Download LECT. 2 STRUCTURAL GEOL

MECHANICAL PROPERTIES
>STRUCTURAL GEOLOGISTS’ CONCERNS:
• MECHANICAL PROPERTIES OF ROCKS RATHER
THAN THEIR ORIGIN
• ROCK WELL-CEMENTED, WELL CONSOLIDATED,
MASSIVE OR NOT, HEAVILY FRACTURED, THINLY
OR THICKLY BEDDED...
> FORCE & ACCELERATION
• FORCE - VECTOR QUANTITY THAT TENDS TO
PRODUCE CHANGE IN THE MOTION OF THE BODY
•UNBALANCED FORCE – CAUSES CHANGE THE
MOTION OF THE BODY
• BALANCE OF FORCE EXISTS WHEN NO
CHANGE IN MOTION OCCURS
> COMPOSITION & RESOLUTION OF FORCES
• FORCE –REPRESENTED BY A VECTOR, I.E.,
LINE OF DIRECTION OF FORCE
• EQUILIBRANT – FORCE TO BALANCE TWO OR
MORE FORCES
• RESOLUTION OF FORCES – PROCESS OF
FINDING THE COMPONENTS OF A SINGLE
FORCE
• COMPOSITION & RESOLUTION OF FORCES
• DIAGRAM (REFER TO DIAGRAM FIG 2)
• OW –FORCE TO BALANCE OX & OY; OW IS
EQUAL TO THE RESULTANT OF TWO FORCES
BUT ACTS IN OPPOSITE DIRECTION
• EQUILIBRANT – FORCE TO BALANCE ONE OR
MORE FORCES
• PROCESS TO FIND RESULTANT OF TWO OR
MORE FORCES IS COMPOSITION OF FORCES
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RESOLUTION OF FORCES (REFER TO FIGURES)
FIG. A : OY & OZ – SAME RESULT AS OX
FIG. B : OW & OV – SAME RESULT AS OX
FIG. C : OT & OU – SAME RESULT AS OX
FIG. D : OP (12 LBS) IMPINGES LM; OQ(11
LBS) USING THE SCALE IN THE FIGURE; OR
HAS VALUE OF ABOUT 5 LBS
• RESULTANT FORCE – SINGLE FORCE THAT
PRODUCES THE SAME RESULT AS TWO OR
MORE FORCES
• FIG. 4 RESOLUTION OF FORCES IN 3-D
• OW LIES IN THE VERTICAL PLANE OZVW; OZ IS
VERTICAL; OV LIES IN THE HORIZONTAL OXVY;
• OW MAY BE RESOLVED INTO 2 COMPONENTS
OZ IS VERTICAL; OV IS LIES IN THE
HORIZONTAL PLANE OXVY;
• OV MAY BE RESOLVED INO OX & OY WHICH
LIE IN THE HORIZONTAL PLANE & AT RIGHT
ANGLES TO EACH OTHER.
• ANY FORCE MAY BE RESOLVED INTO 3
COMPONENTS PARALLEL TO X, Y, & Z AXES
LITHOSTATIC OR CONFINING PRESSURE
• HYDROSTATIC PRESSURE – AN OBJECT IN
WATER AT DEPTH OF ONE (1) MILE IS
SUBJECTED TO PRESSURE OF 337,900 LBS PER
SQ. FT. OR 2,346 LBS PER SQUARE INCH.
• LITHOSTATIC PRESSURE – GRANITE ONE MILE
AT DEPTH IS SUBJECTED TO PRESSURE OF
26,178 POUNDS PER SQUARE INCH.
• LITHOSTATIC PRESSRE INCREASES WITH
DEPTH
EFFECTS OF LITHOSTATIC PRESSRE
• INCREASE IN CONFINING PRESSURE CAUSES
DECREASE IN VOLUME OF ROCKS BUT
INCREASE IN DENSITY
1) DECREASE IN CONFINING PRESSURE CAUSES
INCREASE IN VOLUME BUT DECREASE IN
DENSITY
 DIFFERENTIAL FORCES
• FORCES ACTING ON A BODY ARE NOT
EQUAL ON ALL SIDES
 STRESS – FORCE PER UNIT PER AREA- THE
SAME UNIT AS PRESSURE, BUT ALSO HAS
DIRECTION OF FORCE
• TYPES OF STRESS
1. TENSION (TENSILE STRESS) EXTERNAL
FORCES TEND TO PULL A BODY APART;
2. COMPRESSION – EXTERNAL FORCES TEND
TO AQUEEZ A BODY
3. SHEAR – FORCE THAT ACTS PARALLEL TO A
SURFACE; CAN CAUSE ONE OBJECT TO SLIDE
OVER ANOTHER
4. COUPLE – TWO EQUAL FORCES IN OPPOSITE
DIRECTIONS IN THE SAME PLANE, BUT NOT
ALONG THE SAME LINE.
