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Gollerthan, S.; Young, M. L.; Baruj, A.; Frenzel, J.; Schmahl, Wolfgang W.; Eggeler, Gunther (2009): Fracture mechanics and microstructure in NiTi shape memory alloys. In: Acta Materialia, Vol. 57, Nr. 4: S. 1015-1025
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Abstract

Crack extension tinder static loading in pseudoplastic and pseudoelasticbinary NiTi shape memory alloy (SMA) compact tension (CT) specimens wasexamined. Two material compositions of 50.3 at.\%, Ni(martensitic/pseudoplastic) and 50.7 at.\%, Ni(austenitic/pseudoelastic) were investigated. The SMAs werecharacterized using differential scanning calorimetry to identify thephase transformation temperatures and tensile testing to characterizethe stress-strain behavior. A miniature CT specimen was developed, whichyields reliable critical fracture mechanics parameters. At 295 K, crackspropagate at similar stress intensities of 30 +/- 5 MPa root m intomartensite and pseudoelastic austenite. Integrating the miniature CTspecimen into a small test device which call be fitted into a scanningelectron microscope shows that this is due to cracks propagating intoregions of detwinned martensite in both materials. Investigating apseudoelastic miniature CT specimen in a synchrotron beam proves thatmartensite forms in front of the crack in the center of the CT specimen.i.e. under plane strain conditions.