Open WinRAR choose the "Extract to" button on the toolbar in the window that opens "Keep broken files" is on the bottom right under Miscellaneous. You could also try (from the drop down menu) "Repair archive (Alt+R)" but do this on a copy of the file. If will show error "the archive is either in unknown format or damaged" you're getting means that the file is corrupted, so it just cannot be repaired. But maybe the file is not completely corrupted and some part of the information can be saved. I think there will a RAR recovery items. If you want to try this tool. http://www.oemailrecovery.com/rar_recovery.html
Refer to the following thread for some possible solutions https://community.office365.com/en-us/f/172/t/269398