July 27, 2015 | by Paul Du Bois | views 1260
"Reliable prediction of the behavior of structures made from polymers is a topic under considerable investigation in
engineering practice. Especially, if the structure is subjected to dynamic loading, constitutive models considering
the mechanical behavior properly are still not available in commercial finite element codes yet. In our paper, we
present a new constitutive law for polymers which recovers important phenomena like necking, crazing, strain rate
dependency, unloading behavior and damage. In particular, different yield surfaces in compression and tension and
strain rate dependent failure, the latter with damage induced erosion, is taken into account. All relevant parameters
are given directly in the input as load curves, i.e. time consuming parameter identification is not necessary. Moreover,
the models by von Mises and Drucker-Prager are included in the description as special cases.
With the present formulation, standard verification test can be simulated successfully: tensile and compression test,
shear test and three point bending tests."
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Mechanical
Plastics
Plasticity
Rate Dependency
Yielding/Failure analysis
Automotive
High Speed Testing
LS-DYNA
Research Papers
July 27, 2015 | by Paul Du Bois | views 1545
"Reliable prediction of damage and failure in structural parts is a major challenge posed
in engineering mechanics. Although solid material models predicting the deformation
behaviour of a structure are increasingly available, reliable prediction of failure remains
still open.
With SAMP (a Semi-Analytical Model for Polymers), a general and flexible plasticity
model is available in LS-DYNA since version 971. Although originally developed for
plastics, the plasticity formulation in SAMP is generally applicable to materials that
exhibit permanent deformation, such as thermoplastics, crushable foam, soil and metals.
In this paper, we present a generalized damage and failure procedure that has been implemented
in SAMP and will be available in LS-DYNA soon. In particular, important
effects such as triaxiality, strain rate dependency, regularization and non-proportional
loading are considered in SAMP. All required physical material parameters are provided
in a user-friendly tabulated way. It is shown that our formalism includes many different
damage and failure models as special cases, such as the well-known formulations by
Johnson-Cook, Chaboche, Lemaitre and Gurson among others. "
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Mechanical
Plastics
Plasticity
Rate Dependency
Yielding/Failure analysis
Automotive
High Speed Testing
LS-DYNA
Research Papers
July 22, 2015 | by Paul Du Bois | views 1370
Generating a LS-DYNA material model from cupon-level quasi-static experimental data, developing appropriate failure characteristics, and scaling these characteristics to mesh sizes appropriate for a variety of simulation models requires a regularization procedure. During an Investigation of an anisotropic material model for extruded aluminum, numerical accuracy issues led to unrealistic mesh regularization curves and non-physical simulation behavior. Sensitivity problems due to constitutive material behavior, small mesh sizes, single precision simulations, and simulated test velocity all contributed to these accuracy issues. Detailed analysis into the sources of innaccuracy led to the conclusion that in certain cases, double precision simulations are necesscary for accurate material characterization and mesh regularization.
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Mechanical
Metals
Yielding/Failure analysis
Aerospace and Defense
Automotive
Extrusion
Nonlinear Material Models
LS-DYNA
Research Papers
July 22, 2015 | by Paul Du Bois | views 1442
"Simulation of rubber-like materials is usually based on hyperelasticity. If strain-rate dependency has to be
considered viscous dampers are added to the rheological model. A disadvantage of such a description is timeconsuming
parameter identification associated with the damping constants. In this paper, a tabulated formulation is
presented which allows fast generation of input data based on uniaxial static and dynamic tensile tests at different
strain rates. Unloading, i.e. forming of a hysteresis, can also be modeled easily based on a damage formulation. We
show the theoretical background and algorithmic setup of our model which has been implemented in the explicit
solver LS-DYNA [1]-[3]. Apart from purely numerical examples, the validation of a soft and a hard rubber under
loading and subsequent unloading at different strain rates is shown."
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Mechanical
Rubbers
Hyperelastic
Rate Dependency
Yielding/Failure analysis
Automotive
High Speed Testing
LS-DYNA
Research Papers
July 22, 2015 | by Paul Du Bois | views 1499
"Reliable prediction of the behaviour of structures made from polymers is a topic
under considerable investigation in engineering practice. Especially, if the
structure is subjected to dynamic loading, constitutive models considering the
mechanical behaviour properly are still not available in commercial finite element
codes.
