Stress and Failure, the Mohr Circle Flashcards
What is principle stress (o1>o2>o3)?
Principle stresses act on planes that contain no shear stresses. They are always orientated perpendicular to each other.
What is differential stress?
The difference between the principle stresses.
What is deviatoric stress?
The difference between the principle stresses and the mean stress.
What defines the diameter of the Mohr Circle?
The differential stress, o1-o3
How is the centre of the Mohr circle found?
The mean stress, (o1+o3)/2
What is the highest shear stress? Why is it not 90 degrees?
45 degrees. This is because the angle is 2 theta
How is hydrostatic pressure represented?
By a point, no stresses.
What happens to the Mohr Circle if o1 is zero and o3 is negative?
The circle is on the left of the y-axis, this is called uniaxial tension
What happens to the Mohr Circle if o1 is positive and o3 is zero?
The circle is to the right of the y-axis, this is called uniaxial compression
What is a mode one fracture?
Critical is normal stress, extension fractures
What is a mode two fracture?
Critical shear stress, shear fractures
What is Griffith’s theory for extension fractures?
Theoretical tensile strength of tensile strength is much higher than observed strength
Once a material starts to break under tension why may it continue to grow?
Increase in crack length leads to an increase in stress concentrations so crack grows even faster.
What is Navier Friction criterion?
Shear strength increases with increasing normal pressure expressed by a friction coefficient, u.
Shear strength = cohesion + u normal strength
What is the angle if u (friction coefficient) is o and if it is 1?
If 0 then u is 45 degrees.
If 1 then u is 22.5 degrees.
Why do shear fractures not develop in the same orientation as large shear stresses?
Normal stress also has an influence. The plane with the maximum shear stress has a relatively high normal stress component.
What does an increase in pore fluid pressure result in?
A decrease in effective stress.
What happens to a Mohr Circle when pore fluid pressure increases?
Mohr Circle moves towards the left and can reach the tensile failure curve if the differential stress is low.
