modelling fasteners using zero-length bushes
modelling fasteners using zero-length bushes
(OP)
Hi everyone,
I am currently working on a repair job which is as follows: the part being repaired resembles an L shaped bracket which attaches to the parent structure via fasteners. fatigue issues have arisen due to undesirable design features, and therefore straps on both sides of this joint are to be used to repair the problem area.
my questions is:
what is the best way to represnt the fastener(s) securing the stack which consists of four 2D shell elements (parent flange, bracket flange, aforementioned sandwiched by the two straps).
i believe that modelling the offsets (by creating shells at their true mid planes) is proabably vital if you are concerned about the bending sttifnees of the structure. However, in my case, the structure bending sttifness at htis local rea is not important.therefore, I am trying to keep it simple by minimsing changes to the provided FEM (where parent flange and bracket flange share commomn nodes at fastener interface). i will model the straps without offstes thus creating coincident nodes at the relevant poisitions. now, what is a reasonable way of attaching these straps to the existing joint?
the client has asked to try the common node approach and an alternative approach which represents the bolt/joint stiffness. I was planning to use the Huth stiffness method to assess the in-plane stiffness but i am unsure how to do that when your stack of four elements are all co-incidnetal.for huth sttifness, should I halve the thickness of elements being shared by two bushes? i hope that makes sense. so to summarise, how do i calculcate the in plane translational stiffness(es) for all three bushes of a four element stack, using Huth method? also, how to use axial stiffness AE/L to ensure total axial sttifness of bushes equals axial stiffness of bolt?
thanks
I am currently working on a repair job which is as follows: the part being repaired resembles an L shaped bracket which attaches to the parent structure via fasteners. fatigue issues have arisen due to undesirable design features, and therefore straps on both sides of this joint are to be used to repair the problem area.
my questions is:
what is the best way to represnt the fastener(s) securing the stack which consists of four 2D shell elements (parent flange, bracket flange, aforementioned sandwiched by the two straps).
i believe that modelling the offsets (by creating shells at their true mid planes) is proabably vital if you are concerned about the bending sttifnees of the structure. However, in my case, the structure bending sttifness at htis local rea is not important.therefore, I am trying to keep it simple by minimsing changes to the provided FEM (where parent flange and bracket flange share commomn nodes at fastener interface). i will model the straps without offstes thus creating coincident nodes at the relevant poisitions. now, what is a reasonable way of attaching these straps to the existing joint?
the client has asked to try the common node approach and an alternative approach which represents the bolt/joint stiffness. I was planning to use the Huth stiffness method to assess the in-plane stiffness but i am unsure how to do that when your stack of four elements are all co-incidnetal.for huth sttifness, should I halve the thickness of elements being shared by two bushes? i hope that makes sense. so to summarise, how do i calculcate the in plane translational stiffness(es) for all three bushes of a four element stack, using Huth method? also, how to use axial stiffness AE/L to ensure total axial sttifness of bushes equals axial stiffness of bolt?
thanks





RE: modelling fasteners using zero-length bushes
you Could use beams with a bending stiffness to match the fastener flex from your formula of choice (huth is popular these days)
you could use co-incident 2D plates and CBUSHs.
you could use solid elements and RBEs and CBUSHs, but that seems like overkill.
RE: modelling fasteners using zero-length bushes
RE: modelling fasteners using zero-length bushes
for CBUSHs, input the fastener stiffness calc'd (eg huth).
for multiple layers, I'd use a huth stiffness based on two adjacent layers, so the one fastner could have different stiffness along it length ... clear as mud ?
RE: modelling fasteners using zero-length bushes
can bush elements be of zero-length?and if so I assume that their length has nothing to do with their stiffness?
I am just trying to think of the various possibilities and better understand how such elements work.
Thanks for all your valuable input.
RE: modelling fasteners using zero-length bushes
1st, huth calculates the shear stiffness of the fastener; ie displacement = P/k.
if you model the fastener as a beam, it not the bending stiffness i'm interested in, but rather the displacement of the ends. you can control this displacement to get the result you want (P/k) with I or E.
I thought CBUSHs were zero length. so you would use a rigid RBE to create co-incident nodes, and the CBUSH would have the huth stiffness.
i suggest running some test models to see how things work. either approach (beams or zero length springs and RBEs or zero length springs alone) is valid, though zero length springs alone is trickier to model is a large assembly.
what are you modelling ? if it's a splice joint, a compliance model would be just as good.
RE: modelling fasteners using zero-length bushes
when using cbushes,assuming yz plane is shear plane, K1=axial stiffness,K2=K3=huth,K4=rotational=0 in my case, and K5=K6=? (previously I have been advised to assume k5,k6=1e6 i.e. Very stiff).what is your opinion?
and yes I agree that cbushes are typically of zero-length but,being 1D elements, they can have length? Right?if so,I was interested in the effect this would have (just for my knowledge).
Thanks again.
RE: modelling fasteners using zero-length bushes
K2 = K3 = huth, K1 = K4 = K5 = K6 = whatever, 1, 1000, ...
shouldn't make much difference anyways
never tried CBUSH with length ... try it and see !
RE: modelling fasteners using zero-length bushes
RE: modelling fasteners using zero-length bushes
please refer to the attached file for details on the results, including bdf and f06.