Why are stress different from TPLOT, PLOT and STRESS

[sanshi2000]

I am doing a simple simulation of biaxial stretch based on my user matreial subroutine, as attached, for HGO2000 fiber-enforced material. But the stresses shown by PLOT or recorded by STRESS ALL different with those outputed by TPLOT. Anyone have idea to fix it? Thanks a lot.

[FEAP_Admin]

Try running a patch test where the stress state is homogenous.

Note that in PLOT you are looking at the L2-lumped projection of the stresses, STRESS ALL is reporting stresses at selected points in the element, and TPLOT is reporting stresses based on input parameters which need not match either of the other ways of reporting the stresses.

With a homogenous state of stress all should be the same.

[sanshi2000]

Thank you very much. Yes, they are same for homongeneous stress patch test.

But for the in-homogenous test, given before, I found the conter of stress by PLOT look like strange because all of stress is negative under strech. Addiationally, the output of stress from STESS is also negative.

By the way, could you indicate any references on 'L2-lumped projection'? I don't know what it is.

Thanks a lot. 

[Prof. R.L. Taylor]

The lumped projection is the "row sum" described in the appendix in the first volume of the finite element book by Zienkiewicz and Taylor. 

The outputs from elements in feap are not fully consistent.  Plot and nodal stresses are usually obtained by a row sum algorithm (some variants are necessary for 8-node quads, etc.).  Element stresses are often averaged from the gauss points and printed as a single number.  TPLOT values come from a specific gauss point -- the first one usually, but the first one may depend on how element node numbers are attached.  Renumbering will change the point you get.  The only way to know what comes out is to look at the source code for the element you use.  TPLOT values are put in the array "tt(*)" in increments of the gauss points.  Also what goes in may differ in some elements.  Looking at the elements provided can assist you in adding to any user elements.