How to achieve simple shear with contant mean pressure with periodic boundary?

Hello every one,

I am trying to achieve simple shear loading with the periodic boundary using the function PeriTriaxController().

I want to keep the mean pressure constant throughout the test.

During the test, I first set triax.stressMask=3 and triax.goal = (p_value, p_value, p_value). When I reach this goal, I start the simple shear test with the codes below:

def changeGoalStress():
    s = utils.getStress()
    triax.goal = (0.0, 0.0, 3 * p_value - s[0][0] - s[1][1])

triax.stressMask = 4
triax.maxStrainRate=(0., 0., 10. * rate)

O.engines = O.engines + [PyRunner(command='changeGoalStress()',iterPeriod=5,label='checker')]
O.cell.velGrad=Matrix3(0,0,.01, 0,0,0, 0,0,0) # (du/dx, du/dy, du/dz, dv/dx, dv/dy, dv/dz, dw/dx, dw/dy, dw/dz)

With these codes, I want to achieve shearing by setting du/dz = 0.01 and achieve maintaining mean pressure by setting the triax.goal = (0.0, 0.0, 3 * p_value - s[0][0] - s[1][1])

But things don't go as what I thought. The shear deformation occurs but the boundary in the z direction keep still and the pressure in the z direction can't reach the goal I set.

Could anyone tell me how to achieve simple shear while maintaining mean pressure with periodic boundary?

Thanks a lot.

Long