How does DC in *CONTACT influence the way the friction coefficient transitions between static and dynamic values?
Tagged: lsdyna, LSDYNA Suite, R12, structuralmechanics


January 25, 2023 at 7:34 amFAQParticipant
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 LSDYNA 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 nonzero). When this interaction is treated numerically with a Coulomblike 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.

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