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Summary |
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The dissertation was focused on
modelling mechanical properties of steel at high strain rates. The main
aim was to create a representative material model using a confidental
constitutive equation, which includes the physical terms of the material
model as a specific type of a continuum. Another aim of this work was to
create a methodical complex which integrates effectively material models
for simulation and technological applications with clearly desribed
structure and heating conditions.
This was performed by means of an
applicated research of behaviour and modelling the mechanical properties
of materials at high strain rates, using
the standard tests. Two common types of steel – low carbon steel
TRISTAL and carbon steel ÈSN 41 2050 – wich are especially used in cold
forming technologies, were choosed for the research of the modelling
methodics.
Both types of steel were heat-treated
– spheroidised with strictly determined time and processing conditions. Their
structure was registered before the heat-treatment and also after its
realization. By using the
spectrometer LECO, the specific chemistry of the steels was evaluated.
After taking the useful information about the steels, the basic machanical
tests were performed – in both quasistatic and dynamic conditions. In
quasistatic conditions, the tensile tests were realized, too, and the
basic mechanical parameters - yield stress, yield strength, tensibility
and the hardening exponent - were evaluated. Except all of this, the
compress tests and evaluation of flow stress curves and effort curves were
realized by using the CZR 600 testing machine. The quasistatic compress
tests were carried at three different temperatures – a room temperature,
100°C and 200°C. In dynamic conditions, compress tests using Cam
plastometer and Hopkinson pressure bar test were provided. Especially
experimental results of Taylor Anvil Test with well-founded scientific
analysis of physical parameters in constitutive equations, were used to
create a comprehensive metodology of modelling the mechanical behaviour of
steels at high strain rates. Quantification of coincidence
measure between Taylor Test, simulations and experiments was very
important. As we have found out, something that the constitutive equation
just can not discribe is happening at a specific strain rate and we have
to change its parameters, as far as we want to achieve the coincidence
between experiments and simulations. Only a change of the equation is not
effective, an essencial task
is to change the parameters.
We have discovered that Taylor Test
with its facture comes near practical loading of components in cold
forming processes and therefore it´s ideal for acquirement the material
models of high strain rates deformation.
Main contributions of the dissertation
work are: - a creation of a comprehensive
metodology of modelling the mechanical behaviour of steels at high strain
rates
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