Half Shaft

Folks having trouble with my sisters car. 5 weeks ago the left hand side half shaft cracked at the inside joint with the gearbox. I got a second hand one from a scrap yard and fitted it, but it only lasted until this week when it cracked in the same place again. From talking to other motorheads they say that it is not a common occurence. The wishbone is not bent, has anyone got any ideas. The history of the 94 Opel corsa 1.0 is unknown but he was a bit of a cowboy that we bought the car off. It seems like the shaft is under strain in some sense like the l-h side has been pushed in and causing the shaft to be under pressure. Has anyone had a similar problem with these shafts???