Modal analysis using FEM

The goal of modal analysis in structural mechanics is to determine the natural mode shapes and frequencies of an object or structure during free vibration. It is common to use the finite element method (FEM) to perform this analysis because, like other calculations using the FEM, the object being analyzed can have arbitrary shape and the results of the calculations are acceptable. The types of equations which arise from modal analysis are those seen in eigensystems. The physical interpretation of the eigenvalues and eigenvectors which come from solving the system are that they represent the frequencies and corresponding mode shapes. Sometimes, the only desired modes are the lowest frequencies because they can be the most prominent modes at which the object will vibrate, dominating all the higher frequency modes.

It is also possible to test a physical object to determine its natural frequencies and mode shapes. This is called an Experimental Modal Analysis. The results of the physical test can be used to calibrate a finite element model to determine if the underlying assumptions made were correct (for example, correct material properties and boundary conditions were used).

FEA eigensystems

For the most basic problem involving a linear elastic material which obeys Hooke's Law, the matrix equations take the form of a dynamic three dimensional spring mass system. The generalized equation of motion is given as:^[1]

$[M] [\ddot U] + [C] [\dot U] + [K] [U] = [F]$

where [M] is the mass matrix, $[\ddot U]$ is the 2nd time derivative of the displacement [U] (i.e., the acceleration), $[\dot U]$ is the velocity, [C] is a damping matrix, [K] is the stiffness matrix, and [F] is the force vector. The general problem, with nonzero damping, is a quadratic eigenvalue problem. However, for vibrational modal analysis, the damping is generally ignored, leaving only the 1st and 3rd terms on the left hand side:

$[M] [\ddot U] + [K] [U] = [0]$

This is the general form of the eigensystem encountered in structural engineering using the FEM. To represent the free-vibration solutions of the structure harmonic motion is assumed ^[2], so that $[\ddot U]$ is taken to equal λ[U], where λ is an eigenvalue (with units of reciprocal time squared, e.g., s ^{− 2}), and the equation reduces to:^[3]

[M][U]λ + [K][U] = [0]

In contrast, the equation for static problems is:

[K][U] = [F]

which is expected when all terms having a time derivative are set to zero.

Comparison to linear algebra

In linear algebra, it is more common to see the standard form of an eigensystem which is expressed as:

[A][x] = [x]λ

Both equations can be seen as the same because if the general equation is multiplied through by the inverse of the mass, [M] ^{− 1}, it will take the form of the latter.^[4] Because the lower modes are desired, solving the system more likely involves the equivalent of multiplying through by the inverse of the stiffness, [K] ^{− 1}, a process called inverse iteration.^[5] When this is done, the resulting eigenvalues, μ, relate to that of the original by:

$\mu = \frac{1}{\lambda}$

but the eigenvectors are the same.

Methods of solution

For linear elastic problems that are properly set up (no rigid body rotation or translation), the stiffness and mass matrices and the system in general are positive definite. These are the easiest matrices to deal with because the numerical methods commonly applied are guaranteed to converge to a solution. When all the qualities of the system are considered:

1. Only the smallest eigenvalues and eigenvectors of the lowest modes are desired
2. The mass and stiffness matrices are sparse and highly banded
3. The system is positive definite

a typical prescription of solution is first to tridiagonalize the system using the Lanczos algorithm. Next, use the QR algorithm to find the eigenvectors and eigenvalues of this tridiagonal system. If inverse iteration is used, the new eigenvalues will relate to the old by $\mu = \frac{1}{\lambda}$, while the eigenvectors of the original can be calculated from those of the tridiagonalized matrix by:

[rⁿ] = [Q][vⁿ]

where [rⁿ] is a Ritz vector approximately equal to the eigenvector of the original system, [Q] is the matrix of Lanczos vectors, and [vⁿ] is the n^th eigenvector of the tridiagonal matrix.

Example

The mesh shown below is the frame of a building modeled as beam elements, specifically consisting of 930 elements and 385 nodal points. The building is constrained at its base where displacements and rotations are zero. The next images are that of the first 5 lowest modes of this building during free vibration. This problem can be seen as a depiction of the likeliest deflections a building would take during an earthquake. As expected, the first mode is a swaying of the building from front to back. The next mode is swaying of the building side to side. The third mode is a stretching and compression mode in the vertical y direction. For the fourth mode, the building nearly assumes the shape of half a sine wave. The fifth mode is a twisting mode.

original mesh

mode 1 swaying front to back

mode 1 and original mesh

mode 2 swaying side to side

mode 2 and original mesh

mode 3 stretching and compression

mode 3 and original mesh

mode 4 sine shape

mode 4 and original mesh

mode 5 twisting

mode 5 and original mesh

See also

• Finite element method
• Finite element method in structural mechanics
• Modal analysis
• Seismic analysis
• Structural Dynamics
• Eigensystem
• Eigenmode
• Quadratic eigenvalue problem
• Modal Analysis for deformation simulation

Footnotes

1. ^ Clough, Ray W. and Joseph Penzien, Dynamics of Structures, 2nd Ed., McGraw-Hill Publishing Company, New York, 1993, page 173
2. ^ Bathe, Klau Jürgen, Finite Element Procedures, 2nd Ed., Prentice-Hall Inc., New Jersey, 1996, page 786
3. ^ Clough, Ray W. and Joseph Penzien, Dynamics of Structures, 2nd Ed., McGraw-Hill Publishing Company, New York, 1993, page 201
4. ^ Thomson, William T., Theory of Vibration with Applications, 3rd Ed., Prentice-Hall Inc., Englewood Cliffs, 1988, page 165
5. ^ Hughes, Thomas J. R., The Finite Element Method, Prentice-Hall Inc., Englewood Cliffs, 1987 page 582-584

References

• Clough, Ray W. and Joseph Penzien, Dynamics of Structures, 2nd Ed., McGraw-Hill Publishing Company, New York, 1993.

• Golub, Gene H. and C.F. Van Loan, Matrix Computations, 3rd Ed., The Johns Hopkins University Press, Baltimore, 1996.

• Hughes, Thomas J. R., The Finite Element Method , Prentice-Hall Inc., Englewood Cliffs, 1987.

• Thomson, William T., Theory of Vibration with Applications, 3rd Ed., Prentice-Hall Inc., Englewood Cliffs, 1988.

• Bathe, Klau Jürgen, Finite Element Procedures, 2nd Ed., Prentice-Hall Inc., New Jersey, 1996.

External links

• Frame3DD open source 3D structural modal analysis program
• Example of Wine Glass analyzed in ANSYS