Failure and Mohr's Circle

We use a Mohr stress diagram to map the failure of rocks under stress, by plotting both normal and shear stresses, as well as the greatest and least stresses on the Mohr circle. After we test numerous rocks at different confining pressures, we get a family of failure values that define a failure envelope.

Creation of Joints & Shear Fractures in the Lab

There are 2 basic types of rock strength tests:

1) Tensile strength tests: specimen is pulled along its axis (). Sometimes confining pressure is applied to it’s sides (). The test continues until failure.

2) Compressive strength tests: specimen is compressed along its axis (1) with or without confining pressure applied to it’s sides () until failure.

At failure, the values of the principal stresses are noted and so is the orientation of the plane of failure wrt either or .

These data are plotted in Mohr space.

A single experiment will produce a circle that describes the normal and shear stress (ns) for the plane of failure at the instant of failure.

A number of similar experiments are carried out at different confining pressures to create a series of similar data points.

The location of these points defines a failure envelope.

The envelope defines a region of Mohr space where rock is stable - in no danger of failure. Outside the envelope the rock fails.

Each red star along the failure envelope represents rock failure (e.g., fracture) at different differential stress.

A larger Mohr circle represents a greater difference between the largest 1 and smallest 3 stress.

In Geology, tensile stresses are negative. Rocks are weakest under tension, which plots on the left of zero for a Geology standard.

But it really doesn’t matter. For shear aka s , the plot is symmetrical, and for normal stress n , both standards are useful.

Rock failure (fracture) at a specified 3 and 1.

Tensile Strength Tests Rocks are typically very weak in tension. Rocks are typically 2 to 30 times stronger in compression than in tension.

In geology (say working for USGS) , tensile stresses are negative (-) and compressive stresses are positive (+).

In engineering, (say working for a mining or an environmental company) tensile stresses are positive (+) and compressive stresses are negative (-).

We can visualize tensile failure in Mohr space using the geology convention, and get an idea of what a tensile failure law might look like.

Tensile Strength Tests

Again, compared with compressive tests, rocks are very weak in tension. The ratios of strength in tension in unconfined compression is about 2:1, by may exceed 30:1.

Break a pencil. As we bend it, tension occurs in the outer arc of the bend and compression in the inner arc. Weaker in tension, the pencil snaps (fails) along the outer arc. DEMO: foam pyroxene strand, discuss stress concentration

The state of stress before the experiment starts is . This is represented by a single point, where there is no differential stress.

As tensile stresses build parallel to the length of the sample, differential stress builds.

At the beginning of the experiment, no differential stress (e.g., hydrostatic state of stress).

Tensile failure simply occurs when the tensile strength of the rock is exceeded. The plane of failure is perpendicular to the tensile stress (3).

Increasing tensional stresses, with increases of circle diameter

T0 is the tensile strength of the rock

When tensile strength of the rock is exceeded, the rock breaks perpendicular to the direction of tension (e.g., 3).

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Tensile stresses build up parallel to length of sample. As differential stress increases, the diameter of the Mohr circle increases.

Stress perpendicular to the axis of the rock core is the default direction of 1.

During the test, since tensile stress is negative for the geology standard, it’s the least principal stress (3).

T0 is the tensile strength of the rock

Tensile Strength Law:

A rock will fail by fracturing if the magnitude of least principal stress (3) equals or exceeds the tensile strength of the rock.

3 = To

The fracture is parallel to 1 and perpendicular to 3.

In Mohr space, the radius that connects the center of the differential stress circle with the point of failure lies along the x-axis.

Tensile & Compressive Strength TestsWe can also run triaxial tests (with compressive confining pressure applied to the flanks of the specimen) while at the same time applying a tensile stress along the axis.

Lets explore the relations between differential stress, confining pressure, and fracture strength of a rock in compression and tension, say buried at a divergent margin.

10 MPa

We begin the experiment at a confining pressure of 10 MPa. That’s the compressive part. Then we increase the tensile stresses parallel to the length of the specimen.

When tensile strength of the rock is exceeded, the rock breaks perpendicular to the direction of tension (e.g., ).

