Even though United States currency is strong and durable, it does wear out with constant handling.
All currency in circulation is routinely deposited to Federal Reserve Banks by commercial banks. Worn notes are systematically destroyed by Federal Reserve Banks during ordinary currency processing. The destroyed notes are replaced by new currency provided by the Bureau of Engraving and Printing. The note most frequently replaced is the $1 denomination. There are over four billion $1 bills in circulation, and the life expectancy of each is approximately 18 months. Since larger denominations are handled less, they last longer.
When a note is partially destroyed, the Treasury Department will replace it if clearly more than half of the original remains. Fragments of mutilated currency which are not clearly more than one half of the original whole note may be exchanged only if the Director of the Bureau of Engraving and Printing is satisfied by the evidence presented that the missing portions have been totally destroyed.(from the secret service site).