 ## General

#### How does DC in *CONTACT influence the way the friction coefficient transitions between static and dynamic values?

• FAQ
Participant

Synopsis: The dynamic friction coefficient has no effect unless the decay coefficient DC is also defined. DC serves to smoothly transition from FS to FD over a range of velocities. If DC=0, your net friction coefficient will simply be FS. DC governs the transition from the static friction coefficient to the dynamic friction coefficient, as a function of relative velocity. As points of reference, if … DC*|Vrel| = 0.7, then the friction coefficient is halfway between FS and FD DC*|Vrel| = 4, then the friction coefficient is essentially equal to FD. Full explanation: VREL is the relative velocity of the contacting bodies at the contact point. It is computed by LS-DYNA and has units of velocity. In order for the exponent to be dimensionless, the parameter DC also has units – the inverse of velocity. Contact interaction can involve sticking (when VREL=0) or sliding (when VREL is non-zero). When this interaction is treated numerically with a Coulomb-like friction law we must be able to smoothly transition between the static and dynamic friction coefficients – the exponential involving DC and VREL accommodates this. Please look at the relation for MU and note that it yields FS when VREL=0 – this correctly corresponds to the sticking case. Now, when DC*abs(VREL) reaches a value of about 4.0 the exponential drives the second term down so that MU is within just a couple percent of FD – this corresponds to sliding. When DC*abs(VREL) is between these values we are in the transition state and MU is a combination of FS and FD. There is no “correct” value for DC; its value is the responsibility of the analyst, who must ask himself “above what relative velocity do I consider something to be sliding?”. Using this critical value of VREL the analyst then selects a value of DC, using rule of thumb stated above, such that, DC*abs(VREL) = 4.0. In this way, when VREL attains the critical value we have MU nearly equal to FD. 