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  28.1.1 Solid (continuum) elements Solid (continuum) elements:    are the standard volume elements of Abaqus;    do not include structural elements such as beams, shells, membranes, and trusses; special- purpose elements such as gap elements; or connector elements such as connectors, springs, and dashpots;    can be composed of a single homogeneous material or, in Abaqus/Standard, can include several layers of different materials for the analysis of laminated composite solids; and    are more accurate if not distorted, particularly for quadrilaterals and hexahedra. The triangular and tetrahedral elements are less sensitive to distortion can be used for linear analysis and for complex nonlinear analyses involving contact, plasticity, and large deformations. Choosing an appropriate element Abaqus/Standard solid element library      includes first-order (linear) interpolation elements and second-order (quadratic) interpolation elements in one, two, or three dimensions.      Triangles and quadrilaterals are available in two dimensions      tetrahedra, triangular prisms, and hexahedra (“bricks”) are provided in three dimensions.      Modified second-order triangular and tetrahedral elements are also provided.      Curved (parabolic) edges can be used on the quadratic elements but are not recommended for pore pressure or coupled temperature-displacement elements.      Cylindrical elements are provided for structures with edges that are initially circular.   Choosing between first- and second-order elements      Linear = first order and Quadratic = second order.      First order element o   In first-order plane strain, generalized plane strain, axisymmetric quadrilateral, hexahedral solid elements, and cylindrical elements, the strain operator provides constant volumetric strain throughout the element.   o   This constant s train prevents mesh “locking” when the material response is approximately incompressible      Second order element   o   Second-order elements provide higher accuracy in Abaqus/Standard than first-order elements for “smooth” problems that do not involve severe element distortions. They capture stress concentrations more effectively and are better for modeling geometric features.   o   second-order elements are very effective in bending-dominated problems.       First-order triangular and tetrahedral elements should be avoided as much as possible in stress analysis problems.      If they are required, an extremely fine mesh may be needed to obtain results of sufficient accuracy.   Choosing between full- and reduced-integration elements Choosing between bricks/quadrilaterals and tetrahedra/triangles    a good mesh of hexahedral elements usually provides a solution of equivalent accuracy at less cost.      Quadrilaterals and hexahedra have a better convergence rate than triangles and tetrahedra, and sensitivity to mesh orientation in regular meshes is not an issue.      triangles and tetrahedra are less sensitive to initial element shape, whereas first-order quadrilaterals and hexahedra perform better if their shape is approximately rectangular.      The elements become much less accurate when they are initially distorted.      First-order triangles and tetrahedra are usually overly stiff, and extremely fine meshes are required to obtain accurate results.      As a rule, these elements should not be used except as filler elements in noncritical areas.   Tetrahedral and wedge elements    For stress/displacement analyses the first-order tetrahedral element C3D4 is a constant stress tetrahedron, which should be avoided as much as possible; the element exhibits slow convergence with mesh refinement.      This element provides accurate results only in general cases with very fine meshing.      linear version of the wedge element C3D6  should generally be used only when necessary to complete a mesh, and, even then, the element should be far from any areas where accurate results are needed.     KINEMATIC SPLIT   Include this parameter to change the kinematic formulation for 8-node brick elements only.  Set KINEMATIC SPLIT=AVERAGE STRAIN (default in Abaqus/Explicit) to use the uniform strain formulation and the hourglass shape vectors in the hourglass control. This is the only option available for Abaqus/Standard. Set KINEMATIC SPLIT=CENTROID to use the centroid strain formulation and the hourglass base vectors in the hourglass control in Abaqus/Explicit. Set KINEMATIC SPLIT=ORTHOGONAL to use the centroid strain formulation and the hourglass shape vectors in the hourglass control in Abaqus/Explicit. If SECOND ORDER ACCURACY=YES, the KINEMATIC SPLIT parameter will be reset to AVERAGE STRAIN in Abaqus/Explicit. SECOND ORDER ACCURACY   Set SECOND ORDER ACCURACY=YES to use a second-order accurate formulation for solid or shell elements suitable for problems undergoing a large number of revolutions (> 5) in Abaqus/Explicit. This is the only option available for  Abaqus/Standard. Set SECOND ORDER ACCURACY=NO (default in Abaqus/Explicit) to use the first-order accurate solid or shell elements in Abaqus/Explicit. The SECOND ORDER ACCURACY parameter is not relevant for linear kinematics.
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