The Effect of Friction on Simulated Dynamic Fracturing of Rocks

During underground nuclear tests, rocks may fail by plastic yielding, which limits shear strength, or by tensile fracture, wherever maximum principal stress exceeds tensile strength. A third mode of failure exists due to friction along closed fractured surfaces. There, friction affects slipping and can thus limit stress. In this paper, we study the effect of friction on the simulated dynamic response of rocks to underground nuclear explosions. The coefficient of friction is the ratio of total shear stress applied to a closed fracture surface to normal applied compressive total stress. At low coefficients of friction, the evolving stress field tends to be weakened by frictional slip, which also eases closing of fractures. At high coefficients of friction, the stress field tends to be strengthened, where fractures have closed, but remains weak, where fractures are left open. INTRODUCTION The goal of this work is to evaluate the role of friction along closed rock fractures on simulated containment of underground nuclear explosions. An earlier report 1 provides a complete description of the theoretical formulation and numerical implementation of the material model. This analysis will focus on the Paleozoic layer, just below the cavity, because the effect of friction is most prominent in this region,which shows considerable cracking. After summarizing the material model, the effect of friction on the rock response is discussed, for the simulation, which was performed using the KDYNA code.2 MATERIAL MODEL