Wrong result of umat.plot() for LinearElastic with stretches ≤ 0

[adtzlr]

not only for $\lambda \le 0$ but for the whole range if a stretch $\lambda \le 0$ is included! This happens because all stretches are solved in one scipy.optimize.root()-call by a least-squares problem.

Possible Solution

Try to raise an error if $\lambda \le 0$ is included. There shouldn't be the case where these non-physical stretches are of interest.

Example

import felupe as fem

umat = fem.LinearElastic(E=1, nu=0.3)

x = fem.math.linsteps([1, 0], num=50)

ax = umat.plot(ux=x, bx=x, ps=x)
ax.set_ylim([-1, 1])

While it should be:

import felupe as fem

umat = fem.LinearElastic(E=1, nu=0.3)

x = fem.math.linsteps([1, 1e-6], num=50)

ax = umat.plot(ux=x, bx=x, ps=x)
ax.set_ylim([-1, 1])