Civil Engineering Reference
In-Depth Information
constants are evaluated based on experimental results like tensile or impact tests.
Sometimes, the material behavior during the experimental testing for material
parameter identification purpose is different from its behavior in a real test situ-
ation. The reasons for that could be the difference in strain rates or thermal con-
ditions, in another term the stochastic nature of loads and material parameters. So,
a parameter or two that are used to define the finite element model of the process
are used to tune the numerical finite element results to better match experimental
measurements. Once more, the reader asks himself, what is the purpose of doing
finite element analysis then? To understand the purpose, assume an engineer is
designing a mechanical part-A under specified loading condition. He then needs
preliminary calculations for the part-A initial dimensions. This step can be per-
formed based on experience or using finite element analysis. Then depending on
loading conditions (static, dynamic, thermal), how many experimental tests he
needs to perform to reach the final optimized design? while each iteration requires
manufacturing a new part or assembly. So, it makes sense to perform one or two
experimental testing, while tuning the finite element model parameters to match
the experimental results. Once this finite element is developed, then the engineer
will have the confidence to change any desired part dimensions or loading con-
ditions using the FE model instead of the experimental testing. Once reached an
acceptable solution to the problem in hand using the finite element model, the
engineer then can perform another experimental testing with the new manufac-
tured part to verify the results. This means cutting cost and increasing reliability.
Kay [
32
] published a report of how to estimate the Johnson-Cook parameters
[D
1
-D
5
] using two different experimental tests. The first set of brackets in the
Johnson-Cook fracture model represents the observation that the strain to fracture
decreases as the hydrostatic tension increases. The second set of brackets repre-
sents the effect of an increased strain rate on the material ductility, while the third
set of brackets represents the effect of thermal softening on the material ductility.
Kay [
20
] used different approach to model damage evolution from the one used by
ABAQUS. He did not use the equivalent plastic displacement to model damage
evolution; instead he used Eq. (
7.6
) for damage evolution, while ABAQUS use the
same equation for damage initiation, and then use the equivalent plastic dis-
placement to model damage evolution. The value for equivalent plastic dis-
placement at complete failure (D = 1) can be taken from ABAQUS example
manual (Progressive failure of thin-wall aluminum extrusion). Clausen et al. [
33
]
discussed in details using Split-Hopkinson to estimate Johnson-Cook plasticity
and failure equations parameters of aluminum alloy AA5083. I assumed in the
simulations that the failure equivalent plastic displacement is zero, which agrees
with Kay [
32
].
7.4.1.3 Step Module
The tool is constraint in five degrees of freedom and only the rotation degree of
freedom around the axis of revolution is kept free. Then applying velocity
Search WWH ::
Custom Search