35.0 - Create/delete Contacts
Contacts can be formed between two pieces of different bodies, and the pieces involved in a contact are called the contact ends. Prior to the first cycle in a series of cycles, all contacts are created between pieces (during the initialization of data structures as shown here). This is necessary because without contacts, one cannot calculate the effective stiffness of the system, meaning that a stable timestep cannot be computed.
Contact updating is triggered for pieces that were deemed dirty by their cumulative motion exceeding
their tolerance extent criteria. Such pieces have been remapped in the cell space during cycle point
30.0 (see here for details). Contacts are created when the piece
conformal surfaces overlap and are deleted when the conformal surfaces no
longer overlap. Contact updating only occurs for dirty pieces. Note that one can ensure contacts
are never deleted by setting the state of the contact to always active with the
contact.activate
FISH function.
With the timestep determination mode set to either automatic or scaling (see the
model mechanical timestep
command), the timestep is limited so that any piece may, at most, translate
\(\varepsilon\) from its last position. Given the enforcement of this constraint, one can guarantee
that contacts are created prior to the cycle where forces/moments exist. This is pictorially shown
below. Suppose that two identical balls (identical geometries and \(\varepsilon\) values) exist in
such a configuration that their extents lie infinitesimally close (by a distance of \(\vartheta\)) at
the end of cycle \(N\). Additionally, suppose that both balls have velocities \(\left | \mathbf{v}
\right | = (\varepsilon-\vartheta) / \Delta t\) directed toward one another, where \(\Delta t\) is the
timestep during cycles \(N+1\) and \(N+2\). At the end of cycle \(N+1\), the extents remain unchanged
because neither ball has displaced \(\varepsilon\) from its cycle \(N\) position. The ball locations are
updated during cycle \(N+2\) such that the balls are \(\overline{r}_i + \vartheta\) from one another.
Subsequently, the piece extents are updated and the contact is created prior to the cycle when
forces/moments could potentially develop.
Upon creation, a contact model is assigned to each contact based on the current configuration of the Contact Model Assignment Table (CMAT). As described in the “Model Components” section, the contact geometry is delineated upon creation.
– Next Cycle Point: Force-displacement Law –
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