3. Operands#

3.1. Case where the FOND_FISS operand is filled in#

This case corresponds to a calculation on a mesh crack, defined for post-processing with the operator DEFI_FOND_FISS. By default, the calculation is done automatically for all the nodes at the bottom of the crack.

3.1.1. Operands FOND_FISS/PREC_VIS_A_VIS/GROUP_NO/SANS_GROUP_NO#

◊ FOND_FISS = background, [fond_fiss]

◊ PREC_VIS_A_VIS =/1.D-1, [DEFAUT]

/epsi, [R]

◊/TOUT = “OUI”,

/GROUP_NO = gr_node, [l_gr_node] /SANS_GROUP_NO = gr_node, [l_gr_node]

The FOND_FISS operand makes it possible to provide the fond_fiss concept (created by the DEFI_FOND_FISS command) in which the information necessary for the automatic search for the nodes of the two lips located on normal segments at the bottom of the crack is stored. Attention, the fond_fiss concept must necessarily be defined such that the lips of the crack are initially stuck (CONFIG_INIT =” COLLEE “in DEFI_FOND_FISS [U4.82.01])

In 3D, by default, the calculation of stress intensity factors is done only on the node vertices of the cells composing the crack background (therefore all the nodes for the linear elements, and one node out of two for the quadratic elements). The user has the possibility to:

  • select certain nodes at the top of the crack (keyword GROUP_NO);

  • to exclude knots from the bottom of the crack (keyword SANS_GROUP_NO);

  • to do the calculation on all the middle nodes and vertices of the crack bottom (keyword TOUT).

Maximum distance for calculating stress intensity factors from the crack bottom. In practice, the precision of the results is less good if one is located very far from the bottom of the crack [R7.02.08]. It is therefore advisable to choose \(\mathit{dmax}\) as small as possible (of the order of 3 to 4 elements, or even of the order of the radius of the radiating mesh, if applicable). In the case where at a point \(N\) of the crack bottom, this distance is greater than the distance from the crack bottom to this point at the edge of the lips, the value of the stress intensity factors at point \(N\) are obtained by constant extension. The value selected is that of the closest point at the bottom of the crack and for which the calculation could be carried out.

Operand ABSC_CURV_MAXI is optional. When this operand is not specified, the value of ABSC_CURV_MAXI is automatically calculated from the maximum \(h\) of the sizes of the cells connected to the nodes at the bottom of the crack. These mesh sizes at each bottom node are calculated in the DEFI_FOND_FISS command and are present in the fond_fiss concept [D4.10.01]. It was chosen to take ABSC_CURV_MAXI equal to \(4h\).

If you choose the value automatically calculated for ABSC_CURV_MAXI, however, you must ensure that its value (displayed in the .mess file) is consistent with the dimensions of the structure.

During the automatic search for each node in the crack bottom, the operator selects the nodes that meet the following conditions:

  • distance \(R\) from the bottom of the crack: \(R\) < ABSC_CURV_MAXI,

  • distance \(L\) in relation to his opponent on the other lip:

\(L\) < epsi. ABSC_CURV_MAXI,

  • and in 3D distance \(D\) from a lip node to the line perpendicular to the crack bottom: \(D\) < epsi_fond. \(d\), where \(d\) is the minimum distance between two successive nodes at the bottom of the crack,

where epsi is the precision value provided (keyword PREC_VIS_A_VIS) and epsi_fond is the precision value provided in the PREC_NORM keyword of DEFI_FOND_FISS.

By default epsi is set to \(\mathrm{0,1}\). Increasing the value of PREC_VIS_A_VIS (and/or PREC_NORM in DEFI_FOND_FISS) is equivalent to increasing the number of nodes potentially selected for the calculation.

Note:

Si TYPE_MAILLAGE =” REGLE “, this precision intervenes in the phase of projecting the result onto the cut line: a point is considered to be out of the material if its distance from the structure is greater than epsi. ABSC_CURV_MAXI . It may be necessary to change the default value for this parameter if the crack is represented by a notch.

3.1.2. Operands RESULTAT#

resu is an evol_elas, evol_noli, or mode_meca concept containing the displacement field throughout the model.

Note:

Resu can’t be a type mode_meca concept only if TYPE_MAILLAGE =” REGLE “.

3.1.3. Operand TYPE_MAILLAGE#

◊ TYPE_MAILLAGE =/'REGLE', [default]

/”LIBRE”,

This keyword only makes sense in 3D, for mesh cracks defined by FOND_FISS.

If TYPE_MAILLAGE = “REGLE”, the option to be used by default, the calculation is done by assuming that the knots on the lips of the crack are in normal directions at the bottom and exactly facing each other from one lip to the other. Alarm or error messages are sent if this is not the case.

