New Definition (Material)
The
critical parameter Kc is known as the fracture toughness. When this
parameter measured under certain conditions has been shown to be a true
material constant for a great many materials.
The
Griffith fracture criterion states that a certain combination of excess
pressure pe = p- σ and crack length (or diameter) 2a
is required to keep the fracture open or even increase its dimensions. Of
course, the longer the crack in a given rock structure subjected to constant
overburden (confining) pressure σ the lower the fluid pressure, p,
required. The higher the fracture toughness, Kc, the higher the
fluid pressure, p, required to open and initiate the crack.
Values of Kc and in particular its lower limit
value, KIc for plane strain, have been determined for a large number
of materials following standard test methods and specifications. Many specimen
configurations can be used and standard test methods have been established for
fracture toughness testing of metallic materials. Whereas the development of
such standard methods for toughness testing of rock materials has started only
recentlydue to the relatively recent interest in fracture toughness of rock and
the extreme variety in rock types and applications. Some representative values
of fracture toughness and tensile strength are listed in Table II.
Note that
units of fracture toughness can be expressed as stress times square root of
length; or, expressed in basic units: 1MPa√ m
= 1MNm-3/2 (1psi√ in
= 1000 lb-in-3/2)
A failure
criterion based on fracture toughness accounts for the degradation in
load-carrying ability of a flawed structure. Conversely, if one knows the
design load of a structure and the fracture toughness of the material from
which it is constructed, then one can determine how large a crack that
structure can tolerate without failure; or, how large a fracture fluid pressure
is required to initiate fracture extension. In structural engineering
applications this approach is commonly used to set flaw detection requirements
for structures such as aircraft and pressure vessels.
While this failure criterion seems to be an
obvious improvement over a simple tensile strength criterion which does not
account for structural flaws, the drawback comes in the difficulty in detecting
and measuring flaws before a catastrophic failure occurs. With respect to
underground mining engineering applications flaw size detection still
represents an unresolved problem. Fracture mechanics seems to be applied all
too often in post martens when the cause of failure is being determined rather
than in the design of the component to prevent failure, or, in mining
engineering terms, in the design of optimally controlled fracture development.
Stress analysis techniques have become so powerful with modern finite element
techniques that determining stress is simple; the difficulty is in determining
the size, shape, location, and orientation of cracks - and this holds
especially for cracks in under-ground rock strata.
(Hans-Peter Rossmanith,Rock Fracture Mechanics,p.11)
There is no previous definition.
There is no previous definition.
No comments:
Post a Comment