Skills and concepts that students must have mastered
How the activity is situated in the course
Content/concepts goals for this activity
Understand the concept of using strain markers to estimate strain.
Understand what shear strain is, and be able identify principal strain axes as directions of zero shear strain.
Learn the concept of relative orientation (orientation of the long axis of a fossil relative to the page).
Higher order thinking skills goals for this activity
Other skills goals for this activity
Description and Teaching Materials
This exercise uses strain markers to estimate strain. The students measure angular shear in 15 strain markers that were in different orientations prior to experiencing homogenous simple shear (done in Illustrator). They also measure the orientation of each strain marker, and lengths of particular lines in the strain markers. They then use this data to determine the orientation of the principal axes of the finite strain ellipse; the lengths of these lines before deformation (assuming plane strain and that all strain markers were the same size originally); and from that they calculate stretching and quadratic elongation and construct a Mohr strain diagram first using their points, and then again using the strain equations with their calculated S1 and S3.
Detailed, step-by-step instructions are found in the downloadable file, which is attached as a Word file as well as a pdf with the deformed 'fossils.'
Measuring Strain word document (Microsoft Word 2007 (.docx) 230kB May4 12)
Deformed spirex pdf (Acrobat (PDF) 17kB May4 12)
Teaching Notes and Tips
The next confusing part was how to measure angular strain when they didn't have the originally perpendicular line already drawn in for them. Once we got past that it went pretty smoothly for those who were adept at using a protractor.
If their Mohr strain diagram is not roughly circular, they missed something.