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[Bruno Chareyre <1300167@xxxxxxxxxxxxxxxxxx>]

I don't know UniaxialStrainer, but I don't think the two scripts should give identical results anyway.
A "strainer" should never make bodies cross each other, this is against continuum mechanics (make a compression between two plates, should the top plate cross the bottom plate? If it occures where is the material??).

The real problem I see is that UniaxialStrainer is modifying positions. It should be stricly forbiden in Yade: contact laws are based on velocities, thus playing with positions without a relevant velocity is a way to break most algorithms.
It seems UniaxialStrainer should be simply removed (or someone will fix?).

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https://bugs.launchpad.net/bugs/1300167

Title:
  (Not) Defining velocities in UniaxialStrainer ?

Status in Yet Another Dynamic Engine:
  New

Bug description:
  Hi,

  I have the feeling that there is something "wrong" with
  UniaxialStrainer where the velocities of the "posIds" and "negIds"
  bodies are not defined, and I illustrate it with following examples
  (attached)

  It deals with 2 contacting spheres, with an oblic contact. One sphere is moved towards the 2d sphere, and here both normal and tangential relative displacements occur. 
  - In testUS.py the moving sphere is moved through UniaxialStrainer
  - Whereas in testUSb.py the movement comes from an initial value of velocity (and NewtonIntegrator)

  And, obviously, the two simulations are different (see e.g. the plot
  of fZ sustained by moving sphere). While I would tend to consider
  these two simulations as identical.

  What do you think ?

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