Fem Fundamentals and Chassis Design, spring 2022 course

In order to include a small degree of structural damping (eg. 1% of the critical value) into a MSC.Marc/Mentat harmonic response calculation, the following steps may be followed:

• enter the menu MAIN → MATERIAL PROPERTIES → MATERIAL PROPERTIES;
• preemptively define a modulating table 1/ω
  • menu TABLES, NEW → 1 INDIPENDENT VARIABLE
  • define NAME as modulate_stiffmatmult
  • set Indipendent variable v1 TYPE as frequency
  • define table through FORMULA and type 1/pi/v1, i.e. $g(f)=\frac{1}{\pi f}$
• go back to MAIN → MATERIAL PROPERTIES → MATERIAL PROPERTIES by hitting RETURN;
• select the various model materials, and for each of them enter the submenu STRUCTURAL → DAMPING and activate DAMPING;
• leave alone the MASS MATRIX MULTIPLIER value (0 is ok, otherwise some “structural” damping will be associated to rigid body motions),
• define a STIFFNESS MATRIX MULTIPLIER equal to the desired fraction of the critical value, namely 0.01,
• set a frequency modulating function, namely TABLE, by hitting the TABLE button on the right of the stiffness matrix multiplier value;
• select the just defined modulate_stiffmatmult table as the modulating one, hence hitOK and OK again to return back at the material properties menu
• in this way, I defined damping as a function of the $\alpha$ e $\beta$ coefficients introduced by the Rayleigh proportional damping model, with zero $\alpha$ and hence no contribution of the mass matrix. In particular $\zeta = \frac{1}{2}(\frac{\alpha}{2 \pi f}+2 \pi f \beta)$ with $\alpha=0$ and $\beta= 0.01 \cdot g(f)=\frac{0.01}{\pi f}$, from which $\zeta=0.01$ as desired.
• enter the MAIN → JOBS menu and create a copy of the undamped harmonic response job by hitting the COPY top left button and by setting a new job name;
• enter the job PROPERTIES menu, and reach the ANALYSIS OPTIONS submenu; activate the COMPLEX DAMPING options within the dynamic harmonic section, and then exit withOK
• Enter the JOB RESULTS submenu and deactivate Stress, Equivalent von Mises stress
• substitute them with the AVAILABLE ELEMENT SCALARS
  • Equivalent Real Harmonic Stress , layers MAX & MIN
  • Equivalent Imag Harmonic Stress , layers MAX & MIN
  • the REAL HARMONIC e IMAG HARMONIC stress resultant equivalents for the beam elements, DEFAULT layer, and the Beam Orientatio Vector
• insert from the AVAILABLE ELEMENT TENSORS block
  • Real Harmonic Stress , layers ALL
  • Imag Harmonic Stress , layers ALL
• run the simulation as usual with RUN → SUBMIT
• open the post file as usual with OPEN POST FILE (RESULTS MENU)
• The deformed shape may be visualized according to a given phase within the oscillation cycle (see also the DEFORMED SHAPE SETTINGS menu); in the absence of damping the fase was limited to the 0° and 180° values, cases these that may be represented with the bare variation in sign of the stress and displacement components to be monitored.
• Please note that the real component has a 0° phase ($\cos(\omega t)$ modulation) whereas the imaginary component has a 270° phae ($-\sin(\omega t)$ modulation).
• Please also note that in resonance conditions the imaginary component becomes dominant and reaches the peak values, whereas the real component vanishes (resonant response is in fact ~90° out of phase with respect to the real, 0° excitation).
• Lets e.g. collect the displacement in $z$ direction of the node at the center of the excited wheel contact area:
  • enter the POSTPROCESSING RESULTS menu, with opened t16 result file, and proceed within the HISTORY PLOT submenu
  • define the locations for the response sampling with SET LOCATIONS, hence click on the desired node[s], and finalize with END LIST
  • define the range of the sub-increments to be collected with INC RANGE, and then entering at the prompt 0:1 [enter], 0:397 [enter], 1 [enter], as the sampling beginning, end and step.
  • proceed with the definition of collected response diagrams by entering th ADD CURVES menu, and thenALL LOCATIONS (a single location has been selected); select the Frequency global variable as the abscissa, and the Displacement Z Magnitude nodal variable as the ordinate. The FIT scales the axes to contain all the sample points.
  • By hitting RETURN I may return to the HISTORY PLOT menu, where the label density may be reduced SHOW IDS from '1' to '10'; by entering a '0' value labels are hidden.
• response peaks are now finite (they were theoretically unbounded in the absence of damping), and peaks disappear in correspondence of natural modes that are weakly coupled with the exciting force. In the absence of damping, bounded response at resonance is obtained for strictly uncoupled natural modes only.