Geodynamics - Strain

Lecture 7: Strain Lectures 5 explored faults and brittle deformation across a failure plane. The next set of lectures deals with ductile deformation: stretching, compressing, and twisting materials into different shapes without breaking them. This lecture and lecture 9 treat the mathematical description of ductile deformation called strain. Lectures 11 and 12 describe the relationship between strain and stress. 1. Finite Strain and Infinitesimal Strain Finite strain is characterized by big changes in the shape of a body. Infinitesimal strain is characterized by smaller changes. The next couple of lectures treat infinitesimal strain only because the small changes in shape cause squared and higher-order terms in the strain equation to be very small. These terms are negligible and make the strain equation simpler. 2. Infinitesimal Strain Definition Stress is a second-order tensor that provides a relationship between a normal vector and a traction vector. Strain is also a second-order tensor that provides a relationship between two vectors: a position vector dxi and a displacement vector dui. Consider the following picture: x3 Q' Q dui dxi P ui P' xi x2 x1 Figure 7.1 Assuming that the deformation is continuous, du is related to dx by the equation: du i = ∂u i ∂u ∂u dx1 + i dx 2 + i dx3 i = 1,2,3 ∂x1 ∂x 2 ∂x3 The matrix equation is written: ⎡ ∂u1 ⎢ ⎡ du1 ⎤ ⎢ ∂x1 ⎢du ⎥ = ⎢ ∂u 2 ⎢ 2 ⎥ ⎢ ∂x ⎢⎣ du 3 ⎥⎦ ⎢ 1 ⎢ ∂u 3 ⎢⎣ ∂x1 ∂u1 ∂x 2 ∂u 2 ∂x 2 ∂u 3 ∂x 2 ∂u1 ⎤ ⎥ ∂x3 ⎥ ⎡ dx ⎤ 1 ∂u 2 ⎥ ⎢ ⎥ dx 2 ∂x3 ⎥ ⎢ ⎥ ⎢ ⎥ ∂u 3 ⎥⎥ ⎣ dx3 ⎦ ∂x3 ⎥⎦ The nine-term matrix is the strain tensor. Interpreting the Strain Tensor The strain tensor looks complicated. What it means in terms of stretching, twisting, and rotating a body is not obvious. A few simple examples, however, help illustrate what its components mean in terms of different kinds of deformation. a. Translations In the case of rigid-body translations, du equals zero. The strain tensor has no sensitivity to them. b. Rotations Consider the following figure: x2 Q1 Q1' α P α Q2 Q2' x1 Figure 7.2 ∂u i : ∂x j Two expressions for α can be written in terms of ∂u 2 = − tan α ∂x1 ∂u 2 = tan α ∂x1 Because rotations in infinitesimal strain are small, the equations can be approximated using a small-angle identity: ∂u 2 = −α ∂x1 ∂u 2 =α ∂x1 These equations can be combined to give a single expression for α: 1 ⎛ ∂u ∂u ⎞ α = ⎜⎜ 1 − 2 ⎟⎟ 2 ⎝ ∂x 2 ∂x1 ⎠ This expression can be generalized to represent any rotation: 1 ⎛ ∂u ∂u j ⎞ ⎟ = wij α = ⎜⎜ i − 2 ⎝ ∂x j ∂xi ⎟⎠ The strain tensor can be rewritten to explicitly include the above expression: ∂u i 1 ⎛⎜ ∂u i ∂u j ⎞⎟ 1 ⎛⎜ ∂u i ∂u j ⎞⎟ = + + − ∂x j 2 ⎜⎝ ∂x j ∂xi ⎟⎠ 2 ⎜⎝ ∂x j ∂xi ⎟⎠ = ε ij + wij In this form, the second part of the strain tensor wij represents only rigid-body rotations. It is anti-symmetric. The first part εij represents elongation, compression, and shear. It is symmetric. c. Elongation and Compression Consider the following figure: x3 dx2 P P' Q Q' x2 dx2 + du2 Figure 7.3 Since both dx and du are in the x2 direction, the first part of the strain tensor εij is given by 1 ⎛ ∂u ∂u ⎞ ∂u ε 22 = ⎜⎜ 2 + 2 ⎟⎟ = 2 2 ⎝ ∂x 2 ∂x 2 ⎠ ∂x 2 Consequently, εij i=j can be thought of as a measure of elongation in the direction i. It is equal to a change in length per unit length of a material. For example, if a metal rod of ∆l length l is stretched to a new length l+∆l, the elongation εij is equal to . l l ∆l l + ∆l Figure 7.4 d. Shear Consider the following picture: x3 x3 Q1 Q1' Q1 α P Q2 x2 P α Q2' Q2 x2 Figure 7.5 Just as in the case of rigid-body rotation, two expressions for ϕ can written in terms ∂u of ∂x ij using small-angle approximations: ∂u 3 = tan ϕ = ϕ ∂x 2 ∂u 2 = tan ϕ = ϕ ∂x3 These equations can be combined to give a single expression for ϕ: 1 ⎛ ∂u ∂u ⎞ ϕ = ⎜⎜ 3 + 2 ⎟⎟ = ε 23 2 ⎝ ∂x 2 ∂x3 ⎠ Consequently, while εij i=j is a measure of elongation, εij i≠j is a measure of shear.