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Is it impossible to set perfectly no relative motion using /INTER/TYPE2?

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In figure, two 1T shells are fixed on one edge. (Gap between 2 shells is 1.1)


Interface between top and bottom shells is modeled by using /INTER/TYPE2.

To set no relative motion, setting in type2 is default.



Concentrated load is excited at node number 7 on top shell. Load curve and scale are as follows. (The load has effect like Impact.)



I think that relative displacement between node 7 and 128 is perfectly zero because I use type2 default.

However, in x,y,z direction, relative displacement occurs.





It looks like almost zero but I want perfectly no relative motion. 

How can I set that?


Another question is 


Physically the plate is excited by impact, acceleration of plate is decayed


I don't use damping, so I think acceleration of some node is decayed to steady state response.


But acceleration of node 7 is like this. Response is increased. Why???







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FEM is a numerical technique used to determine the approximated (not exact) solution. Absolutely no relative motion is not possible due to the numerical error (truncation or round-off errors). A slight error due to round-off in one element is likely to result in appreciable error due to the cumulative effect of these errors throughout the domain. This error (0,01%) is insignificant when compared to other sources of error like discretization and formulation errors. In engineering practice, we are happy if FEM results match experiments within 10% due to material and geometry variability and experimental measuring inaccuracies.

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