Contact Detection Methods

After understanding the types of formulations used for contacts in Ansys Mechanical, let us have a deeper insight into the detection of contacts. The contacts are detected using different methods. The two major types are by using integration points and nodal points. Both the methods have their own advantages and disadvantages. So, as an optimal solution, Ansys recently produced an advanced method of Nodal projection method as well. Let us discuss in detail. 

Gauss integration points detection.

At time of contact creation, the user designates two surfaces to be in contact to “contact” and “target” surfaces. The contact surfaces are restrained to penetrate the target surface whereas target surface can penetrate the contact surface as well. The detection points are always present in contact side, whatever the detection type is. So, here in this method, the contact detection points are the Gaussian integration points which are present inside the elements at contact surfaces. These integration points cannot penetrate the target surface. This detection method is the default detection method for Pure Penalty and Augmented Lagrange methods, as these methods allow small amount of penetration. Since the integration points are located inside the element, there will be a small amount of penetration of contact element nodes as well. As there are more integration points per element than nodes, integration point detection method is more effective than nodal detection. Also, one needs to note that the method is not effective for sharp cornered contacts.

 

Nodal point detection.

Contact detection is carried out through the nodes of the elements at surfaces. Thus, fewer in number, detection points are far less as compared to integration points. This detection method is the default type for MPC and Normal Lagrange types of contacts, since this does not allow much penetration as compared to integration points. This type of contact detection provides smoother pressure plots but takes larger time to converge. There are two types in nodal point detection – Nodal – normal from contact and Nodal – normal to target. The only difference in both the types is that contact normal is perpendicular to contact surface in earlier one and contact normal is perpendicular to target surface in later. This usually requires extra calculations to determine correct “Normal” direction.  

Nodal Projection from contact

The recently developed contact detection method by Ansys uses overlapping region of the contact and target surfaces as contact detection location. So, instead of calculating penetration/gap at each integration point or node, it calculates an average value for each overlapping area. Consider the green colored mesh to be of contact area and pink colored mesh to be of surface area. If we put them on top of each other, each unique area becomes a surface projected contact area. 

In most cases, this method provides a more accurate calculation of contact stresses and traction for the underlying elements compared with other settings. This method satisfies moment equilibrium as well if there is an offset between contact and target surfaces with friction. But the method is computationally more expensive because we usually have more contact areas than contact points unlike other methods. The penetration and gap calculated is also an average value, so the contact information calculated at the nodes of corners or edges may penetrate target elements. This is similar limitation observed in integration point detection method too.