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发表于 2009-4-6 14:37:45
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来自 澳大利亚
FLAC manual 3.9.3和3.9.2写得很清楚，可以作为图的注释：
The grid velocities may be assessed either by plotting out the whole field of velocities (using the
PLOT vel command) or by selecting certain key points in the grid and tracking their velocities with
histories (HISTORY xvel or HISTORY yvel). Both types of plot are useful. Steady-state conditions
are indicated if the velocity histories show horizontal traces in their final stages. If they have
all converged to near-zero (in comparison to their starting values), then absolute equilibrium has
occurred; if a history has converged to a nonzero value, then steady plastic flow is occurring at
the gridpoint corresponding to that history. If one or more velocity history plots show fluctuating
velocities, then the system is likely to be in a transient condition. Note that velocities are expressed
in units of displacement divided by number of steps, except in dynamic solution mode.
The plot of the field of velocity vectors is more difficult to interpret, since both the magnitudes and
the nature of the pattern are important. As with gridpoint forces, velocities never decrease precisely to zero. The magnitude of velocity should be viewed in relation to the displacement that would
occur if a significant number of steps (e.g., 1000) were to be executed. For example, if current
displacements in the system are of the order of 1 cm, and the maximum velocity in the velocity
plot is 10−8 m/step, then 1000 steps would produce an additional displacement of 10−5 m, or 10−3
cm, which is 0.1% of the current displacements. In this case, it can be said that the system is in
equilibrium, even if the velocities all seem to be “flowing” in one direction. More often, the vectors
appear to be random (or almost random) in direction and (possibly) in magnitude. This condition
occurs when the changes in gridpoint force fall below the accuracy limit of the computer, which
is around six decimal digits, for the single-precision version (15 digits for the double-precision
version). A random velocity field of low amplitude is an infallible indicator of equilibrium and no
plastic flow.
If the vectors in the velocity field are coherent (i.e., there is some systematic pattern), and their
magnitude is quite large (using the criterion described above), then either plastic flow is occurring
or the system is still adjusting elastically (e.g., damped elastic oscillation is taking place). To
confirm that continuing plastic flow is occurring, a plot or printout of plasticity indicators should
be consulted, as described below. If, however, the motion involves elastic oscillation, then the
magnitude should be observed in order to indicate if such movement is significant; seemingly
meaningful patterns of oscillation may be seen but, if amplitude is low, then the motion has no
physical significance.
For most of the nonlinear models in FLAC, the commands PLOT plas, PRINT state and PLOT state
block display those zones in which the stresses satisfy the yield criterion. Such an indication usually
denotes that plastic flow is occurring, but it is possible for an element simply to “sit” on the yield
surface without any significant flow taking place. It is important to look at the whole pattern of
plasticity indicators to see if a mechanism has developed. A failure mechanism is indicated if there
is a contiguous line of active plastic zones (with a state of 1) that joins two surfaces. The diagnosis is
confirmed if the velocity plot also indicates motion corresponding to the same mechanism. Note that
initial plastic flow often occurs at the beginning of a simulation, but subsequent stress redistribution
unloads the yielding elements so that their stresses no longer satisfy the yield criterion — these
elements show a state number of 2. Only the actively yielding elements (state number of 1) are
important to the detection of a failure mechanism |
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