## Flexible joint property definition

@FLEXIBLE_JOINT_PROPERTY_DEFINITION {
@FLEXIBLE_JOINT_PROPERTY_NAME {FlxPropName} {
@ELASTIC_BRANCH {
@STIFFNESS_MATRIX {k11, k12, k13,... k22, k23,... k55, k56, k66}
}
@MAXWELL_FLUID_BRANCH_DEFINITION {
@RELAXATION_TIME {τ}
@STIFFNESS_MATRIX {k11, k12, k13,... k22, k23,... k55, k56, k66}
}
}
}

### NOTES

1. The physical properties of the flexible joint are defined by means of the generalized Maxwell model, which consists of any number of elastic branches in parallel with any number of Maxwell fluid branches.
2. Keyword @ELASTIC_BRANCH introduces the definition of the elastic branch, which must be defined first. The definition of the elastic branch requires a single input.
1. A symmetric, elastic stiffness matrix of size 6 × 6. Due to symmetry, 21 terms only are defined, corresponding to the line by line definition of the upper half of the matrix.
3. Keyword @MAXWELL_FLUID_BRANCH introduces the definition of a Maxwell fluid branch. Any number of Maxwell fluid branches can be defined; all are optional. The definition of a Maxwell fluid branch requires two inputs.
1. A relaxation time, τ, is defined first.
2. A symmetric stiffness matrix of size 6 × 6. Due to symmetry, 21 terms only are defined, corresponding to the line by line definition of the upper half of the matrix.
4. All stiffness matrices must be positive-definite. Consequently, the eigenvalues of these matrices, denoted κi, i = 1, 2, ..., 6, will be computed.
• If κi < 0 for any i, an error message if printed “Stiffness matrix is not positive-definite.”
• Let κmax denote the maximum eigenvalue of the stiffness matrix; if κi < 0.01 κmax for any i, a warning message is printed “Some eigenvalues of the stiffness matrix are small.”
5. It is possible to attach comments to the definition of the object; these comments have no effect on its definition.