Modal Analysis of an Audi TT using Universal File Format

The experimental tests presented here has been conducted at TH Ingolstadt, Faculty of Mechanical Engineering, Germany under the guidance of Professor J. Bienert, who is gratefully acknowledged.

The experimental campaign consists of impact hammer testing of an Audi TT car body, see a picture depicted below.

The car body was mounted free on four air cushions. Four references impact points were excited with 14 accelerometers as responses. The data acquisition and processing of the Frequency Response Functions (FRF’s) were made with a 24-channel data acquisition system.

Creating an ARTeMIS Modal Project

The FRF’s were stored in the four separate Universal File Format files (Audi_TT_FRF_runX.unv) along with a test geometry in its own Universal File Format file (Audi_TT.unv).

To create a project in ARTeMIS Modal, the Universal File Format option under Create New Project should be selected:

This option opens a dialog where the geometry file and the files with FRF’s can be specified and uploaded:

Once the files are uploaded, the geometry and FRF’s can be seen in the simplified Prepare Data task. The Data Organizer contains all four impact tests listed below each other, and the Geometry window visualize the positions and the directions of the response and reference channels. All the impact positions can be viewed by selecting the first item, All Test Setup, in the Data Organizer, illustrated below.

The 14 response accelerometers are seen in blue, and the impact positions, indicated by a hammer symbol, are colored green.

Estimating Modes using the RFP-Z method

The modal parameters of the car body are computed with the RFP-Z method. The workflow is as follows:

Select a maximum model order to initialize the estimation of the system matrices of the highest dimension for the subsequent computation of a lower model orders
Select the desired frequency band
Select a specific range of the model orders by dragging the mouse down on the stabilization diagram, or simply click the Estimate Models button to estimate all the model orders
Stable modes are found by a study of the natural frequencies, damping ratios and mode shapes (MAC and maximum modal complexity)
Global modes are estimated based on the stable modes in the diagram

The figure below illustrates the analysis of the experimental test of the Audi TT in the frequency range from around 3 Hz up to 200 Hz. Maximum model order was set to 70.

Estimating Modes using the CMIF method

The modal parameters of the car body are also computed with the CMIF method. The workflow is as follows:

Select the desired frequency band
Peak pick the resonance peaks either manually or by clicking the Automatic Mode Estimation button
Global modes are estimated from the Singular Value Decompostion of the FRF’s.

The figure below illustrates the analysis of the experimental test of the Audi TT in the frequency range from around 3 Hz up to 200 Hz

Validating Modes with Other Estimators

The modes obtained from the RFP-Z and CMIF estimators are validated using the standard tools available in ARTeMIS Modal. By selecting the Validation Task, the modes can be compared with the other estimators as well as imported results from any other modal analysis tools.
Below the RFP-Z and CMIF modes are compared by overlaying mode shapes of two identical modes obtained with two different methods. In addition, the Cross-MAC diagram of the modes below 100 Hz of the two methods is shown as well.

Modal Analysis of an Audi TT using Universal File Format

Creating an ARTeMIS Modal Project

Estimating Modes using the RFP-Z method

Estimating Modes using the CMIF method

Validating Modes with Other Estimators

Watch the full demonstration:

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