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Contact Modelling

Theory

An important aspect of the contact algorithm is how Flexcom handles the interaction between the structure and the guide surfaces. Each node that is potentially in contact with a guide surface may be considered to be connected to the surface by means of a non-linear spring which has an orientation normal to the surface. Theoretically, each of these springs should have the force-deflection characteristics shown in the first figure below, whereby the force in the spring is zero until the node makes contact with the surface, at which point the linear spring stiffness is activated. While this formulation is theoretically correct, it is not especially robust, so Flexcom can use a modified spring force-deflection curve shown in the second figure below instead. Adopting a non-linear approach tends to lessen the effects of instantaneous impact, reduce high frequency noise, and consequently allows the solution to proceed at larger time step increments.

Theoretical Contact Model

 

 

Non-Linear Contact Model

In the second figure above, rather than the contact spring stiffness being activated suddenly when contact occurs, the force in the spring is gradually ‘ramped up’. This means that there is some force in the spring before a node penetrates the guide surface, but the magnitude is relatively small. Overall, the contact algorithm provides a robust and accurate engineering approximation. The distance over which the force in the spring is ramped up is symmetric about the Force axis. In other words, the distance from the start of the ramp to the Force axis is the same as the distance from the Force axis to where the ramp meets the linear surface stiffness curve. In Flexcom, these equal distances define an input called the Contact Ramp. This input assumes a value of zero by default, meaning that the spring force-deflection characteristics are as shown in the first figure above. It is difficult to provide meaningful guidance as to what Contact Ramp might represent a “reasonable” value, but a distance of 0.1m (or equivalent in Imperial units) is tentatively suggested as a starting point if you are unsure.

It is also worth noting that the curve used to gradually ramp up the force in the contact spring is governed by a power law equation and the associated ‘power’ value can be specified through the Exponent input in the contact modelling settings. The default value for this input is 1.5, which probably represents an optimum value. However for high stiffness supports you may wish to reduce the Contact Power to a smaller value (e.g. 1.0). Reducing the power has the effect of reducing curvature in the ramp region.

Identifying Contact Nodes

Switching on node numbers in the Model View will allow you to identify Contact Nodes.

Relevant Keywords

•*CONTACT MODELLING is used to specify specialised parameters relating to guide surface contact modelling.