NASA's 'spring tire' improved with 'shape memory alloys'

CLEVELAND — When it comes to designing tires for worlds other than earth, forget elongation and rebound and instead think "stoichiometric" and "shape memory alloys."

Scientists at NASA's Glenn Research Center near Cleveland have developed a non-pneumatic, compliant tire made of a stoichiometric nickel-titanium mesh that they envision will be used on a possible mission to Mars.

Stoichiometry is defined a branch of chemistry that deals with the application of the laws of definite proportions and of the conservation of mass and energy to chemical activity.

The innovation, called the Superelastic Tire, is the latest evolution of the "spring tire" that NASA Glenn and Goodyear developed a few years back, inspired by the Apollo program's lunar-rover tires.

The latest version uses "shape memory alloys" capable of undergoing high strain as load-bearing components, instead of typical elastic materials, NASA Glenn said. This results in a tire that can withstand excessive deformation without permanent damage.

Using shape memory alloy as radial stiffening elements can also increase the tire's load-carrying capacity without a weight increase, NASA Glenn said.

The nickel-titanium alloy undergoes an atomic rearrangement to accommodate deformation, according to Santo Padula, a materials scientists at NASA Glenn. This allows a tire made of the alloy to deform up to 30 times more than one made of a more conventional material and recover its original shape without irreversible "plastic deformation."

This latest design not only offers traction equal or superior to previous designs, it also eliminates the need for an inner frame, which both simplifies and lightens the tire/wheel assembly, NASA Glenn said.

The use of shape memory alloys in this application dates back a few years to a chance meeting in the halls of NASA Glenn between Mr. Padula and Engineer Colin Creager, who was working on tire designs for the Mars Rover.

The two had known each other during their university studies but didn't know they both were working at NASA Glenn. Their chance meeting led Mr. Creager to invite his colleague to the "Slope lab," where the Rover was undergoing testing.

Mr. Padula said he recognized almost instantly that the shape-memory alloys he was working with would be ideal for the mesh tire design he saw at the lab.

The Superelastic Tire offers traction equal or superior to conventional pneumatic tires and eliminates the possibility of puncture failures, thereby improving automobile safety. This tire design also eliminates the need for an inner frame which both simplifies and lightens the tire/wheel assembly.

In addition, the use of shape memory alloys provides enhanced control over the effective stiffness as a function of the deformation, NASA Glenn said, providing increased design versatility.

For instance, the Superelastic Tire can be made to soften with increased deflection, reducing the amount of energy transferred to the vehicle during high deformation events.

In addition, the use of shape memory alloys in the form of radial stiffeners, as opposed to springs, provides even more load-carrying potential and improved design flexibility, allowing this type of compliant tire to operate at increased travel speeds in off-road applications.

NASA Glenn said the design potentially could be considered for use in military or off-road environments where unrestricted mobility is a key operating factor.