I am trying to run a simulation on this small assembly: a copper stud inserted into a plastic housing (nylon 6/10), with no other restrains. Apply torque on the stud and run the simulation to see how much stress is generated by the stud on the housing.

1. I set a global non-penetration contact

2. these following two surfaces are set as pin connectors

3. 3.5 Nm torque is applied on the stud as:

Now here comes the questions:

when I set the fixtures like the following (the 4 side surfaces and the top surface of the plastic housing):

I got the result as the following (the max stress is about 197MPa):

when I fix only the top surface of the plastic housing, like this:

I got the result as the following (the max stress is 350MPa):

which one makes more sense? is there any other problem with my simulation input?

thank you!