Researchers have leveraged robotics and additive manufacturing to enhance the toughness of cement-based materials by incorporating strategically placed hollow tubes, resulting in a product that is over five times stronger than traditional options.
Drawing inspiration from the sturdy outer layer of human bone, scientists at Princeton have created a cement-based material that is 5.6 times more resistant to damage compared to standard variants. This innovative design allows the material to withstand cracking and prevent abrupt failures, setting it apart from the typical brittle cement materials.
In a September 10 article published in the journal Advanced Materials, a research team, headed by Reza Moini, an assistant professor of civil and environmental engineering, along with Shashank Gupta, a Ph.D. candidate in his third year, showcases how cement paste integrated with a tube-like structure can greatly enhance its resistance to crack development and improve its deformation capabilities without sudden breakdowns.
“One major challenge engineers face with using brittle construction materials is their tendency to fail suddenly and catastrophically,” explained Gupta.
In brittle materials commonly utilized in construction and civil engineering, strength indicates the capacity to bear loads, while toughness pertains to the material’s ability to resist cracks and the spread of damage. The method proposed by the team addresses these issues by crafting a material that is tougher than traditional options, all while preserving its strength.
Moini emphasized that the improvement stems from the intentional design of internal structures, balancing the stresses at the crack’s forefront with the overall mechanical properties of the material.
“We apply the theories of fracture mechanics and statistical mechanics to enhance the fundamental properties of materials ‘by design’,” he remarked.
The inspiration for the team’s design came from human cortical bone, the robust outer layer of femurs, known for its strength and fracture resistance. Cortical bone features elliptical tube-like structures known as osteons, which are loosely embedded in an organic matrix. This distinctive architecture helps redirect cracks away from osteons, thereby preventing sudden failures and boosting the material’s overall crack resistance, as Gupta noted.
The bio-inspired design developed by the team integrates cylindrical and elliptical tubes within the cement paste, allowing them to interact with developing cracks.
“Typically, one might expect that adding hollow tubes would make the material less resistant to cracking,” Moini stated. “However, we discovered that by optimizing the geometry, size, shape, and orientation of the tubes, we could enhance interaction between cracks and tubes, improving one property without diminishing another.”
The research revealed that this enhanced interaction initiates a stepwise toughening process, where a crack encounters a tube, gets trapped, and experiences delayed growth. This leads to increased energy dissipation with each interaction.
“The uniqueness of this stepwise mechanism is that each crack’s growth is regulated, preventing a rapid, catastrophic failure,” Gupta explained. “Instead of collapsing all at once, the material endures gradual damage, significantly increasing its toughness.”
In contrast to conventional methods that strengthen cement materials by incorporating fibers or plastics, the Princeton team’s strategy focuses on geometric design. By altering the material’s structure itself, they achieve substantial improvements in toughness without requiring additional materials.
Besides enhancing fracture toughness, the researchers formulated a new way to measure the degree of disorder in materials, which is crucial for effective design. Utilizing principles from statistical mechanics, they introduced parameters to quantify disorder within architected materials, allowing them to create a numerical framework that accurately reflects the architecture’s degree of disorder.
The team explained that their new framework offers a more precise depiction of material arrangements, moving beyond the simple classifications of ordered versus random to embrace a continuum. Moini clarified that their study distinguishes between irregularity and perturbation compared to statistical disorder, like those observed in Voronoi tessellation and perturbation methodologies.
“This framework equips us with a robust tool for designing materials with a customized degree of disorder,” Moini noted. “Leveraging advanced fabrication techniques like additive manufacturing can further facilitate creating more disordered yet mechanically advantageous structures, allowing us to scale these tubular designs for applications in civil infrastructure using concrete.”
Recently, the research team has also pioneered techniques that allow precise creations through robotics and additive manufacturing. By applying these methods to new architectural designs and mixtures of hard or soft materials within the tubes, they aim to broaden the range of potential applications in construction materials.
“We’ve only scratched the surface of the possibilities,” Gupta remarked. “There are numerous factors to explore, including the application of disorder to the tubes’ size, shape, and orientation. These principles might also extend to other brittle materials, leading to the development of structures resistant to damage.”
