Since there is not a single set of values for the contact parameters that will provide an accurate solution for all models, the user must have a good understanding of the functionality of each parameter to determine a proper set of values for the given problem.
You can customize the contact parameters for each pair individually. Select the contact pair in the tree-view, right-click, and choose Settings. Or from the main MES: Surface to Surface Contact dialog (the spreadsheet), click in the Parameters column for that pair. Either method will access the Controls and Parameters for Contact Pair dialog. (Any pair whose parameters have been changed will show Custom in the Parameters column. Otherwise, the Parameters column will show Default.)
Controls and Parameters for Contact Pair Dialog Box
In the Contact problem type drop-down box, select the general type of contact that may occur at any time in the analysis. This selection controls the behavior of the automatic contact stiffness calculation. In some cases, the choice is obvious. In other cases, it may not be clear whether the situation is low speed or high speed. The difference is a matter of solving efficiency. Either selection should give accurate results. The options available are as follows:
In the Contact method drop-down box, select the method of contact that will be used for the selected contact pair. The options are as follows:
Surface-to-surface contact uses the classical friction algorithm: Coulomb friction. In the basic Coulomb friction model, two contact surfaces can resist shear stress up to a certain amount, determined from friction coefficients and normal contact pressure, before they experience relative sliding motion. The Coulomb friction model is defined as τ = μP, where μ is the friction coefficient and P is the normal contact pressure. Surface-to-surface contact provides the ability for defining a static μ s and dynamic μ d friction coefficients. Ideally, once the shear stress exceeds μ s P obtained from the static friction coefficient, the two surfaces start sliding and the shear stress becomes μ d P where the dynamic friction coefficient is used.
The ideal situation (no motion until the shear stress exceeds the static value) is difficult to achieve numerically, as described in Figure 1. Thus, a Tangential stiffness ratio is defined when using friction. The Tangential stiffness ratio is the ratio of the tangential contact stiffness (K t ) to contact stiffness in the normal direction (K, set under the Advanced button. See the page Surface-to-Surface Contact Advanced Controls.) A larger tangential stiffness results in better accuracy but at the expense of requiring more iterations. A smaller tangential stiffness provides better convergence (shorter runtime) with the possibility of reduced accuracy. The recommend value is in the range of 0.01 to 1. As the static friction coefficient becomes larger, a smaller tangential stiffness ratio is recommended.
(a) Model and free body diagram of top block. The applied force F is ramped-up over time until it exceeds the static friction force. |
(b) Plot of friction force Ff versus time (blue curve) and displacement versus time (red curve). Until the friction force Ff reaches the limit of static friction (Fs = μ s xN), there is no motion of the part. Once the static friction is exceeded, the friction force Ff drops from Fs to Fd (= μ d xN), and the part begins to accelerate. |
(c) The graphs in (b) are combined to show the friction force versus displacement. Ideally, there is zero displacement until the friction force reaches Fs, and then sliding begins. Numerically, this is difficult to achieve since any displacement other than zero implies a friction force of Fd. |
(d) To avoid the numerical instability in (c), the part is allowed to displace a small amount (Δs) while the friction force is less than the static limit (Fs). The amount of motion is controlled by the tangential stiffness (Kt = Fs/Δs). Smaller values of Kt allow the part to displace a larger amount before it starts to slide and are easier to converge, while larger values of Kt (steeper slope) allow less displacement before sliding and are more difficult to converge. The tangential stiffness is calculated by Tangential Stiffness ratio x stiffness in the normal direction. |
Figure 1: Tangential Stiffness for Friction |
In frictionless contact, the element stiffness matrices are symmetric. Once friction is involved, the matrix becomes unsymmetric. Using an unsymmetric solver is computationally more expensive than using a symmetric solver. For the sake of efficiency, MES enforces symmetry of element stiffness so that the symmetric solver can be used for frictional contact.
When frictional contact is considered in the analysis, the solution process becomes highly vulnerable from the sliding process. Once the contact surfaces start sliding, the equilibrium state before the sliding is no longer valid and the processor must try to find a new equilibrium state during the solution process. Furthermore, poor convergence is possible due to the symmetry approximation.
In the Contact type drop-down box, select the type of contact that will be used for the selected contact pair.
It is somewhat misleading to describe contact as surface-to-surface. What happens in reality is that the nodes on the second part/surface are connected to the surface of the first part/surface, thereby preventing the nodes from passing through the surface. Optionally, the reverse is true: the nodes on the first part/surface can be connected to the surface of the second part/surface. In no case is the surface of one part prevented from passing through the surface of the second part.
The options for the Contact type are as follows:
The point to surface method is faster, but you must realize that points on the first part could pass through surfaces on the second part; this contact is not detected. Surface-to-surface contact provides better contact detection, but at the expense of generating more contact elements. It indicates that the nodes on the target part/surface cannot pass through the surface defined by the nodes on the master part/surface (just like point-to-surface contact), and the nodes on the master part/surface cannot pass through the surface on the target part/surface. See Figure 2.
Figure 2: Comparison of Contact Types | ||
Three models of dropping a block onto a wedge. Each model uses a different type of contact. |
||
Surface-to-Surface Contact
|
Point-to-Surface Contact (points on the wedge)
|
Point-to-Surface Contact (points on the block)
|
Time = 0 |
||
Equivalent System. The surfaces of both parts are connected through contact. |
Equivalent System. Only the nodes on the wedge exist as far as contact is concerned. |
Equivalent System. Only the nodes on the block exist as far as contact is concerned. |
Time = 0.024 seconds |
||
The surface of the block contacts the surface/node of the wedge. |
The surface of the block contacts the node of the wedge. |
Contact does not occur until the nodes of the block contact the surface of the wedge. The wedge passes through the block. |
Time = 0.12 seconds |
||
The surface of the block slides down the surface of the wedge. |
The block passes in between the nodes of the wedge. |
The block is stuck on the wedge. |
Time = 0.15 seconds | ||
|
|
|
If one of the surfaces in the contact pair is on a part of shell elements, select the side of the shells that will be involved in the contact pair. (The top side is the side opposite of the element normal point defined in the Element Definition dialog.) Contact only occurs when the two bodies approach from the designated direction. For example, it you indicate that contact occurs with the top of the shell, then contact occurs with the second body is moving in the direction from the top toward the bottom. No contact is made if the second body is moving from the bottom toward the top. Another way to visualize this is as follows: the second object makes contact with the chosen side (either the top or bottom) only when it is approaching from outside the shell. It will not detect contact if it passes through the shell to reach the chosen side. See Figure 3.
![]() |
Figure 3: Two Blocks Defined to Contact the Top of the Shell |
Contact with line elements (beams, trusses) can only detect contact when the nodes on the line element are contacting the surface of the first part/surface. In addition, specify whether you want all the nodes on the line elements to be considered for contact (All on line element) of only the end nodes (Only those at ends of elements) in the Nodes Making Contact drop-down box. For example, the bristles on a toothbrush made from beam elements only need to have contact from the nodes on the end of the bristles with the teeth; Only those at ends of elements would be used. But a safety net made from truss elements needs to detect contact between all nodes on the net and the falling object; All on line element would be used.