# Why should I use a contact between two glued bodies in FEA when I can directly merge the mesh nodes between them?

So I am using ANSYS to conduct a FEA analysis. I have two options to built a connection between two bodies; first is I can use a contact (like bonded) between the touching faces and the other option is I can simply merge the topologies of the two faces in contact, so the the two faces will have a single set of nodes and elements between them. I couldn't understand what is the difference between these two options and which one should be used where? I heard from somewhere that if you are interested in finding the amount of force being transferred in that contact, and if you want to know the contact stresses on the faces in contact, then you should choose modelling the connection as a contact in FEA. But still I am not sure if these reasons hold true or not. Please help.

• Adhesives can handle a given level of shear forces and/or separation loads then they fail. If you merge the bodies then you lose the ability to see what the shear/load is at the interface. Jul 1, 2021 at 19:50

Merging the nodes is equivalent of a continuous body (see welded).

Using a contact definition, means that there are two separate surfaces which are connected through a layer/thin film (although this is a simplification). Using the contact definition means that usually there is a surface and adjacent nodes and/or elements are checked against that surface (to measure the distance or penetration).

Figure 1: an algorithm used for penetration of a node against a contact surface (source Autodesk)

By knowing the distance, and setting a spring constant for the definition, it is possible to calculate the forces between the nodes. Changing the constant (i.e. changing the bonding agent) will result in a different force and a different displacement (which in the case of the merged nodes is not possible)

An alternative (naive) approach for the glue, would be to use elements to model the glue layer. However usually the thickness of the layer is a few $$\mu m$$, and that would be harmful for the aspect ratio of elements in the neighborhood (either the glue elements or the structures or both).

## Usage

• Merged nodes: when the faces of the two bodies cannot move relative (or you can assume that the movement is very small) to each other. A prime example is welded materials, or using a rigidly connected fastener. You ca use it even bonded with glue when you don't care about the relative displacement (as a simplification of the structure).
• Contact definition when the faces move relative to each other, and the movement is either significant or is a subject for study. An example is the performance or the loads transferred from a bonded surface. An alternative example is automotive crashworthiness (e.g. when you two cars impacting each other, you need to define contact definitions between the two cars, and also between the different components of each car)

In general contact definition are more expensive, but sometimes is the only way to perform a meaningful simulation.

• Well thanks for answering. But for what purposes should the contact be used and for what purposes can a simple merged topology be used? An example for each would be very much appreciated. And can you also please share the link for the picture you attached? Jun 30, 2021 at 15:56
• Contact should be used if the two parts have different material properties! Jun 30, 2021 at 22:14
• @JonathanRSwift In my experience even when the material have different properties you can merge the nodes and select the elements and give them different properties.
– NMech
Jul 1, 2021 at 3:16
• @JonathanRSwift, why should the contact be used for two parts having different material properties? How is that gonna be different than merging the faces of contact. Jul 1, 2021 at 8:54
• @RameezUlHaq Jonathan IMHO is right, in that you can do it if initially they don't touch and you need to check for penetration or other. The most common case is that the materials will have different properties.
– NMech
Jul 1, 2021 at 10:46