Thomas Leitner

The effect of severe grain refinement on the damage tolerance of a superelastic NiTi shape memory alloy

Nickel-titanium (NiTi) shape memory alloys are widely used for medical components, as they can accommodate large strains in their superelastic state. In order to further improve the mechanical properties of NiTi, grain refinement by severe plastic deformation is applied to generate an ultrafine-grained microstructure with increased strength. In this work comprehensive fracture and fatigue crack growth experiments were performed on ultrafine-grained NiTi to assess its damage tolerance, which is essential for the safe use of this material in medical applications. It was found, that equal channel angular pressing of NiTi for 8 passes route BC increases the transformation stress by a factor of 1.5 and the yield stress of the martensite by a factor of 2.6, without significantly deteriorating its fracture and fatigue crack growth behavior. The fatigue crack growth behavior at high mean stresses is even improved, with lower fatigue crack growth rates and higher threshold stress intensity factor ranges, however, beneficial contributions from crack closure are slightly reduced.

Fatigue Crack-Growth Properties of SPD-Metals

Methods of severe plastic deformation (SPD) can be principally applied to all kinds of metallic materials and are widely used to generate nanocrystalline materials that frequently exhibit improved mechanical and functional properties. The resistance against crack growth is, however, only rarely studied despite its importance for the damage tolerant design of components. In this contribution general tendencies found for the fatigue crack behavior of a large variety of SPD-processed metals and alloys, including pure metals (iron and nickel) but also alloys (NiTi, steels) is given. In general grain refinement leads to a deteriorated fatigue crack growth behavior, which can be mainly attributed to a reduction of crack closure contributions. Another significant factor seems to originate from the frequently observed transition from transgranular to intergranular fatigue fracture possessing a lower crack growth resistance than transgranular crack growth. Based on these general observations strategies to counteract these tendencies are discussed where especially the grain-aspect ratio of the microstructures may play an important role.

Load history effects on fatigue crack propagation: Its effect on the R-curve for threshold

The study deals with the effect of load history during threshold and fatigue crack propagation measurements in the near threshold regime. The compression pre cracking and the stepwise increasing load amplitude test was applied. The effects of pre-cracking as well as the effect of compression and tension overloads after the pre-cracking on the long crack threshold and on the R-curve of the threshold of stress intensity range are analysed.