If the mesh does not meet these conditions, you can use the option TYPE_MAILLAGE =” LIBRE “. The principle of the calculation is then as follows:

  1. determination of the normal directions at the crack bottom for each of the nodes at the bottom,

  2. definition on each of these directions of NB_NOEUD_COUPE points equally distributed between the background and the distance ABSC_CURV_MAXI,

  3. projection of the movement of each lip onto these nodes,

  4. interpolation of the displacement jump.

The calculation with TYPE_MAILLAGE =” LIBRE “may be less accurate than the calculation by default.

3.1.4. Operand NB_NOEUD_COUPE#

/n, [I]

This operand only occurs if TYPE_MAILLAGE =” LIBRE “is specified. It makes it possible to define the number of nodes for projecting the movement of the lips in each of the normal directions. The projection nodes are evenly distributed between the crack bottom and the distance ABSC_CURV_MAXI.

Note:

The projection of the movement of the lips onto the NB_NOEUD_COUPE projection points does not correctly take into account the displacement of the nodes by a quarter (Barsoum elements). It is therefore recommended that you do not use these items if TYPE_MAILLAGE =” LIBRE “.

3.2. Case where the FISSURE operand is filled in#

This case corresponds to a calculation on a non-meshed crack, defined for the calculation and then for post-processing with the operator DEFI_FISS_XFEM.

3.2.1. Operand FISSURE#

Fiss_xfem concept, produced by command DEFI_FISS_XFEM.

3.2.2. Operand RESULTAT#

An evol_elas or evol_noli concept containing the displacement field throughout the model. The mesh is deduced from this concept.

3.2.3. Operand ABSC_CURV_MAXI#

◊ ABSC_CURV_MAXI = dmax [R]

Maximum distance for calculating stress intensity factors from the crack bottom. In practice, the precision of the results is less good if one is located very far from the bottom of the crack [R7.02.08]. It is therefore advisable to choose \(\mathit{dmax}\) as small as possible (of the order of 4 to 5 elements). In the case where at a point \(N\) of the crack bottom, this distance is greater than the distance from the crack bottom to this point at the edge of the lips, the value of the stress intensity factors at point \(N\) are obtained by constant extension. The value selected is that of the closest point at the bottom of the crack and for which the calculation could be carried out.

Operand ABSC_CURV_MAXI is optional. When this operand is not specified, the value of ABSC_CURV_MAXI is automatically calculated from the maximum \(h\) of the sizes of the cells connected to the nodes at the bottom of the crack. These mesh sizes at each bottom node are calculated in the DEFI_FOND_FISS command and are present in the fiss_xfem concept [D4.10.02]. It was chosen to take ABSC_CURV_MAXI equal to \(5h\).

If you choose the value automatically calculated for ABSC_CURV_MAXI, however, you must ensure that its value (displayed in the .mess file) is consistent with the dimensions of the structure.

3.2.4. Operand NUME_FOND#

/number, [I]

Multiple crack backgrounds can be defined in a single fiss_xfem data structure. This operand allows you to select the number of the background on which the calculation must be carried out. By default, only the first background is considered.

3.2.5. Operand NB_NOEUD_COUPE#

/n, [I]

This operand makes it possible to define the number of nodes for projecting the movement of the lips in each of the normal directions. The projection nodes are evenly distributed between the crack bottom and the distance ABSC_CURV_MAXI.

3.2.6. Operand NB_POINT_FOND#

By default for an X- FEM crack, the calculation is done on all the points of the crack bottom, i.e. all the intersection points between the crack bottom and the edges of the mesh. The NB_POINT_FOND operand makes it possible to set a priori the number of post-processing points, in order to limit calculation times. The Nbnofo points are evenly distributed along the crack bottom.

3.3. Operand MATER#

Material type concept containing the elastic characteristics of cracked material. If this optional keyword is entered, the material mat will replace, during the execution of the command, the material used to solve the mechanical problem and present in the fiel_master contained in the sd_result (given under the keyword RESULTAT). The matt material should be homogeneous, isotropic and linear elastic. This material must be constant (mandatory use of DEFI_MATERIAU/ELAS). The MATERest keyword should be used with care.

3.4. Operands INST, LIST_INST, FREQ, LIST_FREQ, TOUT_ORDRE,,,, NUME_ORDRE, LIST_ORDRE, TOUT_MODE, NUME_MODE, LIST_MODE#

Cf. [U4.71.00].

3.5. Operand INFO#

/2,

Message level in the message file: if INFO is 2, the list of all the values calculated for all the nodes processed is provided.

3.6. Operand TITRE#

◊ TITRE = title,

Title that we want to give to the result of the order.