A significant advance is made, however, when the
contribution of tangential elasticity is included. Mindlin (3)
has developed a theory of tangential compliance for two
identical elastic spheres which are pressed together under a
constant normal load. He showed that, for small relative
tangential loads, an annulus of micro-slip is generated at
the boundary of contact. As tangential load increases, the
inner radius of this annulus progressively reduces until,
when the critical value of friction force is reached, the
surfaces break away in gross-slip. In a later paper, Mindlin
and Deresiewicz (4) extended the original theory to cover cases
involving more complex loading.
The authors (5,6) have used a similar approach in making a
detailed study of elastic impact which discloses interesting
deviations from the predictions of the simple theory. To
ensure that the contact area is circular and that normal
motion is not influenced by tangential effects, attention was
restricted to the case of the impact, on an elastic half-space,
of a body whose centre of mass is also the centre of curvature
of a spherical contact surface. Simple examples of such a
body are a sphere or a symmetrical slice from a sphere.
The predictions of this theory agree with those of the simple
rigid body theory when gross-slip persists throughout the
impact, but the occurrence of micro-slip blurs the boundary
between the states of sliding and rolling and changes in
friction force become continous. The interface behaves
somewhat as a pair of mutually perpendicular non-linear springs
which react independently against the body, except that the
stiffness of the tangential 'spring' is influenced by the
normal compliance. Tangential vibration, distinct from the
half-cycle of normal vibration, is excited by the initial
conditions and its frequency, characteristically, depends
upon the mass distribution of the body.