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

Oasys LS-DYNA Environment: User Guide (Version 8.1)
Through-thickness directions at each node are initially normal to the element surface but rotate
with the nodes. Strains are linear through the thickness. Two integration points are sufficient
for linear elastic material, while more points are required for nonlinear material. Stress output
gives stresses at the outermost integration points, not at the surfaces (despite the nomenclature
of post-processors, which refer to top and bottom surfaces), so care is needed in interpretation
of results. For elastic materials, stresses can be extrapolated to the surfaces. For nonlinear
materials the usual policy is to choose four or five integration points through the thickness and
to ignore the error (i.e. the difference in stress between the surface and the outermost integration
point). The location of the outermost integration points are given in the following table:
THICK SHELLS
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Mid Plane 0
(2 points .5774 )
Outermost (3 points .7746 )
Point (4 points .8611 )
(5 points .9062 )
Outer Surface 1.0
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The thick shell element is an eight node brick-shell. The through-thickness direction and
thickness at each corner of the thick shell is defined by the relative position of the two nodes;
rotational degrees of freedom are ignored. Through-thickness deformation is resisted by a
penalty stiffness, although through-thickness stress is taken as zero in the constitutive models.
This means that contact can occur on both faces of the shell, but the compressive stresses
produced by this will not "extrude" the element and may not be reported by post-processing. As
the through-thickness direction of the thick shell element is treated differently to the in-plane
directions it is vital that the topology of the element be defined correctly. The thick shell cannot
be degenerated to a wedge or tetrahedron.
BEAMS
The Hughes-Liu beam is a conventionally integrated element able to model rectangular or
circular cross-sections using an array of integration points at the mid-span of the element.
Alternatively, the user may specify a cross-section integration rule to model arbitrary crosssections.
The beam effectively generates a constant moment along its length, so, as with the
brick and shell elements, meshes need to be reasonably fine to achieve adequate accuracy.
Because of the location of the integration points yielding is detected only at the element centre.
Therefore a model cantilever beam will yield at slightly too high a force because the fully plastic
moment must be developed at the centre of the encastré element rather than at the root.
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