I am setting up a dynamic shock analysis 25 g amplitude and 5.0 msec half sine pulse. I am in IPS units. I enter 386.4 for the base excitation value. Under the Response Spectrum I choose the Acceleration option and then the f(x) button to define my function. I create a new function and attempt to enter the equation and that is where I get hung up.
Based on an example I found in the help files I came up with this equation:
If (time<0.005, 25*sin(pi*time/0.005), 0)
But Mechanica says that is invalid. I also tried this equation which Mechanica will accept but not graph:
I don't think this is what I want either, I should get a half-sine graph if I have entered my equation correctly.
Can someone help me figure out how to input this correctly in equation form into Mechanica? WF 4.0
If your forcing function is dependent on time, then you should run a Dynamic Time, not shock which uses acceleration, velocity, or displacment versus frequency for input. Another point: You may already be aware, but you need to capture all potential affected modes in your modal analysis. If you are not sure which modes are important, then specify (request) modes from something like 0 to 800. That would capture all modes that have any sort of measurable structural displacement, hence any significant stress magnitudes. Hope this helps.
Randy Speed Speed Consulting,LLC www.speedconsulting.com (214) 213-4440
Quoting Stephen Seymour <firstname.lastname@example.org>:
> Greg, > > > > For a dynamic shock analysis your independent variable is frequency not > time. For what you are trying > > to accomplish you would need to run a dynamic time analysis. Mechanica is > probably rejecting your > > equation due to the presence of the time variable. > > > > Hope this helps, > > > > Steve > > > > > > Stephen Seymour, P.E. > > Principal Engineer > > Seymour Engineering & Consulting Group, LLC > > 3600 NW 138th Street > > Suite #102 > > Oklahoma City, OK 73134 >
To add on to what Randy is saying a typical practice is to request a sufficient number of modes such that the cumulative mass participation factor is above 0.80. Also, observe which modes are dominant by comparing the relative magnitude of a particular mode's mass participation to others. Dominant modes will stand out with large amounts of mass participation.
Stephen Seymour, P.E. Principal Engineer Seymour Engineering & Consulting Group, LLC 3600 NW 138th Street Suite #102 Oklahoma City, OK 73134