I'm confused becuase your post title says "open d tuning" but in your post you refer to "open G tuning".

When you retune your A string, are you tuning it up to a G or down to a G. You would have to REALLY tension the A string to break it so I'm guessing you are tuning up.

I think what you want to do is detune the A string a step to acheive G.