1/16/2026 Taylor Parks
Written by Taylor Parks
Mechanical engineering seniors Margaret Dobson, Ben Guo, Maya Ramavarapu, and Kyle Zhou developed a novel testing machine for investigating strain-induced failure in nitinol, a shape memory alloy commonly used in medical devices such as stents and internal braces.
“The team was highly enthusiastic about the idea from the start and also wanted to learn about the fatigue of materials and the nickel-titanium alloy used in stents,” said Professor Huseyin Sehitoglu, who advised the ME 470 team and whose own research focuses on shape memory materials.
Nitinol, or NiTiNOL, alloys are composed of nearly equal amounts of nickel and titanium and are classified according to their weight percentage of nickel, which ranges from 54 to 57. The name indicates nickel-titanium and the U.S Naval Ordinance Laboratory, which first discovered the alloy.
Understanding nitinol’s fatigue life, or the number of loading cycles it can endure before failure, is critical for its use in medical devices. Indeed, this topic hits home for Dobson, whose father received nitinol stents after experiencing a heart attack.
“[Nitinol performance] is a very relevant topic in today’s medical world,” she said. “I want to know if my dad’s stents will last the next thirty years. I’m glad I got to do some research on it and contribute to that field.”
Literature indicates that at very low strain amplitudes, nitinol alloys will exhibit some inconsistent behaviors that are not well understood. Furthermore, a difference of 0.5% in the nickel-titanium composition can noticeably vary the alloy’s behavior, making it harder to predict failure consistently.
To investigate fatigue life, the team designed and fabricated a bespoke machine to test the nitinol wire at 1% strain amplitude.
A brushless motor and electronic speed controller operate a chuck that holds one end of the wire. The other end is fitted into a bushing to hold it in place against the mandrel. The bushing can spin freely so that no torsion is created in the wire during testing. To reduce vibration, the team designed and printed a flexible shaft coupling that joins the chuck to the motor.
“We have a liquid bath assembly to dissipate heat reliably along the wire,” Zhou said of the tank that encloses the wire and mandrel assembly. “We also wanted to test the wire in a relevant environment, so we used a saline solution heated to body temperature by an immersion circulator.”
During testing, sensors relay data to an Arduino to monitor the wire’s cycles. A reflective fiber fed through the mandrel “watches” the wire to detect fracture, at which point the machine stops and provides the final cycle count. Each count becomes one data point.
“We designed the mandrel to be modular,” Ramavarapu said. “To get a full curve [for strain versus number of cycles], we would need to swap out the mandrel to test different strain amplitudes.”
“With our current configuration, we can safely test rotary bending wire fatigue between 3,000 to 12,000 RPM,” Guo said.
At 1% strain amplitude, the students saw a range of 10,000–30,000 cycles as well as outliers below 10,000 and above 100,000 cycles—variations that are considered normal for fatigue testing.
“The team benefited from discussions with Drs. Michael Mitchell and Alan Pelton, both researchers from the biomedical field,” Sehitoglu said. “[The researchers] were impressed with the team’s findings.”
Following graduation, Guo, Ramavarapu, and Zhou intend to continue into graduate school. Dobson will be going into the Edison Engineering Development Program at GE Aerospace.
“The students were exceptional,” Sehitoglu said. “They made remarkable progress quickly and were very fast. Their skills complemented one another well.”
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Huseyin Sehitoglu holds the John, Alice, and Sarah Nyquist Endowed Chair. He serves as Director of the Fracture Control Program and is Chief Editor of the journal Shape Memory and Superelasticity.