Shape 1

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This video shows the diffusion of an initially circular microcrack with isotropic surface energy under no applied stress. The only driving force is the surface energy. Therefore, microcrack will lessen its perimeter length to reduce its surface energy. This evolution process will continue while retaining its circular shape until the perimeter is virtually zero.

Shape 2

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The second video shows an initial ellipsoidal microcrack under the same condition as the previous circular microcrack. As before, the overall perimeter length decreases in effort to reduce the surface energy. However, rather than retaining its shape, the ellipsoid morphs into a circular shape by reducing its aspect ratio. This is because a circle has the least perimeter length compared to any other shape having the same area. The initial aspect ratio is 2, while that value reduces to 1.4 near the end of the video.

Shape 3

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When an elastic stress is applied to an initially circular microcrack with isotropic surface energy, the microcrack can no longer retain its shape. The elastic strain energy on the surface being the driving force, the shape will change in order to reduce the strain energy at the area of stress concentration. A tensile stress is applied in the + and – z direction. As can be seen on this video, the changing of the shape lowers the stress concentration around the 3 and 9 o’clock region, thus reducing the strain energy.

During the initial stage of the evolution, the strain energy is the dominant driving force. The shape change is more focused on reducing the strain energy and hence the aspect ratio increases. After the shape evolves a certain amount, the surface energy becomes dominant and the microcrack begins to retain a constant aspect ratio while reducing its surface.

Shape 4

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The evolution of an initially dumbbell shaped microcrack is shown on this video. The shape change proceeds in a manner to reduce the overall perimeter length and the stress concentration.

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