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Figure 19.57.1. Behavior of the low-density urethane foam
model.
Material Type 58: Laminated Composite FabricParameters to
control failure of an element layer are: ERODS, the maximum
effectivestrain, i.e., maximum 1 = 100 % straining. The layer in
the element is completely removed after
the maximum effective strain (compression/tension including
shear) is reached.
The stress limits are factors used to limit the stress in the
softening part to a given value,
min = SLIMxx strength ,
thus, the damage value is slightly modified such that
elastoplastic like behavior is achieved with
the threshold stress. As a factor for SLIMxx a number between
0.0 and 1.0 is possible. With a
factor of 1.0, the stress remains at a maximum value identical
to the strength, which is similar to
ideal elastoplastic behavior. For tensile failure a small value
for SLIMTx is often reasonable;however, for compression SLIMCx =
1.0 is preferred. This is also valid for the corresponding
shear value. If SLIMxx is smaller than 1.0 then localization can
be observed depending on thetotal behavior of the lay-up. If the
user is intentionally using SLIMxx < 1.0, it is generally
recommended to avoid a drop to zero and set the value to
something in between 0.05 and 0.10.Then elastoplastic behavior is
achieved in the limit which often leads to less numerical
problems.
Defaults for SLIMXX = 1.0E-8.
The crashfront-algorithm is started if and only if a value for
TSIZE (time step size, withelement elimination after the actual
time step becomes smaller than TSIZE) is input .
The damage parameters can be written to the postprocessing
database for each integrationpoint as the first three additional
element variables and can be visualized.
Material models with FS=1 or FS=-1 are favorable for complete
laminates and fabrics, asall directions are treated in a similar
fashion.
For material model FS=1 an interaction between normal stresses
and shear stresses is
assumed for the evolution of damage in the a- and b- directions.
For the shear damage is alwaysthe maximum value of the damage from
the criterion in a- or b- direction is taken.
For material model FS=-1 it is assumed that the damage evolution
is independent of any
of the other stresses. A coupling is present only via the
elastic material parameters and thecomplete structure.
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In tensile and compression directions and in a- as well as in b-
direction, different failure
surfaces can be assumed. The damage values, however, increase
only when the loading direction
changes.
Special control of shear behavior of fabrics
For fabric materials a nonlinear stress strain curve for the
shear part of failure surfaceFS=-1 can be assumed as given below.
This is not possible for other values of FS.
The curve, shown in Figure 19.58.1, is defined by three
points:
a) the origin (0,0) is assumed,b) the limit of the first
slightly nonlinear part (must be input), stress (TAU1) and
strain
(GAMMA1), see below.
c) the shear strength at failure and shear strain at
failure.
In addition a stress limiter can be used to keep the stress
constant via the SLIMS parameter. This
value must be less than or equal to 1.0 and positive, which
leads to an elastoplastic behavior for
the shear part. The default is 1.0E-08, assuming almost brittle
failure once the strength limit SCisreached.
Figure 19.58.1. Stress-strain diagram for shear.
Material Type 60: Elastic With ViscosityThis material model was
developed to simulate the forming of glass products (e.g., car
windshields) at high temperatures. Deformation is by viscous
flow but elastic deformations can
also be large. The material model, in which the viscosity may
vary with temperature, is suitablefor treating a wide range of
viscous flow problems and is implemented for brick and shell
elements.
