Plastic Collapse of a Beam
Plastic Collapse of a Beam
(OP)
All,
I’m working on a simple beam plastic collapse static analysis for some basic benchmarking in Ansys WB. The problem in question is a solid rod (φ = 0.85”, 16” long, σy = 60 ksi) fixed at both ends. A concentrated load acts at the center of the rod at is applied till solver divergence. A mesh size of 0.1” is sufficient for mesh convergence.
Hand Calcs: Wcollapse = 3070 lbf
a) Beam model, Elastic-Perfectly Plastic Material: Wcollapse = 3039 lbf
b) Beam model, Elastic-Plastic Material: Wcollapse = 3125 lbf (At the onset of instability and first bisection, but the solution converges to the applied 5000 lbf load)
c) Brick model, Elastic-Perfectly Plastic Material: Wcollapse = 3230 lbf
d) Brick model, Elastic-Plastic Material: Wcollapse = 3421 lbf (At the onset of instability and first bisection, but the solution converges to the applied 5000 lbf load)
NL Material:
Q: Why does the solution continue bisection and converge while using an elastic plastic model? Is there a method to control this feature? The only warning in the log file at the first bisection is “The preconditioned conjugate gradient solver failed to converge, and therefore no solution was obtained. The equation solver is now being automatically switched to the sparse solver (EQSLV,SPARSE) to allow this analysis to continue". I do not get errors for excessive element deformation or large plastic strain increments typically associated with plastic collapse FEA.
Thank you
I’m working on a simple beam plastic collapse static analysis for some basic benchmarking in Ansys WB. The problem in question is a solid rod (φ = 0.85”, 16” long, σy = 60 ksi) fixed at both ends. A concentrated load acts at the center of the rod at is applied till solver divergence. A mesh size of 0.1” is sufficient for mesh convergence.
Hand Calcs: Wcollapse = 3070 lbf
a) Beam model, Elastic-Perfectly Plastic Material: Wcollapse = 3039 lbf
b) Beam model, Elastic-Plastic Material: Wcollapse = 3125 lbf (At the onset of instability and first bisection, but the solution converges to the applied 5000 lbf load)
c) Brick model, Elastic-Perfectly Plastic Material: Wcollapse = 3230 lbf
d) Brick model, Elastic-Plastic Material: Wcollapse = 3421 lbf (At the onset of instability and first bisection, but the solution converges to the applied 5000 lbf load)
NL Material:
Q: Why does the solution continue bisection and converge while using an elastic plastic model? Is there a method to control this feature? The only warning in the log file at the first bisection is “The preconditioned conjugate gradient solver failed to converge, and therefore no solution was obtained. The equation solver is now being automatically switched to the sparse solver (EQSLV,SPARSE) to allow this analysis to continue". I do not get errors for excessive element deformation or large plastic strain increments typically associated with plastic collapse FEA.
Thank you
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RE: Plastic Collapse of a Beam
RE: Plastic Collapse of a Beam
RE: Plastic Collapse of a Beam
As for the solution method applied to solve the system of nonlinear algebraic equations at each iteration of a load/displacement level, some of them are fast at the expense of a more narrow area of applicability. My advice is to read the Ansys documentation and try out all the equation system solver methods available.
RE: Plastic Collapse of a Beam
To avoid bisections you can simply apply the load more gradually. As in, if you applied a load of 10kN at 1s and the model bisected once and then successfully converged, you could use manual sub-stepping and and apply the load in 5kN increments.
RE: Plastic Collapse of a Beam
Cheers
Greg Locock
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RE: Plastic Collapse of a Beam
I was curious to know how ANSYS solved the BEAM models with a "close" plastic collapse load, as BEAM188/189 does not support multilinear isotropic material models (section b). I completely overlooked the element formulation conditions that are acceptable for BEAM elements.