August 24, 2023 at 1:34 amQusai AlomariSubscriber
I'm trying to model a full RC bridge using LS-DYNA. In this model, one of the pier columns is subjected to the impact of a Single Unit Truck. After that, an explosion charge is detonated.
I'm using the ALE approach to model the Air, Soil, and Explosive, and all structural elements are modeled using solid elements (except steel reinforcement which is modeled using beam elements). Unfortunately, I'm encountering some issues that I couldn't resolve over the last two weeks.
All Lagrangian solid elements are deleted at the very beginning (first two to three time steps), and I get error termination even without an explicit error message. I've tried several things to resolve the problem, as summarized below, but nothing changes:
1- reducing time step scaling factor (TSSF) and (DT2MS).
2- Changing solid element type to constant stress (Elform = 1).
3- Using Types 4 and 5 hourglass controls with a coefficient ranging between 0.01 and 0.05.
4- Reconstructing the mesh.
5- Using IMASCL = 2 in the Control ALE option.
August 24, 2023 at 1:52 pmLoic IvaldiAnsys Employee
don't use mass scaling when you're at an early stage of your simulation
August 24, 2023 at 2:44 pmJim DayAnsys EmployeeI concur with Loic. If problem persists, de-couple Lagrangian from ALE (comment out the coupling keyword(s)), just as an experiment, to see what happens. Also, use double precision LS-DYNA.
August 25, 2023 at 3:36 amQusai AlomariSubscriber
Thank you Loic and Day for your suggestions. The model worked after removing the ALE coupling keywords). So, what could the problem be? As indicated in the figures below, the model consists of two ALE coupling ID's. The first (COUP 700) is used to couple the fluid (i.e., air+explosive) with the structural system. The second (COUP 500) is used to couple the soil with the foundation system.
August 25, 2023 at 10:20 amLoic IvaldiAnsys Employee
Check if the filling of the AMMGs is correct.
Enable the coupling card one by one to check the behavior.
Also, on your coupling cards displayed, normally the master side is the PART representing your ALE domain and then the coupling between AMMG and the structure is handled by MCOUP.
I saw 2 different "master" so maybe this is the issue.
August 25, 2023 at 2:40 pmQusai AlomariSubscriber
ALE parts are masters in the displayed cards. I'm using two masters as the first one (2000) represents the Soil, and the second one (801) represents the air and the explosive. I have also checked the filling AMMG and it looks correct!
August 25, 2023 at 3:13 pmLoic IvaldiAnsys Employee
just in case regarding what Jim said : https://www.dynasupport.com/howtos/general/consistent-units
August 25, 2023 at 3:11 pmJim DayAnsys EmployeeCheck that system of units is consistent. Are you using explosive constants directly from some reference and are you converting those constants from one unit system to another? Check that conversion.
August 25, 2023 at 3:13 pmJim DayAnsys EmployeeDo you get any messages concerning deletion of Lagrangian elements? What failure/erosion criteria are given in your input that may be triggering the element deletion? Wrtie output every time step so you can hopefully see how the erosion evolves.
August 27, 2023 at 2:48 amQusai AlomariSubscriber
The problem has not been resolved yet! I have checked all the units and they are consistent. Regarding the erosion algorithm, for concrete, I'm using CSC with ERODE = 1.1. Steel is modeled using Piecewise Linear Plasticity and plastic strain at failure has been defined.
The messages I received in most of the previous attempts were negative volume warnings (no error messages) and in some other cases, I got out-of-range moments and NaN!
August 28, 2023 at 7:16 amLoic IvaldiAnsys Employee
When you have the time where the instability occurs, output a lot of d3plot just before that time to see what is happening (just be careful to not fill your HDD completely)
August 28, 2023 at 4:57 pmJim DayAnsys EmployeeIt's understood that you need to apply pressure from the explosion to the structure. Since it's not clear what's making the solution unstable, I would recommend simplifying the model just temporarily while debugging. Eliminate the ALE portion of the model and just apply a pressure time history to the structure using *LOAD_SEGMENT_SET which very roughly approximates an explosive pressure. Is that simplified model stable?
August 31, 2023 at 12:38 amQusai AlomariSubscriber
Hello Jim, I got an Error termination with the same element deletion, however, it works for 63 time steps for the first time, and this is just significant progress. I found that the ALE coupling was the source of error. So, I tweaked some parameters such as the number of coupling points.
I still need to figure out the issue that causes the termination right after the explosive charge is detonated.
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