So now let’s look at the different types of contact that we use while modeling bolted connections.
The first one is the bonded contact and the second one is the frictional contact.
Just as its name suggests, bonded contact implies that the two surfaces in contact are bonded to each other,
i.e. they do not separate from each other in the normal direction and they also don’t slide over each other.
In most cases, the contact between the bolt and the nut is defined as a bonded contact,
since there is no significant relative motion between the two when the bolt is sufficiently tightened.
Recall that in the previous section, we discussed that when a bolt is split in half in order to apply bolt preload,
the splitting plane cannot pass through a region where bonded contact is defined.
Defining the bolt preload involves the two split parts of the bolt sliding over each other to adjust the grip length.
But if the splitting plane passes through a region of bonded contact, the two halves of the bolt cannot slide freely
and this is the reason why the splitting plane cannot pass through region of bonded contact.
The second type of contact is the frictional contact.
In this case, contact surfaces cannot penetrate each other, but they can separate from each other.
The normal contact force Fn is equal to zero when the surfaces are not in contact.
The surfaces can also slide over one another provided the applied tangential force exceeds the friction between the two surfaces.
When sliding occurs, frictional forces given by the Coulomb’s law act on both the surfaces.
The frictional force acts in the tangential direction
and acts such that it opposes the relative motion between the two contact surfaces.
So how we decide what type of contact to use?
In bolted joints, contact connections occur primarily in three regions.
First, between the bolt head or the nut and the mating component.
Second is between the two mating components.
And the third region is between the threaded region of the bolt and the nut.
Let’s discuss about the type of contact to be used between the bolt head or nut and the mating component.
When tangential forces are significant, relative sliding may occur between the bodies in contact.
In such cases we use frictional contact.
On the other hand, if tangential forces are not significant
and we do not expect separation to occur between the contact surfaces, we may use bonded contact.
For example, in case of a tension joint, the forces act along the bolt axis
and there is no significant lateral motion between the bolt head and the mating components
or between the nut and the mating components.
Thus, tangential forces are not very significant.
Also, if the bolt has been tightened sufficiently,
we don’t expect separation to occur between the bolt head, the mating component and the nut.
In such a case, bonded contact can be used.
On the other hand, in case of shear joint, tangential forces are very important.
Hence, we use frictional contact between all the three contact pairs.
Bonded contact is linear and hence reduces the computational cost.
So, whenever tangential forces are insignificant, we simplify the model by using a bonded contact.
Frictional contact is computationally costlier than bonded contact because it is nonlinear.
But a useful option that helps to reduce the computation time when using frictional contact is the small sliding option.
In a frictional contact, at the end of every iteration,
the solver checks whether the two surfaces in contact are still within a certain tolerable distance of each other.
This checking after every iteration is an additional step which increases the solve time.
But when small sliding option is used, we specify that the two surfaces will always be within the tolerable distance of each other.
So, the solver does not need to check this after every iteration, which in turn reduces the computational cost.
But we need to be careful when using this option.
The small sliding option should only be used when the gross sliding between the surfaces in contact is very small.
Next, let’s move on to the second contact region, which is between the two mating components.
Generally, frictional contact is used between the two mating components.
Using bonded contact between the mating components makes the use of bolts redundant.
For example, if bonded contact is used between the wall and the bracket here, there will be no relative motion between the two
and stresses developed in the bolts will not be accurate.
This may lead to incorrect judgement and inefficient design.
Finally, let’s discuss the third region of contact i.e. the contact between the threaded portion of the bolt and the nut.
Bolts are designed such that the bolt fails before thread slippage occurs.
Hence, in most situations, the threads are not modeled to improve computational efficiency.
In such cases, bonded contact is defined between the nut and the bolt.
However, in cases where failure of the bolt at or near the threads needs to be investigated,
we can model the thread geometry on the bolt and the nut and define frictional contact between the two.
If we do not want to model the threads geometrically but wish to get an approximate stress distribution at the threaded portion,
it is worth noting that we can use frictional contact with geometry correction between the cylindrical bolt and nut surfaces in contact,
where the threads are modeled virtually via contact rather than from the CAD geometry.
So finally, let’s use everything that we have learned in this lesson
and decide the appropriate contact connections to be used in a real-life example.
Consider we have a bolted joint with a geometry shown here.
Bolts are used to connect two flanges together.
We define frictional contact between the gasket and the two flanges.
Since this assembly is not being subjected to vibration and we are not worried about the bolts getting loosened,
we can safely define a bonded contact between the nut and the bolt surfaces.
But what about the contact between the bolt head or the nut and the flanges?
Due to the geometry of the flanges which is highlighted in red,
there is a gap between the two components being connected together by bolts.
The bolt preload acts in axial direction.
Since, there is no shear load present, we might be tempted to define a bonded contact between the bolt head or nut and the flange.
But in this case, the mating components being shell-like, undergo bending.
Thus, there is relative motion between the bolt head/nut and the mating components.
To decide the type of contact, we look at the objective of the simulation.
If we are interested in the local stresses around the bolts,
then we may want to define a frictional contact between the bolt head or nut and the mating components
since some sliding might occur between them.
However, if we are interested only in the contact pressure on the gasket,
then modeling the contact as bonded contact or as frictional contact with small sliding may suffice.
Thus, we also need to consider the objective of the simulation when choosing appropriate contact connections,
in order to balance our need for accuracy with computational efficiency.