Smart materials and integral sensors enabling intelligent transportation

Gary Kardys | March 21, 2019

Figure 1. Types of smart materials, smart devices, smart structures and applications. Source: Hong Kong Polytechnic UniversitySmart materials and integral sensors have the capacity to improve transportation infrastructure, make roadways safer and bring self-driving vehicles closer to reality.

What are smart materials?

In their book, "Smart Materials and New Technologies for Architecture and Design Professions," Harvard architecture professors D. Michelle Addington and Daniel L. Schodek define smart materials as “a molecule, a material, a composite, an assembly, or a system" responding to one or more environmental states or conditions (e.g., temperature, pressure, strain, light). Intelligent materials are also known as responsive materials. The intelligence is integral to the material and allows the smart material to self-actuate. For example, shape memory alloys alter shape due to a change in temperature, without force from an external actuator or motor. Shape memory materials can be used to open and close vents automatically to adjust airflow and temperature in smart vehicles or transportation structures.

Magnetically responsive smart material or magnetically controlled active materials are being used and researched for smart transportation applications. Magnetostrictive alloys are analogous to piezoelectric materials, which undergo shape changes in response to electrical charge or produce a charge when strained. Magnetorestrictive materials exhibit the Joule effect, which is a shape change under an applied magnetic field. Inversely, they exhibit a change in their magnetization when strained. Giant magnetostrictive materials (GMMs) are based on rare-earth iron alloys such as Terfenol-D, (terbium-iron-dysprosium alloy) and gafenol (gallium-iron alloy). GMMs are used to make solid-state magnetostrictive actuators for active vibration control (AVC) in transportation structures and vehicles. Cedrat Technologies designs and manufactures magnetostrictive and magnetorheological fluid actuators.

Figure 2. Ferrofluid or magnetorheological fluid in magnetic field. Source: PhysicOpenLab.OrgAnother magnetic smart material is magnetorheological fluid (MRF), such as ferrofluids, which stiffen or increase in viscosity when exposed to a magnetic field. Engineers use this effect to design various magneto-rheological fluid actuators for use in valves, brakes, clutches, seals, semi-active dampers and smart hydrodynamic bearings. Lord Corporation, Ferrotec and Macron sell magnetorheological fluid or ferrofluids. MRF has been used in vehicles to provide improved response and dampening where the viscosity and dampening ability is adjusted for road conditions.

Electrorheological (ER) fluids are analogous smart fluids. These smart colloids change viscosity or solidify in response to an applied electrical field. ER fluids can be used in hydraulic valves, clutches, dampeners and shock absorbers. Both magnetorheological and electrorheological fluids become a low-viscosity liquid again when fields are removed. Smart Technology makes smart ER fluid.

Energy harvesting materials are another type of smart material. Photovoltaic materials on the roof and sides of a vehicle can convert sunlight into energy to charge EV batteries while the owner is at work. Many streetlight systems are already smartened through the use of solar power supplies. Thermoelectric smart materials could be used to convert heat energy from an engine, motor or brakes into electrical energy. Magnetoresistive and piezoelectric materials can also be used for vibrational energy harvesting on automobiles, rolling stock and infrastructure.

Monitoring structural health with smart materials

Figure 3. Strain-temperature FBG sensor measures one million points over a 10 km length, which is useful for monitoring the integrity of large structures. Source: LaserFocusWorldSmart materials and in-situ sensors can provide improved monitoring of concrete structures, roadways, and bridges for internal flaws, cracks, corrosion and movement. They could help sense a failure precondition, so a bridge can be fixed before it collapses into another roadway or river. A 2017 article in LaserFocusWorld on fiber based sensors, described a fiber-based sensor with 1 cm resolution, which is quickly able to detect structural problems in bridges and dams. The distributed fiber optic sensor had one million sensing points, for significantly faster detection of structural problems.

