Is the Maximum Stress Servo Velocity a Key Parameter in DEM Modeling?

The stress servo mechanism (SSM) is frequently employed in DEM simulations, but the importance of the parameters in the SSM has been seriously underestimated, and it remains unknown whether this affects the reproducibility of numerical simulations. Therefore, in this note, we focus on explaining the stress servo mechanism in numerical simulations by referring to the existing literature. A series of drained biaxial compression simulations was conducted to investigate how the maximum stress servo velocity (umax), a key parameter within the SSM, influences both the macroscopic mechanical response of the specimen and the evolution of the measured confining pressure. It is found that when the umax is large (≥ 0.05 m/s), the simulation results are consistent with the existing studies and are able to reproduce the stress-strain behaviour of the material. Conversely, if umax is set below a critical threshold (e.g., < 0.05 m/s), the servo mechanism fails to function properly. This inadequacy introduces significant numerical artifacts, distorting the simulated response and ultimately misrepresenting the true mechanical behavior of the material. We therefore advocate for the explicit recognition and detailed reporting of umax as a key parameter in all DEM studies utilizing stress servo control. This study helps to improve the reliability of DEM results and provides a reference for the improvement of numerical simulation methods.