7 Friction basic concepts

wafflecanadianMechanics

Jul 18, 2012 (5 years and 1 month ago)

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Rock Friction
•Basic concepts
•Friction models and analysis of
earthquakes
•Earthquake mechanics
•San Andreas fault
Axial cracks and the formation of
a fault plane
Crustalstrength controlled by faults?
Townendand Zoback, Geology, 2000
Localized brittle failure of intact rock
Subsequent sliding behaviour
Frictional properties
of the two surfaces
in contact
Stiffness of the
loading system
= earthquakes
Coefficient of friction,
µ
:
the ratio of the shear stress to the normal
stress required to:
•initiate a sliding surface (internal friction)
•initiate sliding on the surface (static friction)
•maintain sliding on the surface (dynamic friction)
Is internal friction friction? Mechanically it involves fracture
and the coalescence of cracks –not strictly frictionSavage et al., 1996, Geophys. Res Lett., 23, 487-490.
Normal stress, σyy
Acts perpendicular to surface
Shear stress, τ (σyx)
Acts parallel to surface
Normal stress, shear stress
y
x
Failure depends on the coefficient of friction, µ
and the cohesive strength, C
In Mohr-Coulomb space:
C=0
normal stress,
σn
shear stress, τ
θ
*For friction, C = 0*
Amontons’slaws (1699):
Coefficient of friction is:
1. independent of surface contact area
2. independent of the normal force
n
µ
σ
τ
=
τ
= shear stress, Pa
µ
= coefficient of friction
σ
n
= normal stress, Pa
Friction testing
No law friction law based on micromechanical
processes is currently available due to the
complex interactions of surface contacts, that
constantly evolve with displacement.
Hence friction laws are:
Phenomenologicaldescriptions of friction
behaviour
What are some of the micromechanical
processes that occur during frictional sliding?
Friction controlled by asperities
Hence friction is controlled by:
•area of contact of the asperities (surface
roughness)
•hardness of the material
•shear strength of the material
Sliding of natural rough surfaces:
asperity interactions
Formation of fault gouge
•produced by wear
–shearing of asperities
–fracturing of surrounding material
Usually
has a stabilizing effect on the frictional
properties, hence stable sliding rather than stick
slip.
Hertzianfracture
Movement of asperities during shear
Lawn et al., 1984, J. Mat. Sci. 19, 2561.
0.5mm
Byerlee’s‘Law’for rock friction
Byerlee’sLaw
n
σ
τ
85.0
=
n
σ
τ
6.050
+
=
䱯眠獴牥獳L
䡩杨⁳瑲敳猺
睨敲攺
τ
= shear stress (MPa)
σ
n
= normal stress (MPa)
Byerleefriction at low stresses
large scatter due to surface
roughness –no fracture
n
σµφµτ
)]1([
2
++=
‘Joint dilatancy’