No error is thrown on unstable model in some scenarios

[weikequ]

Describe the bug
In some cases, when using FEModel3D.analyze(), when the model is obviously unstable, no errors are thrown. This happens with the default options, but with sparse=False, errors are sometimes correctly thrown. I've come across a couple different scenarios that I'll go through below.

• Scenario 1: A warning is sent to stderr: MatrixRankWarning: Matrix is exactly singular D1 = spsolve(K11.tocsr(), subtract(subtract(P1, FER1), K12.tocsr() @ D2)), but no error is thrown and code execution continues.
• Scenario 2: No warnings are given, no errors are thrown, code execution continues.
• Scenario 3: Same as Scenario 2, but no errors are thrown when even sparse=False is added

To Reproduce
Scenario 1:

from Pynite import FEModel3D
m = FEModel3D()
m.add_node("N1", 0, 0, 0)
m.add_node("N2", 0, 1, 0)
m.add_material("Steel", 29000, 11400, 0.5, 490 / 1000 / 12**3)
# No supports are added
m.add_section("Section", 10, 100, 150, 250)
m.add_member("M1", "N1", "N2", "Steel", "Section")
m.analyze()

Scenario 2:

from Pynite import FEModel3D
m = FEModel3D()
m.add_node("N1", 0, 0, 0)
m.add_node("N2", 0, 12 * 12, 0)
m.add_node("N3", 15 * 12, 12 * 12, 0)
m.add_node("N4", 15 * 12, 0 * 12, 0)
m.add_material("Steel", 29000, 11400, 0.5, 490 / 1000 / 12**3)
m.add_section("Section", 10, 100, 150, 250)
m.add_member("M1", "N1", "N2", "Steel", "Section")
m.add_member("M2", "N4", "N3", "Steel", "Section")
m.add_section("Section2", 15, 100, 250, 250)
m.add_member("M3", "N2", "N3", "Steel", "Section2")
m.add_node_load("N2", "FX", 50)
# No supports are added
m.analyze()

Scenario 3:

from Pynite import FEModel3D
m = FEModel3D()
m.add_node("N1", 0.0, 0.0, 0.0)
m.add_node("N2", 4.0 , 0.0, 0.0)
m.add_node("N3", 4.0 , 3.0 , 0.0)
m.add_node("N4", 0.0, 3.0 , 0.0)
m.add_material("Steel", E=29000, G=11200, nu=0.3, rho=0.00028356481481481483)
m.add_section("W10x33", A=9.71, Iy=36.6, Iz=171, J=0.58)
m.add_member("M1", "N1", "N2", "Steel", "W10x33")
m.add_member("M2", "N2", "N3", "Steel", "W10x33")
m.add_member("M3", "N3", "N4", "Steel", "W10x33")
m.add_member("M4", "N4", "N1", "Steel", "W10x33")
m.add_node_load("N2", "FX", 50)
# No supports are added
m.analyze(sparse=False)

Expected behavior
An exception should be thrown if we encounter any singular matrices/there is any global instability.

[JBloss1517]

Thanks for reporting this. This isn't the expected or desired behavior. There is a stability check option that is enabled by default when you analyze, but isn't picking it up for some reason.

[JWock82]

I'll have a closer look at this now that I've released v1.1.0.