Shape Memory Effects
Creators: Amy Ai, Florence Grace-Castonguay, Catherine Huang, Stephen Jiang, Ryan Lee, David Li
Supervisor: Peter Yeadon
The studies that are featured below represent a series of explorations into shape memory alloys (SMAs) that change shape in response to changes in temperature. These superelastic alloys are often referred to as nitinol, because they are comprised of nickel and titanium (niti-) and were first discovered at the US Naval Ordnance Laboratory (-nol) by William J. Buehler and Frederick Wang.
Since their discovery in 1959, SMAs have evolved so that they can now respond to a range of activation temperatures. We mostly explored the potential of SMAs that change shape at 40ºC and 70ºC. Heat was introduced to the material via hot air or hot water or candle flame or, for more control, by passing electricity through the SMAs so that heat builds as the material resists the electric current.
Amy Ai
This study explores the potential of using shape memory alloy springs (SMAs) to generate kinetic energy for a jumping motion. The mechanism consists of a nitinol spring, which serves as the actuator, attached to the top of a planar structure that is made of plexiglas. A steel bias spring is included and positioned so that the structure returns to its original shape after activation.
The nitinol spring is 51 mm long, while the bias spring is 63 mm long. This specific arrangement allows the actuator to cycle between hot and cold temperatures, with a shifting balance of forces, enabling continuous jumping motion as the temperature changes.
Florence Grace-Castonguay
This study began with the idea of having two shape memory alloy springs positioned perpendicular to each other, within a flexible ring, in order to make one spring expand when the other spring contracts. I learned that multiple factors affect the ability of the springs to contract, such as the friction between the silicone and the table, the thickness and height of the ring, and the stiffness and the tensile strength of the silicone.
Other spring configurations were explored, whereby the aim was to deform the ring asymmetrically. This led to another experiment involving having one spring contract in the middle of a short ring, making it curl upwards. This is an interesting way to transition from an object that has a lot of friction to something that can rock on the surface, as shown in the video.
Catherine Huang
In the first test, two shape memory alloy springs were connected in sequence, with one end hooking onto the next, and mounted on a vise that controlled the maximum distance the springs could be stretched. When heated, the strength of each spring easily pulled the other without friction.
In the second test, each spring was divided into segments, and switches were added to control the flow of electricity along individual segments: one at a time, a few together, or all at once. The current remained steady, causing the spring to heat up and move continuously to the left or right. However, the switches also heated up and required time to cool down slightly before testing another segment.
Stephen Jiang
The Shapeshifting Crawler explores using shape memory alloy springs as actuators in soft, more fragile mechanisms. The crawler alternates between two primary movements for continuous motion: expand and contract. To contract, a current is passed through the nitinol spring via the copper-plated track; the spring contracts as it heats, pulling in the outer wheels and arms through their pivots. To expand, the current is switched off, allowing the spring to cool; the bent acrylic begins to stretch the spring, resulting in expansion of the overall structure. By alternating between contraction and expansion, the crawler achieves linear motion along the track.
Ryan Lee
This experiment finds the strength limits for a 1 mm diameter SMA wire that is 23 cm long. The wire was loaded with a constant gravitational force, and was tested for the range of angles that its motion can span.
The SMA wire was attached to 20 g aluminum wheels and placed atop a copper tape track with a 27 g load pulling the wheels down. This specific model finds that the minimum angle between the tracks that the wire’s motion can fully span is ~104° (~38° of steepness from each side). It also finds that the maximum angle in which the 27 g load is able to fully pull the wheels together, to reset the cycle after the wire has been fully extended, is ~124° (~28° of steepness from each side).
Interestingly, when the tracks are asymmetrical (one side is at more of an angle than the other side), the wire will extend symmetrically to the middle of the angle that is created between the 2 tracks. For an asymmetric track configuration only, a ~135° angle between the tracks is found to be successful.
David Li
This experiment finds that nichrome can be an external heat source for SMA sheets that might require too much power to heat.
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