Experimental investigation of crack initiation in thin sheets of nitinol

An experimental investigation into the fracture properties of 160-mu m-thick edge-cracked specimens of austenitic nickel-titanium (nitinol) under uniaxial tension is presented. Using the in situ optical technique of digital image correlation (DIC), strain fields directly relating to phase boundary nucleation and propagation of fracture samples were observed for the first time. The shape and size of the saturation and transformation zones as a function of loading near the crack tip were examined. An average plane strain crack initiation fracture toughness (KC) of 51.4 +/- 3.6 MPa root m for fine grained polycrystalline nitinol sheets at room temperature was measured. The extent and nature of the phase transformation obtained from DIC, combined with the relatively high value of KC, underscores the importance of crack tip shielding in the fracture of shape memory alloys. (c) 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.