| The normal strains (ex' and ey') and the shear strain
(ex'y') vary smoothly
with respect to the rotation angle q, in
accordance with the transformation equations given above. There
exist a couple of particular angles where the strains take on
special values.
First, there exists an angle qp where the shear strain ex'y' vanishes. That angle is
given by,
This angle defines the principal directions. The
associated principal strains are given by,
The transformation to the principal directions with their
principal strains can be illustrated as:
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