3 EFFECT OF STRESS RATIO ON FATIGUE-CRACK GROWTH IN 7075-~6 ALUMINUM-ALLOY SHEET b By C. Michael Hudson and Joseph T. Scardina NASA Langley Research Center . Langley Station, Hampton, Va. 'presented at the NatZoWl SympQ&W on Fracture MecQmi@@ GPO PRICE ' $ Hard copy (HC) Microfiche (MF) I 1 ff 653 July 65 I -Bethlehem, P-@*aa June 19-21, 1967 . .