Experimental Modal Analysis of a Brake Disc

1. Experimental Setup & Hardware Configuration

The impact-based modal testing was designed to evaluate the structure under free-free boundary conditions. To decouple rigid body modes from structural modes and minimize external energy dissipation, the disc was supported using low-absorption soft foam elements.

Data acquisition was performed utilizing Siemens acquisition hardware and Simcenter Testlab. Structural excitation was provided by an instrumented impact hammer, while a piezoelectric accelerometer, mounted with beeswax to minimize mass-loading, tracked the dynamic response across 84 Degrees of Freedom (DOFs).

Prior to the test campaign, transducer sensitivities were strictly calibrated within the Testlab channel setup to convert voltage signals into highly accurate physical quantities. The detailed acquisition and H1 FRF estimation parameters are illustrated in Figure 1.

2. Measurement Procedure & Signal Quality Assessment

A moving-accelerometer approach was adopted for the measurement campaign. To improve signal-to-noise ratio and eliminate random noise effects, five distinct impacts were recorded and averaged for each of the 84 acquisition points.

Signal reliability was actively monitored via the coherence function (Figure 2), ensuring values consistently approached unity. Expected local coherence drops were correctly identified near structural antiresonances, where the physical response amplitude approaches the noise floor of the instrumentation. The overall validity of the dataset was confirmed through the combined evaluation of FRF amplitude, phase, and Power Spectral Density (PSD) functions, which were continuously tracked through the measurement interface (Figure 3).

3. Modal Parameter Identification

The post-processing phase relied on a Multiple Degree of Freedom (MDOF) curve-fitting approach within Siemens Testlab to extract the modal parameters from the measured FRFs.

A stabilization diagram (Figure 4) was employed to systematically distinguish true physical structural modes from mathematical artifacts. By progressively increasing the model order, poles demonstrating stable frequency and damping characteristics across successive iterations were successfully isolated.

4. Results & Validation

The MDOF identification procedure successfully extracted 14 distinct structural modes. The robustness of the numerical model was validated by synthesizing the reconstructed FRFs directly from the extracted modal parameters. The synthesis demonstrated a high degree of correlation with the raw experimental measurements in both amplitude and phase across the entire analyzed frequency bandwidth.