Make simple model and verify your relations actually work on that, before starting to do a full blown series of advanced stuff..
You can have multiple design limits:
- stress <350 MPa
- relative displacement <0.005
Please note, that relation length=d8, must be based on your model (d8 was OK for mine). It seems that your feasibility study is not finding a solution...
More knowledge can be found at: Introduction to Creo Simulate - PDSVision