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Random Vibration


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Does Mechanica do dynamic shock and vibraton? I'm new to this tool and have a general need to analyze things that will see half-sinewave shock pulses, sine-sweep vibration, and random vibration (PSD profiles of my own design)? Does Mechanica handle damping? If so, where does one get damping values for various materials?

Lastly, does Mechanica do transient thermal problems?

Thanks in advance for your expert tips! I apologize for the wide open quesitons, but my software (Student Edition) came without documentation on capability.

Yes - to both.

For dynamic analysis (frequency response, shock loading, etc) select Mechanica | Structure | Analyses, and select the appropriate analysis type.

For transient thermal analysis, go to Mechanica | Thermal, and select either steady-state or transient thermal analysis.

However, I have to admit I too am somewhat stumped by how to get the kind of results output I would like from dynamic analysis - for example, I am having trouble producing time-displacement plots. It keeps telling me that there are no suitable measures defined. Does anyone know how to tell Mechanica that displacements or stresses are measures I wish to track, so that I can plot them as a function of elapsed time? Are there any good tutorials for basic dynamic analysis in Mechanica Integrated mode?

(Using Pro/M 2001 Student Edition)

All help greatly appreciated.
Found the answer to my own question!

You have to define a Measure prior to comencing the dynamic analysis. Eg define a number of Datum Points at key vertices of the model, then define one or more Measures such as displacement or stress at selected datum points.

Then, graphs of your selected Measure(s) will be available as a function of elapsed time, frequency, etc.

Pretty obvious, really!
Thanks for the good tips!

What about damping? Did you consider it? If so, where did you get values for the damping constants?

Yes, you can enter damping as well.

As for approipraite values to use - welcome to the Black Art of dynamic analysis!

My background is in structural engineering, rather than in mechanical applications. Basically, one uses experience, together with whatever refrence sources are available. Eg most practical textbooks on dynamics suggest appropriate values of damping for typical applications.

Actual damping depends upon the mechanisms avialable to abosrb and dissipate energy. Eg cracks in concrete, slip in bolted connections, friction between moving components all absorb energy and contribute to damping - not to mention designed damper elements (eg classical dashpot dampers.)

Typical welded steel structures might have damping of 1% of critical or less; maybe 2% for large deflections. Bolted steel structures might be a bit higher - maybe up to 5% of critical. Reinforced concrete structures also tend to have rather more damping than steel strcutures, depending on the extent of cracking etc - perhaps 2% to 5%, say.

If damping is critical to the response of the structure, then you must be very careful about considering a range of damping that will actually be mobilised. This is possibly the greatest unknown in the dynamic analysis of structures. Mass and stiffness can be predicted by normal engineering calculation, but damping is often much more a matter of luck and judgement!

Hope this helps.
There aren't any predefined measures available for many of the Pro/MECHANICA Vibration Analysis types. So the user has to define those prior to running the analysis. For example if you are interested in results plotted against time, define the appropriate measure (e.g. displacement), and then check the Time/Freq Eval box. This will give you the option to choose a time evaluation method. There's a box at the buttom of the measure definition dialogue box. Notice that as you make changes to your measure definition the list analysis types shown in this box change. This box basically tells you what analysis types the measure will be available for.

Hope this helps.
Why don't I see any measures with Random Vibration? The closest I get is Dynamic Time and Dynamic Frequency. When I read the Help it tells me that you can add custom measures to random vibration analysis. Is there another way to create a measure?



You can create the same measures in Random Vibrations as in Time and Frequecy. THe only difference is that the plots that you get are PSD plots which are not really useful by themselves however those same PSD plots can be converted to RMS values. This measure can be assigned in the measures window. THe results will be displayed in the report file at the end of the run as a single value. These RMS values signify the probability that the event you are measuring ( Stress, Velocity, acceleration, etc) will accur within one stardard deviation of a normal probability curve with a probability of that value occuring 68.3% of the time. If you want better probability prediction let's say 3 standard deviation with probabily of 99.73% then you need to multiply the value that mechanica gives you by 3.



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