Just another way of putting it to see the issue:
Case1: E1=E2
If E1=E2=9.0E9, T3 at Node 22 is 7.03E-4 (classical is 7.11E-4)...so this is OK.
Case 2: Change E1 by half and hold E2
If E1=4.5E9 and E2=9.0E9, T3 at Node 22 is 9.08E-4
Case 3: Change E2 by half and hold E1
If E1=9.0E9 and E2=4.5E9, T3 at Node 22 is 9.37E-4
For Cases 2 and 3, I would have expected one of those to have a deflection of 2*7.03E-4 and the other to have a deflection fo 1*7.03E4. But the results are odd instead.
https://www.mystran.com/other/MAT8_issue.dat
Case1: E1=E2
If E1=E2=9.0E9, T3 at Node 22 is 7.03E-4 (classical is 7.11E-4)...so this is OK.
Case 2: Change E1 by half and hold E2
If E1=4.5E9 and E2=9.0E9, T3 at Node 22 is 9.08E-4
Case 3: Change E2 by half and hold E1
If E1=9.0E9 and E2=4.5E9, T3 at Node 22 is 9.37E-4
For Cases 2 and 3, I would have expected one of those to have a deflection of 2*7.03E-4 and the other to have a deflection fo 1*7.03E4. But the results are odd instead.
https://www.mystran.com/other/MAT8_issue.dat