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Armstrong told www.sufc.co.uk: "I'm already feeling a bit nervous but very excited about joining such a big club that did so well last season.
"I saw United quite a bit on television and was impressed with both their football and the blend of youth and experience."
Meanwhile it has emerged that David Kelly is expected to be named as new the Blades number 2 with in the next 24 hours, also ex-Blades defender Kevin Murphy has joined Division Three outfit Kidderminster Harriers on trial.