec 1970

But definitely consider finding a copy of Schijve's book, because there is a discussion of the LIMITATIONS of this practice. I am not confident that you would be using this analogy in the range it's intended to be used without making certain from resources like this, first.

http://www.sparweb.ca

 

[Kristina Sornikova]

Yes, agree, but Miner's rule comes later to calculate damages from raw data processing.

This is for flight test safety, for monitoring the measured loads.

 

[Sparweb]

Goodness. You intend to do this prediction live in flight. That's the other side of crazy where I can't join you.

If you want to witness something fail aerodynamically, do it in a wind tunnel!

http://www.sparweb.ca

 

[Kristina Sornikova]

No Sparweb, it does not mean anything fail, only red flag high load to analyze after landing why happened, testing no stoppage.

 

[WKTaylor]

KS... for my sanity...

What exact specs/standards/regulations/guidance are You doing this work/study PER...????

This question 'just-seems' more related to helo* flight critical mechanical components than structures. Working some field issues with USAF helo*s… I've had interface with engine, transmission, drive-train gearbox and rotor engineers that had to deal with battle damage and 'run-dry' mechanisms... that were intended to 'hold together/operate' for a short time safely-enough before finally/catastrophically failing.

*helo is a nicer-term than chopper

Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[Kristina Sornikova]

WKTaylor,

It is military helo, but I cannot go too much more detail, I am sure you understand.

Like I said, all kind of components are instrumented including drivetrain.

The goal of monitoring limit to make sure sustained fatigue loads at the high levels of 1HR and 10HR endurance are not induced or exceeded during testing.

Anyway, question is simple, if you have a working S-N curve, can you calculate 1HR and 10HR endurance limits from it? If so how?