5. TORSION – TWO ENDS OF A ROD ARE
TWISTED IN OPPOSITE DIRECTIONS
CALCULATION OF STRESS - STRAIN
STRESS = FORCE/AREA
STRAIN = L change/L
STRAIN – CHANGE IN SHAPE OR SIZE OF A
BODY CAUSED BY STRESS.
The resultant of two force vectors acting on the
same point in the same or opposite direction.
•Vector – An arrow used to denote the magnitude
and direction of a vector quantity, such as force or
velocity.
1.Force vector – A vector used to represent
a force:
1.Resultant force – A single force that
produces the same effect as several
forces acting simultaneously.
(A) Resultant force derived from
forces acting in a straight line:
The direction has a magnitude equal to the
algebraic sum of the forces, and acts in the
direction of the greater force.
• STAGES OF DEFORMATION
1) ELASTIC MATERIAL - IF STRESS IS WITHDRAWN
THE BODY RETURNS TO ITS ORIGINAL SHAPE
AND SIZE (STRAIN IS REVERSIBLE)
• IF ELASTIC LIMIT IS EXCEEDED, THE BODY
DOESN’T RETURN TO ITS ORIGINAL SHAPE
• IF STRESS EXCEEDS THE ELASTIC LIMIT, THE
DEFORMATION IS PLASTIC, I.E., PARTIAL
RETURN TO BODY’S ORIGINAL SHAPE
• CONTINUED INCREASE IN STRESS, FRACTURES
DEVELOP & AN OBJECT FAILS BY RUPTURE
• BRITTLE SUBSTANCES – RUPTURE TAKES
PLACE BEFORE PLASTIC DEFORMATION
• DUCTILE SUBSTANCES – HAVE LARGE
INTERVAL BETWEEN ELASTIC LIMIT AND
RUPTURE
> KINDS OF DEFORMATION: 1) ELASTIC –
RETURNS TO ORIGINAL SIZE & SHAPE WHEN
STESS IS REMOVED; 2) DUCTILE – EXCEEDED
STRENGTH = FLOW; 3) BRITTLE – EXCEEDED
STRENGTH = FRACTURE
ROCKS DEFORMED BY:
1) FOLDING; 2) FLOWING; 3) FRACTURING
Photograph of core showing different styles of deformation. Some layers
deform brittlely by faulting with little internal deformation. Other layers
deform ductilely by appearing to flow. Still other layers exhibit deformation
transitional between brittle and ductile. Photo credit: I. Davison.
FACTORS CONTROLLING BEHAVIOR OF
MATERIALS
1. TEMPERATURE – AT HIGH T MOLECULES &
THEIR BONDS CAN STRETCH IN A DUCTILE
MANNER; AT LOW T MATERIALS ARE
BRITTLE
2. CONFINING PRESSURE – AT HIGH CP
MATERIALS ARE LESS LIKELY TO FRACTURE
BECAUSE PRESSURE TENDS TO HINDER
FRACTURING; AT LOW CP MATERIALS ARE
BRITTLE & TEND TO FRACTURE
3. STRAIN RATE – AT HIGH SR MATERIALS TEND
TO FRACTURE; AT LOW SR ATOMS HAVE
MORE TIME TO MOVE & MATERIALS TEND TO
BECOME DUCTILE
• ROCK TYPE – SOME MINERALS LIKE QUARTZ
ARE MORE BRITTLE OTHERS LIKE CLAY ARE
MORE DUCTILE. THIS IS DUE TO CHEMICAL
BONDS THAT HOLD THEM TOGETHER
• WATER – WEAKENS CHEMICAL BONDS; DRY
ROCK-BRITTLE; WET ROCK-DUCTILE
• TIME: FATIGUE & CREEP
BRITTLE-DUCTILE PROPERTIES OF
LITHOSPHERE