|
|||||
 Contact button
Create a contact pair
The analysis of contact behavior is complex because of the requirement to accurately track the motion of multiple geometric bodies, and the motion due to the interaction of these bodies after contact occurs. This includes representing the friction between surfaces and heat transfer between the bodies if required.
In the common physical sense, surfaces that are in contact have these characteristics:
Contact option appearance is ahead:
In the contacted drop down, the element with an higher number of nodes should be selected (between the two corresponding elements). Another recommendations may be applied, just as the contacted element is the more rigid element.
The last statement may be established due to the fact that, in FEM, it is necessary the two elements not to penetrate each other. Thus, user would have more control on the structure. By last, the contacted element could be assigned to a convex element, as a result of managing accurately the distance between the nodes of both of the elements.
In contacting drop down, the element which does not take place as contacted, is selected in the contacting drop down. Contacted element is the one which is limited.
Two passes: this option may be activated or deactivated in the contact properties bar. If this tool is activated, CivilFEM will execute 2 passes when the contact process is done. The first pass consist of evaluate the distance between the contacting nodes to the contacted ones. The second pass consist of evaluate the distance between the contacted and the contacting nodes. If the two passes option is not active, only the first pass is carried out.
Type: this property make available the possibility of defining a: structural, thermal or even seepage contact. This depends on the type of analysis the user is carrying out. Contacts are assigned with the purpose of transmitting the section forces and moments along the elements which contact.
When defining a thermal or seepage analysis, different options are visualized on the type drop down.
Thermal analysis:
Seepage analysis:
A structural contact may be assigned not only in a thermal but also in a seepage analysis. Nevertheless, in a structural analysis, only the structural contact is available. This is because both thermal and seepage boundary conditions are only possible to be defined in their respective analysis.
Interaction properties:
Contacts type:
Regarding the contact types, is principal to establish that both in thermal and seepage cases, only the glue contact option would be available.
1. Glue contact is actually a glue option, which imposes that there is NO relative tangential motion. A novel application of contact is to join two dissimilar meshes. In such a case, by specifying that the glue motion is activated, the constraint equations are automatically written between the two meshes. Nodes in contact via the glue option are not allowed to separate.
A non-penetration constraint is applied through a tying or boundary condition on the displacement components normal to the contact surfaces.
A glue condition suppresses all relative motions between bodies through tyings or boundary conditions applying them to all displacement degrees of freedom of the nodes in contact. For elements with nodes that also have rotational degrees of freedom, the rotations may be additionally constrained to provide a moment carrying glue capability.
Tolerance option is designed by a value which is measured in meters. This value indicate that not only the nodes from the contact surface are going to be taken into account but also the nodes which take place inside the applied distance. That is, the 0 level is located in the contact surface while the tolerance level corresponds to the fixed value in the tolerance blank box (upper level in which CivilFEM will look for nodes).
In regard to Bias factor parameter, the outside contact area is (1. - bias) times the contact tolerance on the inside contact area (1. + bias) times the contact tolerance. This parameter is used in case of both surfaces have a smaller distance on the outside and a larger distance on the inside. This avoids the close nodes from coming into contact and separating again and is accomplished by entering a bias factor. The bias factor should be given a value between 0.0 and 0.99.
On the other hand, Glue contact may be defined in thermal and seepage analysis. The interaction properties bar would be very similar to the structural Glue contact one, however, it would be submitted to change in case of carrying out whether a Thermal/Structural or a Seepage/Structural coupled analysis.
 
As we see, the coupled analysis shapes a different interaction properties window. This is because the structural analysis is solved along with any other analysis type so contact conditions may be defined in both sides of the model. As a result of this, section forces, moments, temperatures and seepages would be taken into account between elements in contact.
2. Frictional or Touching contact is a source of nonlinearity because both the normal and tangential stiffness at contact surfaces change significantly with changing contact status. The numerical objective is to detect the motion of the bodies, apply a constraint to avoid penetration, and apply appropriate boundary conditions to simulate the frictional behavior.
