STUDYING DIFFERENT TYPES OF DAMPING IN DYNAMIC ANALYSIS (USING SUPERPOSITION)
Damping can be defined as energy dissipation mechanism that causes vibration to diminish over time. For example, shock absorbers absorb the shock / vibrations to maintain comfortable ride. It cannot be directly measure since energy dissipation can come from joints and part interaction, fluid interactions, material behavior and other sources. It can be estimated via testing or is specified by industry practice. It is present in most systems and has to be specified in dynamic analysis.
Using of damping for modal analysis types:
Types of Damping input :
Damping= constant damping+ alpha damping+ beta damping
Alpha damping is mass coefficient and beta damping is stiffness coefficient. This both damping are summed up and known as Rayleigh damping.
Following damping values can be defined on system level affecting all parts in “Analysis setting” or on material basis affecting all parts that have material assigned in Engineering Data. MSUP analysis can be applied to lightly damped systems. As MSUP method use undamped nodes, but heavy damping can cause mode and frequency to change. So, light dampening is assumed. Effect of damping are cumulative, where specifying global damping and material damping, after that part consider the effect of sum of material and global damping. In Transient analysis, numerical damping minimize noise due to high frequency but does not represent physical energy dissipation process.
Harmonic response analysis of system under effect of damping:
Effect of constant and beta damping on electronic enclosure exposed to harmonic base excitation within frequency range of 200-350 Hz. The enclosure of the electronic component modeled as point and distributed masses. Here enclosure is modeled as a surface body, meshed with shell elements and material is selected as structural steel. After solving modal analysis we get the natural frequency in range of 200-350 Hz for 10 nodes.?
The first 2 nodes have high Participation factors in Y-direction for 1st two modes in the frequency range of 200-350 Hz. For adding cluster points near natural frequencies for getting peak responses. Adding acceleration 1000mm/s in Y-direction and applying other boundary conditions in Harmonic response. After solving and evaluating frequency response of top and bottom surfaces.
The peak amplitude is at 246Hz and 315Hz which are frequencies of 1st and 2nd mode.
2. Similarly, for top surface the damping ratio,
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The peak amplitude is around 315Hz.
For Beta damping, same conditions as Normal damping but
As 350Hz damping is 3%, so at the frequency of 175Hz damping will be 1.5% and if the frequency is to be extended our frequency range to 700Hz the damping will be double i.e 6%, so during choosing frequency range for Beta damping being selective is important part.
Similarly, for solving and evaluating frequency response of top and bottom surfaces.
For base the Beta damping ratio,
The peak amplitude is at 246Hz and 315Hz which are frequencies of 1st and 2nd mode.
Similarly, for top surface the Beta damping ratio,
The peak amplitude is around 315Hz.
Now comparing the frequency response of base for Damping ratio and Frequency response of base for Beta Damping case,
Now comparing the frequency response of Top surface for Damping ratio and Frequency response of Top surface for Beta Damping case,
In both the charts Red line represents response with damping ratio defined and green line represents response with Beta damping. So Damping ratio is constant within frequency range while Beta damping is linearly proportional to frequency.