Zone

Orientation of Nodes and Faces within a Zone

A zone is a closed geometric domain, with nodes at the vertices and faces forming the boundary of the zone (surfaces in 3D or edges in 2D).

Zones in 3D

The relative orientation of the nodes and faces is shown in the figure below for the basic zone types: brick, wedge, pyramid, degenerate brick and tetrahedron (see zone create for full list). Each face has vertices; these vertices are also identified in the figure. Several FLAC and FISH commands (e.g., zone attach) refer to this orientation.

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Figure 1: 3D zone geometry.

Zones in 2D

The relative orientation of the nodes and faces is shown in the figure below for the basic zone types: quadrilateral and triangle (see zone create2d for full list). Each face (represented by an edge in 2D) has two connected vertices; these vertices are also identified in the figure. Several FLAC and FISH commands (e.g., zone attach) refer to this orientation.

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Figure 2: 2D zone geometry.

Condition Measure of Zones

A zone condition number is a measure of how badly deformed a zone is. Several basic measures are included, and the condition number is taken as their minimum to characterize the worst case. The range of a zone condition number is between 0 and 1. The larger the zone number is, the better the zone geometry is. For detailed information about how zone condition number is calculated, refer to zone geometry-test.

Zone Field Data Names

In FLAC, the FISH zone field data functions allow the user to make queries about the values of a model variable at arbitrary locations in space. This includes zone-based information as well as gridpoint-based information. The FISH functions in this section are state-based. Some of them set the particular data type (and the methods and properties used to get that data), while others retrieve the data once these values have been set.

By default, queries operate as quickly as possible for individual calls. If many (hundreds or more) points of data are to be queried, you can initialize the system to optimize multiple calls. This makes it unnecessary to calculate data more than once in a given zone or gridpoint. As an example, here is a FISH fragment that queries for the gridpoint-based value \(x\)-displacement at a single location in space:

zone.field.name = 'displacement-x'
local result = zone.field.get(vector(3.5,4.3,6.7)) ; for FLAC3D
local result = zone.field.get(vector(3.5,6.7)) ; for FLAC2D

Here is a FISH fragment that performs many queries on a line in space, on the zone-based values of horizontal stress:

array dataset(100)
zone.field.name = 'stress-xx'
zone.field.method.name = ’poly’ ; Use polynomial fit extrapolation
zone.field.init
loop ii (1,100)
   local xp = 5.0 + float(ii)/10.0
   dataset(ii) = zone.field.get(3.5,4.3,xp) ; for FLAC3D
end_loop
zone.field.reset

The available zone field data names are described in the table below. Note that these are the same values available under zone history, for example.


Zone Field Data Names

Name Description
acceleration acceleration magnitude at the gridpoint (only available if model configure dynamic has been specified)
acceleration-x \(x\)-acceleration at the gridpoint (only available if model configure dynamic has been specified)
acceleration-y \(y\)-acceleration at the gridpoint (only available if model configure dynamic has been specified)
acceleration-z (3D only) \(z\)-acceleration at the gridpoint (only available if model configure dynamic has been specified)
condition a measure of how badly deformed a zone is
density the density of the zone
displacement displacement magnitude at the gridpoint
displacement-x \(x\)-displacement at the gridpoint
displacement-y \(y\)-displacement at the gridpoint
displacement-z (3D only) \(z\)-displacement at the gridpoint
extra extra variable value. The extra variable index used will default to 1 and can be changed with the zone.field.extra function. By default, the value will come from the grid point extra variables, but this can be specified using the zone.field.source function. By default, the value will be treated as a scalar floating point type, but this can be specified using the zone.field.type function.
factor-of-safety largest stable factor of safety of a grid point. Calculated during model factor-of-safety and uses the velocity-limit keyword.
pore-pressure pore pressure in zone. By default, this will be the grid point pore pressure but the zone average pore pressure can be specified by using the zone.field.source function.
property-fluid a property of the fluid constitutive model of the zone. The property name must be specified using the zone.field.prop function. By default, it will be assumed the property is a floating point scalar but this can be specified using the zone.field.type function.
property a property of the mechanical constitutive model of the zone. The property name must be specified using the zone.field.prop function. By default, it will be assumed the property is a floating point scalar but this can be specified using the zone.field.type function.
property-thermal a property of the thermal constitutive model of the zone. The property name must be specified using the zone.field.prop function. By default, it will be assumed the property is a floating point scalar but this can be specified using the zone.field.type function.
ratio-local the local unbalanced force ratio at each gridpoint. Like all results, this can be changed to the logarithm of the value by using the zone.field.log function.
saturation the saturation at the gridpoint (only available if model configure fluid has been specified)
strain-increment the strain increment tensor of the zone determined by the current displacement field. Use the zone.field.quantity function to specify which scalar value to retrieve from the tensor.
strain-rate the rate increment tensor of the zone determined by the current velocity field. Use the zone.field.quantity function to specify which scalar value to retrieve from the tensor.
stress the stress tensor of the zone determined by the weighted average of the subzone stresses. Use the zone.field.quantity function to specify which scalar value to retrieve from the tensor.
stress-effective the effective stress tensor of the zone determined by the weighted average of the subzone stresses minus the zone averaged pore pressure. Use the zone.field.quantity function to specify which scalar value to retrieve from the tensor.
strength-stress-ratio the strength-stress ratio of the zone. Not all constitutive models support this calculation. If unsupported, the value returned will be 10. The value returned is generally held to a maximum of 10.
temperature the temperature at the gridpoints. The zone.field.source function can be used to specify that zone-based temperature should be used instead.
timestep-dynamic the local critical dynamic timestep of that particular gridpoint (only available if model configure dynamic has been specified)
unbalanced-force the unbalanced force magnitude at the gridpoint
unbalanced-force-x \(x\)-unbalanced force at the gridpoint
unbalanced-force-y \(y\)-unbalanced force at the gridpoint
unbalanced-force-z (3D only) \(z\)-unbalanced force at the gridpoint
velocity the velocity magnitude at the gridpoint
velocity-x \(x\)-velocity at the gridpoint
velocity-y \(y\)-velocity at the gridpoint
velocity-z (3D only) \(z\)-velocity at the gridpoint

