Do you think Voldemort ever crucio'd Bellatrix?

yes, when they screwed up in Malfoy Manor chapter. But everytime she disobeys or ruins " Her Masters" plan as she sees it. She as wacko as she is, probably thinks she deserves it. So She still loves him just the same. I think she loves Voldemort madami than anyone in her family. Like when she was yelling at Draco and Narcissa. Bella: " Fine Draco if you dont have the guts to finish them off then, I will do it .. take them away" Narcissa:" How dare you talk to my son that way!" Bella:" QUIET! Cissy this madami important than you know!!!" and well...obviously madami than Sirius and Tonks.

Yes, Voldemort shows no mercy. Plus she had to have work her way to the tuktok ranks of the Death Eaters, she couldn't have just been one of his right hand Death Eaters right away and she's bound to have failed a couple times. So she's probably been crucio'd sa pamamagitan ng him before. Was she crucio'd after the prophecy? I'm not sure. Probably because he needed that prophecy really bad. So...

If he shows mercy, voldemort would be no more!

Probably. In the Philosopher's Stone book, when Dumbledore and Harry are discussing Voldemort, Dumbledore says "He shows just as little mercy to his followers as his enemies"
^^ I don't think Voldemort would ever let anything slide when one of his followers doesn't succeed in something he wants doing. Its not something his character would do.

i don,t think so because she has been his most faithful servent ever since hes started

Voldemort wouldn't let anything slide, so of course he used crucio on her before.

Probably. Voldemort shows mercy to no one. He wouldnt hesitate to curse anyone who ever made him mad.

I really think so.

Yes! Thats probably how got most of his Death Eaters...

I just can't help the fact that when I saw this question, I thought 'ooh, kinky ;)'
but seriously, Crucio brings a whole new meaning to sadism...