Here, increasing levels of tension are represented by points () moving further to the left of the origin along the normal stress axis. In other words, bigger negative stresses plot further to the left of zero.

Ultimately, the differential stress is sufficient to break the rock.

T0 is the tensile strength of the rock

As the test goes on, the differential stress () increases (the diameter of the Mohr circle) until failure occurs.

Failure under compressive stress

At increasing confining pressure, we need increased differential stress () for failure.

The increase of differential stress is shown by an change in the Mohr circle diameter.

c 0 tan ( N )Coulomb's Law of Failure:

Dynamic and mechanical models developed by Coulomb (1773) and Mohr (1900).

The law describes the height and slope of the linear envelope. Describes failure of rocks in compression.

Where c = o + tan

= angle of internal frictiontan = coefficient of internal

friction (slope of failure line)c = critical shear stress required

for faultingo = cohesive strength = normal stress

y = b + ax notice tan is the slope

Relationship between stress and fracturing

These tests define a failure envelope for a particular rock.

All of the normal and shearing stresses inside the envelope are stable – no fractures produced.

All of the stresses on or outside the envelope will producing fracturing

When the Mohr circle becomes tangent to the envelope, then the c at that point causes a fracture. 2theregives the failure and the point gives the n

and at failure No fractures are produced by any other combination of c on the circle.

Relationship between stress and fracturing

c 0 tan ( N )Coulomb's Law of Failure:

The slope and straightness of the envelope reveal that compressive strength of a rock increases linearly with increasing confining pressure.

The angle of envelope slope is called, the angle of internal friction ().

The envelope is called the Coulomb envelope.

A law that describes the conditions under which a rock will fail by shear fracturing under compressive stress conditions.

The point of failure on the Coulomb envelope reveals magnitudes of N = 43 MPa

and = s = 47 MPa.

In terms of Coulomb Law of failure, the shear stress value of 47 MPa is the critical shear stress (c) necessary for fracturing to occur.

Part of its magnitude is cohesive strength (0) expressing in units of stress, read directly off of the Mohr y-intercept of the envelope of failure.

The rest of critical shear stress (c) is the stress required to overcome internal frictional resistance to triggering movement on the fracture. This component is labeled: N tan or the coefficient of internal friction.

This value is expressed in terms of the normal stresses acting on the fault plane and the angle of internal friction, which is the slope of the failure envelope

The cohesive strength (0) is a small part of critical shear stress required for shear fracture.

Most shear fractures form when shear stresses on a plane of failure reach a level slightly over 50% of the normal shear stresses acting on the plane.

c 0 tan ( N )

We begin the next experiment at a confining pressure of 40 MPa.

If the confining pressures are in the range of 1 = 3 to 5To (from 3 to 5 times the tensile strength of the sandstone), the failure envelope will flatten slightly as it passes the shear stress axis, and the failure envelope becomes parabolic (dark line).

The two directions of breaking shown are equally likely. Conjugate fractures will form under tension

Mohr Failure Envelope

What happens with higher confining pressures

At very high confining pressures, Coulomb theory is not valid. With increasing confining pressure, rocks behave in a less brittle fashion.

This is apparent in our stress/strain curves, where at higher confining pressures there is a departure from the linear relations between stress and strain.

Analogous to stress/strain, the linear

Coulomb relations between fracture strength and confining pressure breaks down at higher confining pressures – the rock becomes weaker.

The straight-line envelope becomes a concave downwards envelope of lesser slope.

Note change in slope

The von Mises criterion describes deformational behavior above the brittle-ductile transition.

When the critical yield stress is surpassed, the rock will fail by ductile shear along planes of maximum shear stress, oriented at 45° to the greatest principal stress.

Von Mises criterion: brittle to ductile

Measured values of tensile strength, cohesive strength, and internal friction for a few rock types.

Rock failure envelope for a rock marked by low tensile strength, low cohesive strength, and low internal angle of friction.

Rock failure envelope for a rock marked by high tensile strength, high cohesive strength, and high internal angle of friction.