Method for "correcting" stress due to extra (non-modelled) thickness?
Method for "correcting" stress due to extra (non-modelled) thickness?
(OP)
To all
I have an FEM with stress value in an area of interest. Stress is a bit too "high" and is deemed to be the result of the modelling technique used (RBE2,etc).
One does not want to remodel the area as it will most likely create issue. It is known that in that particular area an extra thickness exists due to steel washer.
The question is: Is there a way of "correcting" the predicted stress by taking into account that extra thickness? (using a method than most stress guy will "accept")
I thought of using the ratio (t1/t2)^2 (where t2 is the full "real" thickness)
What I am not sure is the effect of have a "bi-metallic" section (alu + steel)
Any thoughts/ suggestions?
Thanks
Regards
I have an FEM with stress value in an area of interest. Stress is a bit too "high" and is deemed to be the result of the modelling technique used (RBE2,etc).
One does not want to remodel the area as it will most likely create issue. It is known that in that particular area an extra thickness exists due to steel washer.
The question is: Is there a way of "correcting" the predicted stress by taking into account that extra thickness? (using a method than most stress guy will "accept")
I thought of using the ratio (t1/t2)^2 (where t2 is the full "real" thickness)
What I am not sure is the effect of have a "bi-metallic" section (alu + steel)
Any thoughts/ suggestions?
Thanks
Regards





RE: Method for "correcting" stress due to extra (non-modelled) thickness?
RE: Method for "correcting" stress due to extra (non-modelled) thickness?
RE: Method for "correcting" stress due to extra (non-modelled) thickness?
RE: Method for "correcting" stress due to extra (non-modelled) thickness?
Stresses at any element connected to an RBE2 are probably wrong and should be ignored.
RE: Method for "correcting" stress due to extra (non-modelled) thickness?
One option is to "ignore" the stress hot spot with the argument that it is due to the RBE2 and therefore unrealistic. select stress in next element, etc, etc. Never too keen on "wordy" argument.
Are we saying that the fact that there is an extra thickness (due to the washer) as not effect on the local "capability"?
RE: Method for "correcting" stress due to extra (non-modelled) thickness?
If you have modelled a bolted joint with RBEs at a single node or as a wagon wheel around a moodeled hole, then all stresses in all plate/shell elements attached to the rigid element are meaningless (a polite way of saying they are rubbish).
RE: Method for "correcting" stress due to extra (non-modelled) thickness?
you need to be very careful with counting pad-ups as being fully effective. riveted on doublers are only slightly effective (like on a pressure shell a dblr the same thickness as the skin will only pick up 1/3rd of the load (not 1/2, unless it is very large). machined padups, integral with the skin are more effective but not 100% (unless very large).
another day in paradise, or is paradise one day closer ?
RE: Method for "correcting" stress due to extra (non-modelled) thickness?
Spaceship!!
Aerospace Engineer, M.Sc. / Aircraft Stress Engineer
RE: Method for "correcting" stress due to extra (non-modelled) thickness?
The structure being investigated is an aerospace one submitted to very high 'g' level
I dug a bit more into the results and the FEM is predicted very high shear value in the CBUSHs (much higher than the axial load) which must be the source of the high stress.
aerostress82: it looks like you have suggested an (company) modelling "rule" which will most likely impossible to find/confirm
the fixing is modelled with 1 CBUSH (K,1,2,3,5,6 used) and 1 RBE2 on each side (attached to each part!). On the part of interest the RBE2 extend to the head diameter.
RE: Method for "correcting" stress due to extra (non-modelled) thickness?
Your problem is that your model is grossly rigid around the fastener area due to your RBE2 elements there. Which part of the A/C are you modeling? Interiors? Galleys? Primary/Secondary Structure Brackets? Primary/Secondary Structure itself? Wing, Fuselage, Landing Gear?
Every structure has assumptions of its own - so it really matters which part it is that you are modeling to find the most suitable FEA modeling method. Also if following hand calculations (for strength checks, buckling, crippling, fatigue joint & etc) are also existent, let me know that too - because it really changes a lot of things about your FEA. Also, how many years have you been working "professionally" on FEA? Your case is somewhat easy to resolve, but I guess you are new with FEA/stress.
Spaceship!!
Aerospace Engineer, M.Sc. / Aircraft Stress Engineer