Shock Analysis Setup Questions
I'm hoping to gain a better understanding of inputs and settings for a linear transient dynamic analysis using modal superposition with this shell plate example model. The two load cases I'm attempting to run are:
Load Case 1 (LC1): acceleration shock
- input peak acceleration: 15.2 g's
- 11 ms terminal peak sawtooth shock pulse
Load Case 2 (LC2): velocity shock
- initial velocity: 68 in/s
- 11 ms terminal peak sawtooth shock pulse
These are some assumptions and outputs I'd like to have represented:
- time step must capture critical shock input response and the peak structure response after the input finishes
- input shock pulse at same location as SPC
- output displacement with respect to SPC
- 10% global structural damping
- distance units are in inches
Here are some issues and questions from the example model in its current state:
- LC1 results show a displacement "drift" in the direction of acceleration application throughout the time history. Why is this and what can I do to prevent it?
node 613337 is on the cantilevered end of the plate
node 610154 is one of the four SPC/shock input nodes at the center of the rigid elements. I expected this displacement value to be equal to 0 throughout the time history, but it changes based on the time step input.
2. The time step input for both load cases currently has two stages: 0.001 s increments for the first 0.1 s and 0.005 s increments for the remaining time. The severity of the results increases during the second time step stage. I was expecting to see the deflections and stresses continue to oscillate and decrease, but with increased time between results reports. Recommendations or guidance on this?
3. Viewing worst case results: what is the method for locating the peak stress that occurs throughout the transient response?
Model and results files (.fem, .h3d, .out, and .mvw for the Hypergraph images above) are attached. Thanks!
Answers
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Hi Mandy
1) In LC1 you actually apply an acceleration that is positive during the entire event, hence the drift!
So positive acc will increase the velocity in that direction! If you want to brake you also need to apply a negative acc for a certain time as well!
2) I would have started to have constant timestep, maybe smaller than 1ms as well!
3) Here you can try to use the derived timestep feature and select all timestep in one of the Loadcases
Hope this can help a bit!
Kind regards
//johan1 -
Thanks for the response Johan! A couple of follow-on questions:
1. It makes sense that the displacement rate increases along the input acceleration curve, then hits a steady state "drift" after the input acceleration goes down to zero. Do you know if there's a way to output displacement results or post process the results that shows the displacement in relation to one of my "fixed" constraint points?
2. Do you know why the timestep needs to be constant? Can you think of a scenario where it wouldn't be constant? I would like to know why changing the time step mid-run (after the input curve is at zero and remains there) changes the displacement of the input SPCD locations. Also, do you use a rule of thumb for time step size, or have further guidance on that?
3. Do you know of a way to pinpoint which timestep the peak result value is coming from? In HyperView I can turn the max probe on and play through the time history, and the max throughout the time history is displayed, but I don't know how to track which time step it's occurring in apart from advancing through each step until I see the peak displayed.
Thank you!
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Maybe Param,enfmotn,rel is something you might find useful
https://2021.help.altair.com/2021.1/hwsolvers/os/topics/solvers/os/analysis_linear_transfient_c.htm
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Altair Employee