I made a WSF part which is five 9mm diameter spheres joined by cylinders of diameter 2mm. While flexing this piece slightly last night, it broke fairly easily at one of the connections. It seems to me there is a difference between a wall or surface of thickness 2mm and two large volumes which are connected by a cylinder 2mm in diameter. The latter is far weaker.
Anyway, I would recommend a minimum 3mm diameter for cylinders connecting two larger volume pieces. I have not had this size connection break on me.
What is the diameter of the wires used to make springs in the 3D parts database? Do these break when flexed?