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Change CHEXA8 default element to Q1/E21 enhanced assumed strain - #352
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The default CHEXA8 used selective reduced shear integration, which locks in bending and near incompressibility, behaves badly on distorted meshes, and depends on the element's node ordering. It is replaced by the Q1/E21 enhanced assumed strain element (Andelfinger & Ramm 1993), a general purpose formulation that has no preferred direction, no spurious zero-energy modes, passes the patch test on distorted meshes, and relieves shear and volumetric locking. The same enhanced strain operator is used for the stiffness, the thermal load and stress recovery. ISOP = FULL still selects the standard displacement element. PSOLID IN blank is now accepted for CHEXA8, as in Nastran, and IN = BUBBLE selects the enhanced element. The CHEXA20 is unchanged. This builds on the solid thermal recovery and CHEXA corner recovery fixes.
The enhanced assumed strain element had replaced the CHEXA8 for every ISOP blank or REDUCED, including IN = TWO and THREE, so the reduced shear elements could no longer be requested and decks that ask for one explicitly got a different element than in Nastran. The enhanced element is the counterpart of the Nastran bubble-function CHEXA8, so it is now selected only by IN = BUBBLE, or by IN blank with ISOP blank or REDUCED. IN = TWO or THREE again select the 2x2x2 or 3x3x3 reduced shear element with ISOP blank or REDUCED and the standard isoparametric element with ISOP = FULL, identical to upstream. IN blank with ISOP = FULL remains the standard 2x2x2 element, and IN = BUBBLE still selects the enhanced element whatever ISOP says.
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Changed IN = TWO or THREE to select the old reduced shear element when ISOP is blank or REDUCED, and the standard displacement element when ISOP = FULL. |
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The default CHEXA8 used selective reduced shear integration, which locks in bending and near incompressibility, behaves badly on distorted meshes, and depends on the element's node ordering. It is replaced by the Q1/E21 enhanced assumed strain element (Andelfinger & Ramm 1993), a general purpose formulation that has no preferred direction, no spurious zero-energy modes, passes the patch test on distorted meshes, and relieves shear and volumetric locking. The same enhanced strain operator is used for the stiffness, the thermal load and stress recovery.
ISOP = FULL still selects the standard displacement element. PSOLID IN blank is now accepted for CHEXA8, as in Nastran, and IN = BUBBLE or blank selects the enhanced element. The CHEXA20 element is not changed.