ON THE DEVELOPMENT OF UNUSUALLY HIGH FATIGUE CRACK PROPAGATION RESISTANCE IN STEELS: ROLE OF CRACK TIP SHIELDING IN DUPLEX MICROSTRUCTURES

The role of crack tip shielding in retarding fatigue crack growth has been examined in duplex microstructures, with the objective of achieving maximum resistance to fatigue. Specifically, ferritic-martensitic structures have been developed in AISI 1008 steel, where shielding is generated from crack deflection and resultant crack closure. Such techniques yield fatigue thresholds which are 100% higher than in normalized structures, i.e., over 20 MPam, representing the highest thresholds reported for a metallic alloy to date. Effects of the dual-phase microstructures on crack initiation and small crack (10 to 1000 μm) growth are quite different. Structures which show the highest resistance to long crack growth show the lowest resistance to crack initiation and small crack growth, characteristic of behavior influenced by the mutual competition of intrinsic and extrinsic (shielding) “toughening” mechanisms.