/STACK

Description

The input can be either "substack" or by ply, but not both.

Format

Definition

Comments

1. The stack is used in combination with /PLY to create composites properties through the ply-based definition. It is NOT compatible with /PROP/TYPE19 (PLY) option.
   • Each ply in the stack is defined using Pply_ID_i or Pply_ID_j and can only be referenced once in the stack. If ply ids are used more than once an error message will output.
   • Each ply of a shell element can contain a different (≥ 1) number of integration points through its ply thickness. That is, Npt_ply =3 in /PLY.
   • Different integration scheme (uniformly distribution or Gauss integration scheme) could be defined with option I_int inside this property,
2. I_shell, I_sh3n – 4-node and 3-node shell formulation flag.
   • I_shell =1,2,3,4 (Q4): original 4 nodes Radioss shell with hourglass perturbation stabilization.
   • I_shell =24 (QEPH): formulation with hourglass physical stabilization for general use.
   • I_shell =12 (QBAT): modified BATOZ Q4γ24 shell with four Gauss integration points and reduced integration for in-plane shear. No hourglass control is needed for this shell.
   • I_sh3n =30 (DKT18): BATOZ DKT18 thin shell with three Hammer integration points.
   • The hourglass formulation is visco-elastic for Q4 shells.
3. I_smstr - Small strain formulation
   • Small strain formulation is activated from time t = 0, if I_smstr=1. It may be used for a faster preliminary analysis, but the accuracy of results is not ensured. Any shell for which $\Delta t<\Delta t_{\min }$ can be switched to a small strain formulation by Radioss Engine option /DT/SHELL/CST.
   • If I_smstr=1, the strains and stresses which are given in material laws are engineering strains and stresses; otherwise they are true strains and stresses.
4. h_m, h_f, and h_r - hourglass coefficients
   • h_m, h_f, and h_r are only used for Q4 shells. They must have a value between 0 and 0.05.
   • For I_shell = 3, default values for h_m and h_r are 0.1.
5. d_n - Shell numerical damping
   It is only used for I_shell =12 or 24:

   • for I_shell =24, d_n is used for hourglass stress calculation
   • for QBAT, d_n is used for all stress terms, except transverse shear
   • for DKT18, d_n is only used for membrane
6. I_thick - Shell resultant stresses calculation flag
   • If I_thick = 1, the small strain option is automatically deactivated in the corresponding type of element
7. I_drill - Drilling degree of freedom stiffness
   • Drilling DOF stiffness is recommended for implicit solutions especially for Riks method and bending dominated problems
   • I_drillis available for QEPH, QBAT (I_shell = 12, 24), and standard triangle (C0) shell elements (I_sh3n = 1, 2).
8. Anisotropy direction definition.
   The reference vector $\mathrm{V\text{'}}$ is projection of the input vector $V$ on the shell element surface and defined as following according to the flag I_P:

   • If I_P = 0 and skew_ID = 0, the vector $V$ is defined with V_X, V_Y and V_Z.
   • If I_P = 0 and skew_ID ≠ 0, the vector $V$ is the first direction (local X) of the local coordinate system skew_ID.
   • If I_P = 20, the vector $V$ is defined with the node N1 and N2 of the shell elements.

     
     
     Figure 3. I_P = 20

   • If I_P = 22, the vector $V$ is the first direction (local X) of the local coordinate system skew_ID. Vector components V_X, V_Y and V_Z are ignored.

     
     
     Figure 4. I_P = 22

   • If I_P = 23, the vector $V$ is defined with V_X, V_Y and V_Z. The reference vector $\mathrm{V\text{'}}$ is the product of vector $V$ and shell element normal direction $n$ . Local coordinate system skew_ID is ignored.

     
     
     Figure 5. I_P = 23

   Then for each layer, the 1^st material direction (m1) is vector $V^{\prime }$ turned ${\varphi }_i$ degrees (turns positive direction around shell normal $n$ ).

   
   
   Figure 6.

   The hierarchy order to define the reference vector $V$ is:

   • shell element ( ${\varphi }_s$ > 0 in either /SHELL or /SH3N)
   • initial state card (/INISHE/ORTHO)
   • shell property

   In case of reference metrics, the orientation for directions of anisotropy must be defined with the reference geometry, not the initial one.

