Origin 2017 Crack Tips Rapid

Origin 2017 Crack Tips Rapid

At the opening and closing crack tips rapid atomic snap-off and snap-in processes occur which is the origin of the observed sliding friction.

A velocity independent friction force results if rapid processes occur at the sliding interface, involving local slip velocities unrelated to the macroscopic drive velocity.

One important topic in tribology is to understand the origin and nature of the rapid slip events, which generate the sliding friction force 1.

This result follows quite general from the theory of the contact between elastic solids with random surface roughness 2-4.

This results from the fact that for a large system, when the normal force increases the number of asperity contact region increases proportional to the normal force, but the distribution of asperity contact areas and the (contact) stress distribution are unchanged.

It follows that the friction force will be proportional to the normal force independent of the nature of the microscopic frictional interaction at the asperity level, e.g., independent of how the friction force acting on an asperity depends on the asperity contact area.

The atoms on the block interact with the substrate atoms by Lennard-Jones potentials.

The study showed that the friction force is due to lattice pinning: at the opening and (to a lesser extent) the closing crack tips atoms snap-out (and -in) of contact in rapid events, with atom velocities unrelated to the block driving speed, followed by long time periods where the crack tips are pinned (see enclosed movies 1 and 2).

In the rapid slip events, elastic waves (phonons) are emitted from the crack tips (see Fig.1), resulting (for the opening crack) in a larger crack propagation energy than the adiabatic value.

This effect is closely related to lattice trapping, the velocity gap and hysteresis effects observed in model studies of crack propagation in solids.

The MD calculations show that the friction coefficient is proportional to, confirming that for our system the friction is entirely due to the emission of phonons from the opening and the closing crack tips.

The speed of these rapid events is unrelated to the block sliding speed and result in elastic waves (phonons) being emitted from the crack tip.

The result is a nearly velocity-independent sliding friction force.

The effect is closely related to lattice trapping, the velocity gap and hysteresis effects observed in model studies of crack propagation in solids.

Molecular Dynamic Simulations and AFM Experiments at Overlapping Speeds.

Due to mentioned complexity, truly predictive models of friction are yet to be developed.

The variation of surface properties affects the interaction between the wall and lubricant and determines friction.

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