Mechanisms of shock-induced dynamic friction

Abstract

The mechanism of shock-induced dynamic friction has been explored through an integrated programme of experiments and numerical simulations. A novel experimental technique has been developed for observing the sub-surface deformation in aluminium when sliding against a steel anvil at high velocity and pressure. The experimental observations suggest that slight differences in conditions at the interface between the metals affect frictional behaviour even at the very high-velocity, high-pressure regime studied here. However, a clear finding from the experimental work is the presence of two distinct modes of deformation termed deep and shallow. The deep deformation is observed in a region of the aluminium specimen where the interfacial velocity is relatively low and the shallow deformation is observed in a region where the interfacial velocity is higher. A 1D numerical treatment is presented which predicts the existence of two mechanisms for dynamic friction termed ‘asymptotic melting’ and ‘slide-then-lock’. In both modes there is a warm-up phase in which the interface temperature is increased by frictional heating. For high initial sliding velocity, this is followed by the onset of the asymptotic melting state, in which the temperature is almost constant and melting is approached asymptotically. This mechanism produces low late-time frictional stress and shallow deformation. For lower initial sliding velocity, the warm-up terminates in a violent work hardening event that locks the interface and launches a strong plastic shear wave into the weaker material. This slide-then-lock mechanism is characterized by sustained high frictional stress and deep plastic deformation. These predicted mechanisms offer a plausible and consistent explanation for the abrupt transitions in the depth of sub-surface deformation observed in the experiments. A key conclusion arising from the current work is that the frictional stress does not vary smoothly with pressure or sliding velocity. Instead the pressure and sliding velocity determine whether the impulse will be very high or very low. The next generation of friction models for hydrocodes will need to account for these factors.

Modifications and Applications of the HERMES model: June - October 2010

“…For these simulations, no limit was placed on the retarding stress, as per the analyses of [9]. This is in contrast to the analyses of Ball [8] where the limit stress would have been set to 115 MPa, the shear stress at yield in pure shear. In our simulations, then, the disk sticks over the inner two-thirds of its surface.…”

“…Some researchers [8] have advocated limiting the frictional resistance on the surface to be the shear strength of the weaker material, in contrast to the assumptions of gas dynamics. Instead of severe localization of strain near the interface, this technique essentially replaces a boundary layer of extreme distortion with additional energy being p 12 dissipated at the interface.…”

A new model for friction under shock conditions

“…The thermal conduction cannot take place neither and this results in the existence of a very thin boundary layer (few μm) heated by the friction of the interface. As shown by Winter et al [2006], the two competing phenomena that govern the behavior of the surface are the thermal softening, due to the strong heating of the boundary layer, and the hardening of the surface governed by the pressure, the strain and the strain rate. It is important to note that the heat generated can be such that the melt temperature is reached.…”

“…In order to obtain high contact pressure (> 5 GPa), new experiments are necessary, such as those of Winter et al [2006] or Juanicotena [2006]. This article, introduces a new model for friction between metallic materials under these very specific shock conditions.…”

Finite size effects at high speed frictional interfaces

“…Usually experiments are integral in the sense that time dependent material response is measured far from the interface and a model for the local tangential force is calibrated to the observed non-linear response [1][2]. Post experimental analysis depending upon the strength of loading usually reveals severe dynamic deformation characterized by very large plastic strains, subgrain micro/nano structure formation, material mixing and ultimate melting at high velocities [3][4][5][6].…”