New Corvette marks GM’s first use of heat-activated shape memory alloy to replace heavier motorized part
|Corvette’s new shape memory alloy wire replaces a heavier motorized part. Click to enlarge.|
As one of a number of advances to reduce its weight (90 lbs/41 kg lighter than its predecessor), the redesigned seventh-generation Chevrolet Corvette is the first vehicle to use a GM-developed lightweight shape memory alloy wire in place of a heavier motorized actuator to open and close the hatch vent that releases air from the trunk. This allows the trunk lid to close more easily than on the previous models where trapped air could make the lid harder to close.
With about 200 motorized movable parts on the typical vehicle that could be replaced with lightweight smart materials, GM says it is looking at significant mass reduction going forward.
Shape memory alloys—typically made of copper-aluminum-nickel or nickel-titanium—are smart materials that can change their shape, strength, and/or stiffness when activated by heat, stress, a magnetic field or electrical voltage. Shape memory alloys “remember” their original shape and return to it when de-activated.
In the new Corvette, a shape memory alloy wire opens the hatch vent whenever the deck lid is opened, using heat from an electrical current in a similar manner to the trunk lights.
When activated, the wire contracts and moves a lever arm to open the vent, allowing the trunk lid to close. Once the trunk lid is closed, the electrical current switches off, allowing the wire to cool and return to its normal shape, which closes the vent to maintain cabin temperature.
Smart materials like shape memory alloys offer new possibilities for many movable vehicle features. These new materials enable innovative designs and new and improved features at a lower cost than traditional motors and actuators.—Jon Lauckner, GM’s chief technology officer
Shape memory alloy also helps remove unwanted mass, which can help improve vehicle performance and fuel economy. The wire actuator used on the new Corvette is approximately 1.1 pound (0.5 kilogram) lighter than a conventional motorized system.
The shape memory alloy used on the new Corvette represents nearly five years of research and development work on smart materials for which GM has earned 247 patents. And it is just the beginning. We have many more smart material applications in the pipeline that will bring even more improvements to our vehicles going forward.—Paul Alexander, GM smart materials and structures researcher
As a recent example of smart material patent work, in January 2013, the US Patent and Trademark Office (USPTO) published a patent application (2013/0011806 A1) filed by GM and Dynalloy, Inc (a maker of shape memory alloy actuator technology) on an apparatus and method of controlling phase transformation temperature of a shape memory alloy. The device includes a primary wire composed of the shape memory alloy.
An activation source is thermally coupled to the wire and is operable to selectively cause the primary wire to reversibly transform from a Martensitic phase to an Austenitic phase during a cycle. The Martensite phase is a relatively soft and easily deformable phase of the shape memory alloys, which generally exists at lower temperatures. The Austenite phase, the stronger phase of shape memory alloys, occurs at higher temperatures. The temperature at which the shape memory alloy remembers its high temperature form, referred to as the phase transformation temperature, can be adjusted by applying stress and other methods.
The application notes that a loading element is connected to the wire and configured selectively to increase a tensile load on the primary wire when an ambient temperature is at or above a threshold temperature, thereby increasing the phase transformation temperature of the primary wire. In other words, the stress on the primary wire is increased only when the ambient temperature is at or above a threshold temperature.
The primary wire operates under low stress when the ambient temperature is below a threshold temperature such that a longer life cycle can be achieved. This, GM says, enables the use of relatively low cost shape memory alloy wires in certain applications which would otherwise require high cost ultra-high transition temperature shape memory alloy wires.
US Patent Application 2013/0011806 A1: Apparatus And Method Of Controlling Phase Transformation Temperature Of A Shape Memory Alloy