1.3.9. Contacts

Contacts are used for solving the mechanics of two bodies are brought into contact at one or more points. The outputs from a contact analysis are deformations and stresses at the contact surfaces. Note that stresses at the surface can be either, acting normal to the contacting bodies’ surfaces and frictional stresses acting tangentially between the surfaces. Typical applications of contacts are two bodies in collision.

The procedure to simulate contact mechanics between two bodies is by comparing their position between them, being one of the bodies defined as Master geometry and the other as Slave geometry. Master geometry acts as the base body at which positions are referred, and Slave geometry the body at which its positions are calculated. More precisely, RamSeries projects the nodes of the slave geometry onto the master geometry, obtaining a position vector. The modulus of the position vector is the distance between the geometries, and the direction of the vector points if the slave geometry is inside or outside of the master geometry. The contact is obtained by comparing the direction of the vector position and the normals of the master geometry. Therefore, it is necessary that master surfaces’s normals point to slave surfaces for detecting contacts correctly.

1.3.9.1. Contacts configuration

Structural contacts are only available once either Dynamics or Incremental loads analysis options is selected, and Non-linear geometry or non-linear boundary conditions is activated.

Requirements:

  • General data > Analysis > Analysis type > Incremental load analysis

    or

  • General data > Analysis > Analysis type > Dynamic analysis

And
  • General data > Analysis > Geometric constitutive model > Non-linear geometry

    or

  • General data > Analysis > Boundary conditions > Non-linear boundary conditions

Advanced data of contacts can be found in section General data > Advanced > Contacts data.

1.3.9.2. Types of contact

  • Sticking contact: tangential relative motion between bodies is not allowed. In this case, the geometries do not slide one each other, although they can roll between them.

  • Frictionless contact: tangential relative motion between bodies is allowed without friction. In this case, the geometries can slide between them, without no friction force. Take into account that RamSeries do not allow to define a relative friction between the two bodies.

1.3.9.3. Master-Slave contacts between geometries

Ramseries can define two types of relations between geometries:

  • Master-Slave contacts Surf-Line: it is a contact between nodes and elements on different surfaces and line entities that can get in contact during the analysis. It is necessary to provide separately the entities (surfaces and lines) pertaining to either the Master and the Slave groups. This option is used to perform a Contact Analysis among a surface and a line ( Shell - Beam, Solid - Beam). The contact domain must be defined selecting the surface and the beam where there will be contact. It is possible to choose shell’s surfaces or solid’s surfaces. The surface must be assigned as master and the beam must be assigned as slave.

  • Master-Slave contacts Surf-Surf: it is a contact between nodes and elements on different surfaces that can contact during the analysis. It is necessary to provide separately the Master and the Slave surfaces. This option is used to perform a Contact Analysis among two surfaces (Shell - Shell, Solid - Solid or Shell - Solid). One surface will be assigned as master and the other as slave. It is recommended to give different names to the master group and slave group.

It is necessary that master surfaces’s normals point to slave geometries for detecting contacts correctly. If master surface and slave geometry are coincident or separated by some distance, and Normals sense swap option is activated (see advanced contacts data), Ramseries swaps master surfaces’s normals.

1.3.9.3.1. Master-Slave contacts procedure

For contacts analysis, lines and surfaces have to be assigned, therefore, the Groups window can be used to draw master and slave groups more easily.

FIGURE

There is not a fix rule about which surface must be master and which must be slave. However, it is advisable to choose as master surface the more rigid. On the other hand, stresses in slave surface will be more accurate than in master slave, so choosing as slave the surface where a better accuracy is requested, is a good option.

Next, three examples of master and slave surfaces are shown.

FIGURE

FIGURE SURFACES INTERSECTING

This case is not allowed because slave nodes penetrate into master elements.

Fix constraint in slave nodes

If a slave node contacts with a master element but this slave node has defined a fix constraint, the contact will not be created. Therefore, it is advisable to define elastic constraints instead of fix constraints in slave nodes. If the stiffness value of the elastic constraint is \(10^3\) times greater than the material’s stiffness, it will work than a fix constraint.

1.3.9.4. Recommendations for contacts

Defining the optimal definition of which body is the master and which is slave is not trivial. RamSeries gives the following right-hand rules to achieve an optimal definition of them:

  • Analytical rigid surfaces and rigid-element-based surfaces must always be the master surface.

  • A node-based surface can act only as a slave surface and always uses node-to-surface contact.

  • Slave surfaces must always be attached to deformable bodies or deformable bodies defined as rigid.

  • Both surfaces in a contact pair cannot be rigid surfaces with the exception of deformable surfaces defined as rigid (see Rigid body definition).

When both surfaces in a contact pair are element-based and attached to either deformable bodies or deformable bodies defined as rigid, you have to choose which surface will be the slave surface and which will be the master surface. This choice is particularly important for node-to-surface contact. Generally, if a smaller surface contacts a larger surface, it is best to choose the smaller surface as the slave surface. If that distinction cannot be made, the master surface should be chosen as the surface of the stiffer body or as the surface with the coarser mesh if the two surfaces are on structures with comparable stiffness. The stiffness of the structure and not just the material should be considered when choosing the master and slave surface. For example, a thin sheet of metal may be less stiff than a larger block of rubber even though the steel has a larger modulus than the rubber material. If the stiffness and mesh density are the same on both surfaces, the preferred choice is not always obvious.

The choice of master and slave roles typically has much less effect on the results with a surface-to-surface contact formulation than with a node-to-surface contact formulation. However, the assignment of master and slave roles can have a significant effect on performance with surface-to-surface contact if the two surfaces have dissimilar mesh refinement; the solution can become quite expensive if the slave surface is much coarser than the master surface.

For improving stress accuracy in contacts elements, it is recommended to use smaller elements in slave surface than master surface, because doing so, every master element will have more than slave node in contact.

Moreover, master elements normals must be coherent. They can be swapped manually using tools in windows menu Utilities > Swap normals. If analysis is run and master elements normals are not coherent, a warning message appears and contacts may not be detected correctly.

1.3.9.5. Process information

It is possible to know valuable information about contacts during calculation process.

For viewing process info during the calculation, the user has to select in the windows menu Calculate:

Calculate > View process.

Following, an extract of process info is shown:

FIGURE PROCESS INFO

At the beginning of convergence loop, the number of contacts added, edited or removed are shown. A contact is edited when the slave node changes its master element. A contact is removed when is not inside any master element, and a contact is disconnected when traction force is positive.

When Convergence factor is minor than convergence tolerance (See Non-Linear Analysis Data), all contacts with traction force are disconnected and all contacts are added when slave nodes penetrates a master element. Only if the highest penetration is higher than Maximum admissible penetration (See Advanced contacts data), new contacts will be created. The process info shows the number of contacts disconnected or added, as well as the highest penetration and traction force.