3-D 8-Node Coupled Pore-Pressure-Thermal Mechanical
Solid
CPT215 Element Description
CPT215 is a 3-D eight-node coupled pore-pressure-thermal mechanical solid element. The element is defined by eight nodes having four (or optionally five) degrees of freedom at each corner node:
Translations in the nodal x, y, and z directions
One pore-pressure degree of freedom
One temperature degree of freedom (optional)
CPT215 has elasticity, stress stiffening, large deflection, and large strain capabilities. Various printout options are available.
For more details about this element, see CPT215 .
A higher-order version of this element is CPT216.
CPT215 Input Data
The geometry and node locations for this element are shown in Figure 215.1: CPT215 Structural Solid Geometry. A prism-shaped element can be formed by defining the same node numbers for nodes K and L, and nodes O and P. A tetrahedral-shaped element and a pyramid-shaped element can also be formed, as shown in the illustration. (CPT217 is a similar element, but is a 10-node tetrahedron.)
In addition to the nodes, the element input data includes the orthotropic material properties. Orthotropic material directions correspond to the element coordinate directions. The element coordinate system orientation is described in Coordinate Systems.
Element loads are described in Nodal Loading. Pressures can be input as surface loads on the element faces as shown by the circled numbers in Figure 215.1: CPT215 Structural Solid Geometry. Positive pressures act into the element.
For problems that do not consider the optional temperature degrees of freedom, temperatures can be input as element body loads at the nodes. The node I temperature T(I) defaults to TUNIF. If all other temperatures are unspecified, they default to T(I). For any other input temperature pattern, unspecified temperatures default to TUNIF.
As described in Coordinate Systems, you can use the ESYS command to orient the material properties and strain/stress output. Use the RSYS command to choose output that follows the material coordinate system or the global coordinate system.
The effects of pressure load stiffness are automatically included for this element, and the element generally produces an unsymmetric matrix. To avoid convergence difficulty, issue the NROPT,UNSYM command to use the unsymmetric solver.
"CPT215 Input Summary" contains a summary of element input. For a general description of element input, see Element Input.
CPT215 Input Summary
- Nodes
I, J, K, L, M, N, O, P
- Degrees of Freedom
UX, UY, UZ, PRES, TEMP
- Real Constants
None - Material Properties
TB command: See Element Support for Material Models for this element. MP command: EX, EY, EZ, ALPX, ALPY, ALPZ (or CTEX, CTEY, CTEZ or THSX,THSY, THSZ), PRXY, PRYZ, PRXZ (or NUXY, NUYZ, NUXZ), DENS, GXY, GYZ, GXZ, ALPD, BETD, DMPR - Surface Loads
- Pressures --
face 1 (J-I-L-K), face 2 (I-J-N-M), face 3 (J-K-O-N), face 4 (K-L-P-O), face 5 (L-I-M-P), face 6 (M-N-O-P)
- Body Loads
- Temperatures --
T(I), T(J), T(K), T(L), T(M), T(N), T(O), T(P)
- Special Features
Initial state Large deflection Large strain Stress stiffening - KEYOPT(11)
Temperature degree of freedom:
- 0 --
Disabled (default)
- 1 --
Enabled
CPT215 Output Data
The solution output associated with the element is in two forms:
Nodal displacements and pore pressure included in the overall nodal solution
Additional element output as shown in Table 215.1: CPT215 Output Definitions
The element stress directions are parallel to the element coordinate system, as shown in Figure 215.2: CPT215 Stress Output. A general description of solution output is given in The Item and Sequence Number Table. See the Basic Analysis Guide for ways to view results.
Figure 215.2: CPT215 Stress Output
The element stress directions are parallel to the global coordinate system.
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 (ETABLE, ESOL). 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.
Table 215.1: CPT215 Output Definitions
| Name | Definition | O | R |
|---|---|---|---|
| EL | Element Number | - | Y |
| NODES | Nodes - I, J, K, L, M, N, O, P | - | Y |
| MAT | Material number | - | Y |
| VOLU: | Volume | - | Y |
| XC, YC, ZC | Location where results are reported | Y | 2 |
| TEMP | Temperatures T(I), T(J), T(K), T(L), T(M), T(N), T(O), T(P) | - | Y |
| S:X, Y, Z, XY, YZ, XZ | Stresses | Y | Y |
| S:1, 2, 3 | Principal stresses | - | Y |
| S:INT | Stress intensity | - | Y |
| S:EQV | Equivalent stress | - | Y |
| EPEL:X, Y, Z, XY, YZ, XZ | Elastic strains | Y | Y |
| EPEL:1, 2, 3 | Principal elastic strains | - | Y |
| EPEL:EQV | Equivalent elastic strains [3] | - | Y |
| EPTH:X, Y, Z, XY, YZ, XZ | Thermal strains | 1 | 1 |
| EPTH:EQV | Equivalent thermal strains [3] | 1 | 1 |
| ESIG:X, Y, Z, XY, XZ, YZ | Effective stresses | - | Y |
| PMSV:VRAT,PPRE,DSAT,RPER | Void volume ratio, pore pressure, degree of saturation, and relative permeability | - | Y |
Table 215.2: CPT215 Item and Sequence Numbers lists output available via ETABLE using the Sequence Number method. See Element Table for Variables Identified By Sequence Number and The Item and Sequence Number Table in this document for more information. The following notation is used in Table 215.2: CPT215 Item and Sequence Numbers:
- Name
output quantity as defined in the Table 215.1: CPT215 Output Definitions
- Item
predetermined Item label for ETABLE command
- I,J,...,P
sequence number for data at nodes I, J, ..., P
CPT215 Assumptions and Restrictions
The element must not have a zero volume. Also, the element may not be twisted such that the element has two separate volumes (which occurs most frequently when the element is numbered improperly). Elements may be numbered either as shown in Figure 215.1: CPT215 Structural Solid Geometry or may have the planes IJKL and MNOP interchanged.
When degenerated into a tetrahedron, wedge, or pyramid element shape (described in Degenerated Shape Elements), the corresponding degenerated shape functions are used. Degeneration to a pyramidal form should be used with caution. The element sizes, when degenerated, should be small to minimize the stress gradients. Pyramid elements are best used as filler elements or in meshing transition zones.
Stress stiffening is always included in geometrically nonlinear analyses (NLGEOM,ON). It is ignored in geometrically linear analyses (NLGEOM,OFF). Prestress effects can be activated by the PSTRES command.