unphysical initial strain in axisymmetric elements

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unphysical initial strain in axisymmetric elements

Abaqus Users mailing list


 
Dear Community,


I observe strain in my model even before the load arrives and kindly ask your advice.


My simulations model the contact of two bars with different cross-sectional areas when exposed to pulse loading.


I have modelled this process in Abaqus/Explicit as a two-dimensional simulation. The investigation was conveniently done as a parametric study, defining the lateral dimension of the second bar as parameter. Both bars are slim (length 2 m, diameter 2 cm) and elastic. The same incident pulse is fed into all simulations. Abaqus 6.13 was run in double precision.


Five different simulations are run, with different elements:


axisymmetric:
CAX4R: unphysical strain in both bars for all bar aspect ratios
CAX3:  unphysical strain in the first bar for all bar aspect ratios, in the second bar for some bar aspect ratios
CAX6M: unphysical strain in the second bar for some bar aspect ratios


plane strain CPE4R: no unphysical strain


plane stress CPS4R: no unphysical strain




The unphysical initial strain of up to 0.00005 is already observed at an element representing a strain gage on the first bar, i.e., the contact of the bars is uncritical in this regard. The sign of this unphysical strain may be positive or negative.




I downsized my model:
both bars, but no load imposed: strain = 3.4*(10^(-5)) in the first bar
only first bar, no load imposed: strain = 2.0*(10^(-6))




The elements do not distort excessively (no friction, Poisson's ratio = zero, second bar is free to move axially). Hence, hourglassing should not be the cause.


Fringe information: the above simulations are unable to reproduce the reflection and transmission coefficients predicted by theory. The only advice was "Elastic waves are difficult to model explicitly". More on
https://www.researchgate.net/post/How_to_obtain_reflection_and_transmission_coefficients_in_long_bars_with_different_cross-sectional_areas_exposed_to_pulse_loading


Kindly advise me on that unphysical strain and, if possible, also on a tweak to reproduce the reflection and transmission coefficients.


Thank you for generously sharing your expertise,


Frank Richter
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