Dear all,
I am running some simulations on Ls-Dyna (blast loads on concrete structures), and I am getting some problems on the computed hourglass energy.
To begin, I'm getting hourglass energies that represent almost 60% of the internal energy for a specific part of my model.
To control this, since it is know from the litterature that the houglass energy should be kept at max 10% of the internal energy, I already tried to implement some control on this mostly in two ways : viscous form (HID:3) and stifness form (HID:5) and the best results were found to be over the stifness form (HID:5) (either in therms of elements distortion as in hourglass energy to internal energy ratio).
My question now is: Since my problem involves high loads and high strains rates, shouldn't I have to use the viscous form to control the hourglass (at least based on some literature)?
Please, fell free to discuss this topic since it seems to be a very important parameter to check the reliability of the numerical simulation and, most of the times, is really ignored by the researchers.
Rodrigo Mourao
Hi Rodrigo,
Well, I don't know if I have a definite answer to your problem, I just though I would tell you what I experienced when solving impact problems (FOD). I started using Stiffness form (HID4 and 5) and I am finally using HID 6 (Belytschko) with moderate QM as indicated in the manual notes (0.15 since I am dealing with metals). I was finally able to avoid excessive deformation (negative volume) and keep Hourglass energy at approximately 5%. It also helped when I used a finer mesh.
I hope it helps and let me know if you find a solution to your problem.
Best regards and good luck,
Rodolfo
As part of a recent study of steel-plate composite walls (which have a concrete core), I completed a sensitivity analysis of the effect of hourglass controls in LS-DYNA.
We used the Winfrith concrete model and found that hourglass types 4 and 5 reduced hourglass energy by the largest amount (to ~10%) when QM was between 0.01 and 0.20. We were comparing the influence of these parameters to experimentally measured behavior and found that QM = 0.10 was best. Hourglass type 6 with QM = 0.10 gave reasonable results with hourglass energy at about 20%. It should be noted that as displacement increased (especially beyond yield of the section, the hourglass energy increased in our study. If you want more details, see Ch 3 of the linked document.
http://docs.lib.purdue.edu/open_access_dissertations/427/
Dear Rodrigo.
I my case, I usually using Flanagan-Belytschko stiffness form. And I agree with Rodolfo's opinion, impact of the mesh segmentation affects the analysis results. The energy ratio does not necessarily have to be kept at 10%, but it is likely to fluctuate depending on the analysis condition. (check this paper, Zaouk, A. K., Bedewi, N. E., Kan, C. D., & Marzougui, D. (1996, July). Development and evaluation of a C-1500 pickup truck model for roadside hardware impact simulation. In Proceedings of the FHWA Vehicle Crash Analysis Crash Conference, Mclean, VA (pp. 1-31).)
Another important point is mass scaling. check this paper, Pan, F., Zhu, J., Helminen, A. O., & Vatanparast, R. (2006, June). Three point bending analysis of a mobile phone using LS-DYNA explicit integration method. In 9th International LS-DYNA Users Conference, June (pp. 4-6).
Best regards
H Kang
- Badges
- Science topic