CHAPTER 4: FRACTURE
The separation or fragmentation of a solid body into two or more
parts, under the action of stresses, is called fracture.
Fracture of a material by cracking can occur in many ways,
principally the following:
1.Slow application of external loads.
2.Rapid application of external loads (impact).
3.Cyclic or repeated loading (fatigue).
4.Time-dependent deformation (creep).
5.Internal stresses, such as thermal stresses caused by
anisotropy of the thermal expansion coefficient or temperature
differences in a body.
6.Environmental effects (stress corrosion cracking, hydrogen
embrittlement, liquid metal embrittlement, etc.)
CHAPTER 4: FRACTURE
The process of fracture can, in most cases, be subdivided into
the following categories:
1. Damage accumulation.
2. Nucleation of one or more cracks or voids.
3. Growth of cracks or voids. (This may involve a coalescence of
the cracks or voids.)
CHAPTER 4: FRACTURE
Theoretical fracture strength
Early estimates of the theoretical fracture strength of a crystal were made by
considering the stress required to separate two planes of atoms. Figure shows
schematically how the stress might vary with separation. The attractive stress between
two planes increases as they are separated, reaching a maximum that is the
theoretical strength, σt, and then decaying to zero. The first part of the curve can be
approximated by a sine wave of half wavelength x* ,
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CHAPTER 4: FRACTURE
Stress concentration
The reason that the theoretical predictions are high is that they ignore
flaws, and all materials contain flaws. In the presence of a flaw, an
externally applied stress is not uniformly distributed within the material.
Discontinuities such as internal cracks and notches are stress
concentrators. For example, the stress at the tip of the crack, σ max, in
a plate containing an elliptical crack (Figure 14.2)is given by
CHAPTER 4: FRACTURE
CHAPTER 4: FRACTURE
Griffith reasoned that a preexisting crack could propagate under
stress only if the release of elastic energy exceeded the work
required to form the new fracture surfaces. However, his theory,
based on energy release, predicted fracture strengths that were
much lower than those measured experimentally.
Orowan realized that plastic work should be included in the term
for the energy required to form a new fracture surface. With this
correction, experiment and theory were finally brought into
agreement. Irwin offered a new and entirely equivalent approach
by concentrating on the stress states around the tip of a crack.
CHAPTER 4: FRACTURE
Griffith theory
Griffith approached the subject of fracture by assuming that materials
always have preexisting cracks. He considered a large plate with a
central crack under a remote stress, σ ,and calculated the change of
energy, U ,with crack size (Figure 14.3). There are two terms: One is the
surface energy associated with the crack,
CHAPTER 4: FRACTURE
CHAPTER 4: FRACTURE
Orowan theory
Gc is the strain energy release rate
CHAPTER 4: FRACTURE
Fracture modes
There are three different modes of fracture, each having a different
value of Gc. These modes are designated I, II, and III, as illustrated in
Figure 14.4.In mode I fracture, the fracture plane is perpendicular to the
normal force. This is what is occurs in tension tests of brittle materials.
Mode II fractures occur under the action of a shear stress, with the
fracture propagating in the direction of shear. An example is the
punching of a hole. Mode III fractures are also shear separations, but
here the fracture propagates perpendicular to the direction of shear. An
example is the cutting of paper with scissors.
CHAPTER 4: FRACTURE
Irwin ’s fracture analysis
Irwin noted that in a body under tension, the stress state around an
infinitely sharp crack in a semi-infinite elastic solid is entirely described
by *
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CHAPTER 4: FRACTURE
CHAPTER 4: FRACTURE
CHAPTER 4: FRACTURE
Plastic zone size
14.8
CHAPTER 4: FRACTURE