Does anyone know how to prevent VFRICTION subroutines from erroneously dropping CSHEARF to zero when coming to a stop under SMOOTH amplitude?

The Problem | I’ve found that tangential PENALTY contact differs for the 1) VFRICTION and 2) VFRIC subroutines vs. the 3) VFRIC_COEF subroutine and 4) built-in PENALTY contact. All four exhibit an elastic stick regime and nearly identical CSHEARF forces after slipping commences, as expected. However, at the end of a SMOOTH displacement amplitude, both (1) and (2) drop CSHEARF to zero as the slip velocity slows to zero, which seems incorrect, while (3) and (4) maintain the slipping CSHEARF through the end of each amplitude, as expected. For my specific application, the frictional shear force immediately at the end of a SMOOTH displacement amplitude is crucial, and I require features only provided by VFRICTION.

Things I’ve tried | I have separately tried setting *CONTACT DAMPING to zero, increasing the penalty scale factor using *CONTACT CONTROLS ASSIGNMENT, applying the SMOOTH amplitude as a velocity, and applying a strictly sinusoidal amplitude, all to no avail. RAMP loading avoids the issue, but obviously produces bad results.

Example | Attached are two example .inp files (vfriction_coul_inp and vfric_coef_coul_inp) that replicate the erroneous behavior with a SMOOTH displacement amplitude applied to a simple block and constant friction coefficient (μ=0.15). These may be run with vfriction_coul.f and vfric_coef_coul.f from “4.1.30 VFRIC, VFRIC_COEF, and VFRICTION” of the Abaqus Verification Guide.

Here are some key results (Node 111):