Cleveland, OH 44106-7222

Abstract

Plate impact pressure-shear friction experiments have been conducted to study time-resolved frictional characteristics of sliding interfaces under extreme conditions. By utilizing tribo-pair materials comprising hard tool-steel along with low melt point FCC metals such as 7075-T6 aluminum alloys, interfacial normal pressures ranging from 1-2 GPa and slip speeds of approximately 100m/s have been obtained. The resulting relatively large shear tractions (100 to 400 MPa) combined with the high slip speeds generate conditions conducive to interfacial temperatures approaching the near melt regime of the lower melt point metal (aluminum alloy) comprising the tribo-pair. At higher impact speeds transition in interfacial slip occurs from near-melt to the fully melt temperature regime. Under these conditions the interfacial frictional resistance is expected to approach the shearing resistance of the molten aluminum alloy under high hydrostatic pressures and extremely high shear strain rates. The result of the present study indicate that under the present experimental conditions molten aluminum maintains a shearing resistance as high as ~ 100 MPa.

In order to extract dynamic friction parameters under high slip speed, high pressures and elevated temperatures, a Lagrangian finite element model is developed for these pressure-shear dynamic friction experiments. This model accounts for dynamic effect, heat conduction, contact with friction, and full thermo-mechanical coupling. For elements whose temperature is below the melt regime, constitutive equations for finite deformation, isotropic, elastic-viscoplastic solids, formulated at the intermediate configuration within the framework of multiplicative plasticity are used. The formulation reduces to classical hyper-elasticity in the absence of plastic deformations. The elastic response is formulated in non-rate hyperelastic form. Newtonian fluid constitutive law is used for elements whose temperature reaches melt regime.

By measuring the combined normal and transverse motion of the rear surface of the target plate, along with the finite element program presented here, critical frictional parameters such as the applied normal pressure, the interfacial slip resistance, and the interfacial slip speeds can be interpreted. The results of these experiments/finite element analysis provide some new insights into the evolution of interfacial sliding resistance with accumulated interfacial slip, and its dependence on slip velocity, normal pressure and the interfacial temperature.