Limit or Ultimate Load 4

[airmail]

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.

 

[rb1957]

sparweb,

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 ?

 

[Sparweb]

RB1957, that may indeed be the long-term result of this accident. The reg's are in constant flux. Already rules have been re-written and re-interpreted in the wake of the SwissAir crash over Nova Scotia. We could see, in a year or two, new rules on rudder design loads.
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

 

[vorwald]

Some random thoughts as I skimmed the above material

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

 

[RPstress]

One of the things often not very well defined is what to check for in a post-failsafe-failure loadcase. Almost always the limit loads are used, and are checked against failure, with no checks being made against yield, though excessive deflections are avoided.

Minor point: yield in this situation is usually taken to be the t2 stress, which is 0.2% permanent set, not 2%.