Ansys Theory Manual
SHELL181 Element Description SHELL181 is suitable for analyzing thin to moderately-thick shell structures. It is a four-node element with six degrees of freedom at each node: translations in the x, y, and z directions, and rotations about the x, y, and z-axes. (If the membrane option is used, the element has translational degrees of freedom only). The degenerate triangular option should only be used as filler elements in mesh generation. SHELL181 is well-suited for linear, large rotation, and/or large strain nonlinear applications.
Change in shell thickness is accounted for in nonlinear analyses. In the element domain, both full and reduced integration schemes are supported. SHELL181 accounts for follower (load stiffness) effects of distributed pressures. SHELL181 can be used for layered applications for modeling composite shells or sandwich construction. The accuracy in modeling composite shells is governed by the first-order shear-deformation theory (usually referred to as Mindlin-Reissner shell theory). The element formulation is based on logarithmic strain and true stress measures. The element kinematics allow for finite membrane strains (stretching).
However, the curvature changes within a time increment are assumed to be small. See in the for more details about this element.
X o = Element x-axis if ESYS is not provided. X = Element x-axis if ESYS is provided. Single-Layer Definition To define the thickness (and other information), use section definition, as follows:,SHELL, THICKNESS. A single-layer shell section definition provides flexible options. For example, you can specify the number of integration points used and the material orientation. Multilayer Definition The shell section commands allow for layered shell definition. Options are available for specifying the thickness, material, orientation, and number of integration points through the thickness of the layers.
You can designate the number of integration points (1, 3, 5, 7, or 9) located through the thickness of each layer when using section input. When only one, the point is always located midway between the top and bottom surfaces. If three or more points, two points are located on the top and bottom surfaces respectively and the remaining points are distributed equal distance between the two points. The default number of integration points for each layer is three; however, when a single layer is defined and plasticity is present, the number of integration points is changed to a minimum of five during solution. The following additional capabilities are available when defining shell layers.
SHELL181 accepts the (,GENS). When the element is associated with the GENS section type, thickness or material definitions are not required.
You can use the to define thickness as a function of global/local coordinates or node numbers. You can specify offsets. Other Input The default orientation for this element has the S 1 (shell surface coordinate) axis aligned with the first parametric direction of the element at the center of the element, which connects the midsides of edges LI and JK and is shown as x o in. In the most general case, the axis can be defined as.
The cantilever beam and the beam cross-section to be modeled with shells are typical examples of in-plane bending-dominated problems. The use of KEYOPT(3) = 2 is the most effective choice in these circumstances. Reduced integration would require refined meshes. For example, reduced integration for the cantilever beam problem requires four elements through the thickness, whereas the full integration with incompatible modes only requires one element through the thickness. For the stiffened shell, the most effective choice is to use KEYOPT(3) = 0 for the shell and KEYOPT(3) = 2 for the stiffener. When KEYOPT(3) = 0 is specified, SHELL181 uses an hourglass control method for membrane and bending modes. By default, SHELL181 calculates the hourglass parameters for both metal and hyperelastic applications.
To specify the hourglass stiffness scaling factors, use the command. When KEYOPT(5) = 1, the element incorporates initial curvature effects. The calculation for effective shell curvature change accounts for both shell-membrane and thickness strains. The formulation generally offers improved accuracy in curved shell structure simulations, especially when thickness strain is significant or the material anisotropy in the thickness direction cannot be ignored, or in thick shell structures with unbalanced laminate construction or with shell offsets. The initial curvature of each element is calculated from the nodal shell normals. The shell normal at each node is obtained by averaging the shell normals from the surrounding SHELL181 elements.
A coarse or highly distorted shell mesh can lead to significant error in the recovered element curvature; therefore, this option should only be used with a smooth and adequately refined mesh. To ensure proper representation of the original mesh, a nodal normal is replaced by the element shell normal in the curvature calculation if the subtended angle between these two is greater than 25 degrees. SHELL181 includes the linear effects of transverse shear deformation. An assumed shear strain formulation of Bathe-Dvorkin is used to alleviate shear locking. The transverse shear stiffness of the element is a 2x2 matrix as shown below. To define transverse shear stiffness values, use the command.
Transverse shear-correction factors k are calculated once at the start of the analysis for each section. The material properties used to evaluate the transverse shear correction factors are at the current reference temperature during solution. User-field variables and frequency are all set to zero when evaluating the material properties used to calculate the transverse shear correction factors.
