Learning Forum

[mukattash]

Hallo Everyone,

I'm running an explicit LS-DYNA simulation of a masonry wall with a concrete top slab. Before the model was working because the masonry shear wall was with 1x0,3x2,75 m dimension under specific material. the mesh size was 0,01667 in all diretion which was uniform. Now I changed the wall dimesion to 1x0,365 x 2,75 and another material where I decreased the density of masonry bricks. The mesh in y and z direction is equal but x diection is differ and i had to change it because i am having 0,365 wall thickness now. Running the model up to 0.7 sec is ok, after that the ETA starts to increase and the simulation needs more time.  I checked the output file where after this 0,7 sec the time step increases instead of staying constant until the time of termination.

Upon inspection, the top slab is meshed with only one element in the Z-direction, while a transition layer below it has two elements. I suspect element distortion in this thin slab is causing the stable time step (dt) to collapse, as governed by the Courant criterion.I'm not using mass scaling.

Any suggestions, to control the time step , or is that related to CLF condition or Has anyone experienced a similar issue? 

[Armin]

Hello Mukkatash,

You are correct; the small element in the thin slab layer is controlling the time step size and most likely responsible for the behavior you're observing. In explicit dynamics analysis, the critical time step size is governed by the smallest element in the model. I suggest checking the video below, which discusses this topic along with other aspects of time step size controls in explicit dynamics analysis.
Time Step in Explicit Dynamics - Lesson 2

To address this issue, you might consider using shell elements for the thin slab, or you could explore selective mass scaling (also discussed in the video above).

[mukattash]

I have a fixed mass number for the bricks, If I used the concept of mass scling, I am adding or changing the origional value, and I dont want that. Because it is a fixed material parameter,or?