In a September 10 article published in 'Advanced Materials', the research team, led by Reza Moini, an assistant professor of civil and environmental engineering, and Shashank Gupta, a third-year Ph.D. student, demonstrated that a tube-like architecture in cement paste greatly increases its resistance to crack propagation, enabling the material to deform without sudden failure.
"One of the challenges in engineering brittle construction materials is that they fail in an abrupt, catastrophic fashion," explained Gupta.
In civil infrastructure, brittle construction materials rely on strength to bear loads and toughness to resist damage and cracking. The Princeton team's technique addresses these issues by creating a tougher material that does not sacrifice strength.
According to Moini, the breakthrough lies in a careful design of the material's internal structure, balancing stress at the crack front with the overall mechanical response.
"We use theoretical principles of fracture mechanics and statistical mechanics to improve materials' fundamental properties 'by design'," he said.
The researchers took inspiration from human cortical bone-the dense outer shell of the femur. Cortical bone features elliptical tubular components, called osteons, that are embedded in a weaker organic matrix. This structure helps deflect cracks, preventing abrupt failure and increasing overall toughness, Gupta noted.
In their design, the team embedded cylindrical and elliptical tubes within the cement paste to interact with cracks as they propagate.
"One expects the material to become less resistant to cracking when hollow tubes are incorporated," Moini said. "We learned that by taking advantage of the tube geometry, size, shape, and orientation, we can promote crack-tube interaction to enhance one property without sacrificing another."
The researchers found that this interaction initiates a unique stepwise toughening mechanism. The crack becomes trapped by the tube, delaying its progression and causing energy to dissipate at each stage of the process.
"What makes this stepwise mechanism unique is that each crack extension is controlled, preventing sudden, catastrophic failure," Gupta said. "Instead of breaking all at once, the material withstands progressive damage, making it much tougher."
Unlike traditional methods that reinforce cement with fibers or plastics, the Princeton approach uses geometric design. By manipulating the material's structure, the team achieved significant improvements in toughness without additional materials.
To further advance their work, the team introduced a new method to quantify the degree of disorder within the material's architecture. Based on statistical mechanics, they developed parameters that measure the degree of disorder, creating a numerical framework that reflects the material's structural arrangement.
"This approach gives us a powerful tool to describe and design materials with a tailored degree of disorder," Moini said. "Using advanced fabrication methods such as additive manufacturing can further promote the design of more disordered and mechanically favorable structures and allow for scaling up of these tubular designs for civil infrastructure components with concrete."
The team has also developed precision techniques using robotics and additive manufacturing, which allow them to apply new designs and materials in various applications. They plan to explore how these principles could be applied to other brittle materials to improve damage resistance.
"We've only begun to explore the possibilities," Gupta said. "There are many variables to investigate, such as applying the degree of disorder to the size, shape, and orientation of the tubes in the material."
Research Report:Tough Cortical Bone-Inspired Tubular Architected Cement-Based Material with Disorder
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