errr, that depends from server to server, Just Ctrl+G see the dismemberment ratio.
in aikido its 325....well now you just need to know how to make calculations on force, momentum etc and you will discover how much you will hit, normally just try to hit hard enough, but not that hard or you might break yourself.
So depending on the ratio, when a certain join acomulates a certain dmg it will frac or dismember depending on the hit it receives.