Multiscale Investigation of Shape Memory Alloy Fatigue

Fatigue;Shape memory alloys;Experimental mechanics;Aerospace Engineering;Engineering (General);Materials Science and Engineering;Mechanical Engineering;Engineering;Mechanical Engineering

LePage, WilliamPelton, Alan R. ;

University of Michigan

关键词: Fatigue;    Shape memory alloys;    Experimental mechanics;    Aerospace Engineering;    Engineering (General);    Materials Science and Engineering;    Mechanical Engineering;    Engineering;    Mechanical Engineering;   

Others  :  https://deepblue.lib.umich.edu/bitstream/handle/2027.42/146027/wlepage_1.pdf?sequence=1&isAllowed=y

瑞士|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Shape memory alloys (SMAs) are functional materials with two remarkable properties: superelasticity and the shape memory effect. The reversible, solid-to-solid (;;martensitic;;) phase transformations that enable these useful material responses also complicate their durability predictions. SMAs are used extensively in biomedical devices such as stents and root canal files, and also have promising applications for important frontiers, such as weight-saving actuators, active structures for aerospace systems, and damping components for civil structures. Before SMAs can be widely adopted for these new applications, however, their response to cracking and failure must be understood. Complexities from martensitic phase transformations in SMAs confound many existing predictions of damage, fatigue, and fracture mechanics (including Schmid;;s Lawand linear/elastoplastic fracture mechanics). Furthermore, this work;;s findings could extend to other emerging materials with phase transformations, such as Heusler alloys for high-efficiency magnetocaloric refrigeration. To address these knowledge gaps, this work provides new observations and insights on the role of phase transformation during cracking and failure of SMAs. Additionally, new experimental frameworks were developed to characterize cracks in structural materials for unprecedented breadth and precision between the millimeter and nanometer length scales. The early part of this work optimized full-field deformation measurements to enable the high-precision experiments of the later part of this work. Also, it advanced techniques in the broader experimental mechanics community. First, a new method was developed to enhance measurements from digital image correlation (DIC), a powerful technique for measuring material deformation. Also, clear guidelines were presented for optimizing an important sample preparation step for optical DIC (;;speckle patterning;; with paint) The latter part of this work established new insights into SMA cracking by examining the effects of grain size, crystallographic texture, and temperature. There are two types of fatigue in SMAs: functional fatigue (degradation of the phase transformation, especially from reduction in the recovered strain during superelastic cycling), and structural fatigue (the typical fatigue response of metals, with crack initiation and growth). Despite the dramatic enhancement of NiTi;;s functional fatigue resistance with grain size reduction to the nanoscale, there was no improvement in structural fatigue resistance with respect to grain size. Rather, the largest grain size studied had the slowest crack growth rate, which was attributed to roughness-induced crack closure (not observed in the other grain sizes). For all grain sizes, the macroscopic fatigue crack growth correlated well with microscopic crack tip observations: the grain sizes with relatively fast macroscopic crack growth rates exhibited large crack displacements at the microscale, and vice versa. Next, the effects of crystallographic texture on NiTi;;s functional and structural fatigue were characterized. Unlike the grain size study that was complicated by roughness-induced crack closure differences, the three texture conditions had similar fracture surface. Furthermore, there were clear connections between functional and structural fatigue. During cyclic tension, there was about three times as much residual strain accumulation in the sheet;;s rolling direction (RD) compared to the transverse direction (TD), and the crack growth rates of the RD condition were consistently faster than the 45 and TD conditions. Finally, fatigue experiments as a function of temperature characterized the role of martensitic phase transformation on cracking. The slowest crack growth rates were measured in the stable martensite and thermal R-phase martensite.