First, we give an overview of material laws for thermoplastics and show how the
behaviour can be characterized and approximated by using visco-elasticity and
metal plasticity, respectively. Experimental work is presented to point out
important phenomena like necking, strain rate dependency, unloading behaviour
and damage. A constitutive model including the experimental findings is derived.
In particular, different yield surfaces in compression and tension and strain rate
dependent failure, the latter with damage induced erosion, need to be taken into
account. With the present formulation, standard verification tests can be
simulated successfully. Also, an elastic damage model is used to approximate
the unloading behaviour of thermoplastics adequately."
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Mechanical
Rate Dependency
Yielding/Failure analysis
Automotive
High Speed Testing
LS-DYNA
Research Papers
July 22, 2015 | by Paul Du Bois | views 1366
"During the past years polymer materials have gained enormous importance in the automotive industry. Especially
their application for interior parts to help in passenger safety load cases and their use for bumper fascias in pedestrian
safety load cases have driven the demand for much more realistic finite element simulations. For such applications
the material model 187 (i.e. MAT_SAMP-1) in LS-DYNA® has been developed.
In the present paper the authors show how the parameters for the rather general model may be adjusted to allow for
the simulation of crazing effects during plastic loading. Crazing is usually understood as inelastic deformation that
exhibits permanent volumetric deformations. Hence a material model that is intended to be applied for polymer
components that show crazing effects during the experimental study, should be capable to produce the correct volumetric
strains during the respective finite element simulation. The paper discusses the real world effect of crazing,
the ideas to capture these effect in a numerical model and exemplifies the theoretical ideas with a real world structural
component finite element model."
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Mechanical
Plastics
Rate Dependency
Automotive
High Speed Testing
LS-DYNA
Research Papers
June 11, 2015 | by DatapointLabs | views 2055
With the growing interest in 3D printing, there is a desire to accurately simulate the behavior of components made by this process. The layer by layer print process appears to create a morphology that is different from that from conventional manufacturing processes. This can have dramatic impact on the material properties, which in turn, can affect how the material is modeled in simulation. In the first stage of our work, we seek to test an additively manufactured material for mechanical properties and validate its use in ANSYS simulation using the Cornell Bike Crank model.
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Mechanical
ANSYS
Research Papers
Validation
3D Printing
June 09, 2015 | by PolyXtrue | views 1378
Bi-layer flow in a profile coextrusion die was
simulated. Prediction of post-die changes in extrudate
profile was included in the simulation. Mesh partitioning
technique was used to allow the coextrusion simulation
without modifying the finite element mesh in the profile
die. Effect of polymer viscosities on the change in profile
shape after the polymers leave the die is analyzed. It is
found that a difference in the viscosities of the coextruded
polymers can lead to a highly non-uniform velocity
distribution at die exit. Accordingly, post-die changes in
extrudate shape were found to be widely different when
the polymers in the two coextruded layers were changed.
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Rheology
Plastics
Extrusion
PolyXtrue
Research Papers
June 09, 2015 | by PolyXtrue | views 1463
Flow in a flat die with coat hanger type of manifold is
simulated allowing slip on die walls. Flow in the same die
was also simulated by enforcing the no-slip condition on
the walls. With slip on the die walls, the pressure drop,
shear rate, stress, as well as temperature increase in the
die, all were smaller than the corresponding values with
no-slip condition on the walls. For the case with slip on
die walls, since the shear rate is smaller, the elongation
rate in the die is found to be the dominant fraction of the
total strain rate. Due to its high computational efficiency,
the software employed in this work can be effectively
used to design extrusion dies for fluids exhibiting slip on
die walls.
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Rheology
Plastics
Extrusion
PolyXtrue
Research Papers
June 09, 2015 | by PolyXtrue | views 1285
The flow in a coat-hanger die is simulated using the axisymmetric and planar elongational viscosities of a low-density polyethylene (LDPE) resin. Elongational viscosity is found to affect the velocity distribution at the die exit. Also, the predicted pressure drop in the die changed significantly when the effect of elongational viscosity was included in the simulation. However, elongational viscosity had only a minor effect on the temperature distribution in the die. Predicted pressure drop is compared with the corresponding experimental data.
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Rheology
Plastics
Extrusion
PolyXtrue
Research Papers
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