In solving the quasi-static linear elasticity problem, one relates strain to stress.
How much should a collection of elements be strained to account for the stress
related to the external forces? When dealing with a planar simulation, there is no normal stress along the depth
dimension, here z, and there is no shear stress in neither the xz nor the yz planes.
The remaining stresses are then normal stress along x and y and shear stress in the
Quasi-static methods try to solve for equilibrium without taking inertia into account.
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