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Transcript
```Friction:
• Leonardo Da Vinci
• Amonton
• Bowden and Tabor
• Dieterich
From laboratory scale to crustal scale
Figure from http://www.servogrid.org/EarthPredict/
Question: Given that all objects shown below are of equal mass
and identical shape, in which case the frictional force is greater?
Da Vinci law and the paradox
Leonardo Da Vinci (1452-1519) showed that the friction force is
independent of the geometrical area of contact.
Movie from: http://movies.nano-world.org/movies/frictionmodule/en/
The paradox: Intuitively one would expect the friction force to
scale proportionally to the surface area.
Amontons’ laws
Amontons' first law: The frictional force is independent of the
geometrical contact area.
Amontons' second law: Friction, FS, is proportional to the normal
force, FN:
FS  FN

Movie from: http://movies.nano-world.org/movies/frictionmodule/en/
Bowden and Tabor (1950, 1964)
A way out of Da Vinci’s paradox has been suggested by Bowden
and Tabor, who distinguished between the real contact area and
the geometric contact area. The real contact area is only a small
fraction of the geometrical contact area.
Figure from: Scholz, 1990
FN  pAr ,
where p is the penetration hardness.
FS  sAr ,

where s is the shear strength.
Thus:

FS p


.
FN s
Since both p and s are material constants, so is .
The good newsis that this explains Da Vinci and Amontons’ laws.
Static versus kinetic friction
The force required to start the motion of one object relative to
another is greater than the force required to keep that object in
motion.
static
dynamic


Ohnaka (2003)
static  dynamic
Slide-hold-slide - Dieterich
Dieterich and Kilgore, 1994
Static (or peak) friction increases with hold time.
Slide-hold-slide - Dieterich
• The increase in static
friction is proportional to
the logarithm of the hold
duration.
Dieterich, 1972
Monitoring the real contact area during slip - Dieterich and Kilgore
Change in true contact area with hold time - Dieterich and Kilgore
Dieterich and Kilgore, 1994
• The dimensions of existing contacts are increasing.
• New contacts are formed.
Change in true contact area with hold time - Dieterich and Kilgore
Dieterich and Kilgore, 1994
• The real contact area, and
thus also the static friction
increase proportionally to the
logarithm of hold time.
The effect of normal stress on the true contact area - Dieterich
and Kilgore
Dieterich and Kilgore, 1994
Upon increasing the normal stress:
• The dimensions of existing
contacts are increasing.
• New contacts are formed.
• Real contact area is proportional
to the normal stress.
The effect of normal stress on the true contact area - Dieterich
and Kilgore
Indentation yield stress, sy
Acrylic
400 MPa
Calcite
1,800 MPa
SL Glass 5,500 MPa
Quartz 12,000 MPa
The effect of normal stress - Dieterich and Linker
Changes in the normal
stresses affect the
coefficient of friction in
two ways:
• Instantaneous
response, whose trend
on a shear stress
versus shear strain
curve is linear.
• Delayed response,
some of which is linear
and some not.
Instantaneous
response
linear
response
The law of Coulomb - is that so?
Friction is independent of sliding velocity.
Movie from: http://movies.nano-world.org/movies/frictionmodule/en/
Velocity stepping - Dieterich
Dieterich and
Kilgore, 1994
• A sudden increase in the piston's velocity gives rise to a sudden increase in the friction,
and vice versa.
• The return of friction to steady-state occurs over a characteristic sliding distance.
• Steady-state friction is velocity dependent.
Velocity stepping and contact area
Dieterich and Kilgore, 1994
Summary of experimental result
• Static friction increases with the logarithm of hold time.
• True contact area increases with the logarithm of hold time.
• True contact area increases proportionally to the normal load.
• A sudden increase in the piston's velocity gives rise to a sudden
increase in the friction, and vice versa.
• The return of friction to steady-state occurs over a characteristic
sliding distance.
• Steady-state friction is velocity dependent.
• The coefficient of friction response to changes in the normal
stresses is partly instantaneous (linear elastic), and partly delayed
(linear followed by non-linear).