PSI - Issue 66

A.R. Pelton et al. / Procedia Structural Integrity 66 (2024) 265–281 Pelton/ Structural Integrity Procedia 00 (2025) 000–000

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fraction. This paper reviews the understanding of crack initiation and growth through material fatigue strength analysis.

Nomenclature Austentite

Cubic (Pm3m) structure of high-temperature phase in Nitinol

A s A f

Austenite start temperature Austenite finish temperature

da/dN

Crack growth rate; i.e., change in crack length (a) as a function of number of cycles (N)

K

Stress intensity factor

K th Stress intensity threshold Marteniste Montonic (P2 1 /m) structure of low-temperature phase in Nitinol M D

Martensite desist temperature; i.e ., the temperature above which the driving force for phase transformation is equivalent to the driving force for plastic deformation

M f M s

Martensite finish temperature Martensite start temperature

Shape Memory and Superelasticity in Nitinol The mechanical properties of Nitinol depend dramatically on the composition, crystallographic phase, deformation state, and deformation temperature. As such, it is important to understand the effects of metallurgical processing on the resultant phase of the Nitinol. Medical devices are manufactured from slightly Ni-rich Nitinol with a nominal composition of Ni 50.8 Ti 49.2 (ASTM 2018) so as to achieve an optimal transformation temperature from martensite to austenite; commonly medical devices are specified with their Austenite Finish (A f ) temperature (see, for example (ASTM 2016, ASTM 2017)). Figure 1(a) shows the schematic relationship between the constant force strain–temperature curve that illustrates the shape memory effect that designates the major transformation temperatures (M s , M f , A s , and A f ). Figures 1(b,c,d) illustrates the uniaxial tensile stress–strain curves that result at the three specified deformation temperatures TA f , and T>M d (Robertson, Pelton et al. 2012).

Fig. 1(a): schematic displacement–temperature curve under constant tensile force for Nitinol that indicates the transformation temperatures as well as the three general regions for which fatigue data have been generated. The monotonic stress–strain curves at test temperatures: (b) below M f (100% Martensite), with a low stress plateau and a large remnant strain after unloading from 6% strain; (c) above A f (100% Austenite), an increased upper stress plateau (transformation from Austenite to deformed Martensite), an unloading plateau (transformation from Martensite to Austenite) and a return to the original shape; (d) above M d , the temperature above which plasticity is the dominant deformation accommodation mechanism. After (Robertson, Pelton et al. 2012).