Illustrate the normality rule in the principle stresses space

The example shows how to illustrate the normality rule based on the 3D representation the yield surface

Plot the yield surface

from elasticipy.yield_criteria import MohrCoulomb
mc = MohrCoulomb(2, -20)
fig=mc.plot3D(xrange=(-6, 6), yrange=(-6, 6), zrange=(-6, 6))

Plot the surface normals

from elasticipy.tensors.stress_strain import StressTensor

tensile_stress_x = StressTensor.tensile([1,0,0], 1)
tensile_stress_y = StressTensor.tensile([0,1,0], 1)
biaxial_tension  = tensile_stress_x + tensile_stress_y
tension_compress = tensile_stress_x - tensile_stress_y

fig = mc.draw_surface_normal(fig, tensile_stress_x, color='black', auto_scale=True, label='Tensile x')
fig = mc.draw_surface_normal(fig, tensile_stress_y, color='gray', auto_scale=True, label='Tensile y')
fig = mc.draw_surface_normal(fig, biaxial_tension, color='blue', auto_scale=True, label='Biaxial tension')
mc.draw_surface_normal(fig, tension_compress, color='green', auto_scale=True, label='Tensile/compression')