The two attached files provide an example of how to perform a parameter optimization (curve-fitting) for confined compression stress-relaxation.
CCsrlxOpt.feb is the input file for performing the parameter optimization. Execute from the prompt:
CCsrlxFit.feb is the finite element model for biphasic confined compression stress-relaxation.
Model:
Since confined compression is a one-dimensional analysis, only the height (thickness) of the material matters. Therefore a cuboidal model is used here (with a height of 1 mm).
A rigid body is used to represent the indenter. The indenter is connected via a rigid interface to the top surface (z=1) of the cuboid. The fluid pressure is set to zero on that surface, because the indenter is idealized as free-draining. The indenter is given a prescribed displacement (ramp-and-hold).
Optimization:
The optimization is performed on the reaction force Fz of the indenter. In this example, the "experimental" data appearing in CCsrlxOpt.feb was actually generated in FEBio, therefore the parameter optimization should converge to exact values of E=1 MPa and perm=0.001 mm^4/N.s.
The initial guess for E is taken to be 0.5 MPa and a representative scale for this parameter is 1. The initial guess for perm is 5e-4 mm^4/N.s and its representative scale is 1e-3. With this choice of initial guesses, the problem converges nicely.
Note: For some values of the initial guess for perm (e.g., 0.002), the optimization algorithm may fail in this example, because it might produce a negative value for the permeability during the iterative optimization process. (As of the writing of this example, parameter optimization in FEBio does not yet implement bounds on the parameters.)
Gerard
CCsrlxOpt.feb is the input file for performing the parameter optimization. Execute from the prompt:
Code:
febio -s CCsrlxOpt.feb
Model:
Since confined compression is a one-dimensional analysis, only the height (thickness) of the material matters. Therefore a cuboidal model is used here (with a height of 1 mm).
A rigid body is used to represent the indenter. The indenter is connected via a rigid interface to the top surface (z=1) of the cuboid. The fluid pressure is set to zero on that surface, because the indenter is idealized as free-draining. The indenter is given a prescribed displacement (ramp-and-hold).
Optimization:
The optimization is performed on the reaction force Fz of the indenter. In this example, the "experimental" data appearing in CCsrlxOpt.feb was actually generated in FEBio, therefore the parameter optimization should converge to exact values of E=1 MPa and perm=0.001 mm^4/N.s.
The initial guess for E is taken to be 0.5 MPa and a representative scale for this parameter is 1. The initial guess for perm is 5e-4 mm^4/N.s and its representative scale is 1e-3. With this choice of initial guesses, the problem converges nicely.
Note: For some values of the initial guess for perm (e.g., 0.002), the optimization algorithm may fail in this example, because it might produce a negative value for the permeability during the iterative optimization process. (As of the writing of this example, parameter optimization in FEBio does not yet implement bounds on the parameters.)
Gerard
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