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Uniaxial Compression Test

� Uniaxial compression test is one of the

popular test which is done in rock

mechanic laboratories.

� Although this test is very simple, but it’s

has many application in rock problems.

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Effective parameter on UCS

� Internal parameters

� mineralogy

� density

� porosity

� shape and size of minerals

� anisotropy

� External parameters

� effect of loading places

� size of sample

� shape (geometry) of sample

� water

� rate of loading

Uniaxial Compressive Strength

� Standard index property (qu = σσσσu = σσσσc)

� Analogous tests in concrete and soil (unconfined compression test).

� ASTM 4543.

� ISRM 1979, Suggested methods for determining the uniaxial compressive strength and deformability of rock materials. Int J Rock Mech Min Sci Geomech Abs 16(2), sid 135-140.

� ISRM 1999, Draft ISRM suggested method for the complete stress-strain curve for intact rock in uniaxial compression. Int J Rock Mech Min Sci 36(3), sid 279-289.

� Planar ends on NQ size core (d = 47.6 mm)

� Length-to-width ratio: 2 < H/d < 2.5

� Axial loading of cylindrical core specimen

� σσσσu = Max. Force/(ππππd2/4)

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Rock Core Specimens

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Uniaxial Compression Test

GCTS Device ARA Setup at Tyndall AFB, Florida

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Uniaxial Compression Test

� Specimens from drill cores are prepared bycutting them to the specified length and arethereafter grinded and measured. There arehigh requirements on the flatness of the endsurfaces in order to obtain an even loaddistribution. Recommended ratio ofheight/diameter of the specimens is between 2and 3.

� The specimens are loaded axially up to failureor any other prescribed level whereby thespecimen is deformed and the axial and theradial deformation can be measured using aspecial equipment

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Stress-Strain Curve

� Some definition must be explained before the discussing about procedure of uniaxial compression test

� - fracture

� - failure

� - peak strength

� - residual strength

� - brittle fracture

� - strain-softening

� - strain-hardening

� - yield point

Uniaxial Compression Test

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Uniaxial Compression Test

Testing Mechanical Properties1 Uniaxial Compression Tests

Direct test

Load P

Rock cylinder

X-section area = A

Endcap

Strain

Gages E

1

Str

ess

Axial

Strain (εεεε1111))))

Lateral

Strain (ε(ε(ε(ε2222))))

εεεε1111εεεε2222

ν=ν=ν=ν=−−−−εεεε2222/ε/ε/ε/ε1111

1

C0 (failure)

C0 : Uniaxial Compressive Strength

E: Modulus of Elasticity

νννν : Poisson’s RatioA

PC

peako ====

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I II III IV

Indirect method of uniaxial strength determination

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Indirect test (Point Load Test)

Steel

cones rock

Hydraulic

ram

Testing Mechanical Properties1. Uniaxial Compressive Strength Tests

Point Load Index

GCTS Device Roctest Equipment

Fig.8-1

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Point Load Testing

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Point Load Index

� Quick evaluation for uniaxial strength

(field or lab setup)

� ASTM D 5731 procedures

� Little sample preparation (cores, pieces)

� Measure force (P) to crunch intact rock

specimen

� Point Load Index: Is = P/de2 where de= equivalent core diameter

Fig.8-1

Measurement of the Point Load Strength Is and of the indirect C0

A rock core is loaded diametrically between the tips of two

hardened steel cones, causing failure through the development of

tensile cracks parallel to the loading direction.

2

peak

sD

PI =

The load at failure Ppeak is recorded and the point load strength is calculated from:

where D is the distance between the two cone tips.

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Point Load Testing

� Usually a core, diameter, D = 50 mm

� BUT rock sample does not have to be cylindrical

effective diameter, De

225.02e

2e

45.0

250s2500

D

D

P

50

D

D

PI

=

=

Corrections to equivalent core diameter of 50 mm

Measurement of the point load strength Is and of the indirect C0

The uniaxial compressive strength C0 is then indirectly obtained by

using the empirical relationship:

where Is(50) is the point load strength of 50 mm (2 in.) diameter

cores

)50(sICUCS ×=

SSIFI ×=)50(

45.0

50

= e

DF

)175.014( DC +=

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Testing Mechanical Properties (contd.)

