DYNAMIC ANALYSIS

interprets stresses to describe the forces from which they were derived and the relationship between stress and strain

REVIEW

Rotational strain

angular shear

principal axes of the strain ellipse

pure shear simple shear
 
 

DEFINITIONS

READINGS CH. 5

LECTURE

 In Geology, you will always hear of the use of stress instead of force. why?

 In the earth rocks break, faults are created by the concentration of force over surfaces. Usually we're interested in the deformation and the deformation is more readily explained by stresses.

 For example:
 
 

Great stress can be achieved with very small forces. Stresses from near the core/mantle boundary can be reproduced in the laboratory.

Even the greatest forces may not break rocks if the stress is insufficient.
 
 

(stress) [[sigma]] = F/A i.e. kg m /s² m² or Pa, units of pressure
 
 

e.g. a heeled shoe can exert more stress than an elephant.
 
 

e.g. 115 lb/.125 in vs. 10000lb/12x12 in.
 
 

Your car tires are in p.s.i (~36 p.s.i) each p.s.i. is = 1 kg/cm²

 or 1 p.s.i. = 0.07 kg/cm²
 
 

e.g. football player: 122kg/4cm² = (270 lb) / 1 in
 
 

Calculate the pressure at 10 m depth?

 Calculate the lithostatic stress at a depth of 1km in granite?

 lithostatic stress is the stress induced by the overlying weight of rock.
 
 

Representation of stress through the stress ellipse

As we saw with strain, we can describe the state of stress with reference to regular geometric object.
 
 

At depth we have stress from all directions. However, we can show that the three stresses that are at right angles to each other can always be found with which to describe the general stress condition:
 
 

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