   The 2^nd material direction m2 is derived from direction m1 with the angle ${\alpha }_i$ (defined in /PLY).

   If ${\alpha }_i=90°$ , the layer is orthotropic.

9. I_pos - ply position
   • In this property, ply position defined with $Z_i$ and option I_pos.
   • $Z_i$ values are real ply positions in the local Z axis (negative and positive value are allowed and $Z_i$ defines the position of the middle of each ply).
   • $Z_0$ only used with I_pos=2, it indicated distance from the shell element reference middle surface to the bottom surface of the whole ply or stack. For additional information, see I_pos=2
   • I_pos = 0: ply positions are automatically calculated with regards to ply thicknesses.
     • The input thickness “Thick” may be equal to zero as the true shell thickness is always calculated from the ply thicknesses and positions of ply defined by $Z_i$ .
     • If the input thickness “Thick” is different from 0.0, a warning will be displayed if the calculated thickness from the plies is different from the input thickness.

       
       
       Figure 7.

   • I_pos = 1: All ply positions and thickness in the element are user-defined (with in /PLY and $Z_i$ ).
     • Global thickness “Thick” is not checked, as it does not need to be equal to the sum of ply thickness
     • Multiple plies are allowed to have the same space position
     • This option is not recommended unless the number of ply is constant for all elements
   • I_pos = 2: the shell element mid-surface is at $Z_0$ from the bottom of the ply layout.

     
     
     Figure 8.

   • I_pos = 3: the top of the ply layout is coincident with the element mid-surface.

     
     
     Figure 9.

   • I_pos = 4: the bottom of the ply layout is coincident with the element mid-surface.

     
     
     Figure 10.

10. Define by substack.
    • If the substack definition is used, each substack definition begins with the keyword "SUB" and the substack identification number.
    • All the plies until the next keyword "SUB" or the keyword "INT" is encountered belong to the same substack.
    • All substacks must be defined before defining the “INT connections
    • If an element contains plies belonging to several substacks, the connection between the substacks is defined with an “INT” connection: the ply Pply_ID_t of the first substack is connected to the ply Pply_ID_b of the second substack.
    • For additional information, refer to the above examples for this property.
11. Material for each ply.
    • Material for each ply defined with mat_ID in /PLY.
    • Different material laws could be defined for each ply.
    • The material law number given in /PART will be used to define the mass and the sound speed of the composite, as well as the interface stiffness.
12. Element deletion rules used with P_thick_fail and failure models:
    • At the ply level, all P_thickl_{fail_i} defined in the ply and P_thick_fail defined in failure model(s) (/FAIL) are considered to trigger the ply failure.
      • P_thickl_{fail_i} > 0 defines a fraction of failed ply’s thickness. This uses the amount of ply’s thickness assigned to each thickness integration point in the ply.
      • P_thickl_{fail_i} < 0 defines a ratio of failed ply’s integration points. This uses the number of integration point defined in the ply.
    • Minimum of the absolute value defined in the ply or in the failure criterion card is used to trigger the ply failure.
    • In case there is only 1 integration point on each ply, the P_thick_fail defined in failure model(s) are not used (/PROP/TYPE17 (STACK)).
    • When plys’ failure starts, a global element deletion criterion is regarded considering the global value P_thick_fail defined in the stack:
      • P_thick_fail > 0 defines a fraction of failed thickness. This uses the amount of element global thickness assigned to each ply.
      • P_thick_fail < 0 defines a ratio of failed plies. This uses the number of plies.
    • For fully-integrated shells (I_shell=12), the rules described above for under-integrated shells, applies to each Gauss point separately. P_thickl_{fail_i} criterion is checked for all integration points in plys’ thickness for each in-plane Gauss point. The ply is considered as broken only when all Gauss points reach P_thickl_{fail_i} criterion. Then global element criterion is regarded.
    • P_thick_fail or P_thickl_{fail_i} values must the same sign. Otherwise, the P_thick_fail defined in the ply takes the same sign as the value defined in the failure criterion card /FAIL.
    • P_thick_fail rules are not used with failure models defined inside the material laws. It is used only for the failure model defined with /FAIL.