Hourglass — LS-DYNA Support
Contents
1. stiffening of the response and at the same time effectively inhibiting hourglass modes For high velocity impacts viscosity based HG control types 1 2 3 is recommended even for solid structural parts 30 03 2011 11 45 AM Hourglass LS DYNA Support http www dynasupport con howtos element hourglass Type 8 HG control applies only to shell formulation 16 This HG type activates warping stiffness in type 16 shells so that warping of the element does not degrade the solution Type 16 shells will solve the so called Twisted Beam problem correctly ifHG type 8 is invoked Type 6 HG control invokes an assumed strain co rotational formulation for type 1 solid elements and under integrated 2D solids shell types 13 and 15 With the HG type set to 6 and the hourglass coefficient set to 1 0 an elastic part need only be modeled with a single type 1 solid through its thickness to achieve the exact bending stiffness Type 6 HG control should always be used for type 1 solids in implicit simulations in fact this is done automatically in v 970 The hourglass coefficient fort ype 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 over2. Hourglass LS DYNA Support http www dynasupport con howtos element hourglass lof2 Hourglass Preface See the User s Manual HOURGLASS and sections 3 2 and 6 4 of the Theory Manual 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 t ype 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 Notice Triangular shells and tetrahedral solid elements do not have hourglassing modes but have drawbacks with regard to overly stiff behavior in some applications A good way to reduce hourglassing is to refine your mesh The method of loading can affect the degree of hourglassing A pressure loading is preferred over loading individual nodes as the latter appr
3. ly 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 The default hourglass coefficient of 0 1is superseded by any nonzero value given for QH in CONTROL_HOURGLASS I see nothing in the manual to contradict this interpretation The manual does say that the default hourglass type in HOURGLASS is 1regardless of what s given in CONTROL_HOURGLASS Unless I missed something no such note appears with regard to hourglass coefficient The lesson here is that users should specify a nonzero hourglass coefficient wherever HOURGLASS is used Otherwise the user may as you did inadvertently change the intended coefficient by use of CONTROL_HOURGLASS 2 of 2 30 03 2011 11 45 AM
4. oach is more likely to excite hourglassing modes To evaluate hourglass energy set HGEN to 2 in CONTROL_ENERGY and use DATABASE_GLSTAT and DATABASE_MATSUM to report the HG energy for the system and for each part respectively The point is to confirm that the nonphysical HG energy is small relative to peak internal energy for each part lt 10 as arule of thumb For shells only you can fringe hourglass energy density by first setting SHGE 2 in the LS DYNA input deck DATABASE_EXTENT_BINARY Then in LS Prepost choose Fcomp gt gt Misc gt gt hourglass energy For fluid parts the default HG coefficient is generally inappropriate too high Thus for fluids the hourglass coefficient should generally be scaled back several orders of magnitude Use only viscosity based HG control for fluids The default Hc formulation type 1 is generally ok for fluids Please note that in 971 R3 the default hourglass coefficient for ALE parts ELFORM 11 is Le 6 To override that default as might be appropriate for non fluid materials use HOURGLASS and HGID in PART Check the hourglass energy via MATSUM Hourglass types Stiffmess based HG control types 4 5 is generally more effective than viscous HG control for structural parts Usually when stiffness based HG control is invoked I like to reduce the HG coefficient usually in the range of 03 to 05 soas to minimize nonphysical
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