precarious + load

[Sino]

Good morning to all, I would have a doubt about the behavior of a bolted link to which a preload is applied and then also an external load.
I try to explain myself better: I am making a fem verification (patran marc) of three bolted flanges to which I applied a preload (first load case) and a load acting on a flange (second case load).

the doubt concerns the trend of the stresses on the stem of the vines that from the first increment on then tends to descend.
would anyone know how to indicate the correct trend of the stresses on the vines or possibly links, written or otherwise?

Thank you very much
until

 

[cacciatorino]

Good morning to all, I would have a doubt about the behavior of a bolted link to which a preload is applied and then also an external load.
I try to explain myself better: I am making a fem verification (patran marc) of three bolted flanges to which I applied a preload (first load case) and a load acting on a flange (second case load).

the doubt concerns the trend of the stresses on the stem of the vines that from the first increment on then tends to descend.
would anyone know how to indicate the correct trend of the stresses on the vines or possibly links, written or otherwise?

Thank you very much
until

You should understand how you applied the first load on the screws. I guess you want to simulate the shot in the screw stem, due to the screw couple, right? Would you bring a graphic pattern of the problem?

typically the screw preload simulates by applying a negative delta-t to the screw stem: the thermal expansion prevented creates the tension state of traction in the screw itself (clearly it's just a dirty trick to facilitate the work).

in theory: the load in the screw stem should remain constant in the first part of the second load application ramp. when this value equals and exceeds the precarious value of the screws, you should start to see linearly climb even the axial effort of traction in the screws. Here, of course, I'm assuming that the two bolted flanges are subject to a tensile effort, axial and centered on the crown of screws that lock them together.

 

[Sino]

hello hunter and thank you for the answer,
the preload was pinned with a patran tool considering the axial shooting on the screw.
the tool does nothing but divide the mesh of the screw stem and pull the two half.

in the first load case the preload from reliable results, but with the application of an external load on one of the two flanges,
the stress on the vines gradually drops.

Such a thing could be achieved if the flanges were chilled by losing precarious to the screws.
(Unfortunately I can't load files)

 

[cacciatorino]

hello hunter and thank you for the answer,
the preload was pinned with a patran tool considering the axial shooting on the screw.
the tool does nothing but divide the mesh of the screw stem and pull the two half.

in the first load case the preload from reliable results, but with the application of an external load on one of the two flanges,
the stress on the vines gradually drops.

It is necessary to see how this tool works, if it works by blocking the two resulting surfaces of the cutting in space, or by applying a force but leaving the two surfaces free to move axially.

Usually to simulate the screws are used beam elements however, not solid, in order to reduce the computational weight of the problem. this obviously if the object of the study are not the actual screws.

I at the time did a didactic study of two flanges bolted and subject to traction, but the preload of the vines simulated it using beam elements and applying a negative dt, as I told you.

Anyway, without seeing any picture of your problem, it's hard enough to make realistic assumptions.

 

[Vmax]

Regardless of the precarious and vine-representation technique, the problem may be that from a load step to the next the precarious is "spento", for example in certain fems it is necessary to explicitly specify the "lock" option, this could explain the discharge of the screws that in the second load step would see only the external load.

 

[Sino]

Hello, everyone, solved the problem.
in practice as indicated by vmax you must "block" the deformation of the screw so that it is maintained also in the subsequent load cases.
to do this there is a trick, which is in applying a constraint (incastro) on the master node of the mpc created by the tool.
thanks to all
Hi.