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Strain controlled fatigue tests of a pseudoelastic nickel–titanium (NiTi) shape memory alloy (SMA) have been carried out in this investigation. In particular, flat dog-bone shaped specimens, obtained from commercial NiTi sheets, have been analyzed, under pull–pull loading conditions, in two subsequent steps: (i) material stabilization and (ii) fatigue life estimation. The first step was carried out to obtain a stable pseudoelastic response of the SMA, i.e. with no residual deformations upon unloading, and it can be regarded as a preliminary processing condition of the alloy. Results on functional fatigue, i.e. in terms of stabilized pseudoelastic response, and on structural fatigue, in terms of cycles to failure, are reported and discussed. Furthermore, a modified Coffin–Manson approach for fatigue life estimation of SMAs is proposed, which takes into account the strain mechanisms involved during repeated stress-induced martensitic transformations.

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Cohesive zone models are explored in order to study cleavage fracture in adhesive bonded joints. A mode I cohesive model is defined which correlates the tensile traction and the displacement jump (crack faces opening) along the fracture process zone. In order to determine the traction-separation relation, the main fracture parameters, namely the cohesive strength and the fracture energy, as well as its shape, must be specified. However, owing to the difficulties associated to the direct measurement of the fracture parameters, very often they are obtained by comparing a measured fracture property with numerical predictions based on an idealized traction separation relation. Likewise in this paper the cohesive strength of an adhesive layer sandwiched between elastic substrates is adjusted to achieve a match between simulations and experiments. To this aim, the fracture energy and the load-displacement curve are adopted as input in the simulations. In order to assess whether or not the shape of the cohesive model may have an influence on the results, three representative cohesive zone models have been investigated, i.e. exponential, bilinear and trapezoidal. A good agreement between experiments and simulations has been generally observed. However, a slight difference in predicting the loads for damage onset has been found using the different cohesive models.

a b s t r a c tIn this paper new fracture control parameters for Nickel-Titanium (NiTi) based shape memory alloys (SMAs) are proposed, based on a recent literature analytical model on fracture mechanics of SMAs. In fact, the stress induced martensitic transformation, occurring in the crack tip region of NiTi alloys, causes a complex and unusual stress distribution with respect to common engineering materials. For this reason two different stress intensity factors (SIFs) have been defined to describe the stress distribution in both transformed and untransformed regions, i.e. in the martensitic and austenitic phases, respectively. Systematic studies have been carried out to analyze the effects of the main thermo-mechanical parameters of NiTi alloys on the two proposed SIFs, i.e. on the crack tip stress distribution, and comparisons with linear elastic fracture mechanics have been illustrated. Finally, the proposed model was used to analyze different loading conditions of a commercial superelastic NiTi alloys, which demonstrated a marked effect of the temperature on the crack tip stress distribution.

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