In 2017, the article, "Crack monitoring using multiple smart materials" was published in the International Journal of Smart and Nano Materials. The article authors noted that fiber-optic sensors and piezoelectric wafer active sensors (PWAS) are quite common smart materials for structural health monitoring (SHM). Fiber Bragg grating (FBG) and fiber-optic polarimetric (FOP) sensors have been widely researched for structure monitoring over the last twenty years. FBG, FOP and PWAS all respond to strain induced from an applied stress. HBM, Tibercon and FBGS Technologies supply FBG sensors. National Instruments’ "Fundamentals of Fiber Bragg Grating (FBG) Optical Sensing" white paper provides more background on the technology.

Optical fiber-based sensors can also be embedded into a material such as concrete or composite to make a smart material or structure that monitors structural health. The optical fibers laced through bridges, roads or other structural elements would act as a network of nerves. Changes in light signals traveling within the fiber can provide precise details on temperature, strain and other factors. The bridge or structure could automatically send a message to a highway engineer that the road needs repair or should be shut down.

Responsive vehicles with smart materials

Figure 4. The fiber Bragg grating (FBG) sensor reflects a wavelength of light that shifts in response to variations in temperature and strain. Source: National Instruments

Temperature, pressure, strain gauges or force sensors could be integrated into carbon fiber composites in a vehicle to sense stress and environmental changes. Sensors could send a signal to vehicle controls to change operating conditions, trigger actuators (fluid or electric) or smart electro-sensitive material to improve safety and drivability. The smart material triggered might be a shape memory alloy or polymer. The shape can alter the stress or loads on the vehicle based on sensory feedback. For instance, external surfaces or spoilers could alter the shape to reduce or increase downward forces.

Smart glass windows can be used on vehicles where the reflectivity of infrared (thermal) light could be changed automatically to maintain internal temperatures. Electrochromic or electronically tintable windows have been developed for vehicle windows where the visibility and privacy can turn on, off or adjust to a specific transparency level. Research Frontiers has developed SPD-Smart Glass, which can change transparency instantly. Their patented SPD-SmartGlass technology effectively blocks UV and infrared rays regardless of whether the glass is in its clear or tinted state, helping keep cars, planes, yachts, homes, offices and artwork cool and protected. NASA is developing spanwise adaptable wings using shape memory alloys to fold wings from 0° to 70° during flight.

Smart coatings could provide improved reflectivity in roadway coatings, automotive paints, signs and devices to enhance visibility for both human drivers and systems such as lidar and radar in automated vehicles. These smart coatings on ships, aircraft and trains could also adjust drag on different surfaces to alter aerodynamics and lift.

In some applications, the smart material element can be multifunctional. For instance, a composite body panel could be made of smart shape memory wires, linear polyethylene fibers and embedded FBG sensors in an elastomeric matrix. The shape memory alloy wire should provide reinforcement combined with a self-healing property. If the body panel is dented, the embedded FBG sensors would detect strain and activate an electrical power supply. The joule heating applied to the shape memory alloy reinforcement should pop out the dent. Self-healing coatings and materials are also being developed where a resin is exuded by an integral actuator when a material is scratched, and then cures in place once exposed to the air.

Robotic materials: The ultimate smart materials

Figure 5. In-situ sensors, microprocessors and actuators of a robotic material work to change the material properties of the base material. Source: Science Magazine

Robotic, or artificial, materials are a new smart material under development that would combine integral sensing, monitoring and analysis, and then actuation and data transmission.

The 2015 article, "Materials that couple sensing, actuation, computation, and communication," noted that state-of-the-art composites are increasingly integrating sensors and actuators, which permits materials to function autonomously. These materials could create airplane wings and vehicles that adapt their aerodynamic profile or camouflage, infrastructure that detects and repairs damages, or robotic skin and prosthetics that sense touch and subtle textures.

While still in the research phase, these future robotic materials should be more than responsive. Using predictive algorithms and AI, smart robotic materials should be able to anticipate and alter properties before an incident occurs.

Conclusion

Smart materials have an enormous potential as a key technology to transform future transportation systems. Some smart materials like shape memory alloys are already available commercially and can be adapted for smart transportation applications. In other cases, new smart materials such as robotic materials are likely to be developed specifically for intelligent transportation projects.