Chapter 2. Prehension

Chapter 6 - During Contact 211
increased. These stresses produce deformations which are defiied by
the strains (change from former size; Halling, 1976). Elastic solids
follow Hooke's law where stress is proportional to strain, but
independent of rate of strain. For skin, as a viscoelastic material,
stress may be dependent on strain, rate of strain and higher derivatives
of strain. As well, strain may be dependent on stress in a more
complicated way. Thus the ratio of stress to strain may be time
dependent or dependent on the magnitude of the stress (Moore, 1972).
The implications of these factors have yet to be detailed for functional
grasping.
Because of its viscoelastic properties, skin does not follow
classical laws of friction, which hold for most metals (Comaish &
Bottoms, 1971; Moore, 1972). First articulated by Leonard0 da
Vinci, Amonton's laws state that: first, friction is proportional to load,
where F is the force due to friction, p is coefficient of friction, and W
is the normal force; and second, that the coefficient of friction is
independent of the surface area of apparent contact. For a given pair
of surfaces, the coefficient of friction, p, is a constant, independent of
load. Tables exist to document the coefficients of friction for different
materials (see for example, Bowden and Tabor, 1967). As the normal
load increases, so does the frictional force, because of an increase in
the area of true contact (both in the number of contacts and in the
contacting area between asperities; Bowden and Tabor, 1967; Moore,
1972). For viscoelastic materials, due to hysteresiss or deformation,
the area of true contact is not proportional to load, W, but to the 2/3
power of load, W2b (Moore, 1972). The second law does not apply
to elastic and viscoelastic materials, like skin.
When compressed due to grasping, skin becomes thinner under
the force, and wells up around the force, which serves to distribute the
pressure. Modem views of friction recognize the viscoelastic nature
of the two principal components of the friction generated between
unlubricated surfaces in relative motion, adhesion and hysteresis (see
Figure 6.2, for a schematic of a single ridge or asperity).
Assuming no interaction between adhesion at regions of contact
and the hysteresis or deformation factor,
8Hysteresis is the lagging of the effect in the body when the force acting on it is
changed.