A Complete Solution for Modal Pretest Analysis. FE
Model Verification and
Validation using Test-Analysis Correlation
FEMtools Correlation contains tools for:
- Pretest Analysis - Find optimal number and location of test transducers.
- Correlation Analysis - Visual and numerical correlation between two sets of shapes or functions (FEA-Test, FEA-FEA, Test-Test).
If a baseline finite element model is available, then this model
can be used to simulate tests. This provides test engineers with
optimal locations and directions to excite the structure, and to
position measurement transducers. The FE model can be reduced and
converted into a test model. Questions that can be answered with
pretest analysis include:
- How many modes can be expected in a given frequency range?
- What are the optimal location and directions for sensors, exciters and suspensions from a set of candidate locations?
- Create a test model from a reduced finite element model and export in a format readable by modal test packages.
- Determine the directions normal to the surface of curved surfaces from the finite element model and use this information
for decomposing modal test displacements in Cartesian coordinates.
- What are the measurement directions expressed as Euler angles?
- Assess the influence of the accelerometer mass on the modal parameters.
Using the pretest analysis tools in FEMtools Correlation it is
possible to plan an optimal modal test strategy early in the project
and increase quality of modal data for validation and updating of
- Baseline Finite Element Analysis - Analyze mode shapes
in the frequency range of interest. FEA data (model, modes,
FRFs) can be imported or computed using FEMtools Framework or
- Target Mode Selection - Select modes in the frequency
band of interest based on energy considerations. Methods include:
Modal Effective Mass, Kinetic Energy Fraction.
- Selection of Candidate Sensor Locations - Use criteria
like accessibility, cost, geometry (surface, edge or corner
nodes) or any other user-defined criteria to select candidate
locations. A set of fixed sensors can be added.
- Sensor Placement Metrics - These are semi-automatic
methods to find optimal exciter, suspension and measurement
locations and directions. They are based on the observability
of target modes using information on modal displacement or energy
(kinetic or strain). Methods include: Normalized Modal Displacements,
Nodal Kinetic Energy.
- Sensor Elimination Methods - These methods iteratively
eliminate sensors from the set of candidates in a way to optimally
maintain linear independence or orthogonality between mode shapes.
Methods include: Effective Independence Method, Elimination
by MAC, Iterative Guyan reduction.
- Mass Loading Evaluation - This tool
evaluates the effect of accelerometer mass on the modal
- Creation and Export of Test Model - Truncation of the
FE model, conversion to test model and export to a modal test
software. Automatic generation of tracelines between retained
sensor locations. Directions normal to the surface can be obtained
from the FE model. The measurement directions can be
expressed as Euler angles.
- Plan a test strategy early in the project.
- Easily find optimal location for sensors, exciters and suspension.
- Fast creation of a test model from a baseline FE model.
- Increase quality of modal test data for validation and updating of FE models.
Correlation analysis quantitatively and qualitatively compares
2 sets of analysis results data. Usually this is a FEA and a test
database that are imported in the FEMtools database. However, the
tools can be used for FEA-to-FEA and test-to-test correlation as
- Spatial correlation - Compares location in space
between response locations resulting in a table with mapped
degrees-of-freedom. This may require changing orientation and
scaling of the models, which can be done in a manual way or
using automatic tools.
- Visual shape correlation - Visually compare shapes
(static displacement shapes, mode shapes and operational shapes)
using side-by-side, overly and animated displays.
- Global shape correlation - Globally compares shapes
using various criteria. The result is used for shape pairing.
- Local shape correlation - analyzes local spatial
correlation between shapes. Results can be interpreted to localize
modeling deficiencies and serve as guideline for selecting model
- Shape pairing - Creates a table of shape pairs (static,
modal or operational).
- FRF pairing - Creates a table of FRF pairs.
- FRF correlation - Analyzes correlation between FRF
functions, either globally between 2 functions or shape and
amplitude correlation functions for a set of FRF pairs as function
- Correlation coefficients - Calculates values of error
functions from a selection of reference responses. These functions
are used in model updating to monitor the distance between
the updated model and a reference.
Correlation analysis is used for FE model validation,
design of optimal test conditions, evaluate different modeling strategies,
identification of modeling errors, damage detection, ...
Results from correlation analysis are used to define
targets for FE model updating. Similar mode shapes can be identified
in the FE and test database thus providing residues in terms of
resonance frequency differences, MAC, modal displacements.
Another application is to provide the analyst with
information that can only be measured. An example is modal damping,
used in modal superposition methods. Modal damping can be obtained
experimentally and applied to the analytical mode shape that, using
correlation analysis, was found to best match the experimental one.
Modal correlation analysis is also used to scale
test mode shapes obtained by output-only modal analysis. The same
scaling as used by the analytical mode shapes (e.g. unity modal
mass), can be applied to the correlated test modes.
Unlike global correlation analysis, spatial correlation
methods are used to identify areas of better or poorer correlation,
which when linked to structural information, can be interpreted
in terms of 'modeling error'. Depending on how these tools are used,
the results help with the selection of updating variables (parameters),
or are used to assess structural damage.
- FEA-Test, FEA-FEA, Test-Test Correlation.
- Automated or manual model mapping.
- DOF pair table definition, ranking and filtering.
- Static, modal and operational shape correlation analysis
using Modal Assurance Criterion (MAC).
- Mode shape auto- and cross-orthogonality check using full
or reduced system matrices.
- Automated support for double modes (axisymmetric structures).
- Automatic mode shape pairing.
- MAC contribution analysis.
- Spatial shape correlation using Coordinate MAC (CoMAC),
Coordinate Orthogonality Check (CORTHOG), Correlated Shape Difference
and Modal Force Residue analysis.
- FRF correlation (SAC, CSAC, CSF).
- Correlation using local test coordinate systems.
- All definition, editing and analysis accessible via intuitive
menus and dialog boxes or using free format commands for batch
processing and process automation.
- Complete electronic documentation.
- Dedicated graphics viewers for model inspection and results
- Point-and-click interactive selection.
- Direct access to FEA and test data.
- All pretest analysis and correlation tools are programmed
in FEMtools Script language and can be easily customized or
- Customizable user interface.
- Solver-neutral integration with virtually every FEA and
- Computing and OS platform-independent solutions.