Engineers from Northwestern University, Vascotube GmbH, and Exponent Inc., discussed their work at SMST 2019 in which they compared their experimental results of Nitinol fatigue life studies with published data.
They compared probabilistic fatigue lives of diamond samples lasercut from a standard grade of Nitinol tubing manufactured using TM-1 tube processing techniques, to published data in which similar diamonds were lasercut from the same standard grade of Nitinol, plus two other standard grades of Nitinol, and two high-purity grades of Nitinol expressly designed to improve fatigue life. All tubes in the published work were fabricated using TM-2 tube manufacturing techniques. Initial results indicated an effect of the tube processing method on fatigue life, especially comparing behaviors of the standard grades of Nitinol.
To compare effects of tube manufacturing methods TM-1 and TM-2 on fatigue lives of samples made from the cleaner grades of Nitinol, 25mm diameter hollows made from the cleaner materials were drawn to final size using TM-1 methods. Diamonds were fabricated, then fatigued to 107 cycles.
Initial correlation of probabilistic fatigue lives with final microstructures for the standard grades of material showed an inverse power-law relationship with a negative exponent near unity between median fatigue life and transverse median NMI particle diameter. This result was shown to be qualitatively consistent with the nature of the relationship between fatigue life and transverse particle diameter found for steels.
Such relationships for the two cleaner materials were presented and discussed in terms of models describing the effects of NMI particles in Nitinol acting as primary nucleants.
Title: Effects of Tube Processing on the Fatigue Life of Nitinol – Part 2
Mr. Paul Adler, Mr. Chuan Liu, Northwestern University, Evanston, Illinois
Mr. Rudolf Frei, Vascotube GmbH, Birkenfeld, Germany
Dr. Matthew L Bowers, Dr. Paul Briant, Dr. Brad James, Exponent Inc., Menlo Park, California
