Limit or Ultimate Load
Limit or Ultimate Load
(OP)
Hi collegues!
Here goes my question:
Is there any criterion (in order to prove a structure) about using limit or ultimate loads when the load case is a "fail safe case", it means, when a structural element is broken?
Thanks in advance.
Here goes my question:
Is there any criterion (in order to prove a structure) about using limit or ultimate loads when the load case is a "fail safe case", it means, when a structural element is broken?
Thanks in advance.





RE: Limit or Ultimate Load
There is no simple answer.
Check FAR 25.571
Damage-tolerance and fatigue evaluation of structure.
RE: Limit or Ultimate Load
I tend to use ultimate allowables when doing crash analyses.
RE: Limit or Ultimate Load
The thing is that ultimate loads are not "true" loads, because are obtained from limit ones (maximum in the aircraft life) times 1.5. Fail safe cases are also "extreme" loads (the fail of a structural item), so the question is that if it is necessary (or required by authorities) to prove the structure combining both kind of loads.
RE: Limit or Ultimate Load
http://avi
Margins of safety have a purpose: insurance. Insurance against abuse, insurance against aging, insurance against the unknown. Engineers trade this insurance for performance or utility. Aero engineers have been given the liberty to shave the insurance down very thinly: 1.5. Many other industries expect MS > 4. Some > 10.
"Fail safe" is just another way of trading insurance for utility. Now, however you assume something HAS failed, and deal with the consequences.
The analysis requires understanding the process of the failure. It deserves an analysis of deflections and energy absorbtion, not just the beginning and final static states of being.
Steven Fahey, CET
"Simplicate, and add more lightness" - Bill Stout
RE: Limit or Ultimate Load
I didn't know the probable cause of the accident of AA's A300. The question that comes to me now is what "American Airlines Advanced Aircraft Maneuvering Program" mean.
Because I suppose that Airbus had designed their vertical tail plane able to support any rudder pedal input (at limit load and, afterwards, at ultimate load...). I mean, I don't understand why a rudder pedal input involve loads higher than ultimate ones...
Thank you.
RE: Limit or Ultimate Load
SparWeb,
Ah are you talking about Margin of Safety
or Factor of Safety???
RE: Limit or Ultimate Load
(Margin of Safety)= (Reserve Factor) - 1
Being Reserve Factor==> RF=Pallowable/Papplied
I suppose that what you call Factor of Safety is the same as Reserve Factor, isn't it?
So, at the end, we are talking about the same thing.
RE: Limit or Ultimate Load
Steven Fahey, CET
"Simplicate, and add more lightness" - Bill Stout
RE: Limit or Ultimate Load
RE: Limit or Ultimate Load
I'm not trying to be picky, but I'm used to much bigger factors, where adding or subtracting 1 doesn't make a lot of difference. On lifting cables, a factor (reserve) of 12 is quite usual.
John
RE: Limit or Ultimate Load
yes in aerospace we design to the load. The FARs and/or company guidelines define the airplane loads (like manoeuvers), the aero. guys tell us structures guys the external loads applied to the plane, and we calculate the internal loads. Limit load is the maximum load expected in service, and the structure is be designed to ultimate load (1.5*, "factor of safety"). Then the RF (reserve factor) is how much the allowable load exceeds the applied load, and this can be 1.00 or even 0.99.
What this does mean is that we typically have LOTS of cases, covering all the conceivable situations, and lots of "ambulance-chasing" requirements !
RE: Limit or Ultimate Load
HarrisJ, you have understood me. The safety margins (and safety factors for that matter) are very thin in the aircraft business, when compared to what are used in most other industries. But there are several points to consider when comparing aircraft to other machines:
1) The loads on aircraft structures are applied under strictly controlled operating conditions (pilots must be conscientious),
2) Aircraft are very weight sensitive. To design an aircraft with the safety factors used by automotive engineers would make it incapable of flight (and cars are weight sensitive, too),
3) Aircraft wings and fuselages penetrate through air, a very forgiving medium, unlike, for example, a bulldozer through gravel. Most other machines are designed for loads applied when they contact other solid surfaces/structures.
Each industry has developed standards that suit their normal practices. Hoisting cables, for example, must be able to absorb shocks from dropped loads, friction on sheaves, sharp bends, etc, and all with a covering of rust from the salty sea air... no wonder a safety factor of 12 is imposed.
By the way, RB1957, I've always thought of the FAR 25.303 definition of Factor of Safety to be worded awkwardly. Those who find the differences murky could start with reading Bruhn, Chapter A4, and then going on to machine design textbooks like Shigley's and reading what they have to say about safety factors. The assumptions that go into selecting appropriate safety factors are different.
