Question #652185 on Yade changed:
https://answers.launchpad.net/yade/+question/652185

Description changed to:
when i give a [0,0,0] strain to the Yade, the Yade gives back a stress that is 
not zero, and then give a zero strain to Yade, and Yade will give back a 
different stress. 
the question is when i set different young of the balls, i think the stress is 
no doubt different, however, when i give a strain 
like[1e-7,0,0],[0,1e-7,0],[0,0,1e-7] at the beginning of steps, the stress Yade 
get is different.
part of the code is as shown below:

.................................................................................................

O.load('/tmp/cccc.gz') 
O.dt=1e-8 

# see the example of peri3dcontroller, 1.5 is used to speed up the computation
EnlargeFactor=1.5 
EnlargeFactor=1.0
O.engines=[
        ForceResetter(),
        
InsertionSortCollider([Bo1_Sphere_Aabb(aabbEnlargeFactor=EnlargeFactor,label='bo1s')]),
        InteractionLoop(
                
[Ig2_Sphere_Sphere_ScGeom(interactionDetectionFactor=EnlargeFactor,label='ig2ss')],
                [Ip2_CpmMat_CpmMat_CpmPhys()],[Law2_ScGeom_CpmPhys_Cpm()]),
        NewtonIntegrator(),
        
O.cell.velGrad=utils.Matrix3(0,0,0,0,0,0,0,0,0)
O.step()
bo1s.aabbEnlargeFactor=ig2ss.interactionDetectionFactor=1.    


......................................................................................................................................................................


                for b in O.bodies:
                        b.material.young = 1.0001*17e10     #change it to get 
different stress
                        
                        ag = [0]*3      
                        ag[0] = arr[0] - arrcc[0] #arr means current step stain 
and arrcc means last step strain
                        ag[1] = arr[1] - arrcc[1]
                        ag[2] = arr[2] - arrcc[2]

                        ns=100
                        
                        dstrain = utils.Matrix3(ag[0],ag[2],0,0,ag[1],0,0,0,0)  
    # the goal strain xx xy xz, yx yy yz, zx zy zz
                        O.cell.velGrad=dstrain/(ns*O.dt)

                        O.run(ns,True)                                          
     
        
                        stressValue=utils.getStress()
                        stressxx=stressValue[0,0]                               
    # the return stress xx
                        stressyy=stressValue[1,1]                               
    # the return stress yy
                        stressxy=(stressValue[0,1]+stressValue[1,0])/2          
    # the return stress xy = (xy + yx)/2

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