# Shared Flashcard Set

## Details

Structual Geology - Stress/Strain
Stress/Strain
59
Geology
12/09/2009

Term
 Strain
Definition
 the comparison of two states; one pre-deformation and one after deformation
Term
 How do we measure strain
Definition
 3 Ways! 1) Line Length - basically look at the difference 2) Volume (which may increase of decrease based on the material and the amount of strain) 3) Angular Sheer strength - the angular difference between two perp. lines * when we measure strain, we assume the material is homogeonous
Term
 Shapes of Strain Ellipsoids
Definition
 General Strain = L1>L2>L3 Axially Symmetric Extention = L1>L2=L3 Axially Symmetric Shortnening = L1=L2>L3 Plane Strain = L1>L2=1>L3
Term
 Principle Strain Planes
Definition
 L1 is perp to L2+L3 L2 is perp to L1+L3 L3 is perp to L1+L2 L1 is parallel to max extension L2 is parallel to max shortnening
Term
 What is the cigar shape strain ellipsoid and what is the other term for cigar?
Definition
 Axially Strain Extensiion and prolate
Term
 What is the pancake strain ellipsoid?
Definition
 Axially Strain Shortnening and oblate
Term
 What happens when there is general strain?
Definition
 There will be 2 cicular cross section and will intersect at L2. They also dip at specified angles to L1
Term
 What are the types of Plane Strains?
Definition
 Pure shear and Simple shear!
Term
 What is Pure Shear?
Definition
 Pure shear = no shear Ls are at the same lengths! coaxial deformation at undeformed states, L3=L2 L2 remains unchanged lines parallel to L1 and L2 don't rotate lines @ a low angle to L1 + L3 rotate slowly max rotation for lines 45 to L1+L3 two directions of rotation
Term
 What is coaxial deformation?
Definition
 When there is no rotation of strain axes
Term
 What is simple shear?
Definition
 Simple shear is simply that there IS ROTATION no coaxial deformation L2 is remained unchanged we care about SS because we get different structures. all lines rotate except those parallel to the shear plane maximum rotation rate for lines perp. to shear plane all lines rotate in the same direction
Term
 What is structural compatability?
Definition
 Snce structures are not always completely exposed, we rely on the concept of structural compablity to fill in missing gaps. The concept is a code for assessing whether the kinematic displacement are in harmony for a given area.
Term
 Strain path and memory
Definition
 Strain compares two states: "undeformed and deformed" rock memory: give info on strain path, track the path (tracking)
Term
 Spatial and Temporal Relationships
Definition
 may have been through all these stages and may have not...but we really don't know unless we do our own research
Term
 Mechanics and Kinematics
Definition
 Kinematics = Motion history Mechanics = the forces involved
Term
 Dynamics
Definition
 interaction between motion and forces - the relation between stress(mechanics) and strain (kinematics) we study rheology (material behavior) to explain dynamics.
Term
 Stress
Definition
 Intensity of a force (push or pull) USE SIGMA S = F/A the more stress, the smaller the area
Term
 Forces
Definition
 that which change or tends to change that state. rest or state of motions of a body e.g. push or pull F=ma or F=mg   F=(kg)(m2/s2)=Newton
Term
 Types of Forces
Definition
 Body and Surface
Term
 Body Force
Definition
 acts on all volume elements of an object 1) gravitational 2)inertial (meteors) - travel faster than the speed on earth
Term
 Surface Force
Definition
 acts on a single surface w/ in an object
Term
 Determining Stress
Definition
 1) traction = force on a vector; F/A = S on a plane 2) Stress on a surface 3) Stress on a "point"
Term
 Stress on a surface
Definition
 - pair of equal and opposite traction on a surface - each traction can be broken up into components; normal and shear normal stress is perp to surface and shear stress shear stress is parallel to surface force distribution is not equal
Term
 Stress on a "point"
Definition
 on any surface, the distribution of forces may not be uniform, need to examine a smaller area. i.e "point", where the surface is uniform to stress point is often represented as a cube
Term
 Units of Stress
Definition
 Pascal (Pa) = N/m2 kilopascal, mega, and giga all go up by 3 remember: F/A = Stress
Term
 Stress Ellipse/Ellipsode
Definition
 head to head infinite number of surfaces that pass through a point stress ellipsoids, unlike ellipsodes Don't represent the the shape of the deformed object.
Term
 Principle Stess Axes
Definition
 all have normal stress principle plane is defined by stresses (S1S2, S1S3, S2S3)
Term
 Principle Stress
Definition
 S1 = max compressive stress S2 = intermediate S3 = minimum
Term
 Ammonton's law
Definition
 f(normal stress) = shear stress -friction acts parallel to pre-existing surface and acts in a direction opposite to the surface tending to produce slip (shear stress) so...F/A=shear stress however, normal force is also present becase 2 surfaces must stay in contact so F/A = norml stress
