Modal analysis provides important information about the natural frequencies, damping coefficients and mode shapes of a test unit to optimize its design and improve its structural behavior. The modal parameters and mechanical properties of a structure helps users understand its vibration characteristics in operating conditions.
In this case, the modal characteristics of a baseball bat is acquired by performing experimental modal analysis. A hammer impact test is carried out with two uni-axial accelerometers to study its modal behavior. The roving excitation method completely avoids the mass loading effect that is potentially introduced with a roving response procedure. A hard metal tip is chosen to excite higher frequency modes. The hammer impact test module of the EDM Modal suite is used for this test.
Figure 1. Hammer impact modal test of the baseball bat
A geometry mesh configuration of 168 measurement points is uniformly distributed throughout the baseball bat to obtain a good spatial resolution of the mode shapes. The baseball bat is hung with bungee cords to imitate a free-free boundary condition (as shown in the experimental setup). The uni-axial accelerometers are fixed to two points and the modal impact hammer is roved throughout the measurement points. Measuring the excitation force and response acceleration in the radial direction obtains out-of-plane mode shapes.
Figure 2. Baseball Bat Geometry
A sampling rate of 8 kHz is set. A block size of 8192 is selected to ensure the response decays naturally and windowing is not needed. A fine frequency resolution of 0.976 Hz is produced with these configuration settings. Measurements of higher accuracy and reduced noise are obtained by linearly averaging 3 blocks of data at each measurement DOF.
The hammer impact excitation imparts energy across a broad frequency range of 3.5 kHz. With this setup, there will be no leakage and a uniform window can be selected.
Figure 3. Hammer impact measurement of the baseball bat
The coherence plot validates the measurement result, which looks good from the preceding screenshot. The valleys in the coherence plot occur at the anti-resonances which indicates that the response level is relatively lower at these corresponding frequencies. So overall, the inputs and outputs are well correlated in the desirable frequency range.
The FRF measurement shows well identified resonance peaks in the desired frequency band. The well aligned peaks indicates there is no mass loading effect induced in the results.
Figure 4. Modal Data Selection tab showing the overlapped FRFs
The Band Selection tab displays the Complex Mode Indicator Function (CMIF) and the Summed FRFs used to indicate the peaks at the natural frequencies. The Poly-X method is used to curve-fit the FRF’s to obtain flexible modes within the desired frequency range.
Figure 5. Band selection for measured FRFs
The following screenshots display Bending and Hoop modes of the baseball bat associated with stable physical poles.
Figure 6. Bending & Hoop Modes
These results emphasize the strength and efficiency of EDM Modal software to execute sophisticated modal tests on small intricate structures.
To learn more about EDM Modal software, visit: www.crystalinstruments.com/structural-testing