strain Analysis of material behavior, Lecture notes of Geology

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Strain

Deformation

• (^) Deformation-all changes in position and geometry of a rock

body from the original condition

• (^) Includes:

1. translation -movement from one place to another (e.g.,

fault motion)*

2. rotation -spin around an axis (e.g., folding of a rock)*

3. dilation -change in rock volume (expansion or

contraction)

4. strain -distortion of a body—a change in shape

*difficult to analyze

Homogenous and Heterogeneous Strain

Homogeneous-originally straight and parallel lines remain straight and parallel after deformation

Original, pre-deformation condition

Heterogeneous-lines do not remain straight and parallel (most common type of strain)

Strain type is a function of scale!

Homogeneous Strain

(Simple and Pure Shear)

Simple shear

Pure Shear

Stress and strain axes rotate (large strain)

Stress and strain axes remain parallel (small strain)

General shear-combination of simple and pure shear

(most common in nature)

Measures of Strain

Elongation

Angular strain Stretch

Volumetric strain: Δ=(V-Vo)/ Vo

Negative Δ: volume loss (compaction; pressure solution) Positive Δ: volume gain (fracturing)

Strain Analysis

• (^) To quantify the magnitude and/or orientation of the strain

ellipsoid in a rock

• (^) 1. Strain markers: deformed object in rock where original

shape can be inferred a. passive -no mechanical difference with rock (e.g., ooids in carbonates) b. active -mechanical difference with rock (e.g., quartz pebbles in shale)

active passive

Strain Markers (non spherical)

Pure shear

Simple shear