I have this rope that I build using rigidbody2d and pinjoint2d like this:

The first and second circle are staticbody2d, the last circle is a rigidbody2d.
When I increase the gravity or mass of the third circle object, the object fall down and all the chain links are kind of stretched like this:
What I want is all the chain links stick together like an actual rope even if the object are too heavy.
What did I do wrong?