No, they haven't. I need to explain that answer, though. If what you mean really is design, yes, that probably can be done. After all, the designs use circles, so a string and a lawnmower or a board or something like that might replicate the design. I could probably do it myself, given a little time, using a pen and paper.
But to replicate the 'crop circles' is something else. To make one of the very intricate designs, without leaving a trace to show how you got there and did your work minus any footprints or bending any of the crops that aren't part of the design, do it in a short time, and not be seen even when a camera is recording the scene...now, that's a different matter.