wing structure: spar-rib junction in the f.e.m. fine 1

[minny]

Building the fine f.e.m. of a wing structure, I'm modelling longitudinal spars and transv. ribs using 2D elements. All the spars are continuus while the ribs are cutted, I mean each rib is composed of two or three part along chord wise. It's in the real structure as in the f.e.m..
In the real structure a shear clip connect the rib with the spar. How do I model the shear clip in the f.e.m.?
A nothing
B coicident nodes at the spar to rib joint
C ?

thanks in advance

 

[rb1957]

it looks to me as though you're asking two different questions ...

1) the rib/spar joint ... i'd use common nodes (not co-incident) for obth the spar web and the rib web, and use something like "element-grid point force balance" to figure out the load being sheared into the rib, or use the spar elements to show you the change in shear across the rib.

2) the rib/skin connection ... three discrete shear ties ?, maybe it's sufficient to mdel as one continuous tie (common nodes between skin and rib) and use a hand calc afterwards to show how the load builds up in each element.

 

[minny]

ok, for the rib/spar joint i'll use coincident nodes.

for the flange rib/skin joint I don't know if it's better to use:
1) the CBUSH (PBUSH: K1,K2,K3,K4,K5 high and K6=0 with 3 normal to PSHELL)
2) the RBE2

at each fastener location

I have seen same model with solution 1 and same model with solution 2, may you explain the differences? which is more correct?

thanks

 

[minny]

ok, for the rib/spar joint i'll use COMMON nodes.

for the flange rib/skin joint I don't know if it's better to use:
1) the CBUSH (PBUSH: K1,K2,K3,K4,K5 high and K6=0 with 3 normal to PSHELL)
2) the RBE2at each fastener location

I have seen same model with solution 1 and same model with solution 2, may you explain the differences? which is more correct?thanks

 

[rb1957]

two different approaches ...

1) use CBUSH everywhere (rib/spar and rib/skin) to represent the fastener stiffness. give the fastener in-plane stiffness (Huth, or fastener stiffness equation of your choice) and maybe some nominal (small) tension stiffness if you're using Hi-Loks/Hi-Lites (maybe very small tension stiffness for rivets), this approach would use coincident nodes (which'll make it harder to debug the model)

2) use common nodes everywhere, make a free-body of the rib to check on the size of these forces

 

[minny]

dear rb1957

with the approache 1) using CBUSH everywhere you suggest to use coincident nodes, but I have modeled each part on its own middle surface (I absolutely don't want to use any offset) and so I'd use CBUSH that connect two different nodes for each joint, what do you think about the use of CBUSH between two nodes at different location?

thanks

 

[rb1957]

the simple solution would be to add a BEAM element joining the two mid-plane surfaces, i think CBUSH are zero length elements.

you could create a coincident node at one mid-plane, connect this node to the other mid-palne node with an RBE element (a rigid beam) and add a CBUSH between the coincident nodes.

 

[minny]

dear rb1957

the last suggestion look like good but it takes time, the model is huge ... too many fastners. I need a more easy and fast solution. I'm not interested in reading the acting load at each fastener, I'm going to use the shear flow along each fastener's row. So I'm searching for an easy model for the joint. Remember that each surface has to remain at is own middle plane without any offset. What do you thing about the use of CBAR to model each fastener?
(A CBAR with the area of the fastener's cross section and the same material of the real fastener)

thanks

 

[rb1957]

if you're not interested in fastener loads, then just RBE the node mid-thickness nodes together (or maybe a beam element so you can controll the stiffness of a connection if you need to); can't think of any other way to join the nodes.

next time model the rib with the skin nodes (using common nodes for rib and surrounding element).