Oasys LS-DYNA Environment 8.1 VOLUME 3 ... - Oasys Software

Oasys LS-DYNA Environment: User Guide (Version 8.1)
14.0 HOURGLASSING
Certain modes of deformation of LS-DYNA solid and shell elements are zero energy modes and
have no stiffness. They typically give a zig-zag appearance to a mesh (see Figures 14.1 and
14.2); this is known as hourglass deformation. It can swamp the results of an analysis.
Hourglassing can affect brick and quadrilateral shell elements, but not triangular shells or beams.
Hourglass-mode deformation is resisted by a viscosity; this is calculated automatically by the
code. Although it is possible to change the default values (card 1 of each material) it is very
rarely necessary or desirable to do so.
Good modelling practice normally prevents hourglassing becoming significant. The general
principle is to avoid concentrating load on a single node - spread the load over several
neighbouring nodes.
Hourglassing can be a problem with small displacement situations, particularly when dynamic
relaxation is used. In these cases it is often beneficial to opt for the stiffness method of hourglass
control instead of the viscous method - include an *HOURGLASS card with IHQ set to 4.
However, this may over-stiffen results in large deformation problems.
Figure 14.1 Normal Mesh Figure 14.2 Hourglassing
A more generally applicable solution to hourglassing problems is to opt for fully integrated
elements (specified on *SECTION_SOLID or *SECTION_SHELL). These are, however, much
more costly than the default elements.
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