Steven Fahey, CET
RE: Limit or Ultimate Load
In comparison, I was once working on a marine project where the loads were well understood, and there was no danger to life, and we were able to simulate the main life-limiting load very accurately. The weak point in the system was proof tested on every part. So we ended up with a safety factor of 1.05, as we had overdesigned the critical joint by 5% to allow for variation.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Limit or Ultimate Load
RE: Limit or Ultimate Load
1. The structure should not yield in the maximum expected load (including extreme temperatures, environmental conditions accelerations, aerodynamic loads etc)
2. At 1.15 of the maximum expected load the structure is allowed to yield as long as there is effect on the system performance. For example, if a missile fin will yield and will have a permanent 0.5 degree deflection it probably will not affect the missile performance because it is with the manufacturing tolerance.
3. At 1.5 of the maximum expected load the structure should not break. Above it if is allowed to break
RE: Limit or Ultimate Load
But my knowledge is fuzzy about the origins of some of these factors. Most factors appear arbitrary - reasonable, easy to remember, but still arbitrary, and I've not been able to track down how they came about. NACA Reports dodn't give more than hints about them.
Specifically, I am always eager to know how the Fitting Factor (25.625) come about. (15% extra margin on parts substantiated by analysis without tests.)
Also, the seat factor (25.785) of 1.33?
The Normal Category manouvering load factor of 3.8? (Or the formula in 25.337 that calculates it, for that matter)
There are many more, but let's not get bogged down.
Better understanding of how these factors came about would help me and all others practice them.
Steven Fahey, CET
RE: Limit or Ultimate Load
There's no historical background in the article, so I'm still looking...
Steven Fahey, CET
RE: Limit or Ultimate Load
an excerpt for one of the hits ...
As student pilots, we all learn Manoeuvring Speed (Va); it is one of the three speeds that must be memorised for the Private Pilot flight test. We learn that Va is the “…maximum speed [at a particular weight] at which full deflection of the controls can be made without exceeding the design limit load factor and damaging the airplane’s primary structure”(3). We also learn that Va is the maximum recommended speed for turbulent air penetration. This is good information and will, most probably, help us earn pass marks on the private pilot flight test.
As we grow and develop as pilots, we begin to take on new challenges. Developing a deeper understanding of Va becomes essential, particularly if we start thinking about doing any mountain flying or extending our range to distant points from which we will not be able to immediately “return to base” if the weather begins to deteriorate.
The bottom line: Va is the speed below which our aircraft will stall rather than bend or break when we impose or have imposed on us—as in the event of a vertical gust—an increased load. But, as with most things, the devil is in the details.
RE: Limit or Ultimate Load
http:/
and/or this NTSB report:
http://ww
VA is not iron-clad protection against breaking your airplane.
Steven Fahey, CET
RE: Limit or Ultimate Load
a good post. i copied a sample of conventional wisdom. your refs (refering to the A300 rudder) show that unconventional structures (and controls) don't necessarily follow conventional wisdom ... i guess that's one of the problems with being unconventional !?
how soon till we have a "checked roll" manoeuvre ?
RE: Limit or Ultimate Load
The current FAR 25 is for a "checked yaw" manouver where the rudder is suddenly fully displaced in one direction and then returned to center. The rule does not require a subsequent full deflection in the opposite direction. Yet.
Steven Fahey, CET
RE: Limit or Ultimate Load
Ultimate stress is when the material fails, not necessarily equal to 1.5 * limit (yield) stress.
Limit stress is the end of the linear range, when the material exhibits more than 2% permanent deformation.
There are material allowables (tensile, compression, shear) and local geometric property allowables (crimplilng, buckling, etc).
Ultimate stresses are used during crash analysis if the permenant set is acceptible, otherwise use limit stresses.
Margin of safety is applied stress divided by allowable stress minus one
Factors of safties are scale factors applied to the estimated loads, and generally range between 1.5 for well understood loads to 3 or 5 for less understood loads.
Generally, design stress = (Design Loads * Factors of Safties) / (appropriate geometric term). Then MOS = Design Stress / Allowable - 1. MOS is required to be greater than zero. All values less than 0.5 should be checked carefully for correct geometry, loads, and allowables.
Aerospace vehicles operate in a well defined, well tested, operating envelope. The higher factors of safeties used in other applications may be required to offset the unknowns in the loads/operating conditions.
In the perfect world, there were accepted standards for:
a) The NASTRAN representation of typical structural parts
b) The factor of safty requried for different ypes of loads in different operating conditions
c) The interpretation of stresses from NASTRAN
d) The calculation of geomtry dependent allowables
e) Documenting a structural analysis
f) Checking / reviewing a structural analysis
I believe the economic factors are playing too much of a role in the structural design / evaluation process. Possibly, that is the result of previous sucesses in the development of lighter, safer vehicles. However, as engineers, we have a responsibility for doing correct, accurate analysis, even though this responsibilty may not be consistent (or a part of) the latest trend in international management styles.
Beam me up, Scotty
RE: Limit or Ultimate Load
Minor point: yield in this situation is usually taken to be the t2 stress, which is 0.2% permanent set, not 2%.