shear value is negative?

Dear lammps users.

Why the value of the stress has negative value?

And why it hasn’t reached a value after the dislocation is gliding?

Here is my shear input

#–setting the output stress (Mpa)

compute Fx A reduce sum fx

variable xlength equal “lx”

variable ylength equal “ly”

variable area equal “v_xlength*v_ylength”

variable Fshear equal “(c_Fx/v_area)*160217.66208”

variable strain equal “c_Adsp/lx”

variable sigma equal “-v_Fshear”

#–bottom (A) rigid ; upper (B) displace

velocity A set 0 0 0

velocity B set 0 0 0

fix 1 B setforce 0 0 0

fix 2 mobile nvt temp 300 300 0.01

fix 3 A move linear 0.012 0 0 units box

velocity mobile ramp vx 0 0.012 z -60 60 sum yes units box

The attachment is the curve.

Best Regards,


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Stress values are similar to pressure values.
Why should they not be negative sometimes?