The following Tech Tips were previously published in the FEMtools News newsletter:
Scenario-Based Damage Identification (March 2012)
Damage has a direct impact on the modal parameters of structures. However, finding the location and severity of the damage from the modal parameters is a challenging task. This is because damage identification problems are in general highly undetermined, i.e. the number of potential damage locations is much higher than the size of the experimental data set.
In a recent paper written by DDS engineers, a damage scenario-based framework was presented that tries to overcome this problem by both increasing the size of the experimental data set and reducing the number of investigated damage locations. Using a carefully validated FE model of the undamaged structure the effects of a number of damage scenarios are simulated. Eventually, the identification routine detects the ‘fingerprints’ of the damage scenarios in the frequency pattern of the damaged structure.
The scenario-based damage identification framework has been evaluated and showed promising results. It appears to be possible to decompose the measured frequency pattern into the signatures of a series of pre-defined damage scenarios. The scenario-based approach seems to be capable of not only identifying the location of the damage but also the degree of damage.
The complete paper can be downloaded from the following location:
T. Lauwagie, E. Dascotte, A Scenario-based Damage Identification
Framework. Presented at the 30th International Modal Analysis
Conference (IMAC), February 2012, Jacksonville, Florida, USA.
For more information, contact firstname.lastname@example.org
Working with Abaqus Condensed Matrices (October 2011)
ABAQUS can condense stiffness and mass matrices at external nodes and enriched by Craig-Bampton component modes synthesis. This is equivalent to the use of superelements in FEMtools and other finite element programs.
FEMtools 3.5.2 comes with a new interface to import
superelement matrices condensed with ABAQUS.
The superelement reduction can be used to speed-up pretest analysis, dynamic analysis, correlation analysis on large assemblies and even model updating of the residual part of the FE model. Using a wireframe connection between the external nodes, FEMtools allows the visualization of mode shapes using a ?test model? look and feel.
For more information, contact email@example.com
Geometry Updating (May 2011)
The concept of geometry updating was explored in a recent study of the cast iron lantern housing of a gear box. The resonant frequencies and mode shapes of the test structure were measured using impact testing. Next, a set of digital pictures were taken from a number of different angles. By means of photogrammetry, these pictures were converted into a surface model that represented the actual geometry of the lantern housing. This surface model was then compared with an FE-model derived from a CAD-model of the lantern housing. In this way, the regions where there was a substantial difference between the actual geometry and CAD-model could be identified. Finally, the geometry of the FE-model was corrected based on the measured geometry using a mesh morphing technique. For the considered test case, the correction of the geometry provided a significant improvement of the quality of FEM-test correlation of the modal parameters.
The project demonstrated that only a limited number of geometry measurements are needed to update a CAD-based geometry using mesh morphing techniques. With geometry updating it is possible to eliminate most of the uncertainty on the geometry. As such, geometry updating eliminates, or at least reduces, the need for equivalent parameter changes to compensate the effects of geometrical inaccuracies. As the updating process provides parameter changes that are physically more relevant, the application range in which the updated FE-model can be used as a reliable predictive tool for design optimization can be increased.
Improving the accuracy of the FE model to predict a larger number of mode shapes covering a wider frequency range, increases chances to detect damages or manufacturing issues by monitoring the modal parameters. Combined with automated testing and metrology, this opens up the perspective of a modal-based quality inspection tool.
Two technical papers on this subject were presented at the international conferences and are now available for download from the FEMtools website:
T. Lauwagie, E. Dascotte, Geometry-based Updating of 3D Solid Finite Element Models. Presented at the 29th International Modal Analysis Conference (IMAC), February
2011, Jacksonville, Florida, USA.
T. Lauwagie, F. Van Hollebeke, B. Pluymers, R. Zegels, P. Verschueren, E. Dascotte,
The Impact of High-Fidelity Model Geometry on Test-Analysis Correlation and FE Model Updating Results. Presented at the International Seminar on
Modal Analysis 2010 (ISMA), September 20-22, 2010, Leuven, Belgium.
ODS-Based Model Updating (December 2011)
Finite element model updating is a well established method for validating and improving simulation models in structural dynamics. The traditional approach consists of correlating simulation data with the results of an experimental modal analysis (EMA). Natural frequencies and mode shapes extracted from frequency response functions are preferred as references since they are independent of the applied loads.
However, the operational loads or boundary conditions can change the dynamic behavior of a structure, or make it impossible to perform an experimental modal analysis with measured or controllable dynamic loading. In such cases, only operational data can be used as reference data for model updating. Additionally, updating a model using operational data automatically guaranties the validity of the model under the considered operational conditions.
DDS recently introduced a new method in FEMtools for model updating based on Operational Deflection Shapes (ODS) that is able to update the mass, stiffness and damping properties of a structure simultaneously. A technical paper describing the method is available for download:
T. Lauwagie, J. Guggenberger, J. Strobbe, E. Dascotte, Model Updating
using Operational Data. Presented at the International Seminar on Modal
Analysis 2010 (ISMA), September 20-22, 2010, Leuven, Belgium.
