I have used optimization tool but after on second iteration it says no improvement in the model and initial values are the best design.
I have got stress of 161 MPa and I want improve the design for stress of 250 MPa.
Can you share the part that you are looking to optimize. If you need some generic help, then I can suggest the following : -
1. Try changing the material to improve stress response (aluminium, graphene, zirconium etc)
2. Change the dimensions of the part you are designing (thicker impact region, wider surface area, I-beams instead of cylindircal or cuboidal )