thank you for your reply jie liu. My volume constraint is set to 80% of the original volume of the original part, which should result in a reduction of weight? 

I have tried decreasing element size, but did not work. Trying to re-produce the model now with 90 edges rather than radiuses where possible for modelling purposes. Hopefully it works. 

If you don't mind can you describe the part or assembly and the type of loading being applied? Seems like you may be getting very large deformations. Have you tried using nlgeom? Also, what type of elements are you using?

the part is a suspension bell crank. Its held as 'encastre' in the centre circular section. On each opposing ends there are X and Y components applied which result in a magnitude of 5200N force, therefore a 5200 force is being applied at both ends. This force value is applied because thats the force it takes to completely compress the spring. At one end the bell-crank is fixed to a pushrod, at the other a spring damper unit. The loads were applied by using masters and slaves, with a coupling type, as a concentrated load at that point. 

The elements im using are C2D10: 10 node tetrahedron

Hey Luka,

Your optimality conditions are correct. As for elements, I am guessing you are using C3D10 and not C2D10. 

Do you get the error message in the first increment? I am guessing no, as it's able to run correctly with a lower static load. If however, you do get the error in the first increment, the problem maybe due to incorrect input data.

If the message you get is after some plastic deformation has developed, consider the following.

Try the above and rerun the optimization task. Let me know if it works.

Good Luck!