... directions¹

Consider a blob modeling clay--you can deform it by placing between one of your opposable thumbs and a finger; you can deform it by simultaneously squeezing with two sets of opposable digits; you can ``smear'' it by pushing and pulling in opposite directions. These are examples of uniaxial, biaxial, and shear stress.

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... stresses.²

There are actually only six independent stresses in elastic equilibrium--it can be shown that $\sigma_{ij} = \sigma_{ji}$, i.e., the stress is symmetric.

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