Fatigue Analysis

[lLouie]

I have created a new thread about the fatigue test, however I recognised öne thing. Normally, I use nz load for metal part which iş connected to wing, because nz is more critic than the others (X and Y). Must I put X and Y conditons?

I use nCode DesignLife, is there any example for 3 load conditions? (X,Y and Z direction)

 

[rb1957]

if you're doing a fatigue analysis for a wing, yes, vertical loads (nz and aero) are the dominate load, producing Mx (normal bending moment, tip up); but you must also include drag loads (inertia and aero) which produce chordwise bending, Mz. If a strutted wing then also need spanwise loads. You may want to include Cm (wing aero moment = torque).

Start with the loads on the wing ... Cl, Cd, Cm, nz. Spanwise loads, ny?, are important for strutted wings. Chordwise inertia loads, nx?, are likely to be small.

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

 

[SWComposites]

First please define your coordinate system.
Is "z" aligned spanwise?
In general yes, you need to consider the full loading conditions.
Some portions of the wing may be dominated by spanwise bending stress, but there are probably many local areas where the stresses in the other directions are significant. For instance areas around control surface fittings.

 

[lLouie]

Hi,

Thank you so much for your replies.
Z is yaw axis, and X is roll axis.

I use nZ loads now, when I do fatigue analysis, yes ny and nx loads' effects are so low, however when I put the all load together in the FEA which is nCode DesignLife, the life is very low. Must I put the loads seperately? and then I must collect the damages for nz, ny and nx.

 

[rb1957]

normally y spanwise (stbd or port), z (up or down), x follows (aft or fwd).

You say nz loads ... do you mean inertia loads or vertical loads (inertia and flight) ? You can't ignore drag loads. wing axial loads ("fy") are important for strutted wings.

How are you accounting for aero loads ? Cl is normal to the flow direction, and needs to be rotated onto the structural axes. Cd also. Don't forget Cm (as an aero load). It is important to appreciate that aero loads and inertia oppose each other (what we call inertia relief).

As SWC says, it is best to evaluate all loads then say some are negligible; rather than saying "some guys on the internet said it was ok to ignore these loads".

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

 

[lLouie]

Yes, nz is vertical load, and I use pressure load (CFD) for aircraft's surface.
I looked at the nx and ny loads seperatly and their effects are very low, however when I use them together (nx, ny and nz) the life is the worst for Aluminium part.