2. Hardness (also used as Indirect Uniaxial Compressive

Strength Tests)

(a) Shore Scleroscope (b) Schmidt Hammer

Rock

specimen

Graduated window showing the rebound of

pellet or spring

Measurement of the Shore hardness HShore and the Indirect Determination of C0

Shore hardness (HShore) is measured as the extent of

rebound of a steel bullet dropped from a specific height

onto the surface of a rock specimen.

The harder the rock, the higher the bounce.

An empirical correlation between rock hardness and its

uniaxial compressive strength has been obtained based on a

large number of tests on different rocks, and is given by:

62.3H)ft/lb(000066.0)psi(ClogShore

3

dryo+××= γγγγ

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Measurement of the Schmidt hardness HSmdt, and the Indirect Determination of C0

The Schmidt Hammer is a portable tool, similar in principle to the

Shore Scleroscope. It is used exclusively for rock and rock-like

materials and is easy of use in the field. It measures the

rebound off the surface of rock of a spring-driven steel pellet.

An empirical correlation between the Schmidt rock hardness and

uniaxial compressive strength has been obtained based on a large

number of tests on different rocks:

16.3H)ft/lb(00014.0)psi(ClogSmdt

3

dry0+××= γγγγ

( )[ ]wS

HUCS

γ×+×=

0087.016.0109.6

L / 3 L / 3 L / 3

P

P / 2 P / 2

D Rock Core

3

peak

MRD3

LP16T

×π×

××=

(b) Four-point-beam modulus of rupture (tensile strength) TMR.

In this test a long rock core is flexured to failure. There are four

contacting points dividing the core into three sections of equal length.

The middle section is under pure bending. The maximum tensile stress

occurs in the bottom layer of the middle section of the core, and this

is where failure typically occurs as P reaches Ppeak. This test

determines the modulus of rupture (TMR) (beam flexural tensile

strength). From the theory of beams:

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Tensile strength

� Direct method

� Indirect method

Direct Uniaxial Tension Test Setup

steam/water

actuator

Wood’s metal

pull

load cell

specimen threaded bolts

Difficult to attach

sample ends and not

bend the sample

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Tensile Strength (T0) of Rocks

� Direct tensile strength (ASTM D 2936) is

difficult because of end effects.

� Generally replaced by indirect (Brazilian)

split-tension test (ASTM D 3967).

� Length-to-diameter ratios: 2 < H/d < 2.5

� Diametrical compression of rock core

specimens across

Brazilian Split-Tension Test on Rock

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Testing Mechanical Properties (contd.)

3. Tensile Strength Tests

(a) Brazilian (indirect) Test

Flat

platens

Rock

disk

(a) Measurement of the Brazilian tensile strength TB

This is an indirect measurement of the tensile strength of

rock.

A rock disk of uniform thickness is cut from a rock core, and

is loaded diametrically between upper and lower flat (or

rounded) platens in a compression testing machine. Thus, a

compressive line-load is applied to the disk.

When the peak load Ppeak is reached the disk will typically

split along the loaded diameter.

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Theoretical Stresses in Rock during Brazilian Test

Stress Parallel to load is compressive at the edges and tensile in the middle.

Stress perpendicular to the load is greater but rocks are stronger in compression than

tension so the rock splits in tension.

Measurement of the Brazilian tensile strength TB (contd.)

Theoretical analysis shows that uniform tensile stress

develops along this diameter. The tensile strength TB

can be obtained from the elastic solution:

tD

P2T peak

B××π

=

where Ppeak is the load at failure, t and D are the thickness and diameter of the disk respectively.

The ratio of t/D is 1/4 to 1/2.

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Triaxial Compression (ASTM D 2664)

Computerized Compression Frame Rock Triaxial Cell

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σ3=100 MPa

T°C =cte~25°C

Variable Confining pressure

Slope:Additional strain is essential to increase the deformation

(σ1

–σ

3)

ε %

σ3=35 MPa

σ3=10 MPa

σ3=0.1 MPa

R

R

No slope:Deformation occurs without increasing of strain (rock�plastic behaviour)After reaching the ultimate strength, slope=> (-), deformation continues in spite of an decreasing of strain

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Composite failure envelope

you will generate a series of pairs of confining stresses and

associated axial stresses at which samples break…

40 540 500 MPa

150 800 650 MPa

400 1400 1000 MPa

σ1σ3

we use these as σ1 and σ3

and plot Mohr circles…

get a sequence of circles

offset from one another

σ1- σ3σs

σn

500

500

1000 1500

…diameters are stress difference;

…centers are stress sum/2