Term
 Ration between normal and shear stress
Definition
 shear stress/normal stress = f(sliding force) f=tanO O = angle of sliding force
Term
 Whats the difference between Ammonton's law and Byerlee's?
Definition
 Ammonoton's law is theoretical and Byerlee's uses real experiance to get real numbers for f and theda
Term
 Byerlee's Law
Definition
 Ammonton's law w/ real values for f and theda (determined emperically) -irrespective of material. -sliding takes place 41 degrees to max compress tan theda = f= 0.85
Term
 Navier/Coulomb's Criteria
Definition
 -measures how much normal and shear stress is needed to form a fracture in what orientation that fracture forms -this is when there is no pre-existing weakness in the rock shear stress = f(normal stress)
Term
 Stress at a "point"
Definition
 -stress measures the intensity of the forces applied to a "point" at one "instant" we know this by comparing natural rxs to experimental results
Term
 What are the problems with experiments?
Definition
 1) size: same size is very small and typically homogeneous so strength of rx might not compare to nature 2) strain history - is it completely reserved in nature? 3) time - compensate for short period of time by intensifying w/ high temperatures 4) analog materials - materials that behave like rocks dont necessarily represent natural rxs.
Term
 Rheology
Definition
 deformation and flow of matter - experiments helps define rheology material behavior
Term
 confining pressure
Definition
 created w/ some sort of fluid, so same around the entire sample more pressure = greater define deth of deformation
Term
 Types of Stress Tests
Definition
 1) axial compression = S1 -> vertical position; S2=S3 -> confining pressure 2) axial tension= S3 -> vertical; S1=S2 -> confining pressure 3) TWIST SAMPLE
Term
 Differential Stress
Definition
 S1-S3
Term
 Continuum models of material behavior
Definition
 assume material is a continuum. relationship between sigma and strain is idealized into a few simple equations
Term
 Elastic Material
Definition
 BRITTLE ex: spring no permanent deformation no linear relationship between strain and stress equivalent to hooke's law - stress = (young's modulus E) (strain) if stress is applied that is greater than the strength of the material, then it could break = yield strength
Term
 Plastic Material
Definition
 DUCTILE ex: fingerprint in clay permanent deformation after yield strength and non before
Term
 Viscous
Definition
 deals with fluids if stress is applied to a fluid, it flows immediately when stress stops, it stops no yield strength measures by stress vs. rate of strain
Term
 Elastic Viscous
Definition
 aka visco-elastic ex: silly putty represents material behavior in earth -rocks can behave viscously over long periods of time -> creep
Term
 Normal Faults
Definition
 follow coloumb's law has dip angle of 60 degrees from surface
Term
 Thrust Faults
Definition
 follow coulomb's criteria has dip angle of 30 degrees from surface
Term
 Foliation
Definition
 layering primary foliation = bedding surface of layering primary structure and secondary structures
Term
 Structural Cleavage
Definition
 -forms surface perp. to maximum shortnening and parallel to max extension -not discontinuous surfaces. ie not a break in the rock when they form -after cleavage forms, it may act as a fracture. ie there might be sliding along a cleavage -material gets dissolved away and the insoluble material forms cleavage.
Term
 Microlithm
Definition
 area that seperates adjacent cleavage surfaces. ie the part of the rock that has not been altered.
Term
 Veins
Definition
 crack/fracture/joint that is dissolved away
Term
 Joints
Definition
 are always perp to cleavage. fractures w/o motion parallel to fracture surface may form alone or in groups (ie swarms) can identify opening (ie propagation)
Term
 Plumose
Definition
 Vs - that point in the direction of the joint opening
Term
 45 degree rule and Sigmoidal views
Definition
 cleavage and joints ideally form at 45 degrees to the shear zone -because rotation of 45 degrees happen, new joints form on the previous joint to keep the 45 degrees and form SIGMOIDAL VIENS
Term
 Buckling
Definition
 fold from the sides to form fold
Term
 Bending
Definition
 push from upward to form fold "Drape Folds"
Term
 Bend and Buckle folds are accomodated by...
Definition
 1) flexural slip - slickenlines perp to fold hinge (LIMBS) 2) tangential longitudinal strain (HINGES) outer arc may have joints, normal faults, extension.. inner arc may have trust faults, shortening, cleavage, small scale folds
Term
 Cleavage folding
Definition
 crenulation
Term
 Stripping
Definition
 color bedding
Term
 Boudinage
Definition
 layer breaks apart as it gets pulled apart
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