(1) Stress and Strain - short answers Short answers (answer in the space provide
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(1) Stress and Strain - short answers Short answers (answer in the space provided). Include a sketch where appropriate. (a) Define stress. What are the units of stress. (b) Give definitions of the terms (a) Mean stress (b) differential stress (c) deviatoric stress (e) Why is differential stress important in structural geology? (d) Define strain. What are the units of strain? (e) Define strain rate. What are the units of strain rate? What is a reasonable" value for geologic strain rate? (f) What is the difference between a joint and a fault? (g) What is Byd lee's law and what is it used for? (h) Why is pore fluid pressure important when evaluating whether a particular fault will undergo reactivation (i.e. move again)? Illustrate your answer with a schematic Mohr circle construction. (2) General questionsExplanation / Answer
a) In geology stress is the force which is acting on a rock pert unit area and its units are N/m2 which is also called Pascal in SI units
b)
a. Mean stress is arithmetic mean of max. stress and minimum stress
b. Differential stress is the difference between greatest and lowest compressive stress which a rock
experiences
c. Deviatoric stress is a stress component in a particular system which consists of the unequal
principal stresses.
c) In the field of geology the differential stress is mainly used to assess whether the tensile or the shear failure is going to occur in a rock, in Mohr circle
d) Strain is how the rock responds to the stress. Strain has no units as it is the ratio of two lengths
e) Strain rate is the change in strain of a rock with respect to time. The unit is s1. The reasonable values varies between 1012 - 1015 s-1
f) A joint is just a fracture along which there has no movement taken place, usually it is caused by the tensional forces. On the other side a fault is just a fracture in the rock along which there is movement in the rock.
g) Byerlee's law states that with the increased normal stress on the rock the required shear stress also continues to increase, but here the rate of the increase decreases and finally just becomes nearly independent of the rock type.
Used to determine the occurrence of slip along a fault
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