For a single-layer shell with isotropic material, default transverse shear stiffnesses are. In the above matrix, shear-correction factor k = 5/6, G = shear modulus, and h = thickness of the shell. SHELL181 can be associated with linear elastic, elastoplastic, creep, or hyperelastic material properties. Only isotropic, anisotropic, and orthotropic linear elastic properties can be input for elasticity. The von Mises isotropic hardening plasticity models can be invoked with BISO (bilinear isotropic hardening), MISO (multilinear isotropic hardening), and NLISO (nonlinear isotropic hardening) options. The kinematic hardening plasticity models can be invoked with BKIN (bilinear kinematic hardening), MKIN and KINH (multilinear kinematic hardening), and CHABOCHE (nonlinear kinematic hardening). Invoking plasticity assumes that the elastic properties are isotropic (that is, if orthotropic elasticity is used with plasticity, ANSYS assumes the isotropic elastic modulus = EX and Poisson's ratio = NUXY).
Hyperelastic material properties (2, 3, 5, or 9 parameter Mooney-Rivlin material model, Neo-Hookean model, Polynomial form model, Arruda-Boyce model, and user-defined model) can be used with this element. Poisson's ratio is used to specify the compressibility of the material. If less than 0, Poisson's ratio is set to 0; if greater than or equal to 0.5, Poisson's ratio is set to 0.5 (fully incompressible).
Both isotropic and orthotropic thermal expansion coefficients can be input using,ALPX. When used with hyperelasticity, isotropic expansion is assumed. Use the command to specify the global value of reference temperature. If,REFT is defined for the material number of the element, it is used for the element instead of the value from the command. But if,REFT is defined for the material number of the layer, it is used instead of either the global or element value. With reduced integration and hourglass control (KEYOPT(3) = 0), low frequency spurious modes can appear if the mass matrix employed is not consistent with the quadrature rule.
Ansys Theory Reference
SHELL181 uses a projection scheme that effectively filters out the inertia contributions to the hourglass modes of the element. To be effective, a consistent mass matrix must be used. We recommend setting,OFF for a modal analysis using this element type. The lumped mass option can, however, be used with the full integration options (KEYOPT(3) = 2). KEYOPT(8) = 2 stores midsurface results in the results file for single or multi-layer shell elements. If you use,MID, you will see these calculated values, rather than the average of the TOP and BOTTOM results.
You should use this option to access these correct midsurface results (membrane results) for those analyses where averaging TOP and BOTTOM results is inappropriate; examples include midsurface stresses and strains with nonlinear material behavior, and midsurface results after mode combinations that involve squaring operations such as in spectrum analyses. KEYOPT(9) = 1 reads initial thickness data from a user subroutine. You can apply an initial stress state to this element via the command.
For more information, see in the. The effects of pressure load stiffness are automatically included for this element. If an unsymmetric matrix is needed for pressure load stiffness effects, use,UNSYM. A summary of the element input is given in. A general description of element input is given in. Nodes I, J, K, L Degrees of Freedom UX, UY, UZ, ROTX, ROTY, ROTZ if KEYOPT(1) = 0 UX, UY, UZ if KEYOPT(1) = 1 Material Properties command: See for this element. Command: EX, EY, EZ, (PRXY, PRYZ, PRXZ, or NUXY, NUYZ, NUXZ), ALPX, ALPY, ALPZ (or CTEX, CTEY, CTEZ or THSX, THSY, THSZ), DENS, GXY, GYZ, GXZ, ALPD Specify BETD, ALPD, and DMPR for the element (all layers) by issuing the command to assign the material property set.
REFT can be specified once for the element, or it can be assigned on a per-layer basis. See the discussion in for more information. Surface Loads. Temperatures - For KEYOPT(1) = 0 (Bending and membrane stiffness): T1, T2, T3, T4 (at bottom of layer 1), T5, T6, T7, T8 (between layers 1-2); similarly for between next layers, ending with temperatures at top of layer NL(4.(NL+1) maximum).
Ansys Theory Guide
Hence, for one-layer elements, 8 temperatures are used. For KEYOPT(1) = 1 (Membrane stiffness only): T1, T2, T3, T4 for layer 1, T5, T6, T7, T8 for layer 2, similarly for all layers (4.NL maximum). Hence, for one-layer elements, 4 temperatures are used. Special Features for layered shells and for input of homogeneous section stiffnesses KEYOPT(1) Element stiffness. Nodal displacements included in the overall nodal solution. Additional element output as shown in Several items are illustrated in. KEYOPT(8) controls the amount of data output to the results file for processing with the command.
Ansys Autodyn Theory Manual
Interlaminar shear stress is available as SYZ and SXZ evaluated at the layer interfaces. KEYOPT(8) must be set to either 1 or 2 to output these stresses in POST1. A general description of solution output is given in. See the for ways to review results. The element stress resultants (N11, M11, Q13, etc.) are parallel to the element coordinate system, as are the membrane strains and curvatures of the element.
