In this work, beneficial effects of tensile mean strain on fatigue behavior and microstructure of superelastic NiTi (i.e. Nitinol) are studied. Most applications, such as endovascular stents made with NiTi, are subjected to a combination of constant and cyclic loading; thus, understanding the fatigue behavior of NiTi undergoing mean strain loading is necessary. Cyclic strain-controlled fatigue tests are designed to investigate the effects of tensile mean strain on fatigue of superelastic NiTi. Experimental observations show that combinations of large tensile mean strains and small strain amplitudes improve the fatigue life of superelastic NiTi. This behavior arises from reversible, stress-induced phase transformations. The phase transformations cause “stress plateaus” or strain ranges with no change in stress value. Scanning electron microscopy (SEM) of the fracture surfaces of specimens revealed generally short crack growth. Electron backscatter diffraction (EBSD) found the amount of residual martensite to be about ~8%, regardless of loading conditions.
|Advisor:||Stone, Tonya W.|
|Commitee:||Priddy, Matthew W., Shamsaei, Nima|
|School:||Mississippi State University|
|School Location:||United States -- Mississippi|
|Source:||MAI 56/04M(E), Masters Abstracts International|
|Subjects:||Mechanical engineering, Materials science|
|Keywords:||Mean strain, NiTi, Scanning electron microscopy, Superelastic|
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