Too Much Hourglass Energy !

Akhil Sreekumar

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Mar 12, 2023, 1:50:15 AM3/12/23

to LS-DYNA2

Hallo,

Good Morning to Everyone.

This question maybe simple to some people, but i can’t seem to understand it quiet yet

I am trying to copy a research which does a Bullet impact simulation.

I have used Hourglass Values 4,5 and 6 while using Elfrom 1 as reduced integrated wlements need Hourglass damping.

By 4 and 5 i have got similiar results like the research and my Hourglass Energies were 12%

Of Internal Energy ( Rule of thumb is 10 % )

But once i switch to type 6, the Hourglass Energy shoots up very high 10x internal energy values even.

Note that i have used same damping values for all three types !

What can be the reason for this ?

Normally, what does it mean to have too much Hourglass energy ?

If anybody knew information/ Solution , that would be really helpful !!

Wishing you all a Good day !

Best Regards

Akhil

James Kennedy

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Mar 12, 2023, 8:18:35 PM3/12/23

to Akhil Sreekumar, LS-DYNA2

Dear Akhil,

Some notes well worth reading:

https://www.dynasupport.com/howtos/element/hourglass

From that link:

Hourglass (HG) modes are nonphysical, zero-energy modes of deformation that produce zero strain and no stress. Hourglass modes occur only in under-intetgrated (single integration point) solid, shell, and thick shell elements. LS-DYNA has various algorithms for inhibiting hourglass modes. The default algorithm (type 1), while the cheapest, is generally not the most effective algorithm.

A way to entirely eliminate hourglass concerns is to switch to element formulations with fully-integrated or selectively reduced (S/R) integration. There can be a downside to this approach. For example, type 2 solids are much more expensive than the single point default solid. Secondly, they are much more unstable in large deformation applications (negative volumes much more likely). Third, type 2 solids have some tendency to 'shear-lock' and thus behave too stiffly in applications where the element shape is poor.

The hourglass coefficient for type 6 HG control will typically range from 0.1 (default) to 1.0. For elastic material, use 1.0. For other materials, the choice of HG coefficient is not obvious. Even looking at results, it may be difficult to quantify the 'goodness' of the hourglass coefficient used. Too low a value may result in visible hourglass modes of deformation (unlikely). Too high a value may result in overly stiff behavior. It may be necessary to run the model twice to see if the results exhibit any sensitivity to the hourglass coefficient. Checking the hourglass energy is a good idea.

A good way to reduce hourglassing is to refine your mesh.

Akhil Sreekumar

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Mar 13, 2023, 12:37:17 PM3/13/23

to James Kennedy, LS-DYNA2

Thank you for the feedback James , the videos were really helpful.

Best Regards

Akhil

James Kennedy

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Mar 13, 2023, 5:23:43 PM3/13/23

to Akhil Sreekumar, LS-DYNA2

Dear Akhil,

Two presentation (previously suggested for other questions) which have possibly interesting discussions on hour-

glassing, 2D and 3D element formulations, element erosion, etc. involving projectile-plate perforation and penetration:

Balaban, B., and Kurtoglu, I., "An Investigation of AA7075-T651 Plate Perforation Using Different Projectile Nose

Shapes", 10th European LS-DYNA Users Conference, Wurzburg, Germany, May, 2015.

http://www.dynalook.com/10th-european-ls-dyna-conference/7%20Simulation%20IV%20-%20Blast-Penetration/02-Balaban-FNSSSavunma-Sistemleri-P.pdf

Basaran, G., and Gurses, E., "Numerical Study of High Velocity Impact Response of Vehicle Armor Combination

Using LS-DYNA", 11th European LS-DYNA Users Conference, Salzburg, Austria, May, 2017.

http://www.dynalook.com/11th-european-ls-dyna-conference/armor-penetration/numerical-study-of-the-high-velocity-impact-response-of-vehicle-armor-combination-using-ls-dyna

Sincerely,

James M. Kennedy

KBSS2 Inc.

March 13, 2023

Akhil Sreekumar

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Mar 18, 2023, 2:40:09 PM3/18/23

to LS-DYNA2

Hallo James,

Thankyou for all the answers till now, I found it very helpful especially the study on the different Nose shapes.

I have newly found that My Model is Exhibiting Hourglass Energy even when no Hourglass damping value was given.

It was simply ELFORM 1 with no Hourglass.

I have attached a pciture of the Graph Tracking Internal Energy, Hourglass Energy and Contact sliding Energy below, What seemed more peculiar was the Hourglass energy seem to somehow resemble my Contact sliding energy (Negative Value).

My Model has some Initial Penetrations in the Mesh. I have used IGNORE option to solve some of the initial penetration issues and saw a diminishing in this Negative Contact Energy.

After using IGNORE option i have also attached graph of three energies as well, now the negative contact energy seem to resemble the internal energy.

Any thoughts on this interesting Phenomena ?

I was under the impression that it was problem with Hourglass 6 but was contradicted .

Thanks for the Help James. I hope this fascinates you as well.

Best Regards

Akhil

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James Kennedy

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Mar 19, 2023, 5:15:57 PM3/19/23

to Akhil Sreekumar, LS-DYNA2

Dear Akhil,

A default value is usually used when no hourglassing damping value given (look in

d3hsp to verify what is being used).

1. Hourglass Control Types. Hourglass control is viscosity or stiffness that is

added to quadrilateral shell elements and hexahedral solid elements that use

reduced integration. It also applies to formulation 1 tshells. Without hourglass

control, these elements would have zero energy deformation modes which

could grow large and destroy the solution. *CONTROL_HOURGLASS can be

used to redefine the default values of the hourglass control type and coefficient.

If omitted or if IHQ = 0, the default hourglass control types are as follows:

a) For shells: viscous type for explicit; stiffness type for implicit.

b) For solids: type 2 for explicit; type 6 for implicit.

c) For tshell formulation 1: type 2.

These default values are used unless HGID on *PART is used to point to

*HOURGLASS data which overrides the default values for that part.

For explicit analysis, shell elements can be used with viscous hourglass control,

(IHQ = 1 = 2 = 3) or stiffness hourglass control (IHQ = 4 = 5). Only shell forms 9,

16 and -16 use the warping stiffness invoked by IHQ = 8. For implicit analysis,

the viscous form is unavailable.

For explicit analysis, hexahedral elements can be used with any of the hourglass

control types except IHQ = 8. For implicit analysis, only IHQ = 6, 7, 9, and 10 are

available.

If IHQ is set to a value that is invalid for some elements in a model, then the

hourglass control type for those elements is automatically reset to a valid value.

For explicit analysis, if IHQ = 6, 7, 9, or 10, then shell elements will be switched

to type 4 except for forms 16 and -16 shells that are switched to type 8. If IHQ =

8, then solid elements and shell elements that are not form 16 will be switched to

type 4. For implicit analysis, if IHQ = 1-5, then solid elements will be switched

to type 6, and if IHQ = 1, 2, 3, 6, 7, 9, or 10, then shell elements will be switched

to type 4.

Sincerely,

James M. Kennedy

KBS2 Inc.

MRCH 19, 2023

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