CivilFEM tracks the motion of all contact bodies with respect to each other at all times. When two bodies come in contact, software automatically determines the area of contact and calculates the contact normal and friction stresses.
Tolerance and bias factor concepts have been explained in the glue contact theory, so the lesson will focus on the contact properties options of the touching contact.
Relative stress: the user is able to fix the stress from the two elements leave the contact position. Relative stress is related to the traction forces.
The "Limit friction stress" property allows the user to establish a maximum value for the friction stress
 .
This is based on the assumption that the extrapolated and averaged shear (friction) stress in a node is proportional to the applied shear (friction) force. If the shear stress reaches the limit value, then the applied friction force is reduced, so that the maximum shear stress is given by min
 .
In case of activating this property, the user will be able to provide a new value for the friction stress limit. Nevertheless, this value will be 1.0E20 Pa by default.
3. Breaking contact has the capability to break up the glued connection using stress criterion. When a node is released due to breaking glued, its status is changed from being glued to standard contact, permitting separation and friction (touching contact). The contact stresses are calculated using extrapolated stresses for solid elements and as contact force divided by equivalent area for shell elements. Contact break index is available as a CivilFEM result to evaluate the breaking criterion.
Here,
 is the contact normal stress (used in tension only),  the contact tangential stress, and , , , m, and n are user-defined parameters.
The contact stresses are calculated using extrapolated stresses for solid elements and as contact force divided by equivalent area for shell elements.
Touching contact properties have been properly defined in the (2) Frictional or Touching contact paragraph, in the top part of this page.
As an introduction in the (4) Advanced contact , say that most of civil engineering materials follows the same behavior: the fail or slippage happens when submitted to a limit shear stress called cohesion. Moreover, these materials are pressure sensitive: the limit shear stress to produce slippage is pressure sensitive resulting in stronger response in compression than in tension.
Taking into account these properties, slippage will happen when shear stress on contact surface fulfills the following equation:
 
Advanced contact is a particularization of breaking contact. The criterion to release the glue contact is the Mohr-Coulomb criterion with a static friction coefficient and
 in compression. When glue contact is released a touching contact is applied with a dynamic friction coefficient.
Cohesion: soil quality by means of its particles keep contact. Cohesion value vary depending on the soil type. From clays, with an high cohesion, to sands, with C=0.
Static μ: this parameter requires the static friction coefficient value.
Touching contact properties have been properly defined in the (2) Friction or Touching contact paragraph, in the top part of this page.
Once slippage has been produce, the contact surface will have a friction behavior. This friction will be lower than the existing before slippage due to a lower contact between material surfaces.
5. Cohesive contact allows to define a glue contact with a relative motion between contact bodies. When defining the contact (normal and tangential) stiffness, one can either define a constant value or define the contact stress versus relative displacement (in this case the gradient of the table is internally calculated and used for the contact stiffness). Also user can select the automatic mode in any direction and glue condition is applied.
This kind of contact can be useful to simulate the bond-slip behaviour.
The cohesive contact capability is only available with segment-to-segment contact procedure.
Other concepts related to contacts are the "Node to segment contact" and the "Segment to segment contact", from the contact method global solution control. Nevertheless, those ones will be established in a contact toolbar submenu.
In other instance, in the contact option it is available the multiple contact execution by which the user is allowed to add more than a contacted element or more than a contacting element, in the same contact entity.
The previous example two contacts are clearly defined. The first one is based on one supporter shell and four sustained beams while the second one is composed by a supporter beam and the previous mentioned sustained beams.
It would be available, as well, to select more than one contacted element in order to contact to different contacting elements. The same example would be suitable to show it.
Both of the contacted elements are supporting the shame four beams, therefore, only one contact entity might be defined to solve the situation.
|
