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Shock Anlaysis/Entering Functions Help

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Shock Anlaysis/Entering Functions Help

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:

25*sin(pi*0.005)

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

Thanks!
3 REPLIES 3

Shock Anlaysis/Entering Functions Help

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

Shock Anlaysis/Entering Functions Help

Greg,

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 <sseymour@seymourecg.com>:

> 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
>

Shock Anlaysis/Entering Functions Help

Greg,

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.

Steve


Stephen Seymour, P.E.
Principal Engineer
Seymour Engineering & Consulting Group, LLC
3600 NW 138th Street
Suite #102
Oklahoma City, OK 73134