Such generalized strains are available through the SMISC option at the element centroid only. The transverse shear forces Q13, Q23 are available only in resultant form: that is, use SMISC,7 (or 8). Likewise, the transverse shear strains, γ 13 and γ 23, are constant through the thickness and are only available as SMISC items (SMISC,15 and SMISC,16, respectively). The program calculates moments (M11, M22, M12) with respect to the shell reference plane.
By default, ANSYS adopts the shell midplane as the reference plane. To offset the reference plane to any other specified location, issue the command. When there is a nonzero offset (L) from the reference plane to the midplane, moments with respect to the midplane ( ) can be recovered from stress resultants with respect to the reference plane as follows.
X o = Element x-axis if ESYS is not provided. X = Element x-axis if ESYS is provided. The Element Output Definitions table uses the following notation: A colon (:) in the Name column indicates that the item can be accessed by the Component Name method (, ). The O column indicates the availability of the items in the file Jobname.OUT. The R column indicates the availability of the items in the results file.
In either the O or R columns, “Y” indicates that the item is always available, a number refers to a table footnote that describes when the item is conditionally available, and “-” indicates that the item is not available. The following stress solution repeats for top, middle, and bottom surfaces. Nonlinear solution output for top, middle, and bottom surfaces, if the element has a nonlinear material, or if large-deflection effects are enabled (,ON) for SEND.
Stresses, total strains, plastic strains, elastic strains, creep strains, and thermal strains in the element coordinate system are available for output (at all section points through thickness). If layers are in use, the results are in the layer coordinate system. Available only at centroid as a item. Available only if,LOCI is used. Available only if the and,STATE command are used. The equivalent strains use an effective Poisson's ratio: for elastic and thermal this value is set by the user (,PRXY); for plastic and creep this value is set at 0.5.
Not available if the membrane element option is used (KEYOPT(1) = 1). Lists output available through using the Sequence Number method. See in the and in this reference for more information.
The following notation is used in. ANSYS, Inc. Recommends against using this element in triangular form, except as a filler element.
Avoid triangular form especially in areas with high stress gradients. Zero-area elements are not allowed. (Zero-area elements occur most often whenever the elements are numbered improperly.).
Zero thickness elements or elements tapering down to a zero thickness at any corner are not allowed (but zero thickness layers are allowed). If multiple load steps are used, the number of layers cannot change between load steps. When the element is associated with (,GENS), additional restrictions apply. For more information, see. If reduced integration is used (KEYOPT(3) = 0) SHELL181 ignores rotary inertia effects when an unbalanced laminate construction is used, and all inertial effects are assumed to be in the nodal plane (that is, an unbalanced laminate construction and offsets have no effect on the mass properties of the element). For most composite analyses, ANSYS, Inc.
Recommends setting KEYOPT(3) = 2 (necessary to capture the stress gradients). No slippage is assumed between the element layers.
Shear deflections are included in the element; however, normals to the center plane before deformation are assumed to remain straight after deformation. Transverse shear stiffness of the shell section is estimated by an energy equivalence procedure (of the generalized section forces and strains vs. The material point stresses and strains). The accuracy of this calculation may be adversely affected if the ratio of material stiffnesses (Young's moduli) between adjacent layers is very high. The calculation of interlaminar shear stresses is based on simplifying assumptions of unidirectional, uncoupled bending in each direction. If accurate edge interlaminar shear stresses are required, shell-to-solid submodeling should be used.
The section definition permits use of hyperelastic material models and elastoplastic material models in laminate definition; however, the accuracy of the solution is primarily governed by fundamental assumptions of shell theory. The applicability of shell theory in such cases is best understood by using a comparable solid model. The layer orientation angle has no effect if the material of the layer is hyperelastic.
Before using this element in a simulation containing curved thick shell structures with unbalanced laminate construction or shell offsets, validate the usage via full 3-D modeling with a solid element in a simpler representative model. The element may underestimate the curved thick shell stiffness, particularly when the offset is large and the structure is under torsional load. Consider using curved-shell formulation (KEYOPT(5) = 1). The through-thickness stress, SZ, is always zero. This element works best with the full Newton-Raphson solution scheme (,FULL,ON). Stress stiffening is always included in geometrically nonlinear analyses (,ON).
Prestress effects can be activated by the command. In a nonlinear analysis, the solution process terminates if the thickness at any integration point that was defined with a nonzero thickness vanishes (within a small numerical tolerance). If a shell section has only one layer and the number of section integration points is equal to one, or if KEYOPT(1) = 1, then the shell has no bending stiffness, a condition that can result in solver and convergence problems.