More technical papers on different subjects can be found at http://www.femtools.com/products/papers.htm
Free Online Tutorial on Using Reprise License Manager (September 2010)
Reprise Software, the developers of the RLM license manager that is used in FEMtools, has conducted a number of on-line tutorials designed to help licensed software end users get more value out of the Reprise License Manager (RLM). The recorded sessions can now be viewed from the Reprise Software website at http://www.reprisesoftware.com/support/end-users.php.
For more information on tutorials, see the Reprise Software Blog. FEMtools users are encouraged to subscribe to this blog to receive regular news about the RLM license manager.
Mapping Laser Scanning Measurements on a FE Mesh (December 2009)
Laser vibrometry or electronic holography can be used to obtain vibration modes which in turn can be correlated with finite element results. Each experimental mode shape will typically be presented as a dense cloud of scanning points with each point moving in the direction of the laser or camera. Analyzing the correlation of these vibration modes with a finite element model poses some specific problems with respect to mapping the scanned surface onto FE model, identifying and extracting the corresponding translation degrees of freedom, averaging for measurement noise and computing numerical correlation criteria.
DDS has recently developed a custom solution for postprocessing a set of data files containing measured vibration modes with an ANSYS finite element model of a turbine blade. Written in FEMtools Script, this solution automates the entire work flow and reporting of results. It can be integrated into the FEMtools menus or operated in batch mode for processing large quantities of data or as part of an integrated quality inspection system.
For more information, contact firstname.lastname@example.org
Estimating Rigid Body Properties from FRF Measurements: A New FEMtools Add-On Tool (April 2009)
Dynamic Design Solutions announces the upcoming release of FEMtools Rigid Body Properties Extractor, a new tool to obtain the mass (M), center of gravity (CoG) and mass moments of inertia (MoI) from the low-frequency portion of measured accelerances (FRFs). These properties are useful to serve as targets references in updating the finite element model of the tested structure, or to reduce components to lumped masses for model reduction in structural dynamics simulations or motion analysis.
Obtaining the rigid body properties is done in 2 steps. First, the rigid body responses (called "mass lines") must be extracted from the FRFs. The mass line corresponds to the value of the flat part of the response located between the low-frequency suspension modes and the elastic modes. These values are obtained by using a least squares fit of a low order polynomial (e.g. quadratic approximation). Once the mass line values are known, then the rigid body mass properties are obtained in the second step by solving a set of algebraic equations.
Experimental data are imported from a Universal File (UF) or other. The FRFs must satisfy some requirements with respect to the measurement configuration (acceleration/force obtained from a freely suspended structure), and the number and positioning of excitation and response locations. If these conditions can be satisfied, then the proposed method presents a low-cost and fast alternative to traditional pendulum techniques.
The FEMtools Rigid Body Properties Extractor comes as an add-on to any FEMtools configuration. It is an interactive tool that allows the user to display the FRFs, define a frequency band between the suspension modes and elastic modes, and analyze the mass line values. An inertia box visualizes the equivalent mass volume with animation of rigid body modes. The rigid body properties (M, CoG and MoI) are displayed in real-time using tabular format with several error estimates. They can be exported for use in other programs or be used as updating targets in FEMtools Model Updating.
This new exciting add-on is now available for beta evaluation. For more information and to request an evaluation copy, contact email@example.com
Ogden Material Identification using FEMtools Optimization (December 2008)
The Ogden material model is frequently used in finite element programs to simulate the behavior of non-linear elastomers. The values of the material parameters of the Ogden model are highly material dependent. The main challenge in using the Ogden model in finite element simulations, is to find reliable estimates for the values of the Ogden material parameters. The relation between an imposed displacement and the resulting reaction force can be used to identify these material parameters using a mixed numerical-experimental approach. In this approach, the objective is to fit the simulated reaction force curve onto the measured reaction force curve. The computationally most efficient way of doing that is by using a gradient-based optimization strategy.
Such identification routine was implemented using FEMtools Script for the process identification part, FEMtools Optimization for the optimizer routines, and used MSC.Marc to compute the reaction force curves.
More information can be found in the following application note:
Identification of Ogden Material Parameters using FEMtools, Download (PDF, 433 KB)
Finding Optimal Master DOF for Guyan Reduction with FEMtools Pretest Analysis Tools (June 2008)
The pretest analysis tools in FEMtools Correlation are primarily used to find the optimal number and location of transducers for modal testing. One of the methods that are available is the Iterative Guyan Reduction (IGR) method, which is an elimination method to optimize sensors using the modal cross-orthogonality as selection criterion. The method can as well be used for selecting master DOF for Guyan reduction. In an FEA-only context, there are no constraints on the number of master DOFs and their accessibility because they will not serve as test locations. Furthermore, master DOF can include rotational DOF.
Using the IGR tool in FEMtools is a fast and efficient way to select master DOFs for structural components that will be reduced using Guyan reduction.
For more information on this application, contact firstname.lastname@example.org
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