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In preparation for this exercise, students have studied 2D strain, become familiar with strain ellipses, and have plotted 1+e2 vs. 1+e1 for progressive pure shear and simple shear deformations. They have measured 2D strain using a variety of standard lab methods. And they have read about 3D strain and the strain ellipsoid. During class, I have each student make a play-doh cube and mark circles on at least three of the (mutually perpendicular) sides. Then I have each student deform their cube (maintaining an overall rectangular prism shape). I request that they make a different shape than their neighbors' as they deform their play-doh. I ask them to describe what happens to the inscribed circles, and therefore what would be happening to an imaginary sphere within their cube. Next I introduce the idea of a Flinn Plot, as an abstract but elegant means of conveying 3D strain ellipsoid shapes. I describe the axes, point out that the origin is at (1,1), and plot an example, using my own play-doh parallelipiped, deformed like theirs. Each student then calculates (1+e1)/(1+e2) and (1+e2)/(1+e3) for their parallelipiped, and plots their strain ellipsoid on a Flinn Plot on the board. As a class, we examine each deformed block of play-doh and compare it to its corresponding point on the Flinn Plot. I ask the class to generalize about the deformed shapes above the "plane strain" line versus those below the "plane strain" line. Each student thus practices measuring and calculating 3D strain, and plotting that strain on a Flinn Plot. And they have the opportunity to relate some concrete strain shapes to the abstract Flinn Plot. I follow this activity up by having students measure 3D strain in a rock sample and plotting their results on a Flinn Plot. Then we go on to discuss the element of time, and also the behaviors of various strain markers during deformation.
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