I tried to get S11 using python, but couldn't. However, I was able to get the displacment using the same method.

Here's the script:

from odbAccess import openOdb
import os
import numpy as np
odbname = "E:/tulun/tulun/chixing10/chixing10.odb"  
my_odb = openOdb(odbname) 
step = my_odb.steps['wg_rotation'] 
center = my_odb.rootAssembly.instances['SPLINE-1'].nodeSets['SET-1'] 
Frame_number = len(step.frames) 
local_dis_values=step.frames[1].fieldOutputs['S'].getSubset(region = center)
list1 = np.array([])
for node_value in local_dis_values.values:
   list1=np.append(list1, node_value.data)
Cpressure = list1
for i in range(2,Frame_number,80): 
   frame = step.frames[i] 
   dis_field = frame.fieldOutputs['S']
   local_dis_values = dis_field.getSubset(region = center) 
   list2 = np.array([])
   for node_value in local_dis_values.values:
       list2=np.append(list2, node_value.data)
   Cpressure = np.vstack([Cpressure, list2])

The printed outcomes are:

But if I change the outcome into U, the method works somehow.

I wonder why