Contact Stiffness and Damping |
 
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Contact Stiffness and Damping |
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A clashing connection can be considered to operate in a similar fashion to a spring element and (optionally) a damper element in parallel. Specifically, the global stiffness matrix is augmented at appropriate locations by the contact stiffness that you specify, and this tends to oppose the relative penetration of the lines. Once the lines have separated, any stiffness contributions are removed. Similarly, the global load array is augmented at appropriate locations by a damping force term, which is proportional to the relative velocity of the approaching lines. As soon as the lines begin to separate, as the relative velocity changes direction, any damping force terms are switched off.
It may be difficult to quantify what value of contact stiffness represents the physical reality. Generally speaking, the higher the stiffness value chosen, the greater the impact will be, and the smaller the time step that will be required for a robust solution. So you should avoid using excessively high stiffness values, which may lead to convergence difficulties. Conversely, the stiffness value effectively determines the maximum clashing force that may be modelled – the maximum reaction force is equal to the contact stiffness times the average contact radius of the elements which interact with each other. In other words, the maximum relative penetration of the contact elements should not be allowed to exceed the average contact radius. If this occurs, the lines will simply pass though each other. Although in practice this situation would be quite rare, to prevent the situation from occurring, you should ensure that the contact stiffness is sufficiently high. While it is difficult to provide meaningful guidance as to what value might represent a “reasonable” level for all cases, a value of approximately 100 kN/m (or equivalent in Imperial units) is tentatively suggested as a starting point if you are unsure.
Clashing is generally suitable for modelling intermittent contact between adjacent lines. If two lines come into contact, and tend to remain in contact, the Pipe-on-Pipe feature may be more suitable. There is a theoretical limit on the maximum contact force, which is equal to the contact stiffness multiplied by the average contact radius of the contacting elements. Situations where significant contact forces are maintained, for example during a static analysis subject to Current Loading, are better suited to the pipe-on-pipe modelling facility.
Specification of damping is optional, but some level of damping is generally advisable in dynamic analyses to help dissipate high frequency noise.
•*CLASHING is used to specify regions where clashing may occur, and suitable contact stiffness and damping values.