I’m simulating a test where the loading condition is dropping a known weight a known distance until a tether snaps tight and loads the part. I want to use a dynamic step do this, but I don’t know how to create the tether. Ideally I’d skip the tether entirely and instead create the resulting force profile at the attachment point.
The part being tested is plastic, and it’s being constrained by contact with another plastic part. I have true stress-strain info at the given strain rate, but I don’t know how to set up the impulse input.
For that, you can just define a tabular amplitude and assign it to the load. Its magnitude will be multiplied by amplitude value at a given time.
There’s a dynamic step available in PrePoMax. Implicit version should be sufficient although it often suffers from convergence difficulties in this solver. On the other hand, explicit version (meant primarily for high-speed dynamics) is underdeveloped, slow and has some significant limitations in CalculiX.
To actually simulate such a tether you would need a nonlinear spring. In Abaqus, this could he easily done with a connector where you can even define stops (max elongation) and locks (criteria for when the connector’s motion is locked). In CalculiX, you would have to use regular springs with nonlinear stiffness, but perhaps the GAPUNI elements (internally used by compression-only constraint) with their initial clearance would help.
I had a play with Open Radios with a simple model with a tether that had a stop on it -the block accelerates with a gravity load and eventually the stop hits the brake - very quick to set up and u simply have to select the wgt (size/density) of the block, and the characteristics of the tether. This needs contact and i did not get this approach to converge in calculix
I also had a go at using PPmax with dynamic analysis and no tether. I may have over simplified it here but i think the principle is sound. I applied a gravitational force to a block - stopped it spinning and then applied an impulse in the positive z direction after 0.5 seconds. There is a little messing around to get the right impulse and constant load after wards to stop the block. U have to then decide how quickly to stop the block (this will be a function of how long u let it drop and the elastic properties of ur tether ) -i chose 0.01 seconds here then with some simple calculations you can derive some good starting numbers for the impulse and steady state loads to stop it - i did not quite achieve that but I got close enough
Impulse loads can be also applied in transient modal dynamics procedure. It can be much faster because it uses modal superposition instead of full integration of the equations of motion, but the disadvantage is that it’s only applicable to fully linear cases (it’s a linear dynamics procedure): https://www.youtube.com/watch?v=R8HQKI3PTrI
You could compare the results of *MODAL DYNAMIC and *DYNAMIC (implicit) though. Speed difference should be apparent.
However, it’s also important to define proper incrementation in dynamic analyses, especially when using amplitudes - CalculiX doesn’t adjust the automatic incrementation to capture the changes in the amplitudes - you have to take care of it yourself and use sufficiently low time increments.
