Similarities and Differences between Brittle Faults and Ductile Shear Zones Essay Example

of a visco-plastic body, the elastic limits are given by critical values of the equivalence stress according to a proper stress or strain hypothesis. The principal strain hypothesis is only mentioned here, which works similarly to the principal stress hypothesis, and the two simple stress hypotheses already useful for practical applications.

Principal Normal Stress Hypothesis indicates the property of brittleness in materials, in which sensitivity to the tensile component of the stress state predominates, may be described in their elastic range by Hooke’s law or a nonlinear law of elasticity until the limit given by the largest tensile principal normal stress is reached. The influence of the other two principal stresses on the elastic limit is neglected . In seismology, earthquakes are the primary stress failure indicators in the Earth.

When shear stress across a locked fault reaches the failure stress, the fault slips to reduce shear stress on the fault plane, though stress might increase in adjacent regions. This overall average reduction in shear stress on the ruptured fault is the static stress drop . The magnitude of a body force is proportional to the mass of the element on which its acting or its volume limes its density . Several poorly developed shear zones exhibit a very weak stretching lineation that is moderately shearing plunging to horizontal, suggesting there were multiple shearing events within the basement.

Unfortunately, many shear zones can only be observed on a glacially ploshed, near-horizontal surface, making the determination of shear-zone kinematics possible . Shear stress at a locked plate boundary can slowly increase until it reaches the failure stress. The failure stress level can be represented by the static coefficient of

friction. Slip on the fault occur either by the dynamic coefficient of friction during slip. If the dynamic coefficient of friction is the same value or larger, than the static coefficient of friction, then creep is the resultant behavior.

On the other hand, if the dynamic coefficient of friction drops below the static value fast enough, then earthquake usually results . Figure 1: Tensile Stress and Compressional Stress Two of the contributing types of stress are tensile stress and compressional stress. The maximum tensile stress sustained by a material in a tensile test pertains to the tensile strength. The stress that has to be applied to a material, in tension, so as to cause breakage. Obtained by dividing the load by the original cross-sectional area.

Such type of stress may be obtained by dividing the force necessary to cause failure during a tensile test, by the cross-sectional area at break . On the other hand, compressional force moves in the principle of collision and force encounter, which somehow produces impact force . Various components of strain, such as the amount of rotation or the magnitude and orientation of extension, shortening, or stretch, can be defined through direct observations . Figure 2: Shear Stress Force Couple and Deformation

A stress, or force per unit cross-sectional area, is called a “normal stress” when the direction of the force is perpendicular to the plane of the cross section. Such normal stresses are either tensile or compressive, depending on their sign. When the stress lies in the plane of the cross section, it is called a “shear stress”.

Lastly, stresses acting on any plane passed through a point consist of a normal stress

a (compression or tension) and a shearing stress (Soil mechanics problems are normally concerned with compressive stresses. On one particular plane, the normal stress will be the maximum possible value and the shearing stress will be equal to zero. On one plane perpendicular to this plane, the normal stress will be the minimum possible value, with shear stress also equal to zero. On a second plane perpendicular to this plane, the normal stress will have an intermediate value and the shearing stress will also be zero ; hence, termed the principal planes.