The Delft Sand, Clay & Rock Cutting Model, 2019a

The Delft Sand, Clay & Rock Cutting Model.
Figure 8-2: The stress-strain curves for ductile and brittle failure.
8.2. Cutting Process & Failure Criteria.
In granular materials a number of failure mechanisms can be distinguished. For clarity of definitions, the following
definitions are used:
Flow Type. Failure is based on plastic shear failure. Non-destructive, continues. Both the stress-strain curve
according to Figure 8-2 and the non-destructive plastic deformation show ductile behavior. This type of failure
will only occur at very high pressures and/or temperatures. The flow of magma is an example of this.
Tear Type: UCS/BTS=large. Failure based on 100% tensile failure. This type of failure will occur when the
UTS-BTS absolute value is small compared to the UCS value. This is a discontinues mechanism.
Chip Type: UCS/BTS=medium. Failure based on a combination of shear failure and tensile failure, with a
crushed zone near the tool tip. The fractions of shear failure and tensile failure depend on the UCS/BTS ratio.
A large ratio results in more tensile failure, a small ratio in more shear failure. This is a discontinues
mechanism.
Shear Type: UCS/BTS=small. Failure based on 100% shear failure. This type of failure occurs when the
UTS-BTS value is larger and the normal stresses in the shear plane are high, usually at larger blade angles.
This is a discontinues mechanism.
Crushed Type: Cataclastic failure based on shear, similar to the Flow Type and the Shear Type like in sand.
The Crushed Type is based on cataclastic failure, disintegration of the grain matrix. This mechanism will be
identified as pseudo-ductile since it shows ductile behavior in the stress-strain curve of Figure 8-2, but it is
destructive and not plastic.
When cutting in dredging practice, blade or pick point angles of about 60 degrees are used. With these blade angles
often the Chip Type of cutting mechanism occurs. Smaller blade angles may show the Tear Type cutting
mechanisms, while larger blade angles often show the Shear Type of cutting mechanism. The higher the normal
stresses in the rock cut, the less likely the occurrence of tensile failure.
When the pick point starts penetrating the rock, usually very high normal stresses occur in front and below the tip
of the pick point, resulting in crushing of the rock. Destroying the grain matrix. In a stress-strain diagram this
behavior is ductile, but since its also destructive its named pseudo-ductile. Now if the layer thickness is very small,
like in oil drilling, the crushed zone may reach the surface and the whole process is of the Crushed Type. If the
layer cut is thicker, like in dredging, the Chip Type cutting mechanism may occur, a combination of mechanisms.
In the crushed zone and the intact rock a shear plane can be identified based on the minimum deformation work
principle. When the pick point progresses, the shear stress on this shear plane increases. When the shear stress
exceeds the shear strength (cohesion) a brittle shear crack will occur. It is not necessary that the shear stress exceeds
the shear strength over the full length of the shear plane, it only has to exceed the shear strength at the beginning
of the shear crack as in the Nishimatsu (1972) approach. When the pick point progresses, the normal and shear
stresses increase, resulting in a Mohr circle with increasing radius. Now if the radius increases faster than the
normal stress at the center of the Mohr circle, the minimum principal stress decreases and may even become
negative. When it becomes negative it may become smaller than the negative tensile strength, resulting in tensile
failure.
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