Zone Field Quantity Names

The field data names above may return a range of quantities. The options are listed in the following table. The definitions of the field data of a tensor (stress, strain increment, or strain rate) can be found in Stress/Strain Invariants.


Quantity Name Description
intermediate intermediate principal stress or strain (increment/rate)
maximum maximum (most positive) value of the principal stress or strain (increment/rate)
mean mean value defined as the trace of the tensor divided by three
minimum minimum (most negative) value of the principal stress or strain (increment/rate)
norm norm of stress or strain (increment/rate)
octahedral octahedral stress or strain (increment/rate)
shear-maximum maximum shear stress or strain (increment/rate)
total-measure distance from the origin to the tensor point in the principal space
volumetric trace of the stress or strain (increment/rate)
von-mises von Mises measure of the stress or strain (increment/rate)
xx \(xx\)-component of the stress or strain (increment/rate)
xy \(xy\)-component of the stress or strain (increment/rate)
xz \(xz\)-component of the stress or strain (increment/rate)
yy \(yy\)-component of the stress or strain (increment/rate)
yz \(yz\)-component of the stress or strain (increment/rate)
zz \(zz\)-component of the stress or strain (increment/rate)

Below is a simple example for 3D case:

zone.field.name = 'stress-xx'
; Use polynomial fit extrapolation
zone.field.method.name = 'poly'
; Return stress-xx
global value1 = zone.field.get(10,0,-20)
; Reset the stress quantity to 'von-mises'
zone.field.quantity = 'von-mises'
; Return stress-von-mises
global value2 = zone.field.get(10,0,-20)

or (2D case)

zone.field.name = 'stress-xx'
; Use Inv. Distance Weighting approach
zone.field.method.name = 'inverse-distance-weighting'
; Return stress-xx
global value1 = zone.field.get(10,-20)
; Reset the filed name directly to 'stress-von-mises'
zone.field.name = 'stress-von-mises'
; Return stress-von-mises
global value2 = zone.field.get(10,-20)

Strength-Stress Ratio

The strength-stress ratio (SSR) is calculated in some constitutive models as a local indicator of the current stress state’s proximity to failure. Suppose the current effective minimum and maximum principal stresses are \(\sigma_1\) and \(\sigma_3\). The current Mohr circle is plotted in Figure 3. By keeping \(\sigma_3\) fixed, enlarge the Mohr circle so that it is tangent to the shear failure line; the new minimum effective principal stress is denoted by \(\sigma^{\prime}_1\), and the strength-stress ratio is defined as

\[SSR = \begin{vmatrix} \cfrac{\sigma^{\prime}_1 - \sigma_3}{\sigma_1 - \sigma_3} \end{vmatrix} \le 10\]

It is self-evident that if the stress state is in shear failure, SSR = 1. The upper limit of SSR in FLAC3D is set to 10. If the current stress state is in tension failure, the SSR is set to 0. SSR can be plotted as a zone contour value if it is defined in the constitutive model.

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Figure 3: